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36 Cards in this Set

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Model in Kaltoff book for B-catenin activity localization during dorsal specification
-B-catenin proteins are transported by microtubules during cortical rotation tot he dorsal region
-meanwhile, any remaining B-catenin is downregulated by GSK-3 activity in the vegetal region
Model in the Gilbert book for B-catenin acivity localization during dorsal specification
-evoked to explain the fact taht B-catenin seems to initially be homogeneously distributed in ooctyes (b.c.r.)...if this is the case, then you woudl think that in those experiments involving transplantation of cytoplasm from the future dorsal pole from eggs b.c.r. into vegetal blastomeres of 16-celled embryos would have yielded a secondary axis!
-as it is being translated from maternal mRNA, B-catenin is evenly distibuted (bcr) but so is GSK-3 B-catenin activity is effectively suppressed (levels kept rather low)
-however, a GSK-3 inhibitor is transported from the ventral pole to the dorsal pole which allows accumulation of active B-catenin in the dorsal region
-meanwhile, some B-catenin also is transported so that the dorsal region becomes enriched with B-catenin
-so, bcr the vegetal pole cytoplasm can impart dorsal activity to ventral blastomeres because its injection introduces this GSK3 inhibitor as well as soem more B-catenin which will allow B-catenin activity
-acr, B-catenin is downregulated in the vegetal pole due to absence of the GSK3 inhibitor
-a GSK3 inhibitor named Disheveled has been foudn associated with the relevant microtubules and moves accordingly
Clues to the establishment of left-right asymmetry
-the gene situs inversus viscerum+ (or iv+ for short) was identified as playing a role in L-R axis formation...mutations in this gene can lead to the condition of situs inversus, in which the internal organs are incorrectly oriented with respect to other body axes (in this case, their L-R orientations are randomized)
-encodes a subunit of dynein
-certain cilia (actually, called monocilia, have a 9+0(hollow in middle) arrangement of MTs)in an area called Henson's node (for now, think of this as the mammalian and chick version of Spemann's organizer)in iv-/iv- mice are immobile
-another gene that has been implicated is kif3B-encodes a kinesin-cilia are altogether missing in Henson's node
-kinesin is thought to be necessary for cilium assmebly (perhaps by mediating transport of necessary components)while the above-mentioned dynein is requried for their movement
-reserachers have placed fluorescent beads into the fluid around Henson's node, and indeed, in wildtype embryos, the fluid is moved in a net leftward direction, but in the absence of functional monocilia, this does not occur
-researchers have also managed to redirect fluid flow in living embryos and casue situs inversus
-the theory is that signaling molecules are then swept to the left side of the embryo whcih would results in differences in gene expression on the left versus right sides
-the nature of teh "swept" molecules is unknown, but an important gene that appears to be activated in resopnse to the swept signals is nodal+ (yes, sounds familiar, encodes TGF-B family protein)
-general mechanism appears to be faily widespread-in mammals, chicks and amphibians at least
-marks the beginning of morpohogenesis which is the process that gves rise to the organ rudiments and overall body plan
-these processes involve many differnt kinds of cellular movements and behaviors
-invagination, involution, epiboly, delamination, ingression, convergent extension, migration, intercalation, cell shape changes (example, a constriction around the apical region of each of several adjacent cells cold lad to buckling), cell division(may be accompanied by cell growth), changes in cell adhesiveness (to one another and/or to the EC matrix-involved in migration, ingression, delamination and epiboly), programmed cell death (carving out or sculpting of organs-classic example-formation of digits of hands and feet)
ectodermal organs at 5 weeks
the regions are subdivided further
-the prosencephalon develops into the telencephalon:which has two prominent outpockets which will form the future hemispheres of the cerebrum...these two outpockets surround ventricles I and II
-the prosencephalon also develops into the diencephalon, which surruonds ventricle III and which is the rgion that forms the optic cups

-the mesencephalon is still called the mesencephalon, and its lumen is called the aqueduct of Sylvius-it connects ventricles III and IV
-the rhombencephalon surrounds ventricle IV and is subdivided into (1)the metencephalon (more anterior portion of the hindbrain) and (2) the myelencephalon (more posterior and ventral portion of the hindbrain), transitions into the spinal cord
ectodermal organs at 7 weeks
the brain subdivides further...

