Last Part of Gastrulation + Start of Neurulation Flashcards
human gastrulation - what happens after the trophoblast stage w/ the ICM?
a hypoblast delaminates (towards the inside) from the ICM, and the remaining cells of the ICM are now called the epiblast.
what happens subsequently with this new epiblast?
the epiblast delaminates off another cell layer toward the opposite side and the new layer is the amniotic ectoderm
after epiblast delamination, what occurs w/ the hypoblast?
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 cellular movements. result is the “bilaminar stage” embryo.
what eventually happens to the epiblast and amniotic cavity?
what about trophoblast?
embryonic epiblast will give rise to embryo proper
amniotic cavity will eventually spread and cushion the entire embryo
the trophoblast forms structures important for circulation/influx of maternal blood supply
what forms in the posterior portion of epiblast? what movements occur here?
as in birds, primitive streak begins to form in posterior portion of embryonic epiblast
presumptive endoderm and mesoderm involute, then ingress through primitive streak.
what is the primitive knot? what happens to cells migrating into it?
its a node that exists similar to Hensen’s node; cells that migrate thru its primitive pit will contribute to the notochord, whereas cells ingressing thru primitive streak and moving laterally will contribute to endoderm (displaces hypoblast) as well as mesodermal structures (e.g. somites)
what is one major difference in humans (regarding notochord)?
presumptive notochord cells also first intercalate into endoderm, then later separate out to form the notochord
what is the state of the embryo after gastrulation?
embryo is said to be at the trilaminar stage, due to existence of 3 obvious layers - ectoderm mesoderm, and endoderm (compared to bilaminar stage)
describe the state of the fish embryo at the blastula stage.
it is a mound of cells perched atop a yolk mass, consisting of an outer layer of cells (enveloping layer) and an inner mass of deep cells.
also a layer of fused cells atop the yolk mass, called the yolk syncytial layer.
when does gastrulation begin?
begins when enveloping layer undergoes epiboly, accompanied by radial intercalation w/ some of the deep cells.
what is the driving force for epiboly?
MTs and actin/myosin appear to provide driving force for epibolic motion.
experiments w/ magnetism indicate that mechanical stress experienced by cells during involution helps to coordinate epiboly + changes in gene expression required for cell specification
what happens following epiboly, and how?
a germ ring forms around the embryo, consisting of an epiblast and hypoblast layer - controversial whether it forms via involution or ingresion
actually has features more like ingression on dorsal side, and more like involution on ventral (regardless, is inward movement of deep cells).
describe hypoblast/enveloping layer movement.
hypoblast formation begins on dorsal side of embryo but then spreads around the entire circumference of the embryo.
meanwhile, enveloping layer continues to spread until it covers the entire yolk mass.
how does the embryonic shield form?
what would you expect to happen if it were transplanted to a diff. host?
cells in epiblast and hypoblast layer converge mediolaterally at dorsal midline while extending anteriorly, result = embryonic shield (triangular in shape).
since the shield is considered an organizer (STRONG inducer), ectopic transplantation onto a host would result in the formation of extra embryonic axes
what movements occurs w/ the shield?
epiboly drives the base of the “shield” down to the vegetal pole while extension pushes the tip towards the animal pole st a ridge of cells results.
what do the cells of the shield and those adjacent to the shield contribute to?
shield contributes to chordamesoderm (contributed by hypoblast portion of shield)
cells adjacent to the shield will form the somites.
what does the epiblast contribute to?
convergent extension of the epiblast to midline forms the neural keel which will form the neural tube.
remainder of the epiblast becomes epidermis
what will form the head, remainder of trunk and tail?
what does enveloping layer contribute to?
portion of ridge from equator to animal pole forms the head and the remainder of the trunk and tail
enveloping layer contributes to periderm, an epidermal layer that covers the fish at embryonic stages.
what is neurulation, when does it begin and how?
is the formation of brain and spinal cord, begins right after gastrulation
generally begins w/ the formation of the neural plate (region of tail cells in dorsal ectoderm)
what does the plate form, and how?
plate then closes to form a hollow tube called the neural tube (although in fishes, chicks, and mammals, at least some of the tube is solid and hollows out later)
what is the first stage of neurulation? what is an embryo in the process of neurulation called?
neural plate assumes the shape of a keyhole, so an embryo in first phase is called a keyhole stage embryo.
an embryo in the process of neurulation is called a neurula.
how is the keyhole shaped neural plate formed?
keyhole-shaped neural plate is formed when dorsal ectoderm cells move toward the dorsal midline, then toward the anterior portion of the embryo, and these cells also change their shape to be more columnar (dependent on MTs)
describe the next step of neurulation after plate formation
a depression (called the neural groove) forms along midline of the neural plate
ridges of cells called neural folds form along the boundary between the neural plate and surrounding epidermis.
describe movement of the neural plate, and which types are present
neural plate extends along A-P axis, becomes narrower (especially in P region). convergent extension is at work here.
cells intercalate, but not across midline, somehow traps cells and prevents their crossing
what does the keyhole region give rise to?
anterior portion of keyhole region: gives rise to brain
posterior region: gives rise to spinal cord
when does the second phase of neurulation begin? what does it involve?
begins after keyhole stage, involves:
spurt of elongation of neural plate in the A-P axis and
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.
how does closure occur?
appears to involve cell shape changes (cells become wedge-shaped, apical constriction is at least partly responsible for this change)
what else is required for full closure?
presence of epidermis at margins is required for full closure. neural plate cells wedge themselves under these epidermal cells, and this wedging, in combination w/ apical constriction, is thought to allow closure.
what about chick neurulation? how does that (and closure) generally occur?
neurulation begins in anterior portion (or future midbrain), then zips in both directions (starts while P portion is still gastrulating).
there are hinge regions in neural plate that facilitate closure
what shape are cells in the neural plate?
most are columnar
there are 1-3 regions (depends on region, 1 in spinal cord region, 3 in presumptive brain region) that are wedge-shaped to promote closure
*note: in mammals, closure occurs starting at multiple sites (probably 5 in humans)
what then happens to neural tube? what are neural crest cells, what will they become?
neural tube separates from epidermal ectoderm (involves differential expression of E-vs. N-cadherin).
some cells trapped in between + derived from neural tube (neural crest cells) will migrate to remote regions of body, contribute to several areas and cell types
e.g. peripheral NS, parts of skull, and melanocytes
