Last Part of Gastrulation + Start of Neurulation

Question 1

human gastrulation - what happens after the trophoblast stage w/ the ICM?

Answer 1

a hypoblast delaminates (towards the inside) from the ICM, and the remaining cells of the ICM are now called the epiblast.

Question 2

what happens subsequently with this new epiblast?

Answer 2

the epiblast delaminates off another cell layer toward the opposite side and the new layer is the amniotic ectoderm

Question 3

after epiblast delamination, what occurs w/ the hypoblast?

Answer 3

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.

Question 4

what eventually happens to the epiblast and amniotic cavity?

what about trophoblast?

Answer 4

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

Question 5

what forms in the posterior portion of epiblast? what movements occur here?

Answer 5

as in birds, primitive streak begins to form in posterior portion of embryonic epiblast

presumptive endoderm and mesoderm involute, then ingress through primitive streak.

Question 6

what is the primitive knot? what happens to cells migrating into it?

Answer 6

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)

Question 7

what is one major difference in humans (regarding notochord)?

Answer 7

presumptive notochord cells also first intercalate into endoderm, then later separate out to form the notochord

Question 8

what is the state of the embryo after gastrulation?

Answer 8

embryo is said to be at the trilaminar stage, due to existence of 3 obvious layers - ectoderm mesoderm, and endoderm (compared to bilaminar stage)

Question 9

describe the state of the fish embryo at the blastula stage.

Answer 9

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.

Question 10

when does gastrulation begin?

Answer 10

begins when enveloping layer undergoes epiboly, accompanied by radial intercalation w/ some of the deep cells.

Question 11

what is the driving force for epiboly?

Answer 11

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

Question 12

what happens following epiboly, and how?

Answer 12

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).

Question 13

describe hypoblast/enveloping layer movement.

Answer 13

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.

Question 14

how does the embryonic shield form?

what would you expect to happen if it were transplanted to a diff. host?

Answer 14

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

Question 15

what movements occurs w/ the shield?

Answer 15

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.

Question 16

what do the cells of the shield and those adjacent to the shield contribute to?

Answer 16

shield contributes to chordamesoderm (contributed by hypoblast portion of shield)

cells adjacent to the shield will form the somites.

Question 17

what does the epiblast contribute to?

Answer 17

convergent extension of the epiblast to midline forms the neural keel which will form the neural tube.

remainder of the epiblast becomes epidermis

Question 18

what will form the head, remainder of trunk and tail?

what does enveloping layer contribute to?

Answer 18

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.

Question 19

what is neurulation, when does it begin and how?

Answer 19

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)

Question 20

what does the plate form, and how?

Answer 20

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)

Question 21

what is the first stage of neurulation? what is an embryo in the process of neurulation called?

Answer 21

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.

Question 22

how is the keyhole shaped neural plate formed?

Answer 22

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)

Question 23

describe the next step of neurulation after plate formation

Answer 23

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.

Question 24

describe movement of the neural plate, and which types are present

Answer 24

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

Question 25

what does the keyhole region give rise to?

Answer 25

anterior portion of keyhole region: gives rise to brain

posterior region: gives rise to spinal cord

Question 26

when does the second phase of neurulation begin? what does it involve?

Answer 26

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.

Question 27

how does closure occur?

Answer 27

appears to involve cell shape changes (cells become wedge-shaped, apical constriction is at least partly responsible for this change)

Question 28

what else is required for full closure?

Answer 28

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.

Question 29

what about chick neurulation? how does that (and closure) generally occur?

Answer 29

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

Question 30

what shape are cells in the neural plate?

Answer 30

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)

Question 31

what then happens to neural tube? what are neural crest cells, what will they become?

Answer 31

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