Page 475 - Anatomy and Physiology of Farm Animals, 8th Edition

460 / Anatomy and Physiology of Farm Animals

Plasma Apical species, but it is a matter of weeks to
months rather than days. In most farm ani-
segment
membrane
VetBooks.ir (ridge) mals, this period is about 2 months. As a
result, an insult to the germ cells such as a
Acrosomal high fever may require 8 to 10 weeks before
contents normal sperm production is resumed.
Daily production of spermatozoa has been
9
9
Principal estimated as 4.4 × 10 in the ram and 2 × 10
Outer segment
acrosomal in the bull. Eight ejaculations of the bull
membrane within an hour reduced the semen from
4.2 mL at the first collection to 2.9 mL at
the eighth collection, and the number of
Inner spermatozoa was correspondingly reduced
acrosomal from 1.7 billion to 98 million per milliliter.
membrane
Equatorial These data indicate that normal animals
segment produce adequate spermatozoa even when
used often for breeding. However, these
data also suggest that several days of sexual
Nuclear rest may increase numbers of spermatozoa
membrane in animals whose numbers are abnormally
Postacrosomal low. A correlation between daily produc-
region tion and testicular size has been found for
several species. In animals that are sea-
Nucleus
sonal breeders, testicular size and sperm
production both increase during the
breeding season. While females might be
completely anestrus during the nonbreed-
ing season, males often still produce some
sperm.
Figure 25-5. A sagittal section of a bovine sperm
head showing the various anatomic subdivisions.
Source: Hafez and Hafez, 2000. Reproduced with Epididymis
permission of John Wiley & Sons, Inc.
Fully formed spermatozoa released from
excess cytoplasm and cell membrane. The the seminiferous tubules pass through the
sustentacular cells, in which the developing rete testis into the epididymis. The epididy-
spermatozoa are embedded, assist with some mides are the major sites of storage of
of these conversions. For example, susten- spermatozoa, and most spermatozoa are
tacular cells phagocytose residual bodies of contained in the tail of each epididymis.
excess cytoplasm and membrane after Spermatozoa entering the head of the
the release of fully formed spermatozoa. epididymis from the rete testis are immo-
Abnormalities in spermatogenesis can some- tile and incapable of fertilization. During
times be visualized using microscopic evalu- their passage through the epididymis, sper-
ation of the morphology of the spermatozoa matozoa acquire the capacity for motility
in an ejaculate (Fig. 25‐6). and fertilization. It is presumed that these
two characteristics are closely linked and
Rates and Timing of Spermatogenesis most evaluators for a semen sample will
estimate a percent motility of the sperma-
The time required for spermatogenesis tozoa. The functional changes in spermato-
(from spermatogonium to fully formed zoa that occur during their passage through
and released spermatozoa) varies with the epididymis depend in part on epididymal