Page 81 - Anatomy and Physiology of Farm Animals, 8th Edition

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will eventually become the thoracic, abdom- and proximal to distal axes. Abnormalities
of limb development are common congen-
inal, and pelvic cavities.
VetBooks.ir oderm and the adjacent ectoderm make up ital defects, as they do not threaten
The outer layer of the lateral plate mes-
survival of the conceptus.
the somatopleure, which forms part of the
body wall and enters into the formation of
the fetal membranes. The inner layer of the Teratogenesis
lateral plate mesoderm and the endoderm
form the splanchnopleure, which forms Teratogenesis is the formation of a devel-
the wall of the gut (Fig. 3‐6). opmental anomaly. The cause of the abnor-
Limbs begin as small, paddle‐like limb mal development (if known) may be an
buds, with the forelimb forming in agent (infectious or toxic) or other insult
advance of the hindlimb (Fig. 3‐7). These (e.g., radiation or vitamin overdose) and is
consist of the mesodermal precursors of referred to as a teratogen. Although there
cartilage, bone, and muscle covered with are many known agents of developmental
ectoderm. Embryonic muscle cells migrate malformations, including genetic abnor-
from the somites into the growing limb malities, the great majority do not have an
bud, bringing with them the motor axons identifiable cause.
of nerve cells. Signaling molecules from The single most important factor in the
specialized tissues of the developing limb development of an anomaly in response to
will direct its growth so that the limb has a teratogen is the stage at which the insult
clear cranial to caudal, medial to lateral, occurs. Tissues and embryonic structures

(A) (C)

Splanchnopleure

Lateral plate
mesoderm

(B)
Gut
Ectoderm

Coelom

Gut
Coelom
Somatopleure
Endoderm

Figure 3-6. (A) Lateral plate mesoderm splits early in development, creating the coelom (early body
cavity). (B) The external layer associates with overlying ectoderm, while the internal layer associates with
endoderm of the developing gut. (C) Ectoderm plus the external layer of lateral plate mesoderm consti-
tutes the somatopleure; it will become the body wall. The internal mesoderm and endoderm are called
splanchnopleure, which becomes the wall of the gastrointestinal tract.