HUMAN REPRODUCTION :
GAMETOGENESIS
Gametogenesis is the process of gamete (sperm or egg) formation which include spermatogenesis & oogenesis.
Spermatogenesis
Spermatogenesis is a continuous process that results in the formation of sperms, which are then transported by the male sex accessory ducts. This process occurs in the seminiferous tubules at the time of puberty due to a significant increase in the secretion of GnRH and continues throughout life. Increased levels of GnRH act at the anterior pituitary gland and stimulate the secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH).
LH acts on the Leydig cells and stimulates the synthesis and secretion of androgens, which, in turn, stimulate the process of spermatogenesis. FSH acts on the Sertoli cells and stimulates the secretion of some factors that help in the process of spermiogenesis. The spermatogonia present on the inner wall of seminiferous tubules multiply by mitotic division and increase in numbers. Each spermatogonium is diploid and contains 46 chromosomes.
Some of the spermatogonia, called primary spermatocytes, periodically undergo meiosis. A primary spermatocyte completes the first meiotic division (reduction division) leading to the formation of two equal, haploid cells called secondary spermatocytes, which have only 23 chromosomes each. The secondary spermatocytes undergo the second meiotic division to produce four equal, haploid spermatids.
Spermiogenesis, or spermateleosis, is the process of the formation of flagellated spermatozoa from spermatids. Spermiogenesis begins in the seminiferous tubules but is usually completed in the epididymis. After spermiogenesis, sperm heads become embedded in the Sertoli cells and are finally released from the seminiferous tubules by the process called spermiation. The sperms formed by this process are then transported by the male sex accessory ducts.
STRUCTURE OF SPERM
The sperm is a microscopic structure that consists of a head, neck, middle piece, and tail.
Within the head of the sperm is an elongated haploid nucleus, covered by an acrosome that is produced by the Golgi body. The acrosome contains enzymes that are essential for the fertilization of the ovum, and without it, the sperm would not be able to penetrate the ovum.
The neck is short and contains both proximal and distal centrioles.
The middle piece contains numerous mitochondria that produce energy to power the tail’s movement, enabling the sperm to swim and reach the ovum for fertilization.
Low sperm count is known as oligospermia, which can result in infertility. The absence of sperm in semen is referred to as azoospermia, while a condition where sperm are present but completely immotile is known as necrospermia.
Diagram
Oogenesis
Oogenesis is the process of the formation of mature female gametes, also known as ova or eggs. It is a discontinuous process that begins before birth, halts in the middle, and resumes after menarche. During embryonic development, millions of gamete mother cells called oogonia are formed in each fetal ovary. The oogonia then undergo division and enter into prophase-I of the meiotic division, getting arrested at that stage, which is called the primary oocyte. Each primary oocyte is surrounded by a layer of granulosa cells, forming the primary follicle. Many of these follicles degenerate before puberty, leaving only 60,000-80,000 primary follicles in each ovary at puberty.
The primary follicles develop into secondary follicles as they are surrounded by more layers of granulosa cells and a new theca. Soon, the secondary follicle transforms into a tertiary follicle characterized by an antrum, which is a fluid-filled cavity. The tertiary follicle then develops into a mature follicle, also known as a Graafian follicle. The secondary oocyte forms a new membrane called the zona pellucida surrounding it.
When the Graafian follicle ruptures, the secondary oocyte is released from the ovary through a process called ovulation. It is important to note that unlike spermatogenesis, where all four haploid cells are produced, only one haploid cell is formed during oogenesis, and the other three haploid cells formed during meiosis-I remain as polar bodies.
Note
During oogenesis, the secondary oocyte is a female gamete that has completed the first meiotic division and is in the metaphase stage of the second meiotic division. The secondary oocyte will only complete the second meiotic division upon fertilization by a sperm in the fallopian tube. The release of the secondary oocyte (ovulation) is triggered by the surge of luteinizing hormone (LH) during the menstrual cycle.
Menopause marks the end of oogenesis in females, as the ovaries no longer release eggs.
Polar bodies are small, non-functional cells that are produced during oogenesis. They are formed during the meiotic divisions and contain half the number of chromosomes as the original cell. Polar bodies are typically formed during the development of the secondary oocyte, but may also be formed during the first meiotic division.
STRUCTURE OF OVUM
The ovum, also known as the egg cell, is a non-motile female gamete that is typically spherical in shape and is surrounded by one or more protective layers. The size of the ovum can vary depending on the species, and is determined by the amount of yolk present. In mammals, the ovum is typically microlecithal and measures around 100µ in diameter.
The yolk in the ovum provides nutrients for the developing embryo, and its amount is proportional to the size of the egg. The cytoplasm of the ovum also contains organelles such as mitochondria and ribosomes, which are responsible for various cellular functions.
In humans, the lifespan of the ovum within the female reproductive organs is around 48 hours. The nucleus of the ovum is known as the germinal vesicle, and it contains the genetic material necessary for fertilization and development of the embryo.
DIAGRAM
MENSTRUAL CYCLE
The menstrual cycle is a series of cyclic changes that occur in the reproductive tract of primate females, including humans. It begins at puberty in females, which is known as menarche. The menstrual cycle only starts after attaining sexual maturation (puberty), and during ovulation, only one ovum is released per menstrual cycle. The cyclical changes in the ovary and uterus during the menstrual cycle are induced by changes in the levels of pituitary and ovarian hormones. Cyclic menstruation is an indicator of the normal reproductive phase and extends between menarche (the first menstrual cycle) and menopause. The length of the menstrual cycle varies widely in women, but on average, it is completed in 28 days (mensus means a month). The menstrual cycle is absent during pregnancy, may be suppressed during lactation, and permanently stops at menopause.
