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136-406: The mammalian blastocyst hatches before implantating into the endometrial lining of the womb . Once implanted the embryo will continue its development through the next stages of gastrulation , neurulation , and organogenesis . Gastrulation is the formation of the three germ layers that will form all of the different parts of the body. Neurulation forms the nervous system , and organogenesis

272-448: A cell adhesion molecule on the surface of trophoblast cells. An in vitro model of implantation gave evidence to support the hypothesis that L-selectin mediates apposition of the blastocyst to the uterine epithelium by interacting with its ligands. Invasion is the further establishment of the blastocyst into the endometrium. The protrusions of trophoblast cells that adhere into the endometrium continue to proliferate and penetrate into

408-432: A homozygous embryo. The zebrafish embryos are transparent and they develop outside of the uterus, which allows scientists to study the details of development starting from fertilization and continuing throughout development. The zebrafish is also demonstrably similar to mammalian models and humans in toxicity testing, and exhibits a diurnal sleep cycle with similarities to mammalian sleep behavior. However, zebrafish are not

544-407: A seed . Other seed components include the endosperm , which is tissue rich in nutrients that will help support the growing plant embryo, and the seed coat, which is a protective outer covering. The first cell division of a zygote is asymmetric , resulting in an embryo with one small cell (the apical cell) and one large cell (the basal cell). The small, apical cell will eventually give rise to most of

680-605: A solute carrier that appeared to be required for melanin production, and confirmed its function with a Morpholino knockdown. The orthologous gene was then characterized in humans and a one base pair difference was found to strongly segregate fair-skinned Europeans and dark-skinned Africans. Zebrafish with the nacre mutation have since been bred with fish with a roy orbison (roy) mutation to make Casper strain fish that have no melanophores or iridophores, and are transparent into adulthood. These fish are characterized by uniformly pigmented eyes and translucent skin. Transgenesis

816-428: A syncytium known as the syncytiotrophoblast . Between this layer and the blastocyst lies the cytotrophoblast . When the syncytiotrophoblast reaches the basal membrane beneath the decidual cells , it dislodges them to further invade into the uterine stroma. Dislodging is accomplished by degrading the cell adhesion molecules (CAMs) that link the decidual cells, and the associated extracellular matrix . Degradation

952-413: A daily basis. Females are able to spawn at intervals of two to three days, laying hundreds of eggs in each clutch . Upon release, embryonic development begins; in absence of sperm, growth stops after the first few cell divisions. Fertilized eggs almost immediately become transparent, a characteristic that makes D. rerio a convenient research model species . Sex determination of common laboratory strains

1088-497: A dedicated online database of genetic, genomic, and developmental information, the Zebrafish Information Network (ZFIN). The Zebrafish International Resource Center (ZIRC) is a genetic resource repository with 29,250 alleles available for distribution to the research community. D. rerio is also one of the few fish species to have been sent into space . Research with D. rerio has yielded advances in

1224-407: A group, males will mate with whichever females they can find. What attracts females is not currently understood. The presence of plants, even plastic plants, also apparently encourages spawning. Exposure to environmentally relevant concentrations of diisononyl phthalate (DINP), commonly used in a large variety of plastic items, disrupt the endocannabinoid system and thereby affect reproduction in

1360-597: A laboratory animal was pioneered by the American molecular biologist George Streisinger and his colleagues at the University of Oregon in the 1970s and 1980s; Streisinger's zebrafish clones were among the earliest successful vertebrate clones created. Its importance has been consolidated by successful large-scale forward genetic screens (commonly referred to as the Tübingen/Boston screens). The fish has

1496-418: A limited timeframe known as the window of implantation when the uterus is at its most receptive. There are two stages of migration involved in implantation, the first is the migration of the zygote , and the second is the migration of the trophoblast . Fertilization of the oocyte, takes place in the ampulla of the fallopian tube . Cilia on the lining of the tube move the zygote in its migration towards

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1632-454: A mature animal can trace their origin back to one of these layers. For example, the ectoderm will give rise to the skin epidermis and the nervous system, the mesoderm will give rise to the vascular system, muscles, bone, and connective tissues, and the endoderm will give rise to organs of the digestive system and epithelium of the digestive system and respiratory system. Many visible changes in embryonic structure happen throughout gastrulation as

1768-460: A new strain of zebrafish, named Casper, whose adult bodies had transparent skin. This allows for detailed visualization of cellular activity, circulation, metastasis and many other phenomena. In 2019 researchers published a crossing of a prkdc and a IL2rga strain that produced transparent, immunodeficient offspring, lacking natural killer cells as well as B - and T-cells . This strain can be adapted to 37 °C (99 °F) warm water and

1904-592: A number of commercially available global platforms exist for analysis of both gene expression by microarrays and promoter regulation using ChIP-on-chip . The Wellcome Trust Sanger Institute started the zebrafish genome sequencing project in 2001, and the full genome sequence of the Tuebingen reference strain is publicly available at the National Center for Biotechnology Information (NCBI)'s Zebrafish Genome Page . The zebrafish reference genome sequence

2040-628: A potential avenue for preventing disease; however, this has been met with widespread condemnation from the scientific community. ART techniques are also used to improve the profitability of agricultural animal species such as cows and pigs by enabling selective breeding for desired traits and/or to increase numbers of offspring. For example, when allowed to breed naturally, cows typically produce one calf per year, whereas IVF increases offspring yield to 9–12 calves per year. IVF and other ART techniques, including cloning via interspecies somatic cell nuclear transfer (iSCNT), are also used in attempts to increase

2176-460: A sex-specific manner. Zebrafish feeding practices vary significantly across different developmental stages, reflecting their changing nutritional needs. For newly hatched larvae, which begin feeding at approximately 5 days post-fertilization (dpf), small live prey such as Paramecium or rotifers are commonly used until they reach 9–15 dpf. This early diet is crucial for their growth and survival, as these small organisms provide essential nutrients. As

2312-490: A sliding bottom insert, which reduces the depth of the pool to simulate the shore of a river. Zebrafish spawn best in the morning due to their Circadian rhythms . Researchers have been able to collect 10,000 embryos in 10 minutes using this method. In particular, one pair of adult fish is capable of laying 200–300 eggs in one morning in approximately 5 to 10 at time. Male zebrafish are furthermore known to respond to more pronounced markings on females, i.e., "good stripes", but in

