Page 491 - Anatomy and Physiology of Farm Animals, 8th Edition

476 / Anatomy and Physiology of Farm Animals

he ovaries are the source of mature the primary oocyte results in only one
mature ovum and the remaining genetic
Tfemale gametes (ova) and hormones
VetBooks.ir necessary for reproduction. Oogenesis material is extruded as rudimentary cells,
is formation of ova. Estrus (heat) is the
called polar bodies. In most animals, the
period of sexual receptivity in the female. first of the two meiotic divisions is com-
An estrous cycle is the interval from the plete, resulting in the formation of the first
beginning of one estrus to the beginning polar body, before or immediately after
of the next. ovulation (the discharge of an oocyte from
Animals that have only one estrous cycle a follicle). The formation of the second
per year are called monestrous animals, polar body will not occur until after fertili-
while those that have several estrous cycles zation (see Chapter 28).
per year are polyestrous. Many animals
have successive periods of estrus if a preg-
nancy is not established and maintained. Secondary Follicles
The timing of these periods is often
determined by seasonal changes in length In all animals, multiple primary follicles
of day, so these animals are seasonally typically begin further development during
polyestrous. The relatively long period of a single estrous cycle. In monotocous
inactivity in seasonally polyestrous ani- animals (animals not bearing litters and
mals is termed anestrus and is part of the normally having only one offspring per
sexual cycle. gestation, such as the mare and cow), one
follicle usually develops more rapidly than
others, and only one ovum is released at
Oogenesis ovulation. The rest of the developing fol-
licles regress and form atretic follicles.
In the fetus, primordial germ cells migrate Polytocous animals, such as carnivores
from the yolk sac to the developing ovaries, and swine, which normally produce two or
where a single layer of follicular cells sur- more offspring per gestation, usually have
rounds a germ cell destined to become an several follicles that develop and ovulate at
ovum. The central germ cell (now termed approximately the same time. The ova may
an oogonium) enlarges and begins meiosis. all come from one ovary, or some may
(Recall that meiosis entails two cell divisions come from each ovary.
during which the diploid number of chro- The further development of primary
mosomes is reduced by half to the haploid follicles includes enlargement of the oocyte
number.) The oogonium does not com- and replication of the surrounding follicular
plete meiosis; it stops in the first prophase cells. The replicating follicular cells become
before the first division. At this stage, the several layers thick, and this surrounding
developing ovum is a primary oocyte, and group of cells is a granulosa. The granu-
the combination of a primary oocyte and losa cells secrete glycoproteins that cross‐
its surrounding cuboidal follicular cell link to form a protective shell, the zona
(granulosa cell) layer is a primary follicle pellucida, around the oocyte (Figs. 27‐1
(Fig. 27‐1). At birth, the ovaries of most and 27‐2). Cytoplasmic processes of gran-
domestic species contain hundreds of ulosa cells penetrate the zona to permit
thousands of primary follicles waiting to communication and exchange between
continue their development. What deter- them and the oocyte. The initial devel-
mines which of the thousands of primary opment to this point is independent of
follicles is selected to develop further dur- hormonal stimulation by gonadotropins
ing a specific estrous cycle is unknown. (follicle‐stimulating hormone [FSH] and
In sharp contrast to spermatogenesis, luteinizing hormone [LH]).
which produces four spermatozoa from The developing follicle is termed a
each primary germ cell, the maturation of secondary follicle when the oocyte has