Page 75 - Anatomy and Physiology of Farm Animals, 8th Edition

60 / Anatomy and Physiology of Farm Animals

mbryology is the study of the early Development of Germ Layers
E prenatal development of the body. As
VetBooks.ir a descriptive discipline, it has been largely The one‐celled zygote undergoes the first

supplanted in college curricula by devel-
opmental biology, an approach that tends mitotic divisions, known as cleavage,
shortly
after
Cleavage
fertilization.
to focus on the cellular, genetic, and increases the number of cells without
molecular events that underlie the devel- increasing the volume of the developing
opment of the embryo. As biomedically embryo, so that after each cell division, the
significant as these events are, knowledge daughter cells have smaller cytoplasmic
of the morphological changes that are the mass. However, the nuclei of the daughter
subject of descriptive embryology can cells are normal in size and contain a full
make anatomy easier to understand and complement of chromosomes. The cluster
makes the anatomy of birth defects more of small cells resulting from cleavage has a
logical. lobulated appearance resembling a berry;
Development begins with the fertiliza- hence, the name morula (Latin, small
tion of the egg (ovum) by a spermatozoon mulberry) is given to this stage (Fig. 3‐1).
to form a zygote (see Chapter 28). The After this point, further division will begin
ovum and spermatozoon each contribute to increase the size of the conceptus.
half of the nuclear chromosomes to the Within the uterus, the interior of the
newly formed zygote. The cells of the morula develops a cavity, the blastocele,
zygote undergo division, migration, and transforming the morula into a hollow ball
differentiation to become successively a called a blastula. The blastula comprises a
morula, blastula, gastrula, and then neu- layer of cells, the trophoblast, surrounding
rula (Table 3‐1). The zygote and each of the the blastocele, into which a collection of
subsequent stages up to birth may all be cells, the inner cell mass, protrudes. The
described as the conceptus, a convenient inner cell mass eventually forms the body
term that also includes the extraembryonic of the embryo. The trophoblast will
membranes. develop into the extraembryonic tissues,
Developmentally speaking, the embry- including the placenta.
onic period ends when the various organs Because they retain the potential to
and organ systems are established. The become any cell of the embryo (excluding
embryo then becomes a fetus that more the extraembryonic tissues), the cells of the
or less resembles an adult of the same inner cell mass are often described as being
species. This subsequent fetal period pluripotent or multipotent. It is these
primarily entails increase in size and cells of the early embryo, the so‐called
functional differentiation of organs. The embryonic stem cells, which are the sub-
fetus becomes a neonate (newborn ani- ject of such intense interest and debate in
mal) at parturition (birth). the scientific community.

Table 3-1. Approximate Times (in Days) to Establishment of Developmental Stages
Embryo Approximate End
Reaches of Embryonic
Species Uterus Morula Blastula Neurula Period Gestation
Pig 2.5 4 5 14 30 114
Sheep 4 4 6 15 32 150
Ox 4 4 7 19 30–40 280
Horse 4 4 6 18 35–40 340