Chapter 9b Flashcards
The basic rule of amniote cell specification is that germ layer identity (ecto-, meso-, or endoderm) is established before gastrulation starts, but the specification of cell type is controlled by
inductive influences during and after migration through the primitive streak.
Cells migrating through the anterior end of the streak pass down into the blastocoel and migrate anteriorly (forming the endoderm, head mesoderm, and notochord); cells passing through the more posterior portions give rise to the majority of mesodermal tissues.
Migration through Hensen’s node:
- First cells become the pharyngeal endoderm of the foregut. Once deep within the embryo, they migrate anteriorly and eventually displace the hypoblast cells (these become confined to a region in the anterior portion of the area pellucida = germinal crescent)
- Next ones also move anteriorly, but not as far ventrally. They remain between the endoderm and the epiblast to form the prechordal plate mesoderm => the head of the avian embryo forms anterior to Hensen’s node.
- Next ones become chordamesoderm – has two components: the head process and the notochord.
- As the primitive streak regresses, the cells deposited by the regressing Hensen’s node will become the notochord.
Germinal crescent
does not form any embryonic structures, but contain the precursors of the germ cells, which later migrate through the blood vessels to the gonads.
Migration through the primitive streak
Cells migrate to the primitive streak, and as they enter the embryo, the cells separate into two layers.
The deep layer joins the hypoblast along its midline, displacing the hypoblast cells to the sides. These cells give rise to the endodermal organs , as well as to most of the extraembryonic membranes (the hypoblast and peripheral cells of the area opaca form the rest).
The second migrating layer spreads to form a loose layer of cells between the endoderm and the epiblast. This middle layer of cells generates the mesodermal portions of the embryo and the mesoderm lining the extraembryonic membranes.
By 22 hrs of incubation, most of the presumptive endodermal cells are in the interior of the embryo, although presumptive mesodermal cells continue to migrate inward for a longer time.
Regression of the primitive streak and epiboly of the ectoderm
Mesodermal ingression continues
- > the primitive streak starts to regress
- > moving Hensen’s node from near centre of area pellucida to a more posterior position
- > leaves the dorsal axis in its wake (including the notochord).
The notochord is laid down in a
head-to-tail direction, starting at the level where the ears and hindbrain form and extending caudally to the tail bud.
Pharyngeal endoderm + head mesoderm induce
anterior parts of the brain
Notochord induce
the hindbrain and spinal cord
By this time the presumptive endodermal and mesodermal cells have entered the embryo and the epiblast is composed entirely of presumptive ectodermal cells.
While the first two are moving inward, the last proliferate and migrate to surround the yolk by epiboly.
Herculean task, takes 4 days!
the locomotor apparatus of the marginal cells?
Only the cells of the outer margin of the area opaca attach firmly to the vitelline envelope.
These cells can extend enormous (500 microm) cytoplasmic processes onto the vitelline envelope
=> the locomotor apparatus of the marginal cells, by which they pull the other ectodermal cells around the yolk.
As avian gastrulation draws to a close:
- The ectoderm has surrounded the embryo
- The endoderm has replaced the hypoblast
- The mesoderm has positioned itself between these two regions
As a consequence of the sequence in which the head endo-mesoderm and notochord are established, avian (and mammalian, reptilian and teleost fish) embryos exhibit a distinct
anterior-to-posterior gradient of developmental maturity. Anterior has had a ”head start”.
Although the formation of the chick body axes is accomplished during gastrulation, axis specification begins
earlier, during the cleavage stage.
The role of gravity and the PMZ
The conversion of the radially symmetrical blastoderm into a bilaterally symmetrical structure appears to be determined by gravity:
- As the ovum passes through the hen’s reproductive tract, it is rotated for about 20 hrs in the shell gland
- This spinning, 15 revolutions pr. hr, shifts the yolk such that its lighter components (probably containing stored maternal determinants for development) lie beneath one side of the blastoderm.
- This imbalance tips up one end of the blastoderm, and that end becomes the posterior marginal zone, where primitive streak formation begins.
It is not known what interactions cause this specific portion of the blastoderm to become the PMZ, but once it is formed, it controls the other regions of the margin.
- Not only do the cells of the PMZ initiate gastrulation, they also prevent other regions of the margin from forming their own primitive streaks.
- A graft of PMZ tissue (posterior to and including Koller’s sickle) is able to induce a primitive streak and Hensen’s node without contributing cells to either structure.
AP patterning
The patterning of the definitive AP axis occurs differently for the mesoderm and neural ectoderm, but in both cases the process involves timing (the sequential generation of cells from a zone of undifferentiated proliferating cells) and the influence of caudalizing molecules.
While they are still in the epiblast, but close to the primitive streak, the mesoderm cells appear to receive instructions that tell them exactly where they are along the AP axis.
The entire length of the notochord at the midline is derived from cells that are present in Hensen’s node by the full primitive streak stage;
Descendants of progenitor cells gradually leave as the node regresses, laying down the chordamesoderm and the ventral midline of the neural tube (future floor plate of the spinal cord).
AP identities along the axis from the hindbrain to the tail are specified as a function of the time of emergence from the primitive streak and Hensen’s node.
Hox genes
vertebrate homologues of the homeotic (Hom-C) genes of Drosophila.
- Specify the identity of cells along the AP axis
- Four gene clusters (HoxA, HoxB, HoxC, and HoxD)
- Rather than individual Hox genes appearing at particular segmental levels, there is a nested set of Hox gene expression
- ”Anterior” Hox genes are identified with lower numbers, e.g. Hoxb4
- The more posterior cells express more Hox genes than the more anterior cells do.
LR axis formation
The distinction between the sides is primarily regulated by the left-sided expression of two proteins:
- The paracrine Nodal
- The TF Pitx2
Mechanism of Nodal gene expression activation differs among the vertebrate classes.
The embryo ”hatches” from the zona pellucida upon reaching the uterus. During its migration to the uterus,
the zona prevents the embryo from prematurely adhering to the oviduct rather than traveling to the uterus.
Cilia in oviduct push embryo along.
Mammalian cleavage
Meiosis is completed after sperm entry, and the first cleavage begins about a day later.
Cleavages in mammalian eggs are among the slowest in the animal kingdom, taking place some 12-24 hrs apart.
In many (but not all), the first cleavage is a normal meridional division; however, in the second, one of the blastomeres divides meridionally and the other equitorially (rotational cleavage).
Mammalian blastomeres do not all divide at the same time = asynchronous. => no exponential growth, but frequently contain odd numbers of cells.