-the myelencephalon becomes the medulla oblongata-controls the reflexes of the neck, throat and tongue, controls autonomic (involuntary)functions such as breathing and digestion, conduct signals from the brain to the spinal cord

-the metencephalon gives rise to the (1)cerebellum-coordinatino center for posture and movement and (2)the pons-assists in controlling autonomic functions and relays information between the brain and the spinal cord

-the mesencephalon gives rise to the midbrain, which serves to integrate and relay sensory information to the cerebrum

-the diencephalon gives rise to (1)the pineal gland-generates circadian rhythm and, in some vertebrates, modulates the reproductive cycle and (2)the thalamus-serves as a gateway for sensory fibers passing from the spinal cord and brain stem to the cerebral hemispheres and (3) the hypothalamus-contains regulatory centers for sleep, digestion, and body temperature maintenance as well as aggression and other emotional behaviors and (4) the infundibulum-contributes to the osterior lobe of the pituitary (and, in the adult, connects the pituitary to the hypothalamus-note that the anterior lobe of the pituitary orignates from the epithelium that lines the anterior oral cavity

-the telecephalon (term is sometimes used in adults) consists mainly of the two cerebral hemispherse (involved in sensory perception, learning, memory and conscious behavior)
neural crest cells (general)
-able to migrate long distances, contribute to many different structures,a nd cross lines that other ectodermal cells cannot (they are capable of contributing to tissues nromally made by mesoderm, such as connective tissues)

-are found only in vertebrates

-originate when cells at the crests of neural folds come to lie between the epidermis and the neural tube

-they migrate extensively and give rise to things such as
-cartilage in the head
-melanocytes (pigment cells)
-various types of neurons
-gland cells
-smooth muscle cells of the cardiovascular system
specfically, NC cells from the trunk migrate...
-dorsolaterally-these will enter the skin where they differentiate into pigment cells

-or in the ventral direction, and these will differentiate into...
-neurons and glial cells of the dorsal root glanglia, which receive sensory inputs from the skin (this is the fate of NC cells that settle near the neural tube)....or they differentiate into nerve cells of teh autonomic nervous system (innervates internal organs)....or they differentiate into NC cells from the trunk region also differentiate into Schwann cells (myelinate peripheral nerves) as well as the adrenal medulla (part of the adrenal gland that releases epinephrine and norepinephrine during stress)
NC cells form..
-cells from the cranial region (cephalic NC cells) form sensory nerves, bones and connective tissue in the head as well as portions of the eyes, ears and teeth
-NC cells that arise fromt eh area between the head and trunk region give ruse to pigment cells, neurons, cartilage, connective tissue, and muscle and connective tissue-based structes associated with the heart
-transplantation experiments have indicated that the potency to form many of the head and cardiovascular structures is restricted to the cephalic and cardiac NC cells, respectively
-however, the potency to form all of the other aforementioned structures is present along the entire anterior-posterior axis
-two hypothesis:pluripotency hyp and selection hyp
the pluripotency hypothesis
each individual NC cell has the potential to form many NC derivatives, but environmental signals will tell them how to differentiate
the selection hypothesis
any segment of NC's contains a mixed population of already determined cells, each with one possible fate
-this hypothesis would mean that environmental signals would allow only appropriately determined cells to enter certain pathways and/or to divide and differentiate
-evidenc shows it is somehwere in the middle of these two hypotheseses
evidence for the two hypotheses of neural crest cells
-growing out of clones (in vitro clonal analysis)-single cells were cultures in the absence of any known "differentiation cues"-resulting clonse were tested (looked at cell morphology and for maker proteins indicative of differentiated cell types) to see if the clones represented a mixed or homogeneous population
-result?-some clones come out "pure" while others were mixed (had more than 1 cell type)...the mixtures did NOT contain every possible combination
-some cell types were more often found in pure cultures (e.g. melanoctyes, cartilage) while other were only found in clones containing multiple other cell types (e.g. glial cells) so, it is thought that many NC cells are pluripotent initially but that their fates are restricted in a step-wise manner, such that the melanocyte and cartilage fates segregate out first