2448-412: A specialised protein , known as fibroblast growth factor , to ensure their spinal cords heal without glial scarring after injury. In addition, hair cells of the posterior lateral line have also been found to regenerate following damage or developmental disruption. Study of gene expression during regeneration has allowed for the identification of several important signaling pathways involved in

2584-437: A study was published about zebrafishes' capacity for episodic memory . The individuals showed a capacity to remember context with respect to objects, locations and occasions (what, when, where). Episodic memory is a capacity of explicit memory systems, typically associated with conscious experience . The Mauthner cells integrate a wide array of sensory stimuli to produce the escape reflex . Those stimuli are found to include

2720-512: A universally ideal research model; there are a number of disadvantages to their scientific use, such as the absence of a standard diet and the presence of small but important differences between zebrafish and mammals in the roles of some genes related to human disorders. Zebrafish have the ability to regenerate their heart and lateral line hair cells during their larval stages. The cardiac regenerative process likely involves signaling pathways such as Notch and Wnt ; hemodynamic changes in

2856-422: Is 16,596 base pairs. This is within 100 base pairs of other related species of fish, and it is notably only 18 pairs longer than the goldfish ( Carassius auratus ) and 21 longer than the carp ( Cyprinus carpio ). Its gene order and content are identical to the common vertebrate form of mitochondrial DNA. It contains 13 protein -coding genes and a noncoding control region containing the origin of replication for

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2992-562: Is a freshwater fish belonging to the minnow family ( Cyprinidae ) of the order Cypriniformes . Native to South Asia, it is a popular aquarium fish , frequently sold under the trade name zebra danio (and thus often called a " tropical fish " although it is both tropical and subtropical ). The zebrafish is an important and widely used vertebrate model organism in scientific research, particularly developmental biology , but also gene function, oncology , teratology , and drug development , in particular pre-clinical development . It

3128-493: Is a popular approach to study the function of genes in zebrafish. Construction of transgenic zebrafish is rather easy by a method using the Tol2 transposon system. Tol2 element which encodes a gene for a fully functional transposase capable of catalyzing transposition in the zebrafish germ lineage. Tol2 is the only natural DNA transposable element in vertebrates from which an autonomous member has been identified. Examples include

3264-508: Is a spotted colour morph of the zebrafish which arose due to a pigment mutation. Xanthistic forms of both the zebra and leopard pattern, along with long-finned strains, have been obtained via selective breeding programs for the aquarium trade. Various transgenic and mutant strains of zebrafish were stored at the China Zebrafish Resource Center (CZRC), a non-profit organization, which was jointly supported by

3400-411: Is a transmembrane mucin expressed at the apical surface of uterine epithelia. This mucin prevents the blastocyst from implanting in an undesired located on the epithelium. Thus, MUC-16 inhibits cell-cell adhesion. Its removal during pinopode formation has been shown to facilitate trophoblast invasion in vitro . The identity of the molecules on the trophoblast and the endometrial epithelia that mediate

3536-405: Is able to integrate into most cells of the embryo. A known problem with gene knockdowns is that, because the genome underwent a duplication after the divergence of ray-finned fishes and lobe-finned fishes , it is not always easy to silence the activity of one of the two gene paralogs reliably due to complementation by the other paralog. Despite the complications of the zebrafish genome ,

3672-507: Is achieved by the secretion of tumor necrosis factor-alpha from the syncytiotrophoblast, which inhibits the expression of CAMs and beta-catenin . The extracellular matrix is degraded by metalloproteinases such as collagenases , gelatinases and matrix metalloproteinases , and by serine proteases . The collagenases digest types I , II , III , VII and X collagen . The gelatinases exist in two forms; one digesting Type-IV collagen and one digesting gelatin . The extracellular matrix

3808-428: Is also notable for its regenerative abilities, and has been modified by researchers to produce many transgenic strains. The zebrafish is a derived member of the genus Brachydanio , of the family Cyprinidae . It has a sister-group relationship with Danio aesculapii . Zebrafish are also closely related to the genus Devario , as demonstrated by a phylogenetic tree of close species. The zebrafish

3944-658: Is annotated as part of the Ensembl project , and is maintained by the Genome Reference Consortium . In 2009, researchers at the Institute of Genomics and Integrative Biology in Delhi, India, announced the sequencing of the genome of a wild zebrafish strain, containing an estimated 1.7 billion genetic letters. The genome of the wild zebrafish was sequenced at 39-fold coverage. Comparative analysis with

4080-492: Is called the Arias-Stella phenomenon , after the pathologist Javier Arias-Stella . secreted by the uterine glands Not only the lining of the uterus transforms, but the secretion from its glands changes. This change is induced by increased levels of progesterone from the corpus luteum . The target of the secretions is the embryoblast, and has several functions on it. The embryo spends approximately 72 hours in

4216-402: Is considered finished when a bone called the cleithrum becomes visible. In animals that hatch from an egg, such as birds, a young animal is typically no longer referred to as an embryo once it has hatched. In viviparous animals (animals whose offspring spend at least some time developing within a parent's body), the offspring is typically referred to as an embryo while inside of the parent, and

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4352-405: Is degraded by serine endopeptidases and metalloproteinases . The syncytiotrophoblast can then invade into the endometrium taking the embryo with it where it becomes embedded. Eventually, the syncytiotrophoblast comes into contact with maternal blood and forms chorionic villi – the beginning of placentation . Following invasion, the breach in the uterine epithelium made by the blastocyst's entry

4488-517: Is during the early stages of pregnancy, during implantation. Its function as a surrounding tissue is replaced by the definitive placenta . However, some elements of the decidualization remain throughout pregnancy. The compacta and spongiosa layers are still observable beneath the decidua in pregnancy. The glands of the spongiosa layer continue to secrete during the first trimester, when they degenerate. However, before that disappearance, some glands secrete unequally much. This phenomenon of hypersecretion

4624-416: Is enhanced by progesterone , and inhibited by estrogens . During the window of implantation, cell to cell adhesion is inhibited by MUC1 a cell surface glycoprotein, belonging to the glycocalyx . The pinopodes are taller than the microvilli and protrude through the glycocalyx enabling direct contact with the adhering trophoblast. The most important attribute of pinopodes is this removal of glycoproteins from

4760-471: Is fertilized zinc is released in a zinc spark which promotes changes that include the hardening of the zona pellucida preventing polyspermy . Bleeding and spotting are common during the luteal phase of the menstrual cycle, and early stages of pregnancy, but are unrelated to implantation. Implantation bleeding occurs between 7 and 14 days after fertilization, and is a small amount of light vaginal bleeding or spotting that can occur in early pregnancy due to

