Term
| does gastrulation (the conversion of the epiblast from a bilaminar disc into a trilaminar embryonic disc consisting of ectoderm, mesoderm, and endoderm) begin with? |
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Definition
| formation of the primitive streak, which is a linear band of thickened epiblast that first appears at the caudal end of the embryo and grows cranially. at the cranial end, its cells proliferate to form the primitive knot. |
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Term
| what does the epiblast (upper layer of the bilaminar disc) give rise to? |
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Definition
| endoderm (gut+digestive glands), mesoderm (skeletal muscles, vasculature), ectoderm (gives rise to CNS+skin) |
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Term
| how does the notochordal process form? |
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Definition
| mesenchymal cells migrate from the primitive knot to form a midline cellular cord known as the notochordal process. the notochordal process grows cranially until it reaches the prechordal plate, the future site of the mouth. in this area the ectoderm is attached directly to the endoderm without intervening mesoderm. this area is known as the oropharyngeal membrane, and it will break down to become the mouth. at the other end of the primitive streak the ectoderm is also fused directly to the endoderm; this is known as the cloacal membrane (proctodeum), or primordial anus. |
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Term
| what is the purpose of the notochordal process? |
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Definition
| to induce the ectoderm lying on top of it to thicken up and form the neural plate. |
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Term
| what is the process of neurulation? |
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Definition
| the process where the flat sheet of ectoderm cells (those which make up the neural plate) become induced to become tall columnar, which involves *microtubules which extend the long axis (elongates cells). the apex of these cells have *microfilaments which contract and make square cells into triangular cells (cells eventually roll up to form a tube). this eventually forms the neural tube. |
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Term
| where does the neural tube close first? |
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Definition
| around C1, which then extends cranially and caudally. the open end at the cranial side is called the anterior neuropore and the open end at the back end is called the posterior neuropore. these then close by the end of the first month. |
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Term
| when has the basic CNS already formed? |
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Definition
by the end of the first month (a lot of opportunity for things to go wrong if someone was unaware that they
were pregnant) |
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Term
| where are the neural crest cells located? |
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Definition
|
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Term
| where do the somites form? |
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Definition
| on either side of the neural tube in the mesoderm |
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Term
| what has to happen before the neural tube can close? what happens when it does? |
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Definition
| the neural crest cells have to migrate laterally before the neural folds at the top of the curving neural plate can meet. once they do, ectoderm separates from the plate and closes over the top (skin is now continuous over the top). |
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Term
| what do the neural crest cells give rise to? |
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Definition
| the dorsal root ganglia or any other ganglia outside the CNS |
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Term
| when does the brain become divided in to 3 parts? what are the parts? |
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Definition
| at 3 weeks (while the neural tube is still open). the parts are: forebrain (prosencephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon). [3 part brain at 3 weeks] |
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Term
| what do the forebrain (prosencephalon) and hindbrain (rhombencephalon) divide into? when does this occur? |
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Definition
| the forebrain (prosencephalon) divides into the diencephalon and telencephalon. the hindbrain (rhombencephalon) divides into the metencephalon and myelencephalon. this occurs at 5 weeks. [5 part brain at 5 weeks = telencephalon – diencephalon – mesencephalon – metencephalon – myelencephalon] |
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Term
| what happens to the 5 part brain? |
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Definition
| it "accordions" via 3 flexures. |
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Term
| where is the first flexure of the 5 part brain? what is this called? |
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Definition
| the first flexure, called the *cephalic flexure occurs in the middle of the mesenencephalon |
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Term
| where is the second flexure of the 5 part brain? what is this called? |
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Definition
| the second flexure, called the *cervical flexure occurs between the spinal cord and mylencephalon |
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Term
| where is the third flexure of the 5 part brain? what is this called? |
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Definition
| the third flexure, called the *pontine flexure occurs between the metenencephalon and myelencephalon |
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Term
| where does the neural tube form down to? |
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Definition
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Term
| how doe L2, 3,4,5, all the sacrals and all the coccygeals form if they have no neural tube? |
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Definition
| they have ectoderm over a caudal mass (includes mesoderm, neural ectoderm and neural crest cells) w/endoderm at the bottom. there is no folding. |
|
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Term
| how long are the vertebral column and spinal cord the same length? |
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Definition
| 3 months, after that the vertebral column becomes longer |
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Term
| why do we all have tails? |
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Definition
| so our legs can form properly |
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Term
| where is the conus medullaris at 6 months, birth, and in an adult? |
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Definition
| 6 mos: S1. birth: L3. adult: L1. this is important for spinal taps - in a newborn, will have to do it at a lower level |
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Term
| how does the differential in growth between the vertebral column and spinal cord affect the spinal nerves? |
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Definition
