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Physiology of Menstruation, Fertilization, Spermatogenesis, Oogenesis, Pregnancy and Parturition

A typical menstrual cycle lasts 28-29 days, but the length can vary between women and from cycle to cycle.

Physiology of menstruation

A typical menstrual cycle lasts 28-29 days, but the length can vary between women and from cycle to cycle. According to women, the length of their menstrual cycle begins the day after they have their first period and ends the day before their next period. A girl's first period (menarche) typically begins between 11 and 14 years of age. Among the additional development at this point is pubic hair and developing breasts.

Hormones and menstrual cycle

Various glands and hormones are involved in menstruation, which is a complex process. An involuntary release of certain chemicals by the hypothalamus triggers the pituitary gland to release estrogen and progesterone in the ovaries. It is through the activity of one gland or structure that the activity of the others is determined.

This cycle is divided into several phases:
  • Menstruation
  • Ovulation
  • Follicular phase
  • The luteal phase
However, the follicular phase comes before ovulation in the actual process.

Menstruation

Menstruation removes the thickened uterine lining (endometrium) by applying pressure to the uterine cervix. In menstrual fluid as well as mucus, blood and endometrial cells can be found. The average period lasts between three and seven days for most women.

Follicular phase

During the first 48 hours of menstruation, a woman experiences ovulation and ends the follicular phase. Follicle-stimulating hormone (FSH) is released by the pituitary gland when it is activated by the hypothalamus. A hormone secreted by the ovary stimulates the growth of 5-20 nodules (tiny cysts or nodules) on its surface. Inside each follicle is a developing egg. A single egg usually exists in each follicle, while all the others die. Follicle-stimulating hormone (FSH) is released around day 10 of a 28-day cycle. As a result of follicle stimulation, the uterus lining thickens for a possible pregnancy.

Ovulation

An egg is released from one of a woman's ovaries when she ovulates. After the Fallopian tube releases an egg, it may be fertilized by a sperm cell. Ovulation usually occurs approximately two weeks before the period of a woman, during the center of her menstrual cycle. While the duration of the procedure varies from woman to woman and from month to month, it is usually about one month. Those seeking pregnancy should keep an eye on when they will ovulate. 

Around the time of ovulation, the most fertile a woman is, or the most willing to become pregnant, so knowing when she ovulates every month is useful. Monthly menstrual cycles prepare a woman's body for a potential pregnancy. Control of the menstrual cycle is governed by hormones including progesterone, estrogen, luteinizing hormones, and follicle-stimulating hormones. As the ovum (egg) matures and becomes ready for release, hormones play an essential role throughout the menstrual cycle.

Luteal phase

A ruptured follicle forms on the surface of the ovary when the egg bursts from its follicle during ovulation. After transforming into the corpus luteum, the follicle remains in this state for about two weeks. Progesterone and estrogen are also released in small amounts by the structure. After being fertilized by this combination of hormones, fertilized eggs attach to the thickened uterine lining. 

A fertilized egg releases the hormones that maintain the corpus luteum as soon as it implants in the uterine wall. Human chorionic gonadotrophin (HCG) is detected by urine tests for pregnancy. Progesterone is produced in high levels by the corpus luteum to maintain a thickened uterine lining. An untimely pregnancy results in the corpus luteum dying after 28 days. The uterus' lining falls off when progesterone levels drop.

Fertilization

Fertilization, the union of a sperm nucleus from a father and an egg nucleus from a mother to develop an embryo! All organisms reproduce by fusing the chromosomes of two gametes of different sexes that contain half the number of chromosomes typical of their species. Fertilization occurs when two cells exchange genetic material, a process that occurs most frequently in microorganisms and protozoans. 

During fertilization, the first important event occurs when the two gametes fuse, forming a channel through which material can pass from one to another. Advanced plants undergo pollination before fertilization, where pollen is transferred to male gametes or macrospores and established contact with them. An egg that has fused with spermatozoa is usually penetrated by a single sperm. When a zygote is fertilized, a new individual can be formed by cell division. In the egg, several reactions are triggered by the fusion of two gametes. As a result, the membrane(s) of the egg is altered, so that more than one spermatozoon cannot attach to it or penetrate it. Polyspermy occurs when more than one spermatozoon normally enters an egg (polyspermy).