4896-450: Is further facilitated by synthesis of matrix substances, adhesion molecules and surface receptors for the matrix substances. Reproduction in humans is not very efficient. Only around 30% of natural conceptions result in successful pregnancies. Of the failed pregnancies around 85% are due to implantation failure. Implantation failure is considered to be caused by inadequate uterine receptivity in two-thirds of cases, and by problems with

5032-546: Is linked to male infertility. The researchers cloned oestrogen-sensitive genes and injected them into the fertile eggs of zebrafish. The modified fish turned green if placed into water that was polluted by oestrogen. In 2015, researchers at Brown University discovered that 10% of zebrafish genes do not need to rely on the U2AF2 protein to initiate RNA splicing . These genes have the DNA base pairs AC and TG as repeated sequences at

5168-566: Is more local, with scattered records from the Western and Eastern Ghats regions. It has frequently been said to occur in Myanmar (Burma), but this is entirely based on pre-1930 records and likely refers to close relatives only described later, notably Danio kyathit . Likewise, old records from Sri Lanka are highly questionable and remain unconfirmed. Zebrafish have been introduced to California, Connecticut, Florida and New Mexico in

5304-818: Is native to freshwater habitats in South Asia where it is found in India, Pakistan, Bangladesh, Nepal and Bhutan. The northern limit is in the South Himalayas , ranging from the Sutlej river basin in the Pakistan–India border region to the state of Arunachal Pradesh in northeast India. Its range is concentrated in the Ganges and Brahmaputra River basins, and the species was first described from Kosi River (lower Ganges basin) of India. Its range further south

5440-424: Is no longer considered an embryo after birth or exit from the parent. However, the extent of development and growth accomplished while inside of an egg or parent varies significantly from species to species, so much so that the processes that take place after hatching or birth in one species may take place well before those events in another. Therefore, according to one textbook, it is common for scientists to interpret

5576-420: Is of importance in the embryo-uterus crosstalk in regulating the timing of embryonic implantation and uterine receptivity. Adequate concentrations of AEA that are neither too high or too low, are needed for successful implantation. IL-6 and FAAH are both crucial for uterine receptivity and together with AEA there is seen to be a link with adequate endometrial thickness that sustains pregnancy. During adhesion

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5712-549: Is sealed by a fibrin plug . The fibrin plug is a coagulation of a blood clot and cellular debris. Extravillous trophoblasts are cells from the invading villi that migrate into the myometrium of the mother's uterus. These cells remodel the spiral arteries to improve and secure maternal blood flow to the growing embryo. There is also evidence that this process occurs with the uterine veins, stabilizing them to improve drainage of fetal blood and metabolic wastes. Trophoblasts have also been documented to migrate into various tissues in

5848-467: Is that decidualization occurs at a higher degree in conception cycles than in nonconception cycles. Furthermore, similar changes are observed when giving stimuli mimicking the natural invasion of the embryo. The embryo releases serine proteases which causes the epithelial cell membrane to depolarize and activates the epithelial sodium channel . This triggers an influx of calcium ions (Ca ) and phosphorylation of CREB. Phosphorylation of CREB upregulates

5984-459: Is the development of all the various tissues and organs of the body. A newly developing human is typically referred to as an embryo until the ninth week after conception, when it is then referred to as a fetus . In other multicellular organisms, the word "embryo" can be used more broadly to any early developmental or life cycle stage prior to birth or hatching . First attested in English in

6120-461: Is the next phase of embryonic development, and involves the development of two or more layers of cells (germinal layers). Animals that form two layers (such as Cnidaria ) are called diploblastic, and those that form three (most other animals, from flatworms to humans) are called triploblastic. During gastrulation of triploblastic animals, the three germinal layers that form are called the ectoderm , mesoderm , and endoderm . All tissues and organs of

6256-407: Is to reduce gene expression or modify splicing using Morpholino antisense technology. Morpholino oligonucleotides (MO) are stable, synthetic macromolecules that contain the same bases as DNA or RNA; by binding to complementary RNA sequences, they can reduce the expression of specific genes or block other processes from occurring on RNA. MO can be injected into one cell of an embryo after

6392-514: Is unstable to shear stress . Apposition is also reversible allowing repositioning of the blastocyst in the uterus. Adhesion is a much stronger attachment to the endometrium than the loose apposition. The trophoblasts adhere by penetrating the endometrium, with protrusions of trophoblast cells. This adhering activity is by microvilli that are on the trophoblast. The trophoblast have binding fiber connections, laminin, collagen type IV, and integrins that assist in this adhesion process. Mucin-16

6528-735: Is used for addressing fertility concerns in humans and other animals, and for selective breeding in agricultural species. Between the years 1987 and 2015, ART techniques including in vitro fertilization (IVF) were responsible for an estimated one million human births in the United States alone. Other clinical technologies include preimplantation genetic diagnosis (PGD), which can identify certain serious genetic abnormalities, such as aneuploidy , prior to selecting embryos for use in IVF. Some have proposed (or even attempted - see He Jiankui affair ) genetic editing of human embryos via CRISPR-Cas9 as

6664-407: Is usually some vegetation, either submerged or overhanging from the banks, and the bottom is sandy, muddy or silty, often mixed with pebbles or gravel. In surveys of zebrafish locations throughout much of its Bangladeshi and Indian distribution, the water had a near-neutral to somewhat basic pH and mostly ranged from 16.5 to 34 °C (61.7–93.2 °F) in temperature. One unusually cold site

6800-414: Is very rapid, and its embryos are relatively large, robust, and transparent, and able to develop outside their mother. Furthermore, well-characterized mutant strains are readily available. Other advantages include the species' nearly constant size during early development, which enables simple staining techniques to be used, and the fact that its two-celled embryo can be fused into a single cell to create

6936-586: The Carp (including Grass Carp, Ctenopharyngodon idella ) and Milkfish ( Chanos chanos ) which are quite close, both being in the Cyprinidae . However it should also be noted that Danio consistently proves to be a useful model for mammals in many cases and there is dramatically more genetic distance between them than between Danio and any farmed fish. In a glucocorticoid receptor -defective mutant with reduced exploratory behavior , fluoxetine rescued

