Epigenetic Control of Animal Development - Building the Most Complex Structure on Earth - page 126

Figure 3.6 Generalized representation of the
early development in animals from zygote to
gastrulation stage.
Source : From Pearson Education Inc. ( http://
www.devbio.biology.gatech.edu/?page_id=752 ) .

Cleavage

Zygote

Eight-cell stage

Blastula
(hollow ball)

Blastocoel

Cross section
of blastula

Blastocoel

Gastrulation
Archenteron
Endoderm
Ectoderm
Blastopore

Gastrula
Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.

Characteristically, cleavage divisions are not preceded by the normal growth of
the dividing cells. What happens instead is that the zygote (and, depending on spe-
cies, several of its subsequent cell generations) are continually subdivided in the
absence of growth, in a process where daughter cells continue to get half of the cyto-
plasm of the mother. Generally, this process leads to the formation of an embryonic
spherical structure (which is named morula due to its resemblance to a mulberry)
consisting of a spherical layer of cells surrounding a fluid-filled cavity called the
blastula ( Figure 3.6 ), which has the same size as the zygote. As mentioned earlier,
cleavage divisions may be equal or unequal. During unequal division, daughter cells
get different quantities of cytoplasm and different qualities of the parentally provided
epigenetic information, leading to early phenotypic differentiation of cells that are
genetically identical. The causes and mechanisms of this epigenetic differentiation
of early embryonic cells are not fully known and understood. However, some of the
basic mechanisms of cell differentiation are known.

Epigenetic Regulation of Cell Differentiation

Cell differentiation is a typical epigenetic process where, from a single cell and
genotype, tens to hundreds of different cell types and phenotypes develop. The
most obvious and the better-known changes that lead to different types of cells are