Estrus Cycle | Biology for Grade 12 PDF Download

ESTRUS CYCLE
Sequential changes of ovary & uterus are combinedly known as estrus or menstrual cycle. Except primate mammals, the estrus cycle is found in all mammals. Menstrual cycle is found in primates, There is no menstruation at the end of estrous cycle. 

Fig: Estrus cycle of cattleThe estrous cycle runs only during the breeding season. On the basis of estrus cycle these mammals are of 2 types :

(1) Mono estrus animals: Only 1 cycle is found in breeding season.
Ex. Dog, Fox, Dear, Bat

(2) Poly–estrus animals: More than 1 estrus cycles are found in the breeding season.
Ex. Mouse, Pig, Squirrel, Cat

Fig: Estrus cycle

Estrus cycle completes in 4–stages:-

(1) Pre–estrus phase

(2) Estrus phase

(3) Meta–estrus phase

(4) Anestrus/Diestrus phase

Pre Estrus:-
1st step of estrus cycle. In this stage, oogenesis starts in the ovary. In this stage estrogen level is minimum. In low concentration of estrogen some changes occur in uterus.

(i) Blood capillaries become more branched

(ii) Muscles of uterus become more active

(iii) The stratified glandular epithelial layer of uterus known as stratum Basalis now divides actively to form new stratified epithelial layer known as stratum functionale. In this phase no copulation occurs between opposite sexes.

Estrus phase:- 
In this phase secondary follicle develops in the ovary. Concentration of estrogen is increased. Due to high concentration of estrogen following changes occur in the uterus.

(i) The diameter of blood vessels increases

(ii) The circulation also increases

(iii) The muscles of uterus divide to cause further thickening of myometrium.

(iv) Cells of stratum functionale enlarge. New connective tissue & mucus cells are developed.

(v) Mammary glands also get enlarged.

Due to high concentration of estrogen the sex urge develops in the female. This stage is known as heat condition. Copulation occurs only in this phase.

Meta–Estrus Phase:- 
All physiological symptoms which develop in this phase are similar to normal pregnancy even in absence of fertilization, so this phase is known as Pseudo–pregnancy. If fertilization occurs in fallopian tube then this phase extends upto parturition. If fertilization does not occur then this phase becomes short. Keratinization occurs in stratum functionale, myometrium is gradually reduced. Blood vessels gradually constrict. Mammary glands are gradually reduced.

Anestrus/Diestrus phase:- 
Stratum functionale is destroyed by stratum basalis by phagocytosis. Mammary gland, myometrium & blood vessels of uterus become completely normal. So it is the inter phase stage of two estrus cycle & is also known as diestrus phase. In monoestrus animals the anestrus phase is the diestrus phase also.

Formation of ovarian or Graafian follicle 

Ova develop from oogonia present in the cortex of the ovary. The oogonia are surrounded by other cells that form a stroma for them. These stromal cells form the ovarian or Graafian follicle that  surrounds the ovum and protects it.

The stages is formation of Graafian follicle are as follows :

(1) Firstly some cells of the stroma become flattened and surround a primary oocyte (which develops from oogonia). These flattened cells ultimately form the ovarian follicle and are therefore called follicular cells.

 (2) The flattened follicular cells now become columnar. Follicles upto this stage of development are called primordial follicle.

(3) A membrane called the zona pellucida, now appears between the follicular cells and the oocyte.

(4) The follicular cells proliferate now to form several layers of cells to form the membrane granulosa. These cells are now called granulosa cells.

(5) A cavity appears within the membrane granulosa. It is called the antrum. With the appearance of this cavity, the follicle is formed (follicle means a small sac).

(6) The cavity of the follicle rapidly increases in size and gets filled with a fluid called liquor folliculi. Due to increase in the size of the cavity the wall of the follicle (formed by granulosa cells) becomes relatively thin. The oocyte now lies eccentrically in the follicle, surrounded by some granulosa cells that are called as cumulus oophoricus. The cells that attached it to the wall of the follicle are called as discus proligerus or Germ hill.

(7) As the follicle expands, the stromal cells surrounding the membrane granulosa become condensed to form a covering called the theca Interna. The cells of theca interna (Thecal cells) afterwards secrete a hormone called oestrogen.

(8) Outside the theca interna some fibrous tissue become condensed to form another covering called the theca externa. The ovarian follicle is now fully formed and is now called the Graafian follicle.

The granulosa cells lying in the close vicinity of the ovum (secondary oocyte) and zona pellucida, become elongated to form the corona radiata.
After 13 days of menstrual cycle (on 14th day when cycle is ideally for 28 days) Graafian follicle is ruptured & egg is released.

After ovulation the rupured Graafian follicle is called corpus luteum. Soon after ovulation, the granulosa cells of Graafian follicle proliferate & these cells look yellow due to accumulation of pigment called Lutein. These cells are called lutein cells.

Before ovulation the follicle was a vascular but soon after ovulation blood vessels grow & corpus luteum becomes filled with blood. Central part filled with blood is called corpus haemorrhagicum. Lutein cells synthesize the progesterone hormone.

If fertilization occurs in fallopian tube, the corpus luteum then becomes stable for next nine months.  If fertilization does not occur then the corpus luteum starts degenerating after about 9 days of its formation. The degeneration is completed by 14 days to form corpus albicans, which gradually disappears.
Progesterone hormone maintains pregnancy and repairs the wall of uterus to make its surface adhesive to help in implantation.

The total number of follicles in the two ovaries of a normal young adult woman is about four lakhs. However most of them undergo regression and disappear due to death and are disposed off by the phagocytes during the reproductive years of the females. This is termed as follicular atresia. This is responsible for limited number of gamete production in females. Generally, only one ovum is liberated in each menstrual cycle, by alternate ovaries. Only about 450 ova are produced by a human female over the entire span of her reproductive life which lasts till about 40-50 years of age.

Although most of the follicular cells and the oocytes undergo degeneration during follicular atresia, some thecal cells, formed from the stroma and located around the follicle, persist and become active. These are called interstitial cells. These cells secrete small amount of androgen.

In Rabbit copulation occurs in breeding season which extend from  Feb to June.
No specific breeding season is found in human being.
During copulation male ejaculates semen in vagina, this process is called as insemination.
In rabbit ovulation occurs after 12–24 hour of copulation. That means copulation is necessary for ovulation.
Females in which coitus induction is compulsary for ovulation are called as induced or reflex ovulator.
In human female ovulation occurs in presence of FSH & LH. Coitus is not necessary for inducing ovulation.
Such a female is called as spontaneous ovulator.

After copulation oxytocin hormone is secreted from pituitary gland. This hormone promotes the peristalsis in the fallopian tube & uterus. Due to this the semen is sucked into fallopian tube.

Due to action of estrogens and progesterone, the endometrium of uterus is prepared for implantation. By the 6th to 7th day, embryo is implanted into endometrium (most commonly at the fundus).
In rabbit implantation occurs on 6th day.