Generally, only one spermatozoon merges with the egg nucleus. Fertilization results in egg activation, which is crucial to cell division. Although parthenogenesis does not result in fertilization, an egg can be activated through physical and chemical means, without the intervention of a spermatozoon. Parthenogenesis is the normal reproductive method for insects such as aphids, bees, and rotifers.

Spermatogenesis

The process of producing sperms is called Spermatogenesis.

The sperm is a male gamete that fertilizes the ovum (female gamete) to create a zygote. The sperm is produced by the male testis. A process known as spermatogenesis produces sperms.

During the formation of sperms, the following stages take place:
Multiplication phase - Two daughter cells are produced by mitotic division of a single germ cell. Because daughter cells are derived from diploid cells, mitosis produces diploid cells. Spermatogonia are formed by dividing these daughter cells in mitosis. The majority of spermatogonia are diploid; half function as stem cells (by multiplication they will form more spermatogonia) and are referred to as Type A spermatogonia. One-half of them progresses to the next stage, called type B spermatogonia.

Growth phase - During the growth process, spermatogonia of type B grow larger as a result of nutritive material accumulation. Primary spermatocytes are now known as such. These cells are diploid.

Maturation phase - Meiosis happens in the primary spermatocytes. Reductional division during meiosis-1 produces two haploid cells of the secondary spermatocyte. A haploid sperm is produced by equational division in meiosis 2.

Spermatids become motile sperms by undergoing structural changes. Spermiogenesis is responsible for this.

The sperm remains embedded within Sertoli cells during the development process. The seminiferous tubules will release them only when they have matured. Spermiation is the process of releasing them.

Oogenesis

The ovaries undergo oogenesis at their outermost layer. The production of sperm also begins with a single germ cell, the oogonium (plural: oogonia), however, after mitosis, the number of these cells increases, eventually resulting in up to one or two million lines of DNA in the embryo. Primary oocytes are cells that are about to undergo meiosis. The first meiotic division of this cell will occur, but its progress in the prophase stage will be stopped. Prophase is the stage at which all future eggs are during the time of conception. Several follicles develop in an ovary during adolescence due to anterior pituitary hormones. 

Primary oocytes begin to divide after the first meiotic division is completed. The secondary oocyte, referred to as the nucleus, receives only a small amount of cytoplasm and a single set of chromosomes, while the nucleus reabsorbs most of the cellular material and organelles. 

Normally, polar bodies are dead cells of the second type. A meiotic arrest occurs during the metaphase II period. During ovulation, oviducts release secondary oocytes, which then travel to the uterus. During second-oocyte fertilization, meiosis II takes place. Fertilization of the second oocyte involves meiosis II. The meiosis cycle is completed by the production of the second polar body and the fertilized egg, which contains all 46 chromosomes of a human, half of which are derived from the sperm.

Pregnancy

Zygotes are formed when the egg is fertilized by the sperm during pregnancy. After implanting the zygote in the uterus wall, the embryo develops. Nurturing and caring are necessary immediately following implantation. Zygotes develop into embryos that grow into babies. Between the mother and the baby is a layer called the placenta, which handles this responsibility. Embryonic tissue and uterine tissue create the placenta. In pregnancy, this layer is responsible for a multitude of functions, as follows:-
  • The fetus receives oxygen and nutrients from this fluid.
  • By substituting the functions of the digestive system and the excretory system, it also eliminates the waste materials given off by the fetus.
  • For the fetus to grow, the placenta releases several hormones including lactogen, progesterone, estrogen, and human chorionic gonadotropin.
  • By connecting the placenta to the fetus, the umbilical cord facilitates the transfer of growth-promoting substances from the mother to the fetus.
The fertilized mass of cells or embryos separates into the three layers of cells as soon as implantation occurs, revealing the triploblastic nature of human beings. A variety of organs are formed during development, including ectoderm, mesoderm, and endoderm. During human development, after implantation, all these steps lead to the birth of the child, which takes nine months. This process is steady and gradual. As an embryo develops, it is first formed by the heart, followed by its limbs, major organs, hair, and so on. A fully formed fetus is present in the ninth month of pregnancy.

Parturition

At the end of pregnancy, contraction of the uterus at regular intervals causes the baby to be delivered. During ejection, the placenta receives signals from the fully developed fetus, which are interpreted by the fetus as an ejection reflex. It is these signals that cause contractions. The gestation period occurs between pregnancy and childbirth.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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