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76-547: It is one of the three sensory circumventricular organs of the brain. The other four are secretory. The VOLT, median eminence , and subfornical organ are interconnected with the mid- ventral hypothalamus , and together these three structures surround the third ventricle , a complex often called the anteroventral region of the third ventricle ("AV3V" region). This region functions in the regulation of fluid and electrolyte balance by controlling thirst, sodium excretion, blood volume regulation, and vasopressin secretion. The VOLT

152-428: A fruit fly (mature after 10–14 days) can produce up to 900 offspring per year. These two main strategies are known as K-selection (few offspring) and r-selection (many offspring). Which strategy is favoured by evolution depends on a variety of circumstances. Animals with few offspring can devote more resources to the nurturing and protection of each individual offspring, thus reducing the need for many offspring. On

228-640: A CVO. Related to its secretory function, the SCO is partially composed of ependymal cells . These ependymocytes are characterized by elongated cell bodies that contain secretory materials and are covered in cilia. The most prominent of these is the glycoprotein SCO-spondin . One function of the SCO is the secretion of the glycoprotein SCO-spondin, which is released into the third ventricle where it aggregates to create Reissner's fiber . Reissner's fiber

304-445: A circumventricular organ with both neural and secretory functions. The anterior pituitary contains non-neural secretory cells derived from oral ectoderm which are indirectly controlled by "releasing hormones" from the median eminence of the hypothalamus, through the hypophyseal portal circulation. The intermediate lobe (also called pars intermedia ) synthesizes and secretes a hormone stimulating melanocytes under neural control by

380-484: A genetically similar or identical copy of itself. The evolution of sexual reproduction is a major puzzle for biologists. The two-fold cost of sexual reproduction is that only 50% of organisms reproduce and organisms only pass on 50% of their genes . Sexual reproduction typically requires the sexual interaction of two specialized reproductive cells, called gametes , which contain half the number of chromosomes of normal cells and are created by meiosis , with typically

456-523: A male fertilizing a female of the same species to create a fertilized zygote . This produces offspring organisms whose genetic characteristics are derived from those of the two parental organisms. Asexual reproduction is a process by which organisms create genetically similar or identical copies of themselves without the contribution of genetic material from another organism. Bacteria divide asexually via binary fission ; viruses take control of host cells to produce more viruses; Hydras ( invertebrates of

532-407: A much more complex task, but may well be possible to some degree according to current biological knowledge. A synthetic genome has been transferred into an existing bacterium where it replaced the native DNA, resulting in the artificial production of a new M. mycoides organism. There is some debate within the scientific community over whether this cell can be considered completely synthetic on

608-493: A new organism by combining the genetic material of two organisms in a process that starts with meiosis , a specialized type of cell division . Each of two parent organisms contributes half of the offspring's genetic makeup by creating haploid gametes . Most organisms form two different types of gametes. In these anisogamous species, the two sexes are referred to as male (producing sperm or microspores) and female (producing ova or megaspores). In isogamous species ,

684-505: A sensory circumventricular organ (along with the SFO and AP), the vascular organ of lamina terminalis (VOLT) is situated in the anterior wall of the third ventricle . Characteristically of the CVOs, it lacks the tight endothelial blood brain barrier. The vascular organ is further characterized by the afferent inputs from the subfornical organ (SFO), the median pre-optic nucleus (MnPO) region,

760-405: A singular long projection that delves deep inside the hypothalamus. Tanycytes have been evolutionarily linked to radial glial cells of the central nervous system. The tanycytes of the median eminence are often found along the fenestrated peripheral capillaries. They are tightly packed on the capillaries, forming a seal between the third ventricle and the median eminence. This seal can be attributed to

836-413: A wide range of reproductive strategies employed by different species. Some animals, such as the human and northern gannet , do not reach sexual maturity for many years after birth and even then produce few offspring. Others reproduce quickly; but, under normal circumstances, most offspring do not survive to adulthood . For example, a rabbit (mature after 8 months) can produce 10–30 offspring per year, and