-in vivo clonal analysis experiemnts-injection of single NC cells with a flurescent dye indicate that if the injected cell is premigratory, it can contribute to as many as 4 cell types, while migratory NC cells can often give rise to around 2
-in salamander, at least, pigment granules can be found in some premigrtory NC cells, so these are somewhat heterogeneous in nature
-in fact, NC cells are subject to signals along the migration route which helps to explain some of the above findings
local inward buckling an eithelium (layer of cels that are tightly adhered to one another) to create a depression or cavity-think of pushing in a tennis ball
inward movement of an expanding epithelium around an edge, often results in the spreading of a once external layer over the internal surface of the remaining external cells
the spreading of an epithelium to envelope a yolk mass or deeper mass of cells
separation of one layer into 2 separate parallel layers (ex. during hypoblast formation in chick)
the movement of individual cells from the surface layer into the interior of an embryo
convergent extension
the elongation of an epithelium in one dimension and accompanied by a shortening in another (usually perpendicular)dimension
movement of individual cells or smal groups of cells over a substratum of other cells or extracellular martrix materials
wedging of cells between their neighbors
-lateral intercalation-merging of cells within a single layer, often leads to convergent extension
-radial intercalation-occurs between adjacent layers of cells and may cause epiboly because all of a sudden there is a forced increase in surface area so could cause to envelope the embryo more
-involved in gastrulation
-loosely connected groups of cells surrounded by large amounts of extracellular materials, that are capable of migration, and are often epithelial in origin (so the cells are said to undergo an epithelial to mesenchyme transition)
gastrulation in xenopus
-more complex than that you will learn about in sea urchins, due to the large amounts of yolk present
-will result in formation of the 3 germ layers-most important cell layer movements are invagination (formation of the blastopore), involution (the animal pole cells=future endoderm and marginal zone cells, aka future mesoderm, will roll in in layers at the blastopore), convergent extension (of the future mesoderm)and epiboly (spreading of the ectoderm as the presumptive endoderm and mesoderm move inside)...but note that the invagination plays a relatively minor role in frog gastrulation, as it only forms the beginnings of the blastopore
-note that for the sake of simplicity, we have just been referring to the mesoderm in the marginal zone, but in reality, there is a thin layer of putative endoderm outside of the putative mesoderm in this region...this region of putative endoderm will form the bottle cells, which will initiate the process of gastrulatin
-the first event is the formation of the blastopore-a group of cells on the dorsal side change their shape such that they become constricted at their apical ends and also somewhat elongated...these cells are called bottle cells and i is thought that the B-catenin that they inherited allows them to somehow undergo this change...this causes a small groove to form called the blastopore
-the dorsal lip of the blastopore (Spemann's organizer) is the ledge of cells on the animal side of the blastopore
-the putative mesoderm and endoderm on the animal side of the blastopore start to involute inward around the blastopore...this movement starts on the dorsal side but spreads laterally and vegetally such that the blastopore becomes circular
-as the mesoderm and endodermal cells are moving inside and start to line the roof of the blastocoel, the ectoderm cells of the animal cap change their shape (they divide and expand laterally while becoming thinner and also intercalate radially) initiating epiboly (spreading down over the vegetal region)
-a yolky plug (called the yolk plug) of endodermal cells remains exposed at the vegetal pole, although eventually, the blastopore will contract and the yolk plug cells will be forced into the interior where the will form the floor of the gut
-the first mesodermal cells to involute eventually migrate as mesenchyme over the blastocoel roof and will form head structures (head and neck connective tissues, muscles, etc)
