Menstrual Cycle

Menstrual Cycle

Menstrual cycle is marked by a characteristic event repeated every month (28 days with minor variations) in the form of menstrual flow (i. e shedding of endometrium of uterus with bleeding).

Girls get their first period (menarche) on average, between the ages of 11 and 14years. By this age other sexual traits have advanced along with pubic hairs and budding breasts. Menopause is a segment in women’s lifecycle when ovulation and menstruation stops. It occurs between 45 to 50 years of age .It marks the cessation of a woman’s fertility.

A follicle that has double layers of follicular cells is known as the primary follicle. These cells maintain hypertrophy and proliferate to form many layers surrounding the oocyte. Eventually those cells turn out to be recognized as ‘granulosa’ cells.

Fossils having two layers of follicular cells are known as primary follicles, these follicles are formed during the embryonic stage and are the basic unit of the female reproductive system.

Phases of menstrual cycle

The menstrual cycle encompasses the ovarian and uterine cycles. The ovarian cycle describes the modification that arises in the follicles of the ovary, whereas the uterine cycle describes the modification in the endometrial lining of the uterus. Thus, there are 4 levels of menstrual cycle:

Menstrual Phase

It takes place for a period of five days, in the course of this the endometrium breaks down, menstruation starts. The cells of endometrium, secretions, blood and the unfertilised ovum represent the menstrual flow, progesterone production is reduced. 

Follicular Phase

It lasts from sixth to thirteenth day. Endometrium rebuilds, FSH and LH secretion will increase that stimulates follicular development in addition to secretion of oestrogen. It is likewise referred to as the proliferative phase.

Ovulatory Phase

It takes place on the 14th day of the menstrual cycle. Both LH and FSH obtain a peak level. Ovulation takes place.

Luteal Phase

It takes place from the fifteenth until the twenty-eighth day of the menstrual cycle. Corpus luteum secretes progesterone. Endometrium thickens and uterine glands become secretory. It is likewise referred to as the secretory phase.

Hormonal role in menstrual cycle

The menstrual cycle is complicated and is managed by many different glands and the hormones that those glands produce. Hypothalamus in brain causes the nearby pituitary gland to provide certain chemicals, which turn on the ovaries to produce the sex hormones oestrogen and progesterone. Every segment of the menstrual cycle is influenced by female hormones, specifically oestrogen, progesterone, FSH and LH. Variation within the level of those hormones manages the phases of the menstrual cycle.

Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) secreted via means of anterior pituitary. FSH stimulates the growth of ovarian follicles that secrete oestrogen. Progesterone is secreted via means of corpus luteum. FSH and LH regularly will increase in the course of the follicular phase. They stimulate the development of follicles and the release of oestrogen from them. Oestrogen stimulates the proliferation of the endometrium. The level of LH and FSH surge in the center of the cycle. LH induces ovulation. After ovulation, the ruptured follicle develops into corpus luteum, which secretes progesterone, thereby growing the extent of progesterone in the course of luteal phase. Progesterone is needed for the preservation of endometrium for implantation. In absence of fertilisation, corpus luteum degenerates and progesterone level decreases, leading to disintegration of endometrium and menstrual flow.  

Embryo Development 

In human beings the embryo begins to develop about 4 days after an egg is fertilized. Human embryonic development is likewise referred to as embryogenesis. It is characterized via means of processes of cell division and cell differentiation of the embryo that takes place in the course of the early stages of development. Embryogenesis happens very early during pregnancy, even before a baby is a true foetus.

Steps involved in embryogenesis

Fertilisation

Union of a sperm nucleus, of paternal origin, with an egg nucleus of maternal origin, to form the primary nucleus of an embryo.  

Fertilisation in human beings takes place in the ampullary-isthmic junction of the fallopian tube. 

The secretions of the female genital tract take away coating materials deposited at the surface of the sperms, specifically the one at the acrosome. These chemical substances are together known as sperm lysin. 

Due to acrosomal reaction, the plasma membrane of the sperm fuses with the plasma membrane of the secondary oocyte in order that the sperm contents enter the oocyte.

Changes within the membrane of the oocyte block the access of extra sperms. 

Thus, it guarantees that the best sperm can fertilize an ovum. Entry of sperm consists of the completion of the meiotic division of the secondary oocyte. The second meiotic division is likewise unequal and results in the formation of a second polar body and a haploid ovum (ootid). Soon the haploid nucleus of the sperm and that of the ovum fuse collectively to form a diploid zygote. The zygote divides mitotically to produce 2,4,6,8 daughter cells known as blastomeres.

Blastocyst Formation

Because the zygote divides, it forms a blastocyst, or a ball of cells. Embryo with eight-sixteen blastomeres is referred to as morula. At the following level of development, which produces an embryo with  approximately sixty-four cells, a hollow shape is formed in the center of the cell mass. The hollow space is called blastocoel and the embryo is termed the blastocyst. Outer envelope of blastocyst is referred to as trophoblast or trophectoderm. 

Implantation

After development of blastocyst the trophoblast moves via the fallopian tube and attaches to the uterine wall during implantation. When the blastocyst attaches within the uterine wall, cells in the uterus start to grow across the blastocyst, forming the amniotic cavity. Zona pellucida prevents implantation of blastocyst at an abnormal site. 

Conclusion

A menstrual cycle includes a follicular, ovulation and luteal phase. After fertilisation, cleavage produces blastocyst that implants in the endometrium. The female reproductive cycles are managed through hormones launched from the hypothalamus and anterior pituitary and hormones from reproductive tissues and organs.

A human female’s pregnancy starts with fertilization of egg and proceeds via the 3 trimesters of gestation. The first trimester lays down the primary systems of the body, consisting of the limb buds, heart, eyes, and the liver. The 2nd trimester keeps the improvement of all the organs and systems. The 3rd trimester is well-known to show the finest increase of the foetus and culminates in exertions and transport. The exertion technique has 3 stages (contractions, transport of the foetus, and expulsion of the placenta), each propelled with the aid of hormones.