embryonic development
The theory of embryonic development we follow today was proposed 2000 years ago by Aristotle. This idea, called epigenesis, the form of an animal arises gradually from a generally formless egg. The organisms development is largely determined by the genome of the zygote and the organization of the cytoplasm of the egg cell. The first part of the development of a new animal is fertilization. The gametes, the sperm and the egg, are both highly specialized haploid cells that will combine chromosomes to produce the diploid zygote. The union of the sperm with the egg activates the egg, triggering the onset of embryonic development. The mammalian egg is cloaked by follicle cells that were released with the egg during ovulation. The sperm must migrate through this before it reaches the zona pellucida, the extracellular matrix of the egg. To get there the acrosome of the sperm (tip) releases hydrolytic enzymes to the zona pellucida enabling sperm to enter egg. The zona pellucida is made of 3 different glycoproteins. The protein ZP3 functions as a sperm receptor. In non mammals the acrosomal reaction releases hydrolytic enzymes that enable an elongating acrosomal process to penetrate the jelly coat of the egg and to bind to the vitellin
Cells nearest to the ZPA give rise to the posterior structure and those farthest away form the anterior structure. The yolk sac membrane of mammals does not contain yolk but is the site of blood cell synthesis. All vertebrates require an aqueous environment to develop and terrestrial animal evolved to suit reproduction in dry environments. This stage is called the blastocyst. Gastrulation differs from one animal group to another but follows the same fundamental principals. Cell adhesion molecules (CAMs) are glycoproteins on the surface of a cell involved in cell migration and lend to a stable tissue structure. Continued cleavage forms produces a solid ball known as morula. In contrast mammalian cells remain totipotent until they become arranged in a trophoblast. The molecular cues that control pattern formation, called positional formation, tell a cell where it is with respect to the animals body and help determine how the cell and its decedents respond to future molecular signals. The allantois is incorporated into the umbilical cord where it forms blood vessels to transport oxygen and nutrients to the embryo and waste removal. In avian (birds) development, a bird egg in cleavage forms a cap of divided cells called the blastodisc over the undivided yolk. A limb bud consists of a core of mesoderm tissue covered by a layer of ectoderm. The ZPA is necessary for proper pattern formation along the anterior-posterior axis of the limb. These are homologous to the ones of birds. The AER is required for the outgrowth of the limb along the proximal-distal axis and for patterns along its axis.
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