-the mesoderm in the sheet that follows thse initial migratory cells are thought to get somewhat constricted (and they also intercalate) as they pass through the blastopore and this results in convergent extension along the anterior-posterior axis...think of merging of traffic...the bottle cells are then pulled along passively and will conribute to the endodermal roof of the archenteron
-the result of convergent extension is that mesoderm that used to be found in an equatorial ring now forms somewhat of a distorted partially open teepee-shape inside of the embryo (somewhat elongated along the a-p axis)..the notochord forms from the band of mesoderm up the steep side of the teepee...note that the teepee is turned almost 90 degrees from the pic i gave you
-involution created the archenteron, which is the primordial gut (and which is formed by the primitive endoderm), which first displaces and then destroys the blastocoel
-continued intercalation of the mesoderm toward the midline of the embryo along the anterior-posterior axis occurs and eventually, the notochord separates itself out from the somitic (contributes to somites) mesoderm on either side of it, at which time the notochord cells will start to elongate
-meanwhile, as the blastopore lip expands around toward the ventral side, additional mesoderm enters that will eventually form the heart, blood, kidney, bones and parts of other organs
-in the end, ectoderm covers the embryo, endoderm lines the archenteron on the inside, and mesoderm is found between the two
-major work horses are the cells in the deep layer of the IMZ (involuting marginal zone)-removal of this region stops archenteron formation...these cells travel along a fibronectin lattice secreted by the cells of the blastocoel roof...fibronectin pathways also direct the migration of the head mesenchyme cells....furthermore, the IMZ cells are capable of convergent extension even in culture....some of these abilities of the IMZ cells appear to rely upon the presence of the goosecoid protein, the transcriptional factor tht was produced downstream of the Wnt pathway
-a region of ectoderm, called the neural plate, overlies the notochord and will be induced by the notochord to form the neural tube and brain
gastrultion in chicks
-when we last discussed chick embryo, cleavage was complete and had given rise to a disc of tissue, called the blastoderm. The central portion , which is separated from the yolk by the subgerminal space is called the area pellucida. The peripheral portion, which is in direct contact with the yolk is called the area opaca.
-remember also that the blastoderm consists of an upper layer, called the epiblast, a lower layer, called the hypoblast, with a blastocoel found between the two layers (and above the subgerminal space)
-remember that the avian embryo is derived from the epiblast
-gastrulation begins with extensive cell movements in the posterior (=caudal)half of the epiblast
-first, a thickening in the epiblast appears towar the posterior end of the embryo, and it is due to initial involution and then ingression of the involuted epiblast cells into the interior of the blastocoel (these cels will become endoderm), as wellas by migration of the cells from the lateral regions of the epiblast toward the cener
-this thickening is the first sign of the primitive streak, a regions that runs along the anterior posterior axis and where cells are both proliferating and moving inward. It is somewhat analogous to the blastopore in amphibians. Human embryos also have a primitive streak. At the primitive streak stage, the first cells to ingress will form endoderm and extraembryonic mesoderm (which will contribute to extraembryonic membranes)
-the ingressing cells are botle shaped. Unlike the case in amphibians, however, the cels that enter into the embryo do so as individuals (and not as sheets) after having undergone an epihelial-to-mesenchyme transition
-as mentioned above, some of the first cells to ingress will form endoderm. They initially join the hypoblast cels, but later displace the hypoblast cells toward the margin. Ultimately, these first inward-migrating cells will form the foregut. The hypoblast cells will contribute to the extraembryonic membranes
-as these cell movements occur, the primitive streak elongates oward the future head region (p-->a)
-at this point, the primitive streak has become more complicated in structure, and now consists of a furrow, called the primitive groove. Hensen's node is thought of as similar to the dorsal lip, since cells can enter there and also since it has "organizing" activity
-hensen's node will eventually start moving ("regressing) toward the posterior end of the embryo. Cells that migrate from hensen's node into the embryo while it is still in its anterior position will contribute to head structures (head mesenchyme) and the anterior portion of the notochord
