Managing Reproductive Disorders in Dairy Cows
Normal Ovarian Function in Cattle
In postpubertal cattle, two ovarian structures,
follicles and corpora lutea (CL), exist in various stages of growth or regression
during normal reproductive cycles. Unlike males, which continually produce
gametes throughout their life span, females recruit their gametes from a finite
population of oocytes established early during embryonic development. Folliculogenesis
is the process of forming mature follicles capable of ovulation from the pool
of nongrowing, primordial follicles in the ovary. As follicles grow, they
progress through several stages of development (Figure 1). In cattle, less
than 1% of the 100,000 follicles present at puberty will develop to maturity
and ovulate.
The primary functions of ovarian
follicles are: 1) to support and nurture a developing oocyte that is capable
of being fertilized after ovulation, 2) to secrete steroid hormones which
regulate the morphology and function of the reproductive organs as well as
reproductive behavior during estrus, and 3) to provide the precursor cells
that will luteinize and form the corpus luteum (CL) after ovulation (Iranni
and Hodgen, 1992).
Corpora lutea are transient endocrine gland
that form after ovulation from the tissues that previously constituted an
ovarian follicle. Luteal formation begins when increasing peripheral concentrations
of estradiol secreted by a developing follicle indirectly trigger a surge
of luteinizing hormone (LH) to be released from the anterior pituitary. This
LH surge initiates ovulation and luteinization of follicular granulosa and
thecal cells which shifts steroid biosynthesis from estrogens to progestins.
Progesterone is the primary steroid product of the CL, and it is required
for normal implantation and maintenance of pregnancy in cattle. If pregnancy
does not occur or fails to be established, the CL regresses in response to
prostaglandin F2a (PGF2a) secreted by the uterus (Brunner et al.,
1969; Boding, 1974). In nonpregnant cows, luteal regression normally occurs
around Day 16 to 18 after ovulation. Administration of PGF2a or one of its analogues from Day 6 to 16
after ovulation induces luteolysis in nearly all animals, whereas the CL is
refractory to PGF2a-induced luteolysis from Day 1 to 5 after
ovulation.
Follicular Waves
Scientific
studies using transrectal ultrasonography have led to clarification of the
nature of antral follicular development in cattle (Pierson and Ginther, 1984).
The first studies using ultrasound revealed that follicular growth occurs
in waves, each wave culminating with formation of a large follicle (Figure
2).
A
follicular wave begins with emergence of a group or cohort of small antral
follicles just before the day of ovulation. During the next several days,
one of the follicles in this cohort continues to grow and becomes dominant,
thereby suppressing emergence of a new follicular wave. As the dominant follicle
continues to grow, growth of the remaining follicles in the cohort ceases
or slows, and these subordinate follicles eventually undergo atresia. A second
wave of growth emerges on approximately Day 10 after ovulation and, for three-wave
cycles, an additional wave emerges at Day 16 after ovulation. For both two
and three-wave cycles, the ovulatory follicle arises from the final wave (Ginther
et al., 1996).
Under
most circumstances, follicular waves ensure that only one follicle capable
of undergoing ovulation is present at any given time during the estrous cycle.
In general, primiparous and multiparous lactating dairy cows exhibit more
two-wave cycles, whereas nulliparous dairy heifers tend to exhibit more three
wave cycles. Some factors that may influence the number of waves per estrous
cycle in dairy cattle include dietary intake (Murphy et al., 1991) age, parity,
and lactational status (Lucy et al., 1992).