embryogenesis

Embryonic cells differentiate into a variety of different cell types. Image from NCBI.

Embryogenesis is the process by which the embryo is formed and develops. It starts with the fertilization of the ovum (or egg) which, after fertilization, is referred to as 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, 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), two of three primitive tissue types, and a "vegetal pole" (future endoderm), it is also covered with different protective envelopes. 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). Different taxa show different cellular and acellular envelopes englobing the vitelline membrane.

Cleavage

Cell divisions (cleavage).

Cell divisions with no significant growth, producing a cluster of cells that is the same size as the original zygote is called cleavage. The different cells derived from cleavage, up to the blastula stage, are called blastomeres. Depending mostly on the amount of yolk in the egg, the cleavage can be holoblastic (total) or meroblastic (partial)^[1].

Holoblastic cleavage occurs in animals with little yolk in their eggs, such as humans and other mammals who receive nourishment as embryos from the mother, via placenta or milk. On the other hand, meroblastic cleavage occurs in animals whose eggs have more yolk; i.e. birds and reptiles. Because cleavage is impeded in the vegetal pole, there is a very uneven distribution and size of cells, being more and smaller at the animal pole of the zygote^[2].

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:

The end of cleavage is known as midblastula transition and coincides with the onset of zygotic transcription

Blastula and Gastrula

After the cleavage has produced over 100 cells, the embryo is called a blastula^[3]. The blastula is usually a spherical layer of cells (the blastoderm) surrounding a fluid-filled or yolk-filled cavity (the blastocoel).

Mammals at this stage form a structure called the blastocyst^[4], characterized by an inner cell mass that is not present in the blastula. The blastocyst 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. The germ layers are referred to as the ectoderm, mesoderm and endoderm. In diploblastic animals only the ectoderm and the endoderm are present ^[5].

• 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^[6]
  • Ingression - cells move with pseudopods^[7]
  • Invagination - forming the mouth, anus, and archenteron^[8]
  • Delamination - the external cells divide, leaving the daughter cells in the cavity^[9]
  • Polar proliferation

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

In most animals, a blastopore is formed at the point where cells are entering the embryo. Two major groups of animals can be distinguished according to the blastopore's fate. In deuterostomes the anus forms from the blastopore, while in protostomes it develops into the mouth. See Embryological origins of the mouth and anus for more information.

Somitogenesis

Somitogenesis is the process by which somites are produced. These segmented tissue blocks differentiate into skeletal muscle, vertebrae, and dermis of all vertebrates.

Somitogenesis begins with the formation of somitomeres (whorls of concentric mesoderm) marking the future somites in the presomitic mesoderm (unsegmented paraxial). The presomitic mesoderm gives rise to successive pairs of somites, identical in appearance and which differentiate into the same cell types but the structures formed by the cells vary depending upon the anteroposterior (e.g. the thoracic vertebrate have ribs, the lumbar vertebrate do not). Somites have unique positional values along this axis and it is thought that these are specifed by the Hox(homeotic) genes.

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^[10]. 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.

See also

• Cdx2 gene
• Drosophila embryogenesis
• Enterocoely
• Homeobox genes
• Mammalian embryogenesis
• Plant embryogenesis
• Schizocoely

References

1. ^  What is a cell? 2004. A Science Primer: A Basic Introduction to the Science Underlying NCBI Resources. NCBI.
2. ^  Campbell, Neil A.; Reece, Jane B.; Biology Benjamin Cummings, Pearson Education Inc 2002.

External links

• Embryogenesis & MMPs, PMAP The Proteolysis Map-animation
• Development of the embryo (retrieved November 20, 2007)
• Human Embryo (retrieved November 20, 2007)
• Video of embryogenesis of the frog Xenopus laevis from shortly after fertilization until the hatching of the tadpole; acquired by MRI (DOI of paper)