Estrus Cycle

Upon reaching puberty, female animals exhibit rhythmic patterns of sexual behavior. This behavior change, marked by increased sexual receptivity, is referred to as estrus, a term derived from the Greek word. The sequence of physiological changes that starts with one estrus period and stays at the onset of the next is known as the estrus cycle. The day estrus occurs is designated as day zero of this cycle.

1.0What is Estrus Cycle?

The estrus cycle is regulated through intricate interactions among several hormones in the brain and ovaries, including progesterone and estrogen. During the cycle, the Follicle (egg) matures, and ovulation—the release of the egg—takes place when progesterone levels decrease, and estrogen levels increase. Following ovulation, a structure known as the corpus luteum develops on the ovary, which then secretes progesterone.

2.0Phases of Estrus Cycle

The oestrus cycle comprises four phases: pro-estrus, estrus, metestrus, and diestrus.

Pro-estrus

This stage involves follicle growth, the corpus luteum (CL) regression, and an increase in estrogen production.

• Elevated estrogen levels enhance the blood supply to the genital tract, leading to edema in the genitalia.
• There is a slight enlargement of the uterus; the endometrium becomes congested and edematous with increased glandular secretory activity.
• The vaginal mucosa exhibits hyperemia.
• In bitches, the epithelial cell layers increase in number, and the superficial layers undergo cornification.
• Additionally, there is vulval edema, hyperemia, and premenstrual bleeding during this phase.

Estrus

This period has a clear beginning and end, which are precisely measurable and serve as reference points to determine the cycle length.

• During this time, there is an increase in mucus secretion from the uterine, cervical, and vaginal glands. Additionally, the vaginal epithelium and endometrium exhibit hyperemia and congestion, and the cervix becomes relaxed.
• Ovulation occurs in all species, except cows, where it occurs 12 hours after estrus ends.
• The discharge during this period is clear and stringy. 

Metestrus 

• After estrus ends, the granulosa cells from the ovulated Follicle transform into lutein cells.
• This leads to the formation of the corpus luteum.
• There may be a minimal, blood-tinged discharge (menstrual bleeding).
• Should fertilization take place, there is an increase in mucus secretion from the uterine glands.

Diestrus 

• The Corpus Luteum (CL) and progesterone dominate during this phase.
• There is growth in both the mammary glands and the uterine myometrium.
• The uterine glands secrete a dense, sticky fluid known as uterine milk, which nourishes the zygote.
• If fertilization does not occur, the Corpus Luteum regresses approximately 15-16 days later.

Anestrus 

• Extended duration of sexual inactivity and dormancy of the genital tract.
• Limited development of follicles.
• Inactive and regressed corpus luteum.
• Sparse and thick secretions.
• It narrowed the cervix and pale appearance of the vaginal lining.

Causes of anestrus

• Season
• Stress
• Nutrition
• Pregnancy
• Presence of Offspring

3.0Signs of Estrus Cycle

• Heightened restlessness and increased activity levels
• Reduced consumption of feed and lower milk production
• Vocalizing when separated from the group
• Minor elevation in body temperature by 0.1°C
• Presence of clear vulval discharge, often referred to as a 'bulling string'
• Indications of rubbing or sores around the tail head area
• Demonstrates mounting behavior on other cows, especially from the front

Estrus- cycle, estrus and time of ovulation in some of the domestic animals

4.0Difference between Estrus Cycle and Menstrual Cycle

5.0Estrus Cycle Overview

• The estrus cycle, the reproductive system in most mammals, begins after puberty when reproductive hormones activate in sexually mature females.
• The cycle lasts approximately four days and is divided into phases: proestrus, estrus, metestrus, and diestrus. These phases can be identified by examining the types of cells present in a vaginal smear.
• The estrus cycle involves recurring changes in reproductive hormone levels due to ovarian activity, which is influenced by pituitary hormones. Gonadotropins, including Follicle-stimulating hormone (FSH), stimulate the growth and development of ovarian follicles, where ovaries are produced, and promote the secretion of the estrogen hormone by the ovaries.
• Luteinizing hormone (LH) is crucial for forming the corpus luteum, which produces hormones in the ovaries post-ovulation and regulates estrogen and progesterone.