Page 76 - Anatomy and Physiology of Farm Animals, 8th Edition

Embryology / 61

Zygote Blastula
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2-Cell stage
Morula

4-Cell stage

Figure 3-1. Development from zygote to blastula.

It is the blastula stage that is collected cell lines that will give rise to all of the tis-
from donor animals for embryo transfer. sues in the adult body and can be described
As the inner cell mass develops, a cavity as a trilaminar embryo.
forms dorsal to it (but still within the Each of the three germ lines will form
trophoblast); this is the amniotic cavity. specific kinds of tissues (Table 3‐2). The
The portion of the inner cell mass facing ectoderm on the dorsal surface of the
the amniotic cavity is the epiblast, and the embryo will become epidermis and nerv-
portion adjacent to the blastocele is the ous tissue. The endoderm is the germ cell
hypoblast. line destined to become the lining of the
Sometime around the end of the second gastrointestinal and respiratory systems,
week of development, the epiblast begins the epithelial parts of glands associated
to thicken with proliferating cells on the with the digestive system, and parts of the
long axis of the embryo. This thickening is reproductive system. The mesoderm gives
the primitive streak, and here the epiblast rise to muscle, the skeleton, and parts of
cells migrate into the interior of the the urinary, cardiovascular, and reproduc-
embryo, taking up residence deep to the tive systems.
surface layer of cells (now called ectoderm) The trilaminar embryo begins as a
and displacing the hypoblast to create a flattened, disk‐like shape, but after gas-
deep layer, the endoderm. Between the trulation, the embryo’s sides, head region,
ectoderm and endoderm the third and and tail region begin to fold inward so
final germ cell layer, the mesoderm, is that the endoderm is drawn into the inte-
established. This migration of cells is gas- rior of the embryo, the ectoderm covers
trulation (Fig. 3‐2) and the embryo at this its exterior, and the entire body assumes a
stage is called a gastrula. At this point the more recognizable, cylindrical shape
embryo has established the three primary (Fig. 3‐3).