Reproduction in Animals

Menstrual cycle

The menstrual cycle is a recurring cycle that occurs in female primates, such as humans, monkeys, and chimpanzees. This cycle involves the development of an egg cell (oocyte) in the ovary, the preparation of the uterus for the possible implantation of a fertilized egg (zygote), and the shedding of the uterine lining if pregnancy does not occur.

Phases of the menstrual cycle

Ovarian Cycle

The ovarian cycle involves three main phases: follicular phase, ovulation, and luteal phase.

Follicular Phase:

• The cycle begins with the development of primary ovarian follicles. This is stimulated by Follicle Stimulating Hormone (FSH) secreted by the anterior pituitary gland.
• Luteinizing Hormone (LH) also plays a role by stimulating the secretion of oestrogen from the follicular cells, which in turn stimulates the growth of follicles into Graafian follicles.
• As oestrogen levels increase, FSH is inhibited, but LH levels remain unchanged.
• A peak in oestrogen levels triggers the release of both LH and FSH, leading to ovulation.

Ovulation:

• During ovulation, the Graafian follicle bursts, releasing a secondary oocyte (egg) into the fallopian tube.
• The released egg can be fertilized by sperm if mating occurs.

Luteal Phase:

• After ovulation, the ruptured follicle transforms into the corpus luteum, which secretes progesterone and some oestrogen.
• These hormones maintain the structure of the endometrium (uterine lining) to prepare for implantation.
• If pregnancy does not occur, the corpus luteum disintegrates, leading to a decline in progesterone and oestrogen levels, which initiates the next cycle.
• If pregnancy occurs, the corpus luteum is maintained during the first trimester to support the pregnancy until the placenta takes over.

Uterine Cycle

The uterine cycle consists of three phases: menstruation, proliferative phase, and secretory phase.

Menstruation:

1. Menstruation is the shedding of the uterine lining when pregnancy does not occur.
2. It is characterized by lower levels of sex hormones (oestrogen and progesterone), leading to the breakdown of the endometrium.
3. Menstrual bleeding, known as the menstrual flow, occurs and typically lasts for 3 to 5 days.
4. Although menstruation is commonly associated with the absence of pregnancy, implantation bleeding can sometimes occur, which is lighter and happens early in pregnancy.

Proliferative Phase:

1. This phase occurs after menstruation (days 6-13) and is marked by increased secretion of oestrogen from the ovaries.
2. Oestrogen stimulates the regeneration of the endometrial lining and prepares the uterus for the upcoming ovulation.
3. Fertile cervical mucus is also produced during this phase to aid sperm passage.

Secretory Phase:

1. This phase coincides with the luteal phase of the ovarian cycle (days 15-28).
2. The corpus luteum secretes high amounts of progesterone to prepare the uterus for potential implantation.
3. The endometrium becomes more receptive to a fertilized egg, and blood flow increases to the uterine lining.
4. If implantation does not occur, progesterone and oestrogen levels drop, and the cycle restarts with menstruation.

Oestrus cycle

The oestrus cycle is found in most non-primate mammals, such as dogs, cats, and cows. It differs from the menstrual cycle in several ways. The oestrus cycle has specific periods of sexual receptivity known as oestrus, which are often preceded and followed by phases where the female is not sexually receptive (called anoestrus).

Phases of the oestrus cycle

1. Pro-oestrus:
   • Follicles in the ovary begin to develop, and oestrogen levels rise. The uterus lining starts to develop, but the female is not yet sexually receptive.
   • Some animals may experience vaginal bleeding during this phase.
2. Oestrus:
   • This phase is characterized by sexual receptivity. Ovulation occurs during this period, and the female may display behavioral signs of being in heat, such as increased activity or vocalizations.
   • The female is most likely to mate and conceive during this phase.
3. Metestrus:
   • After ovulation, the corpus luteum forms and produces progesterone, causing the uterine lining to prepare for potential pregnancy.
   • Sexual receptivity decreases, and the female is no longer in heat.
4. Dioestrus:
   • If pregnancy does not occur, the corpus luteum regresses, and the cycle returns to anoestrus. This phase involves a period of sexual inactivity.
5. Anoestrus:
   • This is the resting phase of the cycle, during which the female is not sexually receptive.
   • Anoestrus may be influenced by environmental factors such as light exposure and seasonal changes.

Differences between the menstrual cycle and the oestrus cycle:

Aspect	Menstrual Cycle	Oestrus Cycle
Cycle Type	Occurs in primates (e.g., humans, monkeys, chimpanzees)	Occurs in most non-primate mammals (e.g., dogs, cats, cows)
Sexual Receptivity	Females can be sexually active at any time in the cycle, even outside the fertile window	Females are sexually receptive only during the oestrus phase (heat)
Endometrium Disposition	If conception does not occur, the endometrium is shed through menstruation	If conception does not occur, the endometrium is reabsorbed by the body
Hormonal Regulation	Luteinizing Hormone (LH) regulates the menstrual cycle	Follicle Stimulating Hormone (FSH) regulates the oestrus cycle
Ovulation	Ovulation occurs once during the cycle, generally around the middle of the cycle (day 14)	Ovulation occurs during the oestrus phase
Hormonal Secretions	Oestrogen and progesterone play significant roles in regulating the cycle	Oestrogen stimulates the oestrus phase; progesterone is produced after ovulation
Behavioural Signs	No distinct external signs of fertility during the cycle	Behavioral signs like lordosis reflex indicate fertility during oestrus
Cycle Length	Typically lasts 28 days, but can vary from 21 to 35 days	The cycle can vary greatly in length depending on the species
Postpartum Oestrus	Does not occur in humans (unless specifically induced)	Some species experience postpartum oestrus immediately after birth
Reproductive Hormones	Regulated by LH, FSH, oestrogen, and progesterone	Regulated primarily by oestrogen and progesterone after ovulation
Fertility Period	Fertile window occurs during ovulation (mid-cycle)	Fertility is only during the oestrus phase