Page 78 - Anatomy and Physiology of Farm Animals, 8th Edition

Embryology / 63

VetBooks.ir (A) Cross-sectional view Ectoderm

Neural tube

Endoderm
(B) Lateral view
Ectoderm

Endoderm
Cranial Caudal

Figure 3-3. Embryonic folding. (A) The sides of the embryonic disk grow in such a way as to fold
inward, creating a roughly circular cross‐sectional aspect to the body. (B) Viewed from the side, the cra-
nial and caudal ends of the embryo fold under, resulting in a C‐shaped body. Notice how the endoderm
lines the tubular gastrointestinal tract after embryonic folding.

Principles of Differentiation the molecular and genetic process by which
some cells influence other, adjacent cells to
The cells of the zygote and morula have the commit to a particular developmental fate.
potential to become any of the hundreds of The inducing substance is a protein expressed
cell types that constitute the body and the on the surface of the inducing cell or released
extraembryonic membranes (e.g., placenta); by it into the extracellular environment and
this property is called totipotency. Once for which a complementary receptor exists
development has progressed to the blastula on the cell being induced. There is a strict,
stage, however, individual cells are no longer temporal sequence to induction and the
able to become any cell type. Differentiation differentiation of embryonic tissues. That is,
(also called determination or commit- one group of cells is induced to become a
ment) is the process by which a cell alters its particular tissue type, the presence of which
genetic expression to become more special- induces another differentiation of adjacent
ized. During differentiation, sets of genes cells, which induces formation of another tis-
are activated or inactivated, and the cell sue, etc…. This sequence is dictated by care-
assumes a more specialized appearance and fully timed activation and repression of
function according to the expression of its specific genes, a series of steps controlled
genes. A cell that has undergone a change in partly by genetic preprogramming and partly
genetic expression that reduces its potency by environmental influences. Failure of any
(i.e., directs it to become more specialized) step in the sequence is likely to have far‐
is said to have become determined, or reaching influences on subsequent steps,
committed. Differentiation is usually an since any given developmental event is usu-
irreversible process during normal develop- ally dependent on the correct expression of
ment. It typically occurs in steps bridged by the previous step.
many generations of cells; there are there-
fore many intermediate levels of differentia-
tion between the totipotent zygote and, for Neurulation
example, a fully differentiated bone cell.
As different types of tissue in the embryo Beginning around the second week of
interact by their proximity, one type may development, the mesoderm on the mid-
induce another to differentiate. Induction is line of the developing embryo condenses