Oestrous cycle – Class 12 | Chapter – 1 | Short Notes Series PDF

Oestrous cycle: Oestrous cycle  or Estrus cycle occurs in non-primate mammalian females. It is the cyclic changes occurring during the reproductive phase of an animal. Oestrus cycle is controlled by various hormones. The Oestrous cycle starts at the onset of sexual maturity and stops during pregnancies. The frequency and duration of the Oestrous cycle vary in different animals. It is similar to the menstrual cycle in humans and other primate females. It occurs in animals, such as dogs, cows, sheep, deers, horses, tigers, etc.

Differences in lengths of estrous cycles among species are determined primarily by duration of the luteal phase. Luteal phases of larger mammals are long compared to species of lesser body stature. Many small animals are subject to predation, and cannot afford the luxury of lengthy nonpregnant cycles (moreover, they are usually litter-bearing, have short gestations, abbreviated or no lactational anestrus, and their young attain puberty quickly). Extinction can quickly besiege those species with extended reproductive cycles (eg., rhinoceroses and elephants).

Stages of Oestrous cycle

The Oestrous cycle can be divided into four stages:

• Proestrus: It is the follicular phase where follicles start developing and secrete oestrogen. The uterus lining or endometrium starts to develop, and the vaginal epithelium proliferates.
• Estrus: It is also known as the period of heat. In this phase, the female is sexually receptive. Ovulation occurs in this phase. The female gets attracted to males and is ready for mating. It might be visible through various signs such as reddening of the labia.
• Metestrus: It is the luteal phase where the corpus luteum develops from the ruptured follicle after ovulation.
• Diestrus: The corpus luteum produces progesterone hormone, and it prepares the uterus for implantation. In the absence of pregnancy, the corpus luteum degenerates. Here, the uterus lining is not shed but reorganised for the next cycle.
• Anoestrus: This is the resting phase or the phase of sexual inactivity. It is induced by pregnancy, lactation, illness, age and seasons or time of the year in seasonal breeders. In seasonal breeders, it is controlled through the exposure to light that influences the release of melatonin from pineal glands.

During proestrus the CL regresses (progesterone declines) and a preovulatory follicle undergoes its final growth phase (estradiol increases). Ovulation usually occurs during estrus (cows ovulate during metestrus). Proestrus and estrus comprise the follicular phase. Corpora lutea develop during metestrus and function at optimum during diestrus. Metestrus and diestrus make up the luteal phase.

When does sexual activity start?

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By Team Learning Mantras