-as hensen's node moves posteriorly, cells entering at the level of hensen's node will contribute to the more posterior ortions of the notochord (specifically true for those cells moving from the anterior side of the node)as well as to the somites (segmented blocks of mesoderm that will form body and limb muscles, and portions of the vertebral column and dermis, specifically true for cells moving from the lateral portions of the node). It is often said taht hensen's node leaves the notochord and somites "in it's wake" as it regresses
-the ectodermal cells remain on the outside and spread by epiboly and will form epidermis and neural tube, brain etc. However, note that one portion of the neural plate (will form the neural tube), that is the median strip of it, is actually contributed by cells that ingressed from hensen's node
-the result of gastrulation is an out layer of ectoderm, a layer of endoderm (contiguous with hypoblast at the lateral margins), and mesoderm in the middle
-note that neurulation (formation of the brain and neural tube)has already begun in the anterior of the embryo while gastrulation is still occurring at the very posterior end (anterior ahead b/c hensen's node a-->p)
-the three embryonic germ layers as well as the hypoblast all contribute to 4 exraembryonic membranes that support development
-yolk sac:immediately surrounds the yolk. Its cells will digest the yolk and its blood vessels will carry the nutrients into the embryo
-amnion:encloses the embryo in a protective fluid-filled sac
-chorion:outermost membrane, thought to cushion the embryo from mechanical shock and also serves to facilitate gas exchange
-allantois:extens from the embryonic archenteron, functions as a disposal sac for uric acid waste produced by the embryo. Also, participates in gas exchange between the embryo and its environment
human gastrulation
-most similar to chick
-after the trophoblast stage, a hypoblast delaminates (toward the inside) from the ICM and the remaining cels of the ICM are now called the epiblast
-subsequently, the epiblast delaminates off another cell layer toward teh opposite side and this new layer is the amniotic ectoderm
-the hypoblast cells spread out to surround the blastocoel, herein referred to as the primitive "yolk" sac-but THERE IS NO YOLK MASS IN IT-however, the embryo behaves as if there is in terms of its cellular movements. The result is the so-called bilaminar stage embryo
-the embryonic epiblast will give rise to the embryo proper
-eventually, the amniotic cavity will spread and cushion the entire embryo
-the trophoblast forms structures important for circulation/influx of the maternal blood supply
-as in birds, the primitive streak begins to form in the posterior portion of the embryonic epiblast
-presumptive endoderm and mesoderm involute and then ingress through the primitive streak
-hensen's node exists (is sometimes also called the primitive knot) and cells that migrate through its primitive pit will contribute to the notochord, while cells ingressing through the primitive streak and moving laterally will contribute to endoderm (displaces hypoblast)and as well as mesodermal structures (such as somites)
-one major difference is that in humans, the presumptive notochord cells also first intercalate into the endoderm and then later separate out to form the notochord
-after gastrulation, the embryo is said to be at the trilaminar stage due to the existence of 3 obvious layers-ectoderm, mesoderm, endoderm
gastrulation in fish
-remember, no true blastocoel
-remember that when we last left the fish embryo (specifically zebrafish), it was a mound of cells perched atop a yolk mass, and the mound consisted of an outer layer of cells, called the enveloping layer, and an innder mass of deep cells. There is also a layer of used cells atop the yolk mass, and this layer is called the yolk syncytial layer
-gastrulation begins when the enveloping layer udnergoes epiboly, accompanied by radial intercalation with some of the deep cells
-microtubules appear to provide the driving force for the epibolic movement
-next, a germ rings forms around the embryo, consisting of an epiblast and a hypoblast layer-controversial whether it occurs by involution or ingression, but it is the deep cells that form the germ ring in any case. Hypoblast formation begins on the dorsal side of the embryo but then spreads around the entire circumference of the embryo. Meanwhile, the enveloping layer continues to spread until it covers the entire yolk mass.
-cells in the epiblast and hypoblast layer converge mediolaterally at the dorsal midline while extending anteriorly, and the result is the formation of the embryonic shield (triangular in shape).
