Page 505 - Anatomy and Physiology of Farm Animals, 8th Edition

490 / Anatomy and Physiology of Farm Animals

regnancy is the condition of a female fraction of the inseminated spermatozoa
reach the uterine tube, the first arriving
Panimal while young are developing
VetBooks.ir within her uterus. The interval, the gesta- spermatozoa typically do not accomplish
fertilization.
tion period, extends from fertilization
of the ovum to the birth of the offspring. Oxytocin, a peptide hormone from the
It includes fertilization, the union of the neurohypophysis (see Chapter 13), is
ovum and sperm; early embryonic develop- released as a result of a neural reflex and
ment in the lumen of the female reproductive promotes contraction of the smooth mus-
tract; implantation of the embryo in the cle in the female tubular genitalia to assist
uterine wall; placentation, the deve lopment with spermatozoa transport. Tactile stimu-
of fetal membranes; and continued growth lation of the female reproductive tract can
of the fetus. result in oxytocin release, and both natu-
Normal gestation periods vary greatly ral mating and artificial insemination can
from species to species (see Table 27‐1), result in increased myometrial activity.
and there is considerable variation between Oxytocin is also important for “milk letdown”
individuals within each species. If the young in lactation (Chapter 29) and as with milk
are carried throughout a normal gestation letdown, the neural pathways that stimulate
period, it is a full‐term pregnancy. A pre- oxytocin release from the neurohypophysis
mature birth is delivery of a viable fetus can be triggered with nontactile (auditory,
before fetal development is complete. visual, etc.) stimuli as well as dampened by
Termination of pregnancy with delivery of stressors that increase sympathetic tone.
a nonviable fetus is abortion. Therefore, it is not surprising that in the
sow, the mere presence of the boar, rather
than the act of mating, results in the release
Fertilization of oxytocin.
Spermatozoa must remain in the female
Spermatozoa Transport and Viability reproductive tract for some period after
ejaculation before they are capable of
In most cases, fertilization occurs in the fertilization. The physiologic process that
uterine tube next to the ovary; during conveys the capacity to fertilize the ovum
natural copulation, spermatozoa are depos- is termed capacitation. Capacitation
ited in the vagina (most species) or uterus includes changes in or removal of compo-
via the cervix (mare, sow, and bitch). Sperm nents of the outer acrosome and plasma
motility, contractions of the female tubular membranes of the spermatozoa so that
genitalia following insemination, and vary- acrosomal enzymes can later be released
ing degrees of postmating uterine inflam- and activated. Part of the natural capacita-
mation are all important factors for the tion process requires exposure of the
transport and survival of spermatozoa in spermatozoa to female reproductive tract
the female reproductive tract. After ejacu- secretions, but capacitation of spermatozoa
lation the spermatozoa become motile. can be done in vitro using experimentally
Based on the calculated speed of bull sper- derived protocols and solutions.
matozoa, it would take 1.5 hours for them The presence of semen in the female
to swim the distance from the site of reproductive tract results in an immunologic
insemination (vagina) to the site of fertili- reaction and inflammation. Interestingly,
zation (uterine tube) in a cow, but the mus- repeated insemination rarely elicits an
cular activity of the female reproductive immunity to male antigens present in
tract allows the first spermatozoa to reach semen, thus permitting reproductive suc-
the site of fertilization about 2.5 minutes cess. Under normal conditions, viability and
after deposition. In the mare, it takes survival times of spermatozoa in the repro-
approximately 4 hours for spermatozoa to ductive tract of domestic animals is only a
reach the uterine tube. While only a small matter of hours and is highly correlated to