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7072-634: The Ministry of Science and Technology of China and the Chinese Academy of Sciences . The Zebrafish Information Network ( ZFIN ) provides up-to-date information about current known wild-type (WT) strains of D. rerio , some of which are listed below. Hybrids between different Danio species may be fertile: for example, between D. rerio and D. nigrofasciatus . D. rerio is a common and useful scientific model organism for studies of vertebrate development and gene function. Its use as

7208-401: The anal fin origin. The zebrafish can reach up to 4–5 cm (1.6–2.0 in) in length, although they typically are 1.8–3.7 cm (0.7–1.5 in) in the wild with some variations depending on location. Its lifespan in captivity is around two to three years, although in ideal conditions, this may be extended to over five years. In the wild it is typically an annual species. In 2015,

7344-460: The blastocyst hatches, attaches, adheres, and invades into the endometrium of the female's uterus . Implantation is the first stage of gestation , and, when successful, the female is considered to be pregnant . An implanted embryo is detected by the presence of increased levels of human chorionic gonadotropin (hCG) in a pregnancy test . The implanted embryo will receive oxygen and nutrients in order to grow. For implantation to take place

7480-421: The blastocyst nearer to the endometrium and immobilize it. During this stage the blastocyst can still be eliminated by being flushed out of the uterus. Successful implantation is co-dependent on the viability of the embryo, and the receptivity of the uterus. A critical involved factor is the developmental synchrony between the embryo and the uterus. The synchrony gives a short period of receptivity known as

7616-491: The cytoskeleton of decidual cells . This, in turn, dislodges the decidual cells from their connection to the underlying basal lamina , which enables the blastocyst to perform the succeeding invasion. The window of implantation is a limited timeframe for the successful attachment of the blastocyst. In humans uterine receptivity is optimum on days 20-24 of the secretory phase of the menstrual cycle when luteinizing hormone levels are at their peak. The crosstalk between

7752-459: The epidermis or outer covering of a plant, ground tissue will give rise to inner plant material that functions in photosynthesis , resource storage, and physical support, and vascular tissue will give rise to connective tissue like the xylem and phloem that transport fluid, nutrients, and minerals throughout the plant. In heart stage, one or two cotyledons (embryonic leaves) will form. Meristems (centers of stem cell activity) develop during

7888-422: The formation of the placenta . They are: migration and hatching, pre-contact, attachment, adhesion, and invasion. The first four stages are similar across the species with the process of invasion being variable. These three stages of apposition , attachment , and invasion are also alternatively termed contact (apposition), adhesion (attachment), and penetration (invasion), and can only take place during

8024-469: The lateral line signals by McHenry et al. 2009 and visual signals consistent with looming objects by Temizer et al. 2015, Dunn et al. 2016, and Yao et al. 2016. The approximate generation time for Danio rerio is three months. A male must be present for ovulation and spawning to occur. Zebrafish are asynchronous spawners and under optimal conditions (such as food availability and favorable water parameters) can spawn successfully frequently, even on

8160-436: The organ-on-a-chip program, an endometrium-on-a-chip has been developed to model the functioning of the endometrium that could more clearly identify causes of implantation failure. Organoids have also been developed to model the endometrium and its role in implantation. In women with more than three implantation failures in assisted reproduction , a review of several small randomized controlled studies estimated that

8296-453: The pre-implantation embryo . In humans, following the stage of hatching that takes place around four to five days after fertilization , the process of implantation begins. By the end of the first week, the blastocyst is superficially attached to the uterine endometrium. By the end of the second week, implantation has completed. There are five recognized stages of implantation in mammals, including two pre-implantation stages that precede

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8432-432: The replication crisis in biomedical research. Genetic differences in wild-type lines among populations maintained at different research institutions have been demonstrated using both Single-nucleotide polymorphisms and microsatellite analysis. Due to their fast and short life cycles and relatively large clutch sizes, D. rerio or zebrafish are a useful model for genetic studies. A common reverse genetics technique

8568-410: The 32-cell stage, reducing gene expression in only cells descended from that cell. However, cells in the early embryo (less than 32 cells) are permeable to large molecules, allowing diffusion between cells. Guidelines for using Morpholinos in zebrafish describe appropriate control strategies. Morpholinos are commonly microinjected in 500pL directly into 1–2 cell stage zebrafish embryos. The morpholino

8704-487: The United States, presumably by deliberate release by aquarists or by escape from fish farms . The New Mexico population had been extirpated by 2003 and it is unclear if the others survive, as the last published records were decades ago. Elsewhere the species has been introduced to Colombia and Malaysia. Zebrafish typically inhabit moderately flowing to stagnant clear water of quite shallow depth in streams, canals, ditches, oxbow lakes , ponds and rice paddies . There

8840-399: The absence of an immune system makes the use of patient derived xenografts possible. In January 2013, Japanese scientists genetically modified a transparent zebrafish specimen to produce a visible glow during periods of intense brain activity. In January 2007, Chinese researchers at Fudan University genetically modified zebrafish to detect oestrogen pollution in lakes and rivers, which

8976-423: The activity of proteases, including MMPs , plasminogen activators, and cathepsins . It is unknown whether the cytokines involved in hatching are pro-inflammatory or anti-inflammatory, or which proteases are involved. However, it is well accepted that the pro-inflammatory cytokines are dominant during implantation. Cytokines are also present in the uterine milk which might regulate the development and function of

9112-434: The artificial interaction produced between LEF1 and Catenin beta-1 /β-catenin/ CTNNB1 . Dorsky et al. 2002 investigated the developmental role of Wnt by transgenically expressing a Lef1/β-catenin reporter. There are well-established protocols for editing zebrafish genes using CRISPR-Cas9 and this tool has been used to generate genetically modified models. In 2008, researchers at Boston Children's Hospital developed

9248-443: The blastocyst are; To enable implantation, the uterus goes through changes in order to be able to receive the conceptus. Receptivity includes changes to endometrial cells in the formation of pinopodes that help to absorb uterine fluid; changes in the thickness of the endometrium and its blood supply development, and the formation of the decidua . Collectively these changes are known as plasma membrane transformation , and bring

9384-414: The blastocyst but there is no evidence to support their involvement in hatching. Leukemia inhibitory factor (LIF) is a pro-inflammatory cytokine expressed in the endometrium during the luteal phase of the menstrual cycle , with the highest expression seen during the window of implantation. LIF plays a role in adhesion and invasion. Assisted zona hatching may take place in assisted reproduction, where

