Page 512 - Anatomy and Physiology of Farm Animals, 8th Edition

Pregnancy and Parturition / 497

These include: (1) providing negative endometrial cup formation, the mare
feedback to the hypothalamus to inhibit
VetBooks.ir any further estrous cycles; (2) inhibiting will not cycle again until after the
endometrial cups are no longer actively
the smooth muscle of the uterus to permit
the attachment and development of the secreting eCG. Unfortunately, the lifes-
pan of the endometrial cups is approxi-
fetus; and (3) assisting with maintenance of mately the length of the breeding season
the contractility of the cervix to protect the which means that the breeding season
uterine environment. has typically ended by the time the
Plasma levels and sources of progesterone mare may be fertile again. Pregnancy
differ among species and among stages of loss after endometrial cup formation is
pregnancy. In all domestic animals, the a major economic loss to equine repro-
initial source of the necessary progester- duction programs as they represent an
one is the corpus luteum, and in some spe- entire breeding lost for that mare.
cies (e.g., cow and sow), the corpus luteum
remains the primary source throughout Relaxin
pregnancy. In other species (e.g., mare
and ewe), the initial corpus luteum can be Relaxin is a protein hormone secreted by
removed after secondary sources produce the corpus luteum in some species (sow
enough progesterone to maintain preg- and cow) and the placenta in others (bitch
nancy. These sources include secondary or and mare). The primary function of relaxin
accessory corpora lutea and the placenta in is preparation for parturition and ulti-
the mare; the placenta is the secondary mately lactation. Relaxin contributes to the
source in the ewe.
opening of the cervix and the relaxation of
the muscles and ligaments associated with
Equine Chorionic Gonadotrophin the birth canal to facilitate the passage of
the fetus. In some species, peak secretion
of relaxin occurs just prior to parturition.
The mare appears to be unique among Gradual increases in relaxin during the
domestic species in that the equine placenta latter part of gestation also facilitate
is the source of a protein hormone that acts mammary gland development to prepare
similarly to the luteinizing hormone, the for lactation.
source of which is the pituitary. Secretion
of equine chorionic gonadotrophin (eCG,
formerly known as pregnant mare serum
gonadotrophin, or PMSG) begins after Pregnancy Diagnosis
about a month of gestation and continues
until about 4 months of gestation. During If accurate records of estrus periods and
this period, follicular development occurs breeding dates are available, the earliest
on the ovary of the pregnant mare, and eCG indication of pregnancy in most animals is
promotes the luteinization of these folli- the failure to have another estrous cycle at
cles. These accessory corpora lutea provide the expected time. Such an absence of
secondary sources of progesterone. estrus is not, however, proof of pregnancy.
Trophoblastic cells of fetal origin found A nonpregnant animal may miss an estrous
in specialized structures termed endome- cycle because of failure of the corpus
trial cups are the source of the eCG. These luteum to regress normally or some other
structures are seen as small, raised circular reproductive abnormality. An animal may
areas with a central depression on the also have a delay of one or two estrous
endometrial surface of the pregnant uterus. cycles if an initial conception is followed by
The trophoblastic cells are found in asso- inability to sustain that pregnancy.
ciation with the endometrium in these Palpation of the reproductive tract via
areas. If pregnancy loss occurs after the rectum in the mare and cow can be