Page 507 - Anatomy and Physiology of Farm Animals, 8th Edition

492 / Anatomy and Physiology of Farm Animals

in the male results in four haploid sperma- mechanisms fail and polyspermic fertili-
zation occurs, the typical result is early
tozoa (see Fig. 25‐3). In sharp contrast, the
VetBooks.ir female will develop only a single mature embryonic death. This is believed to be
rare in most domestic species when fertili-
gamete (also called the ovum, egg, or
oocyte) and the genetic material that is zation occurs in vivo, but the incidence of
discarded is referred to as a polar body polyspermy is higher when fertilization is
(Fig. 28‐1). The first polar body, which done in vitro.
results from the first meiotic division, also A maternal pronucleus is formed in the
accompanies the ovulated ovum within the ovum by the enclosure of the maternal
zona (see Chapter 27). chromosomes in a nuclear membrane. The
The zona pellucida is a semipermeable head of the sperm enlarges, becoming the
membrane that helps protect the ovum male pronucleus. The two pronuclei come
and has receptor sites for attachment of together and fuse membranes, forming
spermatozoa during fertilization. A series one cell with the genetic material of both
of species specific zona pellucida glyco- parents (Fig. 28‐1). The new cell is ready
proteins facilitate the binding of sperma- for cleavage and formation of a morula.
tozoa. The zona protein ZP3 varies among Cleavage and the subsequent development
species, but this variation is the primary into a morula and blastula are described in
reason that spermatozoa from one species detail in Chapter 3.
cannot bind to and fertilize ova from During early development, the embryo
other species. is not attached to the epithelium lining the
During or just after binding and attach- female reproductive tract. During this
ment to the zona, spermatozoa undergo a period, embryos obtain their nourishment
series of events termed the acrosome from fluids and nutrients secreted by
reaction. As part of this reaction, locally glands in the walls of the reproductive
released acrosome enzymes digest a pas- organs (e.g., endometrial glands in the
sage through the zona. This passage permits walls of the uterus). Progesterone stimu-
spermatozoa to swim their way to the lates secretion by these glands, and blood
vitelline membrane of the ovum, which is levels of progesterone are relatively high
accomplished in a matter of minutes. during this period, as it is being secreted
Multiple spermatozoa may attach to the from ovarian corpora lutea.
zona of a single ovum, but only one sper- Maternal recognition of pregnancy is
matozoon will ultimately be responsible detection of a developing embryo by the
for fertilization. maternal reproductive tract, which pre-
After penetration of the zona, the cell vents regression of the progesterone‐
membrane of the single spermatozoon that secreting corpora lutea and termination of
will accomplish fertilization attaches to the pregnancy. Several mechanisms have
and fuses with the vitelline membrane of been identified in different species, but in
the ovum. This initiates the second meiotic general, the mechanisms involve secretory
division by the ovum, which results in products (e.g., proteins or steroids) from
formation of the second polar body. The the developing embryo that act locally to
fusion of the spermatozoon with the ovum trigger a response by the maternal repro-
also stimulates release of cytoplasmic ductive tract. In most cases, the embry-
granules by the ovum and depolarization onic secretory products inhibit the uterine
of the plasma membrane of the egg. These secretion of prostaglandin F (PGF ).
2α
2α
events bring about changes in the chemical Recall that uterine secretion of PGF is the
2α
nature of the zona pellucida which, key hormonal signal that causes luteolysis
together with changes in the vitelline in most domestic species. In litter‐bearing
membrane, prevent polyspermy, which is animals, a minimal number of developing
the entry of more than one spermatozoon embryos appears to be required to recog-
into the ovum (Fig. 28‐1). When these nize pregnancy and prevent regression of