9520-571: The blastocyst itself are essential for the breakdown of the egg-coat. Mechanisms in the latter are indicated by the fact that the zona pellucida remains intact if an unfertilized egg is placed in the uterus under the same conditions. Among the known molecular regulators that promote hatching are predominantly proteases that are stimulated by various growth factors . The blastocyst also produces cytokines , both pro-inflammatory and anti-inflammatory, that have crucial roles during implantation and other stages of pregnancy. Both types of cytokines modulate

9656-564: The blastocyst penetrating the lining of the uterus during implantation. By day 13 the penetration site in the endometrium has usually been closed by a fibrin plug but increased blood flow into the syncytiotrophoblast spaces can sometimes cause bleeding at that site. Implantation bleeding may be accompanied by symptoms such as cramping, nausea, breast tenderness, and headaches. Implantation bleeding can be distinguished from period bleeding by color, clotting, strength and duration of flow. Zebrafish The zebrafish ( Danio rerio )

9792-435: The cell surfaces of the uterine epithelial cells. MUC16 has also been shown to disappear from the cell surfaces with the development of the pinopodes. Some studies have reported that pinopodes entrap cilia, which prevents embryo movement, and during implantation allows close contact and adherence of the embryo. Pinopodes bring uterine fluid and its macromolecules into the cells by the process of endocytosis . This decreases

9928-441: The cells such as RNAs and proteins actively promote key developmental processes such as gene expression, cell fate specification, and polarity. Before implanting into the uterine wall the embryo is sometimes known as the pre-implantation embryo or pre-implantation conceptus . Sometimes this is called the pre-embryo a term employed to differentiate from an embryo proper in relation to embryonic stem cell discourses. Gastrulation

10064-493: The cells that make up the different germ layers migrate and cause the previously round embryo to fold or invaginate into a cup-like appearance. Past gastrulation, an embryo continues to develop into a mature multicellular organism by forming structures necessary for life outside of the womb or egg. As the name suggests, organogenesis is the stage of embryonic development when organs form. During organogenesis, molecular and cellular interactions prompt certain populations of cells from

10200-402: The creation of a zygote, a single cell resulting from the fusion of gametes (e.g. egg and sperm). The development of a zygote into a multicellular embryo proceeds through a series of recognizable stages, often divided into cleavage, blastula, gastrulation, and organogenesis. Cleavage is the period of rapid mitotic cell divisions that occur after fertilization. During cleavage, the overall size of

10336-452: The cross-talk is conveyed by receptor - ligand -interactions, both integrin-matrix and proteoglycan ones. Proteoglycan receptors are found on the surface of the decidua, and their counterparts, the proteoglycans, are found around the trophoblast cells of the blastocyst. This ligand-receptor system is also present just at the implantation window. The blastocyst signals to the endometrium to adapt further to its presence, for example by changes in

10472-783: The damaged heart are sensed by ventricular endothelial cells and their associated cardiac cilia by way of the mechanosensitive ion channel TRPV4 , subsequently facilitating the Notch signaling pathway via KLF2 and activating various downstream effectors such as BMP-2 and HER2/neu . In 2011, the British Heart Foundation ran an advertising campaign publicising its intention to study the applicability of this ability to humans, stating that it aimed to raise £50 million in research funding. Zebrafish have also been found to regenerate photoreceptor cells and retinal neurons following injury, which has been shown to be mediated by

10608-424: The dedifferentiation and proliferation of Müller glia . Researchers frequently amputate the dorsal and ventral tail fins and analyze their regrowth to test for mutations. It has been found that histone demethylation occurs at the site of the amputation, switching the zebrafish's cells to an "active", regenerative, stem cell-like state. In 2012, Australian scientists published a study revealing that zebrafish use

10744-492: The detrimental effects of inbreeding depression . Inbreeding depression is predominantly caused by the homozygous expression of recessive deleterious alleles. For zebrafish, inbreeding depression might be expected to be more severe in stressful environments, including those caused by anthropogenic pollution . Exposure of zebrafish to environmental stress induced by the chemical clotrimazole, an imidazole fungicide used in agriculture and in veterinary and human medicine, amplified

10880-456: The different germ layers to differentiate into organ-specific cell types. For example, in neurogenesis, a subpopulation of cells from the ectoderm segregate from other cells and further specialize to become the brain, spinal cord, or peripheral nerves. The embryonic period varies from species to species. In human development, the term fetus is used instead of embryo after the ninth week after conception, whereas in zebrafish , embryonic development

11016-448: The effects of inbreeding on key reproductive traits. Embryo viability was significantly reduced in inbred exposed fish and there was a tendency for inbred males to sire fewer offspring. Zebrafish are common models for research into fish farming , including pathogens and parasites causing yield loss or spreading to adjacent wild populations. This usefulness is less than it might be due to Danio ' s taxonomic distance from

11152-457: The embryo and the endometrium takes place during this time. The endothelial epithelial cells lining the uterus are the first cells to detect signals from the blastocyst, and they are transduced into downstream signalling pathways. In humans the window of implantation is only available for 24–36 hours. The endometrial microbiome has been indicated as having an important role in successful implantation in controlling endometrial cell function, and

11288-404: The embryo begins its existence attached to the inside of the archegonium on a parental gametophyte from which the egg cell was generated. The inner wall of the archegonium lies in close contact with the "foot" of the developing embryo; this "foot" consists of a bulbous mass of cells at the base of the embryo which may receive nutrition from its parent gametophyte. The structure and development of

11424-424: The embryo does not change, but the size of individual cells decrease rapidly as they divide to increase the total number of cells. Cleavage results in a blastula. Depending on the species, a blastula or blastocyst stage embryo can appear as a ball of cells on top of yolk, or as a hollow sphere of cells surrounding a middle cavity . The embryo's cells continue to divide and increase in number, while molecules within

11560-695: The embryo from being rejected by the mother, trigger the final decidualization and prevent menstruation. Preimplantation factor is secreted by trophoblast cells ahead of placenta formation. The embryo differs from the cells of the mother, and would be rejected as a parasite by the immune system of the mother if it did not secrete, immunosuppressive agents. Such agents include platelet-activating factor , human chorionic gonadotropin , early pregnancy factor , prostaglandin E2 , interleukin-1 alpha , interleukin 6 , interferon -alpha, leukemia inhibitory factor and colony-stimulating factor . Other factors secreted by