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912-467: Is a long fibrous projection that travels caudally through the Sylvian aqueduct and terminates in the spinal cord. This fiber is thought to contribute to the maintenance of the patency of the Sylvian aqueduct. While the function of the subcommissural organ remains under investigation, it may be part of the mechanism of aldosterone secretion and CSF detoxification, along with osmoregulation . The SCO

988-423: Is a sensory CVO situated on the underside of the fornix and lacking a BBB , the absence of which characterizes the circumventricular organs. Protruding into the third ventricle of the brain, the highly vascularized SFO can be divided into 3–4 anatomical zones, especially by its capillary density and structure. The central zone is composed exclusively of the glial cells and neuronal cell bodies. Conversely,

1064-457: Is classified as a secretory organ. The median eminence is rich in fenestrated capillaries, allowing for the passage of proteins and neurohormones . More specifically, the median eminence allows for the transport of neurohormones between the CSF and the peripheral blood supply. The major cell type that makes up the median eminence are specialized ependymal cells known as tanycytes . These contribute to

1140-411: Is considered a secretory organ and its activity shows circadian oscillations . Its main function – secretion of the hormone melatonin – rests when there is no input from the primary circadian pacemaker in the suprachiasmatic nuclei . Melatonin production is controlled by the previously mentioned circadian timing and is suppressed by light. Pineal tumors can affect sexual development, but

1216-430: Is employed to exploit suitable conditions for survival such as an abundant food supply, adequate shelter, favorable climate, disease, optimum pH or a proper mix of other lifestyle requirements. Populations of these organisms increase exponentially via asexual reproductive strategies to take full advantage of the rich supply resources. When food sources have been depleted, the climate becomes hostile, or individual survival

1292-523: Is innervated by many systems, the most common of which is associated with the serotonergic system , which influences water and sodium intake. During water deprivation, it will also reduce its innervation to the SCO. The reduction of input to the SCO causes a marked decrease in RF production. This finding implies that the subcommissural organ and its associated Reissner's fiber are integral parts of fluid electrolyte balance and water homeostasis. The pituitary gland

1368-405: Is involved in controlling thirst . Circumventricular organs#Sensory organs Circumventricular organs ( CVOs ) ( circum-: around  ; ventricular: of ventricle ) are structures in the brain characterized by their extensive and highly permeable capillaries , unlike those in the rest of the brain where there exists a blood–brain barrier (BBB) at the capillary level. Although

1444-527: Is jeopardized by some other adverse change in living conditions, these organisms switch to sexual forms of reproduction. Sexual reproduction ensures a mixing of the gene pool of the species. The variations found in offspring of sexual reproduction allow some individuals to be better suited for survival and provide a mechanism for selective adaptation to occur. The meiosis stage of the sexual cycle also allows especially effective repair of DNA damages (see Meiosis ). In addition, sexual reproduction usually results in

1520-464: Is one of the three sensory circumventricular organs providing information to other brain regions. VOLT capillaries do not have a blood–brain barrier , and so neurons in this region can respond to circulating factors present in the systemic circulation . Neurons in the VOLT are osmoreceptors sensitive to the sodium content and osmotic pressure of blood. Neurons of the lamina terminalis project to

1596-514: Is sometimes substituted for autogamous pollination (not necessarily leading to successful fertilization) and describes self-pollination within the same flower, distinguished from geitonogamous pollination , transfer of pollen to a different flower on the same flowering plant , or within a single monoecious gymnosperm plant. Mitosis and meiosis are types of cell division . Mitosis occurs in somatic cells , while meiosis occurs in gametes . Mitosis The resultant number of cells in mitosis

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1672-498: Is subdivided into lobes – the anterior pituitary , the intermediate pituitary , and the posterior pituitary (also known as the adenohypophysis and neurohypophysis (or neural lobe), respectively). Each one functions as a separate endocrine organ . The pituitary neural lobe consists of axonal projections that directly extend from cell bodies in the hypothalamus through the infundibulum . Under neurohumoral control, it secretes oxytocin and vasopressin , thereby qualifying it as

1748-460: Is the biological process by which new individual organisms – " offspring " – are produced from their "parent" or parents. There are two forms of reproduction: asexual and sexual . In asexual reproduction, an organism can reproduce without the involvement of another organism. Asexual reproduction is not limited to single-celled organisms . The cloning of an organism is a form of asexual reproduction. By asexual reproduction, an organism creates

1824-413: Is the growth and development of embryo or seed without fertilization . Parthenogenesis occurs naturally in some species, including lower plants (where it is called apomixis ), invertebrates (e.g. water fleas , aphids , some bees and parasitic wasps ), and vertebrates (e.g. some reptiles, some fish, and very rarely, domestic birds ). Sexual reproduction is a biological process that creates

1900-501: Is thought that the VOLT may also play a role in the regulation of LH secretion via a negative feedback mechanism. It is also hypothesized that the VOLT may be the mechanism through which pyrogens function to initiate a febrile response in the CNS. Finally, VOLT neurons have been observed to respond to temperature changes indicating that the organum vasculosum of the lamina terminalis is subject to different climates. The subfornical organ

1976-409: Is twice the number of original cells. The number of chromosomes in the offspring cells is the same as that of the parent cell. Meiosis The resultant number of cells is four times the number of original cells. This results in cells with half the number of chromosomes present in the parent cell. A diploid cell duplicates itself, then undergoes two divisions ( tetraploid to diploid to haploid), in

2052-563: The brainstem , and even the hypothalamus . Conversely, the vascular organ of the lamina terminalis maintains efferent projections to the stria medullaris and basal ganglia . As a major player in the maintenance of the mammalian body fluid homeostasis, the VOLT features the primary neurons responsible for osmosensory balance. These neurons, in turn, feature angiotensin type I receptors, which are used by circulating angiotensin II to initiate water intake and sodium consumption. In addition to

2128-616: The central nervous system and peripheral blood. Additionally, they are an integral part of neuroendocrine function. Highly permeable capillaries allow the CVOs to act as an alternative route for peptides and hormones in the neural tissue to sample from and secrete to circulating blood. CVOs also have roles in body fluid regulation, cardiovascular functions, immune responses , thirst , feeding behavior and reproductive behavior . CVOs can be classified as either sensory or secretory organs serving homeostatic functions and body water balance . The sensory organs include

2204-430: The ciliate Paramecium aurelia , have more than two "sexes", called mating types . Most animals (including humans) and plants reproduce sexually. Sexually reproducing organisms have different sets of genes for every trait (called alleles ). Offspring inherit one allele for each trait from each parent. Thus, offspring have a combination of the parents' genes. It is believed that "the masking of deleterious alleles favors

2280-896: The order Hydroidea ) and yeasts are able to reproduce by budding . These organisms often do not possess different sexes, and they are capable of "splitting" themselves into two or more copies of themselves. Most plants have the ability to reproduce asexually and the ant species Mycocepurus smithii is thought to reproduce entirely by asexual means. Some species that are capable of reproducing asexually, like hydra , yeast (See Mating of yeasts ) and jellyfish, may also reproduce sexually. For instance, most plants are capable of vegetative reproduction  – reproduction without seeds or spores – but can also reproduce sexually. Likewise, bacteria may exchange genetic information by conjugation . Other ways of asexual reproduction include parthenogenesis , fragmentation and spore formation that involves only mitosis . Parthenogenesis

2356-424: The pineal gland varies greatly among mammals. The most commonly used classification for this gland takes into account its location relative to the diencephalon and the third ventricle of the brain, as well as its size and shape. Under these conditions, the human pineal gland is classified as type A. A type A pineal gland rests proximally to the posterior section of the diencephalon. It is located within 1-2mm of

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2432-463: The subfornical organ . The area postrema is located in the caudal medulla oblongata near the junction of the brainstem and the spinal cord . In humans and in most other mammals that have been studied, it consists of swellings on either wall of the fourth ventricle. In rodents and lagomorphs , however, the area postrema forms a midline structure dorsal to the obex . When viewed histologically for its capillary distribution and morphology,

2508-407: The supraoptic nucleus and paraventricular nucleus to regulate the activity of vasopressin -secreting neurons. In a situation of lowered blood volume, secretion of renin by the kidneys results in the production of angiotensin II , which stimulates receptors in the VOLT and subfornical organ to complete a positive feedback loop. These neurons also project to the median preoptic nucleus which

2584-416: The vascular organ of lamina terminalis (VOLT — also known as the organum vasculosum of the lamina terminalis (OVLT)), the median eminence , the pituitary neural lobe , and the pineal gland . The circumventricular organs are midline structures around the third and fourth ventricles that are in contact with blood and cerebrospinal fluid , and they facilitate special types of communication between

2660-485: The SFO’s integral role as a regulator of energy homeostasis. The subcommissural organ (SCO) is a small secretory organ located on the ventral surface of the posterior commissure near the anterior entrance of the cerebral aqueduct . It differs from other CVOs in that it does not have highly permeable capillaries. Its role as a neuroendocrine structure associated with the ventricular system qualifies it for classification as

2736-564: The VOLT neurons. This can be seen through the predominantly excitatory effects of ANG on the VOLT through the TRPV1 receptor. In this context, it is worthy to note the VOLT neurons typically feature a resting membrane potential in the range of -50 to -67 mV with input resistances ranging from 65 to 360 MΩ. Despite a solid understanding of the VOLT’s role in the maintenance of body fluid homeostasis, other functions are less understood. For example, it

2812-571: The afferent projections of the SFO are considered less important than the various efferent connections, it is still notable that the subfornical organ receives synaptic input from the zona incerta and arcuate nucleus . Study of subfornical organ anatomy is still ongoing but evidence has demonstrated slow blood transit time which may facilitate the sensory capability of SFO, enabling increased contact time for blood-borne signals to penetrate its permeable capillaries and influence regulation of blood pressure and body fluids. This observation coincides with

2888-403: The anatomy of the VOLT is not yet fully comprehended. As previously mentioned, the vascular organ of lamina terminalis features neurons responsible for the homeostatic conservation of osmolarity. In addition, the fenestrated vasculature of the VOLT allows the astrocytes and neurons of the VOLT to perceive a wide variety of plasma molecules whose signals may be transduced into other regions of

2964-479: The angiotensin receptors, the neurons of the VOLT are also characterized by the presence of a nonselective cation channel deemed the transient receptor potential vanilloid 1, or TRPV1 . Though there are other receptors within the TRPV family, a study by Ciura, Liedtke, and Bourque demonstrated that hypertonicity sensing operated via a mechanical mechanism of TRPV1 but not TRPV4 . Despite a significant amount of data,

3040-434: The area postrema has numerous subregions separated according to capillary permeability, rates of blood flow , and duration of blood transit through respective capillary beds. Relatively little is known about the function of the area postrema in humans. However, there is strong evidence that the area postrema acts as the chemoreceptor trigger zone for vomiting, which is triggered by the presence of noxious stimulation from

3116-419: The area postrema, the subfornical organ, and the vascular organ of lamina terminalis, all having the ability to sense signals in blood, then pass that information neurally to other brain regions. Through their neural circuitry , they provide direct information to the autonomic nervous system from the systemic circulation . The secretory organs include the subcommissural organ (SCO), the pituitary gland,

Vascular organ of lamina terminalis - Misplaced Pages Continue

3192-422: The blood. There is also evidence that the area postrema is the site at which angiotensin stimulates glucose metabolism , presumed efferent neural activity, blood pressure control, and thirst. The area postrema also has integrative capacities that enable it to send major and minor efferents to sections of the brain involved in the autonomic control of cardiovascular and respiratory activities. Classified as

3268-472: The boundaries between life and machines until the two overlap to yield "truly programmable organisms". Researchers involved stated that the creation of "true synthetic biochemical life" is relatively close in reach with current technology and cheap compared to the effort needed to place man on the Moon. Sexual reproduction has many drawbacks, since it requires far more energy than asexual reproduction and diverts

3344-429: The brain, thereby eliciting autonomic and inflammatory reactions. In experiments, mammalian VOLT neurons were shown to transduce hypertonicity by the activation of the TRPV1 nonselective cation channels. These channels are highly permeable to calcium and are responsible for membrane depolarization and increased action potential discharge. Stated simply, an increase in osmolarity results in a reversible depolarization of

3420-406: The central nervous system. The most important set of fibers involved are the unmyelinated postganglionic sympathetic fibers from the superior cervical ganglia , which also form the bilateral nervi conarii. The second set of fibers enters the pineal gland anteriorly via the commissural peduncles. The third set of fibers is myelinated and forms the ventro-lateral pineal tract. The pineal gland

3496-436: The evolution of a dominant diploid phase in organisms that alternate between haploid and diploid phases" where recombination occurs freely. Bryophytes reproduce sexually, but the larger and commonly-seen organisms are haploid and produce gametes . The gametes fuse to form a zygote which develops into a sporangium , which in turn produces haploid spores. The diploid stage is relatively small and short-lived compared to

3572-411: The expression of a wider range of traits and thus making the population more able to survive environmental variation. Allogamy is the fertilization of flowers through cross-pollination, this occurs when a flower's ovum is fertilized by spermatozoa from the pollen of a different plant's flower. Pollen may be transferred through pollen vectors or abiotic carriers such as wind. Fertilization begins when

3648-431: The fact that SFO neurons have been shown to be intrinsically osmosensitive. Finally, it has been established that SFO neurons maintain resting membrane potential in the range of -57 to -65 mV. The subfornical organ is active in many bodily processes including, but not limited to, osmoregulation, cardiovascular regulation, Both hyper- and hypotonic stimuli facilitated an osmotic response. This observation demonstrated

3724-622: The fact that the SFO is involved in the maintenance of blood pressure. Featuring an AT1 receptor for ANG, the SFO neurons demonstrate an excitatory response when activated by ANG , therefore increasing blood pressure . The induction of the drinking response via the SFO can be antagonized, however, by the peptide, ANP . Additional research has demonstrated that the subfornical organ may be an important intermediary though which leptin acts to maintain blood pressure within normal physiological limits via descending autonomic pathways associated with cardiovascular control. Recent research has focused on

3800-399: The formation of a life stage that is able to endure the conditions that threaten the offspring of an asexual parent. Thus, seeds, spores, eggs, pupae, cysts or other "over-wintering" stages of sexual reproduction ensure the survival during unfavorable times and the organism can "wait out" adverse situations until a swing back to suitability occurs. The existence of life without reproduction is

3876-401: The function of a few genes involved with imprinting, other Japanese scientists combined two mouse eggs to produce daughter mice and in 2018 Chinese scientists created 29 female mice from two female mice mothers but were unable to produce viable offspring from two father mice. Researches noted that there is little chance these techniques would be applied to humans in the near future. There are

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3952-402: The gametes are similar or identical in form ( isogametes ), but may have separable properties and then may be given other different names (see isogamy ). Because both gametes look alike, they generally cannot be classified as male or female. For example, in the green alga, Chlamydomonas reinhardtii , there are so-called "plus" and "minus" gametes. A few types of organisms, such as many fungi and

4028-692: The grounds that the chemically synthesized genome was an almost 1:1 copy of a naturally occurring genome and, the recipient cell was a naturally occurring bacterium. The Craig Venter Institute maintains the term "synthetic bacterial cell" but they also clarify "...we do not consider this to be "creating life from scratch" but rather we are creating new life out of already existing life using synthetic DNA". Venter plans to patent his experimental cells, stating that "they are pretty clearly human inventions". Its creators suggests that building 'synthetic life' would allow researchers to learn about life by building it, rather than by tearing it apart. They also propose to stretch

4104-402: The haploid stage, i.e. haploid dominance . The advantage of diploidy, heterosis, only exists in the diploid life generation. Bryophytes retain sexual reproduction despite the fact that the haploid stage does not benefit from heterosis. This may be an indication that the sexual reproduction has advantages other than heterosis, such as genetic recombination between members of the species, allowing

4180-469: The hypothalamus. It is not commonly included among circumventricular organs. The pituitary gland is located in the sella turcica of the sphenoid bone at the base of the skull. The median eminence (ME) is located in the inferior portion of the hypothalamus and is ventral to the third ventricle. While some publications do not list the ME as a CVO, when it is considered to be a circumventricular organ, it

4256-402: The integrity of the genome to be maintained and offspring health to be protected. Scientific research is currently investigating the possibility of same-sex procreation, which would produce offspring with equal genetic contributions from either two females or two males. The obvious approaches, subject to a growing amount of activity, are female sperm and male eggs . In 2004, by altering

4332-442: The laboratory. Several scientists have succeeded in producing simple viruses from entirely non-living materials. However, viruses are often regarded as not alive. Being nothing more than a bit of RNA or DNA in a protein capsule, they have no metabolism and can only replicate with the assistance of a hijacked cell 's metabolic machinery. The production of a truly living organism (e.g. a simple bacterium) with no ancestors would be

4408-519: The lateral hypothalamus with fibers terminating in the supraoptic (SON) and paraventricular (PVN) nuclei, and the anteroventral 3rd ventricle (AV3V) with fibers terminating in the VOLT and the median preoptic area . It seems that the most essential of all these connections is the SFO’s projections to the paraventricular hypothalamic nucleus. Based on their functional relevance, the SFO neurons can be branded as either GE, featuring nonselective cation channels, or GI, featuring potassium channels. While

4484-442: The mechanism has yet to be established. Other peptides aside from melatonin have been detected in the pineal. They are most likely associated with a type of innervation deemed "pineal peptidergic innervation." These include vasopressin, oxytocin, VIP , NPY , peptide histidine isoleucine, calcitonin gene-related peptide, substance P and somatostatin. Reproductive behavior Reproduction (or procreation or breeding )

4560-450: The median eminence, and the pineal gland. These organs are responsible for secreting hormones and glycoproteins into the peripheral blood using feedback from both the brain environment and external stimuli. Circumventricular organs contain capillary networks that vary between one another and within individual organs both in density and permeability, with most CVO capillaries having a permeable endothelial cell layer, except for those in

4636-406: The midline of the brain. The pineal gland starts to develop during the second month of gestation. In the average adult, the dimensions are as follow: 5-9mm in length, 1-5mm in width and 3-5mm in thickness. Its average weight is 100–180 mg. The pineal gland consists of a central core made up of small lobes and a cortex that possesses a diffuse distribution of neurons . The principal cell type of

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4712-445: The organ's ability to selectively allow macromolecules to pass from the central to the peripheral neuroendocrine systems. Ventromedial subregions of the bilateral hypothalamic arcuate nucleus display relatively high capillary permeability, indicating this nucleus may have moment-to-moment regulatory roles for sensing and neurally conveying hormonal signals. Tanycytes line the floor of the third ventricle and can be characterized by

4788-484: The organisms from other pursuits, and there is some argument about why so many species use it. George C. Williams used lottery tickets as an analogy in one explanation for the widespread use of sexual reproduction. He argued that asexual reproduction, which produces little or no genetic variety in offspring, was like buying many tickets that all have the same number, limiting the chance of "winning" – that is, producing surviving offspring. Sexual reproduction, he argued,

4864-582: The other hand, animals with many offspring may devote fewer resources to each individual offspring; for these types of animals it is common for many offspring to die soon after birth, but enough individuals typically survive to maintain the population. Some organisms such as honey bees and fruit flies retain sperm in a process called sperm storage thereby increasing the duration of their fertility. Organisms that reproduce through asexual reproduction tend to grow in number exponentially. However, because they rely on mutation for variations in their DNA, all members of

4940-469: The pineal is the pinealocyte sensu stricto. This type of cell has a prominent nucleus and a granular appearance. The level of vascularization in the pineal gland is high. It receives a large supply of blood from branches of the posterior choroidal arteries that derive from cerebral arteries in the posterior mesencephalon . The pineal gland is innervated by fibers from the peripheral parasympathetic and sympathetic systems , in addition to fibers from