| while the spinal cord and vertebral column are developing at the same rate the nerves come out at roughly right angles. when spinal cord slows down and vertebral column slows down, nerves at the bottom are “trapped”. |
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Term
| what are all the nerve roots called below the conus medullaris at L1? how does this affect lower back trauma? |
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Definition
the cauda equina - the lower a herniated disc is, the greater chance it has of affecting more than 1 nerve
root |
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Term
| what is the filum terminale? what does it become past S2? |
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Definition
| the nonneural part of the spinal cord which continues w/the cauda equina - composed mainly of pia mater and ependymal cells. past S2, it becomes wrapped in dura mater and is then called the coccygeal ligament (attaches the cord to the coccyx). |
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Term
| where does the spinal cord usually end in an adult? |
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Definition
| at L1. arachnoid and dura continue down to S2, creating a space called the lumbar cistern - where spinal taps are done. |
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Term
| what does the neural tube look like histologically once it has closed? |
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Definition
| the neural tube is a pseudostratified columnar tube (1 cell thick, but many nuclei). the cells in this start in the lumen of the tube and extend outward (forming an internal luminal and external membrane). the nuclei move back and forth in every cell (when the nuclei move close the lumen, the cell will divide). |
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Term
| what are the alar and basal plates? |
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Definition
| on either side of the limiting sulci (dents) on opposite sides of the neural tube lumen. the alar plate (more posterior) gives rise to the sensory nerves (dorsal horn) and the basal plate (more anterior) gives rise to the motor nerves (anterior horn). [in the lumbar and thoracic areas the visceral sympathetics also form intermediate columns] |
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Term
| what do the cells comprising the neural tube do once the tube closes? |
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Definition
| dividing rapidly. they move back and forth between the internal and external membrane and as they divide they lose contact w/one membrane until the 2 daughter cells are separated (which then connect to the membranes and repeat the process). |
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Term
| how do the cells of the neural tube differentiate over the course of development? |
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Definition
| the cells of the original neural plate give rise to an (intermediate) mantle layer which forms the grey matter and glial cells (except for microglia). the layer outside that becomes the marginal zone and has a lot of fiber tracts (becomes white matter). this occurs in stages: for the first 3.5 months, neuroblasts are only made, which become nerve cells. after 3.5 months, glial cells start appearing. once all this differentiation takes place, the cells which are left remain to line the central canal (ependymal layer). |
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Term
| what are the precursors of the glial cells? what do the glial cells give rise to? |
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Definition
| spongioblasts are the precursors to glial cells (and arise from the ependymal layer). the glial cells then become astrocytes, oligodendrocytes, and those which are left become ependymal cells. |
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Term
| what do the neural crest cells become? |
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Definition
| dorsal root ganglia (and all other neurons outside the CNS) for the first 3.5 mos. after 3.5 mos, the neural crest cells become spongioblasts which give rise to satellite cells and schwann cells (myelinate neurons outside the CNS) |
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Term
| where do the coverings of the brain (pia, arachnoid, dura) arise from? |
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Definition
| mesoderm, which comes in to the CNS w/the blood vessels and *microglia cells (phagocytize dead material in the nervous system). |
|
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Term
| what do the astrocytes do? |
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Definition
| insulate/support/nurture nerve cells by transferring nutrients from the blood vessels to the nerve cells |
|
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Term
| what do the oligodendrocytes do? |
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Definition
| oligodendrocytes myelinate multiple nerves within the CNS - in the PNS, schwann cells can only myelinate 1 part of 1 axon. *both schwann cells and oligodendrocytes can protect multiple nerves (they don't have to myelinate), but oligodendrocytes can myelinate multiple nerves. |
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Term
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Definition
| associations of nerves w/in the CNS "there is no such thing as a naked nerve" |
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Term
| what characterizes brainstem (myelencephalon, mesencephalon and rhombencephalon) cellular development? |
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Definition
| similar to the neural tube of the spinal cord, there is an ependymal layer - the neuroblasts of which give rise to the mantle zone. all the neurons in the mantle zone of the brainstem give rise to the deep nuclei of the brainstem and those which migrate further out to the marginal layer become cortex. |
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Term
| how does the medulla organize? |
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Definition
| the alar plates (sensory) separate at midline and roof of the neural tube thins out and becomes non-neural inbetween. the opposing basal plates (motor) remain joined at the bottom, forming a triangular space. the sulcus limitans still separates the alar and basal plates on either side (thus, distal to the limiting sulcus will be sensory and proximal to limiting sulcus will be motor). these neurons do not form a nice big solid mass like in the spinal cord, instead they have deep individual nuclei. |
|
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Term
| how does the pons organize? |
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Definition
| the same as the medulla, only the alar plates enlarge more bulbously to form the superficial cerebellar cortex |
|
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Term
| where do the olivo nucleus in the medulla/pontine nuclei come from? |
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Definition
| the olivo nucleus in medulla and pontine nuclei in pons come from the alar plate and migrate downwards |
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Term
| how does the midbrain organize? |
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Definition
