Gametogenesis- the process of meiosis and cytodifferentiation that converts germ cells into mature male and female gametes, spermatozoa and definitive oocytes respectively.
– timing between sexes differs
– diploid —– haploid
– 2n —– 1n
First meiotic division- involves DNA replication and recombination and yields two haploid 2n daughter cells.
– Primary oocyte or spermatocyte – the 4n cell coming forth from the 2n replication
– The first meiotic cell division produces two secondary spermatocytes in the male or a secondary oocyte and a first polar body in the female.
– the chromosomes condense into compact, double-stranded structures
– in late stage, the double chromosomes of each homologous pair match up, centromere to centromere, to form a joint structure called a chiasma.
– The chiasma allows for crossing-over which accounts for an increase in genetic variability.
- the primary oocyte enters a phase of meiotic arrest during the first meiotic prophase.
– the four-stranded chaisma structures are organized on the equator of a spindle apparatus similar to the one that forms during mitosis.
– one double-stranded chromosome of each homologous pair is distributed to each of the two daughter nuclei.
– The centromeres of the chromosomes do not replicate, and therefore the two chromatids of each chromosome remain together; which makes the resulting nuclei haploid, but 2n.
– They contain the same amount of DNA as the parent germ cell, but half the number of chromosomes.
Second meiotic division – the double-stranded chromosomes divide, yielding 4 haploid 1n daughter cells.
– no DNA replication occurs in the 2nd meiotic division
– the 23 double-stranded chromosomes condense during the 2nd meiotic prophase, and then line up during the 2nd meiotic metaphase.
– The chrosomal centromeres then replicate
– During anaphase the double-stranded chromosomes pull apart into 2 single stranded chromosomes and one is delivered to each daughter nucleus.
– the 2nd meiotic division produces two definitive spermatocytes or spermatids in males and in females it produces one large definitive oocyte and another dimunitive polar body. The first polar body may undergo the 2nd meiotic division and produce a third polar body.
– In the female, the oocyte enters another phase of meiotic arrest during the second meiotic metaphase before the replication of the centromeres. Meiosis does not resume until the oocyte is fertilized.
– at puberty the testes begin to secrete increased amounts of testosterone.
– Stimulates the development of many secondary sex characteristics
– triggers the growth of the testis
– triggers the maturation of the seminiferous tubules
– triggers spermatogenesis
– cells to undergo spermatogenesis arise from the spermatogonia via mitosis
– these cells are gradually translocated between the Sertoli cells from the basal to the luminal side of the seminiferous epithelium while spermatogenesis takes place.
– During this migratory phase, the primary spermatocytes pass without interruption through both meiotic division resulting in the four spermatids.
– The spermatids undergo the changes that converts them into mature sperm while they complete their migration to the lumen of the seminiferous epithelium.
– maturing spermatocytes and spermatids are connected to the Sertoli cells by tight junctions, gap junctions, and by a specialized junction termed tubulobulbar complexes.
– Tubulobulbar complexes- extend into the Sertoli cells; thought to provide a mechanism by which the excess cytoplasm is transferred to the Sertoli cells.
– As the cytoplasm is removed, the spermatids undergo the changes that convert them into spermatozoa.
– Finally the junctions with the Sertoli cells break, thus releasing the spermatozoa into the tubule lumen. (termed spermiation)
– head- contains the nucleus and is capped by an apical vesicle (acrosome) filled with hydrolytic enzymes.
– Midpiece- contains large, helical mitochondria and generates the power needed for swimming
– Tail- contains the microtubules that form the propulsion system.
– Errors are not at all uncommon
Spermatogenesis is continuous from puberty until death
– gamates are produced in synchronous waves in each local area of the germinal epithelium
– about four waves of synchronously differentiating cells can be observed in any given region of human tubule epithelium at any given time.
In the human male, each cycle of spermatogenesis takes about 64 days.
– spermatogonial mitosis- 16 days
– first meiotic cell division- 8 days
– second meiotic cell division- 16 days
– spermiogenesis- 24 days
· Sperm produced in the seminiferous tubules are stored in the epididymis, which is a special duct connected to the vas deferens.
- During ejaculation, the sperm are propelled through the vas deferens and the urethra and are mixed with nourishing fluid consisting of secretions from the seminal vesicles, prostate, and bulbourethral glands.
- As many as 200 million spermatozoa are ejaculated
- Only a few humdred succeed in reaching the ampulla.
– Capacitation- terminal step of functional maturation that prepares a spermatozoa to fertilize an oocyte; consists primarily of changes in the acrosome that prepare it to release the enzymes necessary to penetrate the zona pellucida of the oocyte; thought to take place in the female genital tract and to require contact with the secretions of the oviduct.
- the total number of primary oocytes is produced in the ovaries by the fifth month of fetal life.
- Germinal vesicle – thought to protect the DNA during the long period of meiotic arrest.
- Follicle cells- surround the primary oocyte in a single, squamous layer; oocyte and follicle cells together are termed primordial follicle.
Menarche- (Female puberty)
– Menstrual cycle- responsible for producing monthly a single female gamate and a properly conditioned uterus to receive a fertilized embyro.
– Monthly maturation of primordial follicle
– Concurrent proliferation of the uterine endometrