birds development Flashcards
amniotes
reptiles and bird; egg-laying or placental
amniote egg
characterized by a set of membranes (amnions) that allow for embryonic development on land
amnion
surrounds the embryo, forming the fluid-filled amniotic cavity within which the developing embryo floats; prevents desiccation
yolk sac
supports nutrient uptake and circulatory system development
allantois
collects metabolic waste products
chorion
outer enclosure that contains blood vessels important for gas exchange
cleavage and early development in birds
-fertilization in oviduct before addition of albumin (reservoir of water and proteins) and shell
-cleavage occurs on first day before egg is laid
- eggs are telolecithal (have yolk on one end)
- cleavage is mesoblastic (incomplete) –> forms blastoderm –> blastoderm absorbs water from the albumin and transfer it underneath, forming the subterminal cavity
- central, deep cells of the blastoderm die, leaving behind a single-cell layer called the epiblast and enlarged subterminal cavity
-cells from the edges and some cells from the epiblast layer eventually form the underlying hypoblast layer
bird cleavage
-no, not chicken breast
-cleavage is meroblastic (incomplete) and discoidal, forming a blastoderm
blastoderm dev and onset of gastrulation
- the one cell thick epiblast initially forms the area pellucida
-the peripheral ring where deep cells survive becomes area opaca
-marginal zone cells are area boundaries
-in the posterior marginal zone (PMZ), an epiblast thickening forms the Koller’s sickle
-at the same time, cells delaminate across the pellucida epiblast to initially form hypoblast islands, then spread to form primary hypoblast
-a sheet of cells from the posterior opaca region migrate anteriorly under the epiblast forming secondary hypoblast
-the primary and secondary hypoblast cells eventually together form hypoblast (?)
what does the primitive streak establish?
the primitive streak defines major body axes as it moves posterior to anterior. cells move in the streak down from dorsal to ventral and the streak position at the midline divides right and left
where does the primitive streak form initially?
the primitive streak begins to form in the posterior epiblast at Koller’s sickle
what is the primitive streak equivalent to?
the primitive streak is the functional equivalent of the amphibian dorsal blastopore lip
parts of the primitive streak
primitive streak begins by forming the primitive groove in the posterior epiblast. the anterior end of the primitive groove is called Hensen’s node. they create the streak by extending toward the anterior
how does the primitive streak form?
Hensen’s node and the primitive groove create the streak by extending toward the anterior. this formation occurs by convergent extension; the initial shape is wide and narrows during extension
primitive streak movement
Formation is via convergent extension. Eventually the streak will reach 60-75% across the area pellucida. the streak then regresses as Henson’s node moves posteriorly, leaving notochord behind as it goes
how does the primitive streak create cell specification?
when the streak reaches to about 50% across of the pellucida, cells begin to migrate down through the groove by EMT and become endoderm and mesoderm. cells migrating through the node become notochord, so Henson’s node is essentially the chick organizer. cells engrossing at different positions along the streak (A/P axis) take on different mesodermal and endodermal fates. Epiblast cells that remain epiblast near the streak become neural tissue, with epidermis formatting at a distance
Gravity establishes A/P axis
-in the developing egg, the blastoderm is lighter/less dense than the yolk, so it tends to be positioned on top because of gravity
-slow egg rotation in the reproductive tract causes the blastoderm to tilt such that one side will be elevated relative to the other
-the upper end invariably becomes posteriors here the streak originates
left/right axis specification
-sonic hedgehog (shh) is assymetrically expressed left of Hensen’s node
-TGF-B signaling (Activin/ BMP4) signaling inhibits Shh transcription on the right
-Shh activates Cerberus –> activates Nodal –> activates Pitx2 which directs asymmetric development
-lefty expression at the midline blocks cerberus from moving into the right side
anamniotes v amniotes
The anamniotes are an informal group of craniates comprising all fishes and amphibians, which lay their eggs in aquatic environments. They are distinguished from the amniotes (reptiles, birds and mammals), which can reproduce on dry land either by laying shelled eggs or by carrying fertilized eggs within the female.