| similar to the medulla/pons, however the roof doesn't actually thin out. instead, the alar plates remain conjoined as the neural tube expands (limiting sulcus remains as well). the top (dorsal/sensory/alar) neurons spread out to form the tectum at the top, then some move ventrally and out and around the basal plates to form the substantia nigra and red nucleus. the basal plates remain in position and shrink to form the visceral and somatic red nucleus. the lumen of the neural tube continues to shrink and forms an aqueduct |
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Term
| what characterizes forebrain cellular development and organization? |
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Definition
| the limiting sulcus ends at the midbrain and is called the hypothalamic sulcus. also, all of the nerves from here on out are sensory (there is no basal plate). the initial neurons of the neural tube migrate to the *mantle zone and give rise to the deep nuclei* (thalamus, hypothalamus, caudate nucleus, and mamillary bodies). the second migration (**directly from the ependymal zone**), to the distal part of the marginal zone then forms the cortex. the cortex differentiates and gives rise to the fiber tracts that runs down toward the hypothalamus - cuts the corpus striatum in half to create the caudate nucleus and lentiform nucleus (becomes the globus pallidum and putamen). |
|
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Term
| where do the 3 major commissures (connections between the hemispheres) arise from? |
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Definition
| the most anterior wall of the forebrain (called the lamina terminalis) |
|
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Term
| what is the sequence of commissure formation in the forebrain? |
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Definition
| the anterior commissure (white commissure) forms at 2 months and connects the sides of the olfactory system. the hippocampal commissure forms at 3 months and joins the limbic system. then late in the 3rd month, the corpus callosum (largest, most superior) joins the 2 sides of the neocortex and this will function as the primary association cortex. |
|
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Term
| why does the brain form in a C-shape? |
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Definition
| b/c the deep nuclei (caudate nucleus, lenticular, etc) form slowly and the external cortex forms quickly. |
|
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Term
| how does the cortex initially appear? what is the sequence of fold formation? |
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Definition
| smooth. the first indication of a fold is the lateral fissure at 3 months. all the primary fissures are present by 6 months. the gyri are not obvious until about 9 months (may not form in anomalies like down's syndrome) - these form to crowd in more neurons (increased surface area). |
|
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Term
| what is the migration pattern in the cerebellum (starting from the neural tube)? |
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Definition
| first, the neuroblasts go from the ependymal zone to the mantle and give rise to the deep nuclear groups (dentate, villiform, etc). second, cells from the ependymal zone (closest to the neural tube lumen) migrate to the marginal zone and give rise to the cortex (purkinje layer) while others simply stay in the marginal zone and form the granular and golgi layer (not differentiate until they get to their endpoint) [multi-layered cortex in the cerebellum]. |
|
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Term
| what is the general differentiation of the nerve cells? |
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Definition
| differentiation occurs in cranial to caudal priority, with the big motor efferents (anterior horn) forming first, followed by the afferent dorsal root ganglion nerve cells (sensory), followed by the intermediate neurons (3 big commissural first, then the association neurons), finally followed by the preganglionic sympathetics (last to form w/in the spinal cord - postganglionic cells form in the periphery). |
|
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Term
| what characterizes the process of myelination in the CNS? |
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Definition
| at 4 mos, myelination occurs w/in the spinal cord segment. by 6 mos, the dorsal columns will myelinate - at 7 mos the big ascending (sensory) tracts in the lateral column (spinothalamics, spinocerebellar) form. between birth and 2 yrs, the big descending motor tracts (corticospinal, rubrospinal, vestibulospinal) form. |
|
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Term
| what characterizes the role of cell death during CNS development? |
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Definition
| only the cells which make meaningful connections survive, this occurs mostly during cell differentiation - but also at a very slow rate during the rest of human life. |
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Term
| where does the choroid plexus come from? |
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Definition
| the tela choroidea layer. when the 4th ventricle thins out, the intermediate zone does not go up into the roof - instead, the choroid plexus forms as a combination of **pia and ependymal** cells (tufts of cells which bulge out into the 4th ventricle). |
|
|
Term
| when does the first choroid plexus form in the 4th ventricle? |
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Definition
| around 7 weeks (even before there are any openings in the subarachnoid space). the foramen of magendie opens a couple days later to relieve the resultant pressure from CSF production. |
|
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Term
| when does the second choroid plexus form in the lateral ventricle? |
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Definition
| around 8 weeks, and it comes out through a choroid fissure in the medial wall. it grows fastest initially, and by 9 weeks it is the major CSF producer, but the **4th ventricle then overtakes it again and is then again the major CSF producer**. |
|
|
Term
| when does the third choroid plexus form in the third ventricle? |
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Definition
|
|
Term
| what is the communication of CSF in the brain once all the choroid plexi have formed? |
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Definition
| CSF is produced in highest amounts in the 4th ventricle, then goes to subarachnoid space and around the spinal cord. the 3rd and 4th ventricle communicate via the aqueduct of sylvius (cerebral aqueduct). the lateral ventricle communicates w/the 3rd ventricle via the foramen of monro (interventricular foramen). |
|
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Term
| where are the CSF cisterns? |
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Definition
| the cisterna magna arises where the foramen of magendie opens (around 8 wks). the pontine cisterns form around 5 months on each side of the pons via the opening of the foramen of luschka. |
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Term
| when does the connection between the central canal of the brain and spinal cord close? |
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Definition
| shortly before birth, causing all CSF to circulate between the brain+spinal cord via the subarachnoid space. |
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