Embryogenesis

Embryogenesis is the process by which the embryo is formed and develops. It starts with the fertilization of the ovum, which is then called a zygote. The zygote undergoes rapid mitotic divisions with no significant growth (a process known as cleavage) and cellular differentiation, leading to development of an embryo. It occurs in both animal and plant development, but this article addresses the common features among different animals.

The zygote
The egg cell (and hence the fertilized egg) is always asymmetric, having an "animal pole" (future ectoderm and mesoderm) and a "vegetal pole" (future endoderm), it is also covered with different protective envelopes. The first envelope, the one which is in contact with the membrane of the egg, is made of glycoproteins and is called vitelline membrane (zona pellucida in mammals). Different taxa show different cellular and acellular envelopes.

Cleavage
The zygote undergoes rapid cell cycles with no significant growth, producing a cluster of cells that is the same size as the original zygote. Depending mostly on the amount of yolk in the egg, the cleavage can be holoblastic (total) or meroblastic (partial). The different cells derived from cleavage (up to the blastula stage) are called blastomeres.

In holoblastic eggs the first cleavage always occurs along the vegetal-animal axis of the egg, the second cleavage is perpendicular to the first. From here the spatial arrangement of blastomeres can follow various patterns, due to different planes of cleavage, in various organisms:

Blastula and Gastrula
Blastulation begins after the cleavage has produced 128 cells, in this stage the embryo is called a blastula. The blastula is usually a spherical layer of cells (the blastoderm) surrounding a fluid-filled or yolk-filled cavity (the blastocoel).

In mammals blastulation leads to the formation of the blastocyst, which must not be confused with the blastula; even though they are similar in structure their cells have different fates.

During gastrulation cells migrate to the interior of the blastula, consequently forming two (in diploblastic animals) or three (triploblastic) germ layers. The embryo during this process is called a gastrula.


 * Among the different animals, different combinations of the following processes occur to place the cells in the interior of the embryo:
 * Epiboly (expansion of one cell sheet over other cells).
 * Ingression (cells move with pseudopods)
 * Invagination
 * Delamination (the external cells divide, leaving the daughter cells in the cavity)
 * Polar proliferation


 * Other major changes during gastrulation:
 * Heavy RNA transcription using 'zygotic' genes; up to this point the RNAs used were maternal (stored in the unfertilized egg).
 * Cells start major differentiation processes, losing their pluripotentiality.

Organogenesis
At some point after the different germ layers are defined, organogenesis begins. The first stage in vertebrates is called neurulation, where the neural plate folds forming the neural tube. Other common organs or structures which arise at this time include the heart and somites, but from now on embryogenesis follows no common pattern among the different taxa of the animal kingdom.

In most animals organogenesis along with morphogenesis will result in a larva. The hatching of the larva, which must then undergo metamorphosis, marks the end of embryonic development.

Reference

 * What is a cell? 2004. A Science Primer: A Basic Introduction to the Science Underlying NCBI Resources. NCBI.

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