Question

Describe the processes involved for the formation of mature spermatozoa, and a secondary oocyte. Include a review of all hormonal regulation involved in the processes of spermatogenesis and oogenesis, as well as the actual formation and maturation of the sex cells from spermatogonia and primary oocytes. Maturation of the sex cells should discuss the supporting cells for both types of sex cells, as well as a review of the organs and structures hosting the maturation and development. Your response should consider formation, development, and maturation for the two sex cells until the end of the process, to be considered as: 1) fully functional, mobile spermatozoa in the epididymis, and 2) just after ovulation for the secondary oocyte.

Answer 1

Spermatogenesis is the process by which the male gametes called spermatozoa (sperms) are formed from the primitive spermatogenic cells (spermatogonia) in the testis .It takes 74 days for the formation of sperm from a primitive germ cell. Throughout the process of spermatogenesis, the spermatogenic cells have cytoplasmic attachment with Sertoli cells. Sertoli cells supply all the necessary materials for spermatogenesis through the cytoplasmic attachment.
STAGES OF SPERMATOGENESIS
Spermatogenesis occurs in four stages:
1. Stage of proliferation
2. Stage of growth
3. Stage of maturation
4. Stage of transformation.   
1. Stage of Proliferation
Each spermatogonium contains diploid number (23 pairs) of chromosomes. One member of each pair is from maternal origin and the other one from paternal origin. The 23 pairs include 22 pairs of autosomal chromosomes and one pair of sex chromosomes. Sex chromosomes are one X chromosome and one Y chromosome.
During the proliferative stage, spermatogonia divide by mitosis, without any change in chromosomal number. In man, there are usually seven generations of spermatogonia. The last generation enters the stage of growth as primary spermatocyte.
During this stage, the spermatogonia migrate along with Sertoli cells towards the lumen of seminiferous tubule.
2. Stage of Growth
In this stage, the primary spermatocyte grows into a large cell. Apart from growth, there is no other change in spermatocyte during this stage.
3. Stage of Maturation
After reaching the full size, each primary spermatocyte quickly undergoes meiotic or maturation division, which occurs in two phases:
First phase
In the first phase, each primary spermatocyte divides into two secondary spermatocytes. The significance of the first meiotic division is that each secondary spermatocyte receives only the haploid or half the number of chromosomes. 23 chromosomes include 22 autosomes and a X or a Y chromosome.
Second phase
During this phase, each secondary spermatocyte under- goes second meiotic division, resulting in two smaller cells called spermatids. Each spermatid has haploid number of chromosomes.
4. Stage of Transformation
There is no further division. Spermatids are transformed into matured spermatozoa (sperms), by means of spermeogenesis and released by spermination.   
Role of Hormones in Spermatogenesis
Spermatogenesis is influenced by many hormones, which act either directly or indirectly.
Hormones necessary for spermatogenesis are:
i. Follicle-stimulating hormone (FSH)
ii. Testosterone
iii. Estrogen
iv. Luteinizing hormone (LH)
v. Growth hormone (GH) vi. Inhibin
vii. Activin.
i. Follicule-stimulating hormone
Follicule-stimulating hormone is responsible for the initiation of spermatogenesis. It binds with Sertoli cells and spermatogonia and induces the proliferation of spermatogonia. It also stimulates the formation of estrogen and androgen-binding protein from Sertoli cells .
ii. Testosterone
Testosterone is responsible for the sequence of remaining stages in spermatogenesis. It is also responsible for the maintenance of spermatogenesis. Testosterone activity is largely influenced by androgen-binding protein.
iii. Estrogen
Estrogen is formed from testosterone in Sertoli cells. It
is necessary for spermeogenesis.
iv. Luteinizing Hormone
In males, this hormone is called interstitial cell- stimulating hormone. It is essential for the secretion of testosterone from Leydig cells.
v. Growth Hormone
Growth hormone is essential for the general metabolic processes in testis. It is also necessary for the pro- liferation of spermatogonia. In pituitary dwarfs, the spermatogenesis is severely affected.
vi. Inhibin
Inhibin is a peptide hormone and serves as a transforming growth factor. It is secreted by Sertoli cells. In females, it is secreted by granulosa cells of ovarian follicles. Its secretion is stimulated by FSH.
Inhibin plays an important role in the regulation of spermatogenesis by inhibiting FSH secretion through feedback mechanism. FSH secreted from anterior pituitary induces spermatogenesis by stimulating Sertoli cells. It also stimulates the secretion of inhibin from Sertoli cells. So, when the rate of spermatogenesis increases, there is a simultaneous increase in inhibin secretion also. Inhibin in turn, acts on anterior pituitary and inhibits the secretion of FSH, leading to decrease in the pace of spermatogenesis.

The germ cells undergo rapid mitotic division and
by 20 weeks, the number reaches about 7 million. While majority of the oogonia continue to divide until 7th month of gestation, some enter into the prophase of first meiotic division and are called primary oocytes. These are surrounded by flat cells from the stroma (pregranulosa cells) and are called primordial follicles. The primary oocytes continue to grow through various stages of prophase (leptotene, zygotene, pachytene and diplotene) and ultimately reach to the stage of diplotene or else become atretic. Primary oocytes are then arrested in the diplotene stage of prophase of first meiotic division, until ovulation.
Total number of oocytes at 20 weeks of intra­ uterine life is about 6–7 million. At birth, the total number of primordial follicles is estimated to be about 2 million. The primary oocytes do not finish the first meiotic division until puberty is reached.
At puberty, some 400,000 primary oocytes are left behind, the rest become atretic. During the entire reproductive period, some 400 are likely to ovulate. Thus, the important feature is the tendency of the sex cells to undergo degeneration. The degeneration starts in the intrauterine life and continues throughout childhood and the childbearing period. As a result, no more follicles with ova can be detected in menopausal women.
mAtURAtion
The essence of maturation is reduction of the number of chromosomes to half. The primary oocyte remains in diplotene phase until shortly before ovulation unless it undergoes atresia. It is the midcycle LH surge that initiates the resumption of meiosis­1. The primary oocyte undergoes first meiotic division giving rise to secondary oocyte and one polar body. The two are of unequal size, the secondary oocyte contains haploid number of chromosomes (23, X) but nearly all the cytoplasm. The small polar body also contains haploid number of chromosome (23, X) but with scanty cytoplasm. The formation of secondary oocyte occurs with full maturation of Graafian follicle just prior to ovulation.