11696-580: The embryo itself in the other third. Most IVF procedures fail because of implantation failure accounting for almost half of all pregnancy failures. Inadequate uterine receptivity may be caused by abnormal cytokine and hormonal signaling as well as epigenetic alterations . Recurrent implantation failure is a cause of female infertility . Therefore, pregnancy rates can be improved by optimizing endometrial receptivity for implantation. Evaluation of implantation markers may help to predict pregnancy outcome and detect occult implantation deficiency. As part of

11832-424: The endometrium using gelatinases A ( MMP-2 ), and B ( MMP-9 ). Trophoblasts invade the uterus attempting to reach maternal blood supply, for setting up the foundation for fetal blood flow. They also secrete preimplantation factor , a peptide that helps their invasion and placenta formation. As these trophoblasts penetrate, they fuse with their neighbours, terminally differentiating into a multinucleated tissue,

11968-466: The endometrium. Human chorionic gonadotropin (hCG) is an autocrine growth factor for the blastocyst, while insulin-like growth factor 2 , stimulates its invasiveness. Human chorionic gonadotropin not only acts as an immunosuppressive, but also signals to the mother that she is pregnant , preventing luteolysis of the corpus luteum and menstruation by sustaining the function of the corpus luteum. Secretions loosen decidual cells from each other, prevent

12104-457: The ends of each intron . On the 3'ss (3' splicing site), the base pairs adenine and cytosine alternate and repeat, and on the 5'ss (5' splicing site), their complements thymine and guanine alternate and repeat as well. They found that there was less reliance on U2AF2 protein than in humans, in which the protein is required for the splicing process to occur. The pattern of repeating base pairs around introns that alters RNA secondary structure

12240-420: The expression of COX2 , which leads to the release of prostaglandin E2 (PGE2) from epithelial cells. PGE2 acts on the stroma cells activating cAMP-related pathways in stromal cell leading to decidualization. The decidua can be organized into separate sections, although they have the same composition. After implantation the decidua remains, at least through the first trimester. However, its most prominent time

12376-441: The extraembryonic membranes ( fetal membranes ). The blastocyst is still enclosed in the egg-coat known as the zona pellucida , and for it to be able to implant into the uterine wall it must rid itself of this covering. This stage is known as zona hatching , and when there is sufficient dissolution the blastocyst is able to initiate the apposition stage of implantation. Lytic factors in the uterine cavity, as well as factors from

12512-503: The fields of developmental biology , oncology , toxicology , reproductive studies, teratology , genetics , neurobiology , environmental sciences , stem cell research, regenerative medicine , muscular dystrophies and evolutionary theory . As a model biological system, the zebrafish possesses numerous advantages for scientists. Its genome has been fully sequenced , and it has well-understood, easily observable and testable developmental behaviors. Its embryonic development

12648-409: The first species to be sold as GloFish (fluorescent colored fish). In late 2003, transgenic zebrafish that express green , red, and yellow fluorescent proteins became commercially available in the United States. The fluorescent strains are trade-named GloFish ; other cultivated varieties include "golden", "sandy", "longfin" and "leopard". The leopard danio, previously known as Danio frankei ,

12784-407: The function of the local immunity system that prevents pathogen growth. This is associated with the secretion of protective substances. Pinopodes are formed at the beginning of the window of implantation, and are found in many species. They are mushroom-like protrusions from the apical cell membrane of uterine epithelial cells. Pinopodes are formed by the swelling of these epithelial cells, and

12920-429: The fusing together of a number of microvilli, to reach a maximum size. They appear between day 19 and day 21 of gestational age , and are fully formed at day 20. This corresponds to a fertilization age of approximately five to seven days, which corresponds well with the time of implantation. Pinopodes only persist for a maximum of two days, and are seen as the ultrastructural markers of receptivity. Their development

13056-634: The heavy strand. In between a grouping of five tRNA genes, a sequence resembling vertebrate origin of light strand replication is found. It is difficult to draw evolutionary conclusions because it is difficult to determine whether base pair changes have adaptive significance via comparisons with other vertebrates' nucleotide sequences. T-boxes and homeoboxes are vital in Danio similarly to other vertebrates. The Bruce et al. team are known for this area, and in Bruce et al. 2003 & Bruce et al. 2005 uncover

13192-548: The initial interaction between the two remain unidentified. However, a number of research groups have proposed that MUC1 , a member of the mucin family of glycosylated proteins, is involved. MUC1 is a transmembrane glycoprotein expressed at the apical surface of endometrial epithelial cells during the window of implantation in humans and has been shown to be differentially expressed between fertile and infertile subjects during this time. MUC1 displays carbohydrate moieties on its extracellular domain that are ligands of L-selectin ,

13328-551: The largest collection of plant reproductive tissue, with more than a million samples stored at −18 °C (0 °F). Fossilized animal embryos are known from the Precambrian , and are found in great numbers during the Cambrian period. Even fossilized dinosaur embryos have been discovered. Zona hatching Implantation , also known as nidation , is the stage in the mammalian embryonic development in which

13464-411: The larvae develop, from 15 dpf onwards, they are typically transitioned to a diet that includes brine shrimp nauplii and dry feeds, which are more nutritionally balanced and easier to manage in laboratory settings. For larvae aged 25 dpf, feeding rates can range from 50% to 300% of their body weight (BW) per day, depending on their size and growth requirements. As zebrafish grow into juveniles (30–90 dpf),

13600-453: The mid-14c., the word embryon derives from Medieval Latin embryo , itself from Greek ἔμβρυον ( embruon ), lit. "young one", which is the neuter of ἔμβρυος ( embruos ), lit. "growing in", from ἐν ( en ), "in" and βρύω ( bruō ), "swell, be full"; the proper Latinized form of the Greek term would be embryum . In animals, fertilization begins the process of embryonic development with

13736-709: The most common aquaculture species. Because the most common are salmonids and cod in the Protacanthopterygii and sea bass , sea bream , tilapia , and flatfish , in the Percomorpha , zebrafish results may not be perfectly applicable. Various other models – Goldfish ( Carassius auratus ), Medaka ( Oryzias latipes ), Stickleback ( Gasterosteus aculeatus ), Roach ( Rutilus rutilus ), Pufferfish ( Takifugu rubripes ), Swordtail ( Xiphophorus hellerii ) – are less used normally but would be closer to particular target species. The only exception are

13872-460: The mother – will prevent initiation of the mesoderm or endoderm development processes in this species.) In 1999, the nacre mutation was identified in the zebrafish ortholog of the mammalian MITF transcription factor. Mutations in human MITF result in eye defects and loss of pigment, a type of Waardenburg Syndrome . In December 2005, a study of the golden strain identified the gene responsible for its unusual pigmentation as SLC24A5 ,