-epiboly drives the base of the shield down to the vegetal pole while extension pushes the tip toward the animal pole such that a ridge of cells results
-the portion of the ridge from the equator of the animal pole forms the head and the remainder of the trunk and tail
-the epiblast will contribute to teh ectoderm of these structures, whilet he hypoblast will form the endoderm and mesoderm
-formation of the brain and spinal chord, begins right after gastrulation
-generally, begins with the formation of the neural plate-region of tall cells in the dorsal ectoderm
-the plate then closes to form a hollow tube caled the neural tube (although in fishes, chicks and mammals, at least some of the tube is initially solid and hollows out later)
-an embryo in the process of neurulation is called a neurula
neurulation in amphibians in detail
-the neural plate assumes the shape of keyhole, and so an embryo in the first phase is called a keyhole stage embryo
-the keyhole-shape neural plate is formed when dorsal ectoderm cells move toward the dorsal midline and then toward the anterior portion of the embryo, and these cells also change their shape to become more columnar-this shape change is dependent upon microtubules
-a depression, called the neural groove, forms along the midline of the neural plate, and ridges of cells called neural folds form along the boundary between the neural plate and the surrounding epidermis
-the neural plate extends in the A-P axis, but becomes narrower, especially in the posterior region. Overall, convergent extension appears to be at work here. Cells intercalate, but not across the midline, whcih somehow seems to trap the cells
-the anterior portion of the keyhole region will give rise to the brain and the posterior region will give rise to the spinal cord
-the second phase of neurulation begins after the keyhole stage, and involves (1)a spurt of elongation of the neural plate in the A-P axis, (2)the very rapid closure of the neural tube via curling over of the neural folds and fusion of epidermal ectoderm above the neural tube along the dorsal midline, (3)the neural tube then separates from the epidermal ectoderm, but some cells trapped in between and derived from the neural tube, called neural crest cells, will migrate to remote regions of the body and contribute to the peripheral nervous system, parts of the skull, and melanocytes (pigment cells)
closure of the neural tube via curling over of the neural folds and fusion of epidermal ectoderm above the neural tube along the dorsal midline
-closure apears to involve cell shape changes (the cells become wedge-shaped, and apical constriction is at least partly responsible for this change)
-in addition, the presecene of epidermis at the margins is required for full closure. The neural plate cells wedge themselves under these epidermal cells, and this wedging, in combination with apical constriction, is thought to allow the tube to close
-note that in chicks, in which neurulation begins in the anterior portion while the posterior portion is still gastrulating, there are hinge regions in the neural plate that facilitate closure. While most cells of the neural plate are columnar, there are 1-3 regions (depending on which part of the tube we are talking about, 1 in the spinal cord region, and 3 in the presumptive brain region) that are wedge-shaped and that promote closure
-note that in humans, neural tube closure begins int he neck region and then proceeds in the anterior and posterior directions
Neurulation part II-focus on Xenopus
-classic story is that the chordamesoderm (makes the notochord and somites)and/or the notochord itself are the primary inducers of neural tube formation but the story is more complicated than that, as it turns out...
-a formation of the neural tube really involves a series of inductions, occurring in both the lateral plane (called the planar induction)and the vertical plane (called the vertical inductino)
-evidence for vertical induction-transplantation of chordamesoderm from an early neurula under the blastocoel roof of early gastrulas results in neural tube induction in a region-specific manner (see fig 12.20)
-evidence for planar induction-in exogastrula, the chordamesoderm does not underlie the ectoderm. Notochord and somites develop in this situation, and the ectoderm does express neural-specific marker (e.g. N-CAM), presumably in response to signals traveling from the notochord adn somites laterally through a "neck of tissue" to the ectoderm
-evidence against simple planar induction-in exogastrula, despite the presence of N-CAM in the ectoderm, it does not appear to be capable of forming a complete neural tube (which goes to show that marker genes aren't the end of the story!)
