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The difference between a mammalian embryo and an embryo of a lower chordate animal is evident starting from blastula stage. Due to that fact, the developing mammalian embryo at this stage is called a blastocyst, not a blastula, which is more generic term. There are also several other differences from embryogenesis in lower chordates.
Embryogenesis continues with the next stage of gastrulation, when the three germ layers of the embryo form in a process called histogenesis, and the processes of neurulation and organogenesis follow. The entire process of embryogenesis involves coordinated spatial and temporal changes in gene expression, cell growth, and cellular differentiation.
These genes play a role in germ line development to localize nanos mRNA to the posterior and localize germ cell determinants. Drosophila progeny with mutations in these genes fail to produce pole cells and are thus sterile, giving these mutations the name 'grandchildless'. The genes oskar, nanos and germ cell-less (gcl) have important roles ...
The entire process of embryogenesis can be described with the aid of two maps: an embryo map, a temporal sequence of 3-dimensional images of the developing embryo, showing the location of cells of the many cell types present in the embryo at a given time, and an embryogenic tree, a diagram showing how the cell types are derived from each other ...
The egg cell is generally asymmetric, having an animal pole (future ectoderm). It is covered with protective envelopes, with different layers. The first envelope – the one in contact with the membrane of the egg – is made of glycoproteins and is known as the vitelline membrane (zona pellucida in mammals).
The blastocyst is a structure formed in the early embryonic development of mammals.It possesses an inner cell mass (ICM) also known as the embryoblast which subsequently forms the embryo, and an outer layer of trophoblast cells called the trophectoderm.
While the cuboidal hypoblast cells delaminate ventrally, away from the embryonic pole, to line the blastocoele, the remaining cells of the inner cell mass, situated between the hypoblast and the polar trophoblast, become the epiblast and comprise columnar cells. In the mouse, primordial germ cells are specified from epiblast cells. [4]
The mammalian equivalent to the chick hypoblast is called the anterior visceral endoderm (AVE) [10] and creates an anterior region by secreting antagonists of Nodal. [5] In the mouse, (the most studied mammalian model organism for this) the hypoblast restricts Nodal activity using Cerberus and Lefty1 while birds use only Cerberus. [5]