14008-516: The mother. Due to this, trophoblasts have been implicated in a phenomenon known as fetomaternal microchimerism where fetal cells establish cell lines in maternal tissues. Pre-implantation blastocysts have been shown to be capable of secreting growth factors, hormones and trypsin-like proteases to participate in the hatching process. During invasion the blastocyst secretes factors for a multitude of purposes. It secretes several autocrine factors, targeting itself and stimulating it to further invade

14144-467: The normal exploratory behavior. This demonstrates relationships between glucocorticoids, fluoxetine, and exploration in this fish. Zebrafish have been used as a model for studying DNA repair pathways. Embryos of externally fertilized fish species, such as zebrafish during their development, are directly exposed to environmental conditions such as pollutants and reactive oxygen species that may cause damage to their DNA . To cope with such DNA damages,

14280-445: The numbers of endangered or vulnerable species, such as Northern white rhinos , cheetahs , and sturgeons . Cryoconservation of genetic resources involves collecting and storing the reproductive materials, such as embryos, seeds, or gametes, from animal or plant species at low temperatures in order to preserve them for future use. Some large-scale animal species cryoconservation efforts include " frozen zoos " in various places around

14416-428: The opportunity, adults eat hatchlings, which may be protected by separating the two groups with a net, breeding box or separate tank. In captivity, zebrafish live approximately forty-two months. Some captive zebrafish can develop a curved spine. They can range from a few centimeters to a few inches, and provide movement in a freshwater fish tank. The zebra danio was also used to make genetically modified fish and were

14552-429: The process, such as Wnt signaling and Fibroblast growth factor . In probing disorders of the nervous system, including neurodegenerative diseases, movement disorders, psychiatric disorders and deafness, researchers are using the zebrafish to understand how the genetic defects underlying these conditions cause functional abnormalities in the human brain, spinal cord and sensory organs. Researchers have also studied

14688-584: The recommended feeding rate decreases to about 6–8% of their BW per day, with a focus on high-quality dry feeds that meet their protein and energy needs. Upon reaching adulthood (over 90 dpf), zebrafish typically require a feeding rate of around 5% of their BW per day. Throughout these stages, it is essential to adjust the particle size of the feed: less than 100 μm for newly hatched larvae, 100–200 μm for those between 16 and 30 dpf, and larger particles for juveniles and adults. This structured approach to feeding not only supports optimal growth and health but also enhances

14824-551: The reliability of experimental outcomes in research settings Zebrafish are hardy fish and considered good for beginner aquarists. Their enduring popularity can be attributed to their playful disposition, as well as their rapid breeding, aesthetics, cheap price and broad availability. They also do well in schools or shoals of six or more, and interact well with other fish species in the aquarium. However, they are susceptible to Oodinium or velvet disease, microsporidia ( Pseudoloma neurophilia ), and Mycobacterium species. Given

14960-430: The rest of the embryo varies by group of plants. Since all land plants create embryos, they are collectively referred to as embryophytes (or by their scientific name, Embryophyta). This, along with other characteristics, distinguishes land plants from other types of plants, such as algae , which do not produce embryos. Embryos from numerous plant and animal species are studied in biological research laboratories across

15096-509: The role of two of these elements in oocytes of this species. By interfering via a dominant nonfunctional allele and a morpholino they find the T-box transcription activator Eomesodermin and its target mtx2 – a transcription factor – are vital to epiboly . (In Bruce et al. 2003 they failed to support the possibility that Eomesodermin behaves like Vegt . Neither they nor anyone else has been able to locate any mutation which – in

15232-410: The scope of embryology broadly as the study of the development of animals. Flowering plants ( angiosperms ) create embryos after the fertilization of a haploid ovule by pollen . The DNA from the ovule and pollen combine to form a diploid, single-cell zygote that will develop into an embryo. The zygote, which will divide multiple times as it progresses throughout embryonic development, is one part of

15368-406: The side of the body, which are reminiscent of a zebra's stripes, and which extend to the end of the caudal fin . Its shape is fusiform and laterally compressed, with its mouth directed upwards. The male is torpedo -shaped, with gold stripes between the blue stripes; the female has a larger, whitish belly and silver stripes instead of gold. Adult females exhibit a small genital papilla in front of

15504-414: The sides of the yolk (8 h panel) and begin forming a head and tail (16 h panel). The tail then grows and separates from the body (24 h panel). The yolk shrinks over time because the fish uses it for food as it matures during the first few days (72 h panel). After a few months, the adult fish reaches reproductive maturity (bottom panel). To encourage the fish to spawn, some researchers use a fish tank with

15640-448: The stromal cells that are always present in the endometrium, and make up a new layer, the decidua . The rest of the endometrium, in addition, expresses differences between the luminal and the basal sides. The luminal cells form the stratum compactum of the endometrium, in contrast to the basalolateral stratum spongiosum , which consists of the rather spongy stromal cells. Decidualization expands if pregnancy occurs, further developing

15776-495: The structures of the mature plant, such as the stem, leaves, and roots. The larger basal cell will give rise to the suspensor, which connects the embryo to the endosperm so that nutrients can pass between them. The plant embryo cells continue to divide and progress through developmental stages named for their general appearance: globular, heart, and torpedo. In the globular stage, three basic tissue types (dermal, ground, and vascular) can be recognized. The dermal tissue will give rise to

15912-524: The surface of the endometrium produces a kind of rounded cells, which cover the whole area toward the uterine cavity. This happens about 9 to 10 days after ovulation. These cells are called decidual cells , which emphasises that the whole layer of them is shed off in every menstruation if no pregnancy occurs, just as leaves of deciduous trees . The uterine glands, on the other hand, decrease in activity and degenerate around 8 to 9 days after ovulation in absence of pregnancy. The decidual cells originate from

16048-449: The torpedo stage, and will eventually produce many of the mature tissues of the adult plant throughout its life. At the end of embryonic growth, the seed will usually go dormant until germination. Once the embryo begins to germinate (grow out from the seed) and forms its first true leaf, it is called a seedling or plantlet. Plants that produce spores instead of seeds, like bryophytes and ferns , also produce embryos. In these plants,

16184-413: The type of trophoblast cells , and structures of the placenta across the different species of mammals. Of the five recognised stages of implantation including two pre-implantation stages that precede placentation , the first four are similar across the species. The five stages are migration and hatching, pre-contact, attachment, adhesion, and invasion. The two pre-implantation stages are associated with