The menstrual cycle is divided into four phases: the follicular (proliferative) phase, ovulatory phase, luteal phase, and menstrual phase.
FOLLICULAR (PROLIFERATIVE) PHASE OR POST-MENSTRUAL OR PRE-OVULATORY PHASE The follicular phase follows the menstrual phase and lasts for about 9-10 days (from 6 to 13th day of menstrual cycle). During this phase, the following changes occur: Under the stimulation of FSH-RF of hypothalamus, there is an increased secretion of FSH from the anterior pituitary. FSH stimulates the change of a primary follicle of the ovary into a Graafian follicle. Follicular cells of the Graafian follicle secrete estrogens. The proliferative phase consists of the growth of endometrium, fallopian tube, and vagina. The follicular phase ends with ovulation.
OVULATORY PHASE OR FERTILITY PHASE The ovulatory phase involves the ovulation from the Graafian follicle of the ovary. It occurs at the end of the proliferative phase, i.e. 14th day or midday during the menstrual cycle and coincides with the beginning of the next phase. It is caused by increased turgidity and contraction of smooth muscle fibers around the Graafian follicle. The ovum is received by the fimbria of the fallopian tube and is viable for two days. Ovulation is controlled by the increased level of LH in the blood. The egg at that time is in the secondary oocyte state. LH also starts the change of empty Graafian follicle into corpus luteum and secretion of progesterone from corpus luteum. During ovulation, the secondary oocyte remains surrounded by its zona pellucida and corona radiata. There is no significant change in uterine endometrium during the ovulatory phase. In animals, ovulation follows three patterns:
- Fix or spontaneous ovulators: In these animals, ovulation takes place at a fixed time in the midway of the cycle. There is no need for coitus for ovulation. E.g., Primates (Human, Ape, and Monkey).
- Induced or reflex ovulators: In these animals, copulation or coitus is necessary for ovulation, e.g., rabbit.
- Seasonal ovulators: Ovulation occurs in the breeding season, e.g., frog.
LUTEAL PHASE
The luteal phase is the final phase of the menstrual cycle and lasts for about 12-14 days. It begins after ovulation and ends with the start of the next menstrual period.
After ovulation, the ruptured follicle forms a structure called the corpus luteum, which produces progesterone and some estrogen. These hormones prepare the uterus for implantation of a fertilized egg and help maintain the uterine lining.
If fertilization does not occur, the corpus luteum degenerates, leading to a drop in progesterone levels, and the start of the next menstrual period.
MENSTRUAL PHASE OR BLEEDING PHASE
This phase lasts for about 3-5 days and occurs from the 1st to the 4th day of the menstrual cycle.
If the ovum remains unfertilized, the corpus luteum starts to degenerate, leading to a decline in the level of progesterone in the blood. As a result, the uterine tissues fail to be maintained, and the unfertilized ovum, along with ruptured uterine epithelium, about 50 – 100 ml of blood, and some mucus, is discharged through the vaginal orifice, known as menstrual flow or menstruation.
The decrease in the level of progesterone and estrogen in the blood stimulates the hypothalamus and anterior pituitary to release FSH-RF and FSH, respectively, which initiate the follicular phase of the next menstrual cycle.
FERTILIZATION AND IMPLANTATION
Fertilization is the process of the fusion of a sperm with an ovum, resulting in the formation of a diploid cell. This process activates the secondary oocyte to complete the division and typically occurs in the upper part of the oviduct (ampulla).
During fertilization, a sperm comes in contact with the ovum’s zona pellucida layer and induces changes in the membrane that prevent the entry of additional sperm. This ensures that only one sperm can fertilize an ovum, as polyspermy can be detrimental. When the acrosome of the spermatozoa touches the surface of the egg, the cytoplasm of the egg bulges forward, forming a receptive cone or fertilization cone, which is the region where the sperm enters the egg.
The secretions of the acrosome help the sperm enter the cytoplasm of the ovum through the zona pellucida and the plasma membrane, which induces the completion of the meiotic division of the secondary oocyte. The second meiotic division is also unequal and results in the formation of a second polar body and a haploid ovum (ootid). The haploid nucleus of the sperm and that of the ovum then fuse to form a diploid zygote.
Cleavage is the mitotic division of the zygote that forms 2, 4, 8, 16 daughter cells called blastomeres. Cleavage occurs more readily in the active cytoplasm, and in humans, it is equal holoblastic. The morula is a solid ball of 32 cell stage without a cavity that is formed after the 5th cleavage and 31 cell division. It looks like a little mulberry. Morula changes to blastula due to rearrangements of blastomeres, and this process is called blastulation.
The mammalian blastula with a large blastocoel is called a blastocyst in humans, and it has three parts: the trophoblast, inner cell mass, and blastocoel. The blastomeres in the blastocyst are arranged into an outer layer called the trophoblast and an inner group of cells attached to the trophoblast called the inner cell mass. The trophoblast layer then gets attached to the endometrium, and the inner cell mass gets differentiated as the embryo.
After attachment, the uterine cells divide rapidly and cover the blastocyst. As a result, the blastocyst becomes embedded in the endometrium of the uterus, which is called implantation (or nidation), and it leads to pregnancy. Implantation generally occurs between the 6th to 9th day after fertilization. The site of implantation determines the portion of the placenta. In humans, implantation is of interstitial type in which the embryo is buried in the uterine epithelium, which completely surrounds it.
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