Estrous Cycle Learning Module
Reproductive performance of the cow and heifer is one of the most important factors that influence ranch profitability. Understanding the biological mechanisms associated with getting a cow or heifer bred can be a significant management tool for increasing realized income. This learning module summarizes the endocrinology and physiology of the bovine (beef cow) estrous cycle.
Endocrinology of the Estrous Cycle
Estrous cycles give females repeated opportunities to become pregnant throughout their productive lifetime. The cycle is regulated by the hypothalamic-pituitary-gonadal axis, which produces hormones that dictate reproductive events. The reproductive axis is composed of the hypothalamus, pituitary, and the ovary (see the animated diagram illustrating the interaction of the structures and hormones).
The hypothalamus is a specialized portion of the ventral brain. Its primary function is to produce gonadotropin-releasing hormone (GnRH) in response to circulating estrogen, or to cease GnRH production in response to progesterone. The pituitary is composed of the anterior and posterior lobes. In this module, we will discuss the function of the anterior lobe as it relates to reproduction in beef females. The anterior pituitary is located directly beneath the hypothalamus in a small depression of the sphenoid bone. It produces the gonadotropins follicle-stimulating hormone (FSH) and luteinizing hormone (LH) in response to GnRH and estrogen. FSH and LH production is inhibited by progesterone. The third portion of the reproductive axis consists of the ovaries, located in the pelvic cavity of the cow. Follicles are structures on the ovarian surface that contain ova (egg) and produce estrogen. Follicles range in size and maturity at different stages of the cycle, but usually only one is selected to ovulate. A corpus luteum (CL) is a structure that forms from the previous cycle's ovulation point. The CL is responsible for progesterone production. Both estrogen and progesterone are produced following FSH and LH stimulation of the ovary. The uterus is also found in the pelvic cavity. It likewise contributes to reproductive control, as it produces prostaglandin F2a(PGF2a).
The second animated diagram illustrates the relationships between the hormones that control the estrous cycle of the bovine female. The sequence of hormonal release essentially begins with the synthesis and release of GnRH from the hypothalamus. This polypeptide hormone is transported to the anterior pituitary through a highly specialized capillary network called the hypothalamo-hypophyseal portal system. GnRH functions to stimulate the anterior pituitary to produce and release FSH and LH. FSH and LH are transported through systemic blood circulation to the ovaries, where they initiate a series of morphological changes that lead to ovulation and pregnancy if fertilization occurs.
The primary hormones produced by the ovary are estrogen and progesterone. These hormones are transported by the blood stream to "target" tissues to cause a reaction. Estrogen is produced by the follicle, which is located on the ovary. As the follicle grows, more estrogen is produced. As increasing amounts of estrogen are released into the blood stream and travel to the anterior pituitary, it acts in a positive feedback fashion, stimulating pulsatile LH release. It also affects the nervous system of the cow, causing restlessness, phonation, mounting, and most importantly, the willingness to be mounted by other animals. Estrogen causes the uterus to contract, allowing sperm to be transported through the female reproductive tract more efficiently after insemination. Other effects of high estrogen concentrations in the blood include increased blood flow to the genital organs and the production of mucus by glands in the cervix and vagina. These characteristics are all signs of estrus, or sexual receptivity.
Progesterone produced by the CL prevents cyclicity by acting on the anterior pituitary in a negative feedback fashion; therefore, decreasing the release of FSH and LH. It prepares the uterus for reception of fertilized ova and subsequent pregnancy. It also helps the cow maintain pregnancy by suppressing uterine contractions and promoting development of the uterine lining.
A fifth hormone important in female reproduction is prostaglandin F2a(PGF2a).This hormone is secreted by the endometrium of the uterus and also affects structures on the ovary, helping to initiate ovulation by causing the demise of the CL, which results in withdrawal of progesterone's negative feedback mechanism. High circulating concentrations of progesterone stimulate the production and release of PGF2a from the uterus.
Physiology of the Estrous Cycle
The estrous cycle of the cow is generally about 21 days long, but it can range from 17 to 24 days in duration. Each cycle consists of a long luteal phase (days 1-17) where the cycle is under the influence of progesterone and a shorter follicular phase (days 18-21) where the cycle is under the influence of estrogen. The cycle begins with standing heat, or estrus. This time of peak estrogen secretion can last from 6 to 24 hours, with ovulation occurring 24 to 32 hours after the beginning of estrus.
Ovulation marks the beginning of the luteal phase, and is the culmination of a process called oogenesis, in which germ cells mature under the proper conditions. Germ cells are contained in thousands of tiny structures called follicles that contain receptors for FSH, which in turn stimulates the growth and maturation of responsive follicles. Most follicles develop in patterns referred to as follicular waves (animated diagram illustrating follicular waves). The first one or two waves consist of a group of follicles being recruited and grown to 3-5 mm in diameter. These follicles then grow to 6-9 mm in diameter in response to FSH, and one of the follicles may even reach 12-15 mm. However, these follicles generally don't become ovulatory due to the inhibitory action of progesterone on the anterior pituitary's production of FSH. Instead, these first waves of follicles undergo atresia (regression or death) after being broken down and invaded by connective tissue. It isn't until the second or third follicular wave that one follicle is selected to become the ovulatory, or Graafian, follicle. This follicle matures to a diameter of 16-18 mm and consists of an ovum or egg surrounded by many layers of cells, around which forms a central cavity filled with fluid and encompassed by several thin cell layers.
This animated diagram illustrates the morphological changes that occur on the ovary throughout the cow and heifer's estrous cycle. Once ovulation has occurred and the egg is released, the cells on the ovary that made up the ovulatory follicle differentiate to form luteal cells. This occurs under the influence of LH, and results in a structure called a corpus luteum (CL). The CL is responsible for progesterone production. Peak of progesterone production occurs about day 12 when the CL is fully mature. As previously noted, progesterone inhibits LH and FSH release by the anterior pituitary and prevents ovulation by inhibiting follicular development and maturation.
After about 14 days of progesterone influence, the uterus begins to release pulses of prostaglandin (PGF2a) into its venous drainage to the ovaries. Prostaglandin lyses the luteal tissue of the CL and causes its regression, resulting in a rapid decline of circulating progesterone and removing progesterone's negative feedback on the anterior pituitary. By about day 17, the luteal phase of the estrous cycle comes to an end.
The follicular phase begins with the removal of the blocking action of progesterone, which allows for greater amplitude and frequency of GnRH pulses. Greater GnRH results in more FSH and LH production, which in turn supports follicular development of the ovulatory follicle in the follicular wave. This Graafian follicle produces increasing amounts of estrogen, which causes positive feedback to the anterior pituitary. Once estrogen levels reach a threshold level, a surge of LH (at least 10 times greater than tonic levels) results in ovulation.
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