amniotes
amniotes also include mammals, which are egg-laying (few) or placental
mammal cleavage
-mammals exhibit rotational holoblastic cleavage
-second cleavage division characterized by orthogonal division planes, one meridional and one equatorial
-blastomere pairs appear to rotate relative to each other
-cell divines are asynchronous
-activation of zygotic gene expression begins early (relative to number of cleavage divisions), and many zygotic genes are required for progression of cleavage
mammal cleavage timeline
cleavage divisions are relatively slow, occurring 12-24 hrs apart
blastomere compaction
-around the 8 cell stage, loosely arranged blastomeres form close contacts with each other and begin to undergo compaction
-this coincides with the expression of adhesion molecules such as E-cadherin
-outside cells form tight junctions that “seal” the embryo and inside cells form gap junctions that allow for exchange of small molecules
morula to blastocyst
- at 16 cells, the embryo is a morula with clear inner and outer cells
- outer morula cells become trophoblast cells, and inner morula become inner cell mass cells: these are both characteristic of the blastocyst
trophoblast
a layer of tissue on the outside of a mammalian blastula, supplying the embryo with nourishment and later forming the major part of the placenta.
morula
a solid ball of cells resulting from division of a fertilized ovum, and from which a blastula is formed.
inner cell mass cells
The inner cell mass of the blastocyst is composed of two kinds of cells: those that will become the mature organism (the epiblast), and those that will develop into the placenta, the chorion, and the amniotic membranes
developmental adaptations for internal embryonic development
- mammalian eggs don’t rely on yolk, get nutrients directly from mother –> adaptation required to restructure lower oviduct into uterus
-embryos evolved trophoblast to interact with mother
-trophoblast forms chorion induces uterus to form decide, both parts of placenta
-the ICM will form epiblast for the embryo, the amnion, and the allantois
-the ICM will also form hypoblast cell types that create a yolk sac
human gastrulation similar to chick
- the ICM is initially localized to one side of the blastocoel
- the ICM then forms two layers; epiblast and hypoblast
-hypoblast cells migrate away and line the blastocoel; this forms the fluid-filled yolk sac
-epiblast cells for a second layer of epiblast cells that form the amnion; fluid collects between the amnion and epiblast to form the amniotic cavity - these events result in two distinct embryonic cavities separated by a bilaminar germ disc (two layer blastodisc) composed of epiblast and hypoblast
-this arrangement is like the chick blastodisc, except with a yolk sac instead of yolk
-like in the chick blastodisc, gastrulation begins in the posterior with a primitive groove and node that extends anteriorly
-the process of ingression, EMT, and migration are largely conserved
how are regional embryonic identities established?
- established by conserved homeotic genes
-the homeotic (Hox) genes that impart segment identity in flies are conserved in mammals
-the Hox genes exist in mammals in four sets (Hoxa to Hoxd) - as in flies, gene expression in the embryo correlates with gene position within the cluster
-note that timing of positional gene expression is different in mammals; anterior develops before posterior, so expression varies accordingly
how genes specify vertebral identity along the A/P axis
- the mouse and chick have a similar number of overall vertebrae, but diff proportions of cervical and thoracic vertebrae
- in both animals, the cervical/thoracic boundary is established by end and beginning of Hox5 and Hox6, respectively
- thus, identity codes are likely the same, with positional information being slightly different
-examination of the thoracic/lumbar boundary supports this model, as it coincides with the Hox9/10 transition in both animals
Homeotic transformation of vertebral identity in mouse
- hox gene mutations in mice demonstrate homeosis and the anteriorization rule
-when Hox10 is mutated (knocked-out), posterior lumbar vertebrae develop ribs like anterior thoracic vertebrae
-when Hox11 is mutated, posterior sacral vertebrae develop as anterior lumbar vertebrae
blastodisc
a blastula having the form of a disk of cells on top of the yolk in the eggs of reptiles and birds.
