Page 82 - Anatomy and Physiology of Farm Animals, 8th Edition

Embryology / 67

• Incomplete development: this sort of
abnormality results when the structure
VetBooks.ir begins to form but fails to reach its

normal endpoint. This results in apla-
sia (complete or nearly failure to form)
or hypoplasia (inadequate formation).
• Persistence of embryonic structures:
there are many embryonic structures
that form during development but do
not normally persist into adulthood.
Persistence of a structure that failed to
undergo apoptosis or the appropriate
Limb buds development into a more mature form is
sometimes of no clinical significance, or
it can be life‐threatening if it interferes
Figure 3-7. Lateral view of early embryo with with vital structures.
limb buds. Embryonic stem cells are capable of
developing into any of the many cell
types of the body. Unlike most differen-
often have a critical period during which tiated tissues, they can survive in long‐
exposure to a teratogen is most likely to term culture (outside the body).
produce a developmental anomaly. The Embryonic stem cells are profoundly
critical period is usually associated with interesting to biomedical researchers, as
initial induction or differentiation of a cell it appears that cultured stem cells can be
type or structure. induced to differentiate into a wide vari-
Very early developmental abnormal- ety of tissue types. This technique offers
ities (in the first 2 weeks of gestation) promise of a novel way of treating dis-
are very often lethal and result in spon- eases involving loss of normal tissue.
taneous abortion. Teratogenesis at later A few of the many possibilities include
stages of the embryonic period is more using stem cells to create (1) hematopoi-
likely to result in structural malforma- etic cells to regenerate normal blood
tions. Insults during the fetal period precursors for treatment of blood and
are more generally manifest as func- immune disorders, (2) cartilage and
tional disorders and/or retardation of bone to replace damaged skeletal com-
growth. ponents, and (3) neuronal precursors to
Anomalies that result from altered for- replace the neurons lost in Parkinson’s
mation (those taking place during the disease or Alzheimer’s disease. Research
embryonic period) usually fall under one with human embryonic stem cells is
of several categories: fraught with contentious ethical consid-
• Developmental failure: when structures erations, since these cells are generally
fail to form, there is said to be agenesis acquired from aborted embryos and
of those structures. fetuses. Research using animal embryos
• Developmental excesses: a developmen- tends to be less ethically problematic,
tal excess usually takes the form of either but it will naturally lead to intense
increased size or number. Polydactyly debate as techniques developed in ani-
(increased numbers of digits) is a com- mal models are extrapolated for use in
mon example of this sort of anomaly. human medicine.