Page 508 - Anatomy and Physiology of Farm Animals, 8th Edition

Pregnancy and Parturition / 493

the corpora lutea. This number is about before implantation. In litter‐bearing
animals, this migration permits the spac-
four for the sow.
VetBooks.ir while still enclosed in a zona pellucida (see ing of embryos so that each has adequate
Embryos develop to the blastula stage
room for development and ensures that
Chapter 3). The zona is shed (hatching) each uterine horn contains some embryos.
prior to attachment of the embryo to the Contact, or paracrine communication,
uterine wall for placentation. The outer- between embryos and the uterine epithe-
most layer of cells of the blastula is the lium is necessary for pregnancy recogni-
trophoblast, and it is from these cells that tion, and in litter‐bearing animals, each
the fetal membranes will be formed. All uterine horn must contain embryos to
ungulates, including ruminants, also have permit this recognition. Embryos of non-
a period of rapid elongation of the tropho- litter‐bearing species also migrate within
blast prior to uterine attachment. the uterus before attachment. In the mare,
Early embryonic death (death of the migration back and forth between uterine
embryo prior to attachment to the uterine horns before attachment is necessary to
wall) is responsible for a significant num- prevent luteolysis and loss of the develop-
ber of reproductive failures in domestic ing embryo.
animals. Some studies report that up to Placentation is development of the
30% of fertilized embryos die before extraembryonic membranes, or placenta.
developing into a fetus. Possible causes of The placenta is an arrangement of mem-
embryonic death include inherited lethal branes with sites for exchanges between
factors, infections, nutritional deficien- the maternal and fetal circulations so that
cies, inappropriate levels of maternal nutrition from the dam can reach the fetus
hormones, and defects in the ovum or and waste products from the fetus can be
spermatozoa before fertilization. transferred to the dam. In domestic animals,
the terms fetal membranes and placenta
are used interchangeably, although tech-
Implantation and Placentation nically the fetal membranes are called the
fetal placenta to distinguish them from
Implantation is attachment of a blastula maternal components of the placenta. In
to the uterine epithelium and penetration some species a portion of the endome-
of the epithelium by embryonic tissue. The trium is also shed at parturition. This is the
degree of penetration by embryonic tissues maternal placenta, or decidua. The fetal
varies among species. In most domestic placenta includes the chorion, allantois,
animals, the degree of penetration is much amnion, and vestigial yolk sac.
less than in rodents and primates, whose The chorion, the outermost membrane,
penetration extends into the connective is in contact with the maternal uterine
tissue beneath the epithelium. Implantation endometrium. The next layer (moving
in domestic animals is considered to be from outermost inward), the allantois, is
noninvasive and primarily the result of a continuous layer that encloses a sac, the
formation of cell‐to‐cell junctions between allantoic cavity (Fig. 28‐2). The chorion
embryonic tissues and the uterine epithe- and the outer layer of the allantois fuse to
lium. These junctions involve binding of form the chorioallantois. The amnion is
membrane proteins in embryonic tissues the innermost membrane, closest to the
to receptors on maternal epithelium. After fetus. It is a fluid‐filled cavity that contains
fertilization, attachment occurs in the sow the fetus. The amnion is fused with the
at about 11 days, in the ewe about 16 days, inner layer of the allantois. The allantoic
in the cow about 35 days, and in the mare cavity, sometimes called the first water
about 55 days. bag, is continuous with the cranial extrem-
Developing embryos migrate (i.e., move ity of the bladder by way of the urachus,
about) within the lumen of the uterus which passes through the umbilical cord.