5016-446: The pollen is brought to a female gamete through the pollen tube. Allogamy is also known as cross fertilization, in contrast to autogamy or geitonogamy which are methods of self-fertilization. Self- fertilization , also known as autogamy, occurs in hermaphroditic organisms where the two gametes fused in fertilization come from the same individual, e.g., many vascular plants , some foraminiferans , some ciliates . The term "autogamy"

5092-555: The process forming four haploid cells. This process occurs in two phases, meiosis I and meiosis II. Animals, including mammals, produce gametes (sperm and egg) by means of meiosis in gonads (testicles in males and ovaries in females). Sperm are produced by spermatogenesis and eggs are produced by oogenesis . During gametogenesis in mammals numerous genes encoding proteins that participate in DNA repair mechanisms exhibit enhanced or specialized expression. Male germ cells produced in

5168-401: The rostral and caudal areas are mostly made of nerve fibers while few neurons and glial cells can be seen in this area. Functionally, however, the SFO may be viewed in two portions – the dorsolateral peripheral division and the ventromedial core segment. The SFO has many efferent projections, shown to broadcast efferent projections to regions involved in cardiovascular regulation including

5244-446: The species have similar vulnerabilities. Organisms that reproduce sexually yield a smaller number of offspring, but the large amount of variation in their genes makes them less susceptible to disease. Many organisms can reproduce sexually as well as asexually. Aphids , slime molds , sea anemones , some species of starfish (by fragmentation ), and many plants are examples. When environmental factors are favorable, asexual reproduction

5320-420: The subcommissural organ. Furthermore, all CVOs contain neural tissue, enabling a neuroendocrine role. Although the choroid plexus also has permeable capillaries, it does not contain neural tissue; rather, its primary role is to produce cerebrospinal fluid (CSF), and therefore is typically not classified as a CVO. The sensory organs are the area postrema, the vascular organ of the lamina terminalis , and

5396-521: The subfornical organ as an area particularly important in the regulation of energy. The observation that subfornical neurons respond to a wide range of circulating energy balance signals, and that electrical stimulation of the SFO in rats resulted in food intake supports the SFO’s importance in energy homeostasis. Additionally, it is assumed that the SFO is the lone forebrain structure capable of constant monitoring of circulating concentrations of glucose. This responsiveness to glucose again serves to solidify

5472-441: The subject of some speculation. The biological study of how the origin of life produced reproducing organisms from non-reproducing elements is called abiogenesis . Whether or not there were several independent abiogenetic events, biologists believe that the last universal ancestor to all present life on Earth lived about 3.5 billion years ago . Scientists have speculated about the possibility of creating life non-reproductively in

5548-405: The term "circumventricular organs" was originally proposed in 1958 by Austrian anatomist Helmut O. Hofer concerning structures around the brain ventricular system , the penetration of blood-borne dyes into small specific CVO regions was discovered in the early 20th century. The permeable CVOs enabling rapid neurohumoral exchange include the subfornical organ (SFO), the area postrema (AP),

5624-582: The testes of animals are capable of special DNA repair processes that function during meiosis to repair DNA damages and to maintain the integrity of the genomes that are to be passed on to progeny. Such DNA repair processes include homologous recombinational repair as well as non-homologous end joining . Oocytes located in the primordial follicle of the ovary are in a non-growing prophase arrested state, but are able to undergo highly efficient homologous recombinational repair of DNA damages including double-strand breaks. These repair processes allow

5700-427: The tight junctions observed between tanycytes and functions to restrict the travel of molecules between the median eminence and the third ventricle. The median eminence is also closely linked to the transport of GnRH between the median eminence and the anterior pituitary. Neuronal projections of GnRH neurons actually end at the median eminence, allowing for its release into the portal blood system. The morphology of

5776-420: Was like purchasing fewer tickets but with a greater variety of numbers and therefore a greater chance of success. The point of this analogy is that since asexual reproduction does not produce genetic variations, there is little ability to quickly adapt to a changing environment. The lottery principle is less accepted these days because of evidence that asexual reproduction is more prevalent in unstable environments,

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