-both planar and vertical induction are probably involved
-disinhibition allows Spemann's organizer to induce CNS formation
neurulation part II-both planar and vertical induction are probably involved, at least in Xenopus
-planar signals emanating from the dorsal lip in the early gastrula appear to predispose the adjacent ectoderm such that it will be competent to respond to vertical induction by the chordamesoderm during the late gastrula/early neurula stage
-in the early gastrula stage, the dorsal lip partially inhibits the expression of an epidermal-specific marker, called Epi1 (whose eventual absence is correlated with the ability to carry out the neural fate)in the adjacent putative neural ectoderm
-later, the chordamesoderm appears to totally inhibit the expression of Epi1 in the overlying ectoderm
-explanted dorsal ectoderm still expressing some Epi1 forms epidermis, but those no longer expressing Epi1 are capable of forming neural plate
-therefore, stepwise inhibition of Epi1 expression in dorsal ectodermal cells is correlated with a stepwise determination toward neural plate formation
-in response to vertical contact with chordamesoderm, the dorsal ectoderm will express large amounts of N-CAM and XIHbox6+ mRNAs (neural markers, the first is a cell adhesion molecule and the second a possible transcription factor). The ventral ectoderm responds only weakly (expresses a little), again suggesting that earlier signaling events make the dorsal ectoderm more competent to respond
disinhibition allows Spemann's organizer to induce CNS formation
-there is ample evidence to suggest that the TGF-B homologue, BMP-4 (stands for bone morphogenetic protein 4, because members of the BMP family were first ID'd for their ability to promote bone formation) is a signal which promotes ventral mesoderm and epidermis development while inhibiting dorsal mesoderm and CNS development-evidence on page 300
-therefore, BMP-4 can be thought of as an antagonist to Spemann's organizer
-Spemann's organizer antagonizes the effects of BMP-4 by secreting two proteins, called chordin and noggin, which bind to BMP-4 and prevent it from activating its receptor
-goosecoid, a transcription factor in the dorsal lip, is thought to stimulate the expression of noggin and chordin, which leads to inactivation of BMP-4, allowing the formation of dorsal mesoderm which is then ablet o induce CNS formation via poorly understood mechanisms (vertical and lateral)
-humans have noggins and chordins, but their exact roles are still being investigated
-one of the molecules that the notochord secretes is sonic hedgehog (SSH) which binds to Patched/Smoothened receptors leading to a cascade of events ending in transcriptional activation in the nucleus (see handout). The concentration gradient of SSH helps the neural tube to differentiate in a regional-specific manner. Mutations in sonic hedgehog in humans can lead to severe abnormalities (e.g., Holoprosencephaly, which in severe form, may have a one-eyed phenotype with a nose-like proboscis located above the eye).
neural tube
-consists of an outer covering called the external limiting membrane (a basement membrane) which surrounds a layero f neuroepithelial cells (forms the wall of the neural tube)
-these cells serve as stem cells that give rise to (1)neuroblasts (neuron precursor cells), (2)glioblasts (glial cell precursors, glial cells provide support/protection/insulation for the neurons), (3)and ependymal cells (form the lining of the lumen of the spinal cord and brain)
-the neuroblasts (the first cells to be produced by the neuroepithelium)migrate toward the external limiting membrane and where they form a layer of nodividing cells (the mantle layer)
-here the neuroblasts will mature by forming neurites (cellular extensions that transmit info away from the cell body, the longest of which is the axon), as well as dendrites (processes which transmit info to the cell body)
-next, the neuroepithelial cells give rise to the glioblasts, which then differentiate into glial cells (e.g. oligodendrocytes in the central nervous system)
-the neurons will develop such that their cell bodies will remain in the mantle layer (later called the gray matter). Outside of the mantle layer is the marginal layer, consisting of sprouting neurites. This marginal layer will later be termed the white matter, due to the fact that the axons int he layer are myelinated giving it a whitish appearance
-neuroepithelial cells that remain in dthe adult spinal cord are called ependymal cells, and they surround the central lumen of the spinal cord, now called the central canal. Whether these ependymal cells give rise to new neurons in adulthood is still being debated
the brain
-develops from the cranial part of the neural tube (clearly, we are focusing on vertebrates)
-early development is similar to what we jsut went over for the rest of the spinal cord but the central canal dilates and forms fluid filled spaces called ventricles (brain regions will develop around these ventricles)
-some clusters of mantle cells move into the marginal layer and form islands of grey matter called nuclei while other mantle cells move to the periphery so that grey matter eventually surrounds the white matter
-the first three brain regions to form are the prosencephalon-(forebrain), the mesencephalon (midbrain) and the rhombencephalon (hindbrain).
-these major regions will subdivide further