Gastrulation in Chicks Flashcards
review of chick embryo state at gastrulation - describe the major features
complete cleavage gives rise to a disc of tissue, called the blastoderm. central portion, separated from the yolk by subgerminal space is called the area pellucida. the peripheral portion, in direct contact with the yolk is the area opaca.
what does the blastoderm consist of? where is the blastocoel?
blastoderm consists of an upper layer, the epiblast (gives rise to avian embryo), lower layer, the hypoblast (extraembryonic endoderm), w/ blastocoel in the middle of the layers (above the subgerminal space)
what does gastrulation begin with?
begins w/ extensive cell movements in the posterior (or caudal) half of the epiblast
first, what happens w/ the epiblast near the P end of the embryo? what is this caused by?
a thickening in epiblast appears towards P end, due to initial involution and then ingression of the involuted epiblast cells into the interior of the blastocoel (these cells become endoderm), as well as by migration of cells from lateral regions of epiblast towards center
what is this thickening a sign of? describe it.
thickening is the first sign of the primitive streak, a region running along the A-P axis and where cells are both proliferating and moving inward. somewhat analogous to the blastopore in amphibians.
human embryos also have a primitive streak.
what cell movements occur at the primitive streak stage? what is the result?
at primitive streak stage, 1st cells to ingress will form endoderm + extraembryonic mesoderm (will contribute to extraembryonic membranes).
what shape are the ingressing cells? how do they enter the embryo?
ingressing cells are bottle shaped. however, unlike in amphibians, the cells that enter into the embryo do so as individuals (not sheets) after having undergone an epithelial-to-mesenchyme transition.
what happens with the 1st group of inward-migrating cells?
will initially join the hypoblast cells, but later displace them towards the margin. ultimately, these first inward-migrating cells will form the foregut. hypoblast cells will contribute to extraembryonic membranes.
what occurs with the primitive streak as cell movements occur?
streak gets more elongated toward the future head region.
at this point, prim. streak has become more complex; now consists of a furrow (primitive groove), flanked by two ridges, called the primitive ridges.
what do the ridges end in? describe it.
primitive ridges end anteriorly in a thickening known as Henson’s node, contains a funnel-shaped depression called the primitive pit, the anterior end of the prim. groove.
Hensen’s node is thought of as similar to dorsal lip; cells can enter there and it has “organizing” activity.
what happens to Hensen’s node? what about cells that migrate into it?
will enventually start moving (“regressing”) toward posterior end of embryo.
cells that migrate from Hensen’s node into embryo while it’s still in anterior position will contribute to head structures (head mesenchyme) + anterior portion of notochord.
what happens to cells that enter Hensen’s node as it moves posteriorly?
cells entering at the level of the node will contribute to more posterior portions of the notochord (specifically true for cells moving from anterior side of node) + somites (segmented blocks of mesoderm, will form body + limb muscles, portions of vertebral column, true for cells moving from lateral positions of node).
what actions do ectodermal cells take?
remain on the outside, spread by epiboly and will form epidermis and neural tube, brain, etc.
note that the median portion of the neural plate (plate forms neural tube) is actually contributed by cells that ingressed from Hensen’s node
what is the result of gastrulation?
outer layer of ectoderm, layer of endoderm (contiguous w/ hypoblast at lateral margins), and mesoderm in the middle.
when does neurulation begin?
neurulation (formation of brain + neural tube) already has begun in anterior end of embryo while gastrulation is still occurring at very posterior end.