16320-399: The underlying endometrium. Ultimately, the inner cell mass (also embryoblast), inside the trophoblast layer, is aligned closest to the decidua . If the inner cell mass is not aligned with the decidua at apposition, it has the ability to freely rotate within the trophoblast and achieve this alignment. Apposition is only a weak interaction of the trophectoderm with the uterine epithelium that

16456-403: The use of adjunct low molecular weight heparin improves live birth rate by approximately 80%. Luteal phase support can include the use of progesterone and human chorionic gonadotropin (hCG) to improve the chances of a successful implantation. Zinc is crucial in pre-conception, (and successful pregnancy), and its deficiency can lead to incompetent blastocyst development. Once an egg

16592-412: The uterine cavity before implanting. In that time, it cannot receive nourishment directly from the blood of the mother, and must rely on secreted nutrients into the uterine cavity, e.g. iron and fat-soluble vitamins. In addition to nourishment, the endometrium secretes several steroid -dependent proteins, important for growth and implantation. Cholesterol , and steroids are also secreted. Implantation

16728-456: The uterine glands, the zona compacta and the epithelium of decidual cells lining it. The decidual cells become filled with lipids and glycogen and take the polyhedral shape characteristic of decidual cells. Factors from the blastocyst also trigger the final formation of decidual cells into their proper form. In contrast, some decidual cells in the proximity of the blastocyst degenerate, providing nutrients for it. An indication of embryonic influence

16864-543: The uterus must become receptive. Uterine receptivity involves much cross-talk between the embryo and the uterus, initiating changes to the endometrium. This stage gives a synchrony that opens a window of implantation that enables successful implantation of a viable embryo. The endocannabinoid system plays a vital role in this synchrony in the uterus, influencing uterine receptivity, and embryo implantation. The embryo expresses cannabinoid receptors early in its development that are responsive to anandamide (AEA) secreted in

17000-476: The uterus. During this migration the zygote undergoes a number of cell divisions that creates a ball of 16 compacted blastomeres called a morula . The morula enters the uterus after three or four days, and as it does a cavity called the blastocoel is formed in the morula to produce the blastocyst . The blastocyst contains the inner cell mass that will go on to develop into the embryo proper, and an outer cell layer of trophoblasts that will develop into

17136-409: The uterus. AEA is produced at higher levels before implantation and is then down-regulated at the time of implantation. This signaling is of importance in the embryo-uterus crosstalk in regulating the timing of embryonic implantation and uterine receptivity. Adequate concentrations of AEA that are neither too high or too low, are needed for successful implantation. There is an extensive variation in

17272-474: The volume of the uterus, taking the walls closer to the blastocyst floating in it. Thus, the period of active pinopodes might limit the implantation window. Pinopodes continue to absorb fluid, removing most of it during the early stages of implantation. The endometrium increases thickness, becomes vascularized and its glands grow to be tortuous and boosted in their secretions. These changes reach their maximum about seven days after ovulation . Furthermore,

17408-412: The window of implantation, and involves much crosstalk between the blastocyst and the endometrium at this stage. The endocannabinoid system plays a vital role in this synchrony in the uterus, influencing uterine receptivity, and embryo implantation. The embryo expresses cannabinoid receptors early in its development that are responsive to anandamide (AEA) secreted in the uterus. This signaling

17544-845: The world to learn about topics such as stem cells , evolution and development , cell division , and gene expression . Examples of scientific discoveries made while studying embryos that were awarded the Nobel Prize in Physiology or Medicine include the Spemann-Mangold organizer , a group of cells originally discovered in amphibian embryos that give rise to neural tissues, and genes that give rise to body segments discovered in Drosophila fly embryos by Christiane Nüsslein-Volhard and Eric Wieschaus . Creating and/or manipulating embryos via assisted reproductive technology (ART)

17680-748: The world, including in the UK's Frozen Ark , the Breeding Centre for Endangered Arabian Wildlife (BCEAW) in the United Arab Emirates, and the San Diego Zoo Institute for Conservation in the United States. As of 2018, there were approximately 1,700 seed banks used to store and protect plant biodiversity, particularly in the event of mass extinction or other global emergencies. The Svalbard Global Seed Vault in Norway maintains

17816-460: The zebrafish reference genome revealed over 5 million single nucleotide variations and over 1.6 million insertion deletion variations. The zebrafish reference genome sequence of 1.4GB and over 26,000 protein coding genes was published by Kerstin Howe et al. in 2013. In October 2001, researchers from the University of Oklahoma published D. rerio's complete mitochondrial DNA sequence. Its length

17952-473: The zebrafish to gain new insights into the complexities of human musculoskeletal diseases, such as muscular dystrophy . Another focus of zebrafish research is to understand how a gene called Hedgehog , a biological signal that underlies a number of human cancers, controls cell growth. Inbred strains and traditional outbred stocks have not been developed for laboratory zebrafish, and the genetic variability of wild-type lines among institutions may contribute to

18088-409: The zona pellucida may be artificially pierced to facilitate hatching. Following zona hatching, the very first loose connection or contact between the blastocyst and the endometrium is called apposition. Apposition is usually made where there is a small crypt in the endometrium, and also where there has been enough breakdown of the zona pellucida to allow the blastocyst trophoblast to directly contact

18224-467: Was found in other teleosts , but not in tetrapods . This indicates that an evolutionary change in tetrapods may have led to humans relying on the U2AF2 protein for RNA splicing while these genes in zebrafish undergo splicing regardless of the presence of the protein. D. rerio has three transferrins , all of which cluster closely with other vertebrates . When close relatives mate, progeny may exhibit

18360-435: Was only 12.3 °C (54.1 °F) and another unusually warm site was 38.6 °C (101.5 °F), but the zebrafish still appeared healthy. The unusually cold temperature was at one of the highest known zebrafish locations at 1,576 m (5,171 ft) above sea level, although the species has been recorded to 1,795 m (5,889 ft). The zebrafish is named for the five uniform, pigmented, horizontal, blue stripes on

18496-445: Was shown to be a complex genetic trait, rather than to follow a simple ZW or XY system. The zebrafish embryo develops rapidly, with precursors to all major organs appearing within 36 hours of fertilization. The embryo begins as a yolk with a single enormous cell on top (see image, 0 h panel), which divides into two (0.75 h panel) and continues dividing until there are thousands of small cells (3.25 h panel). The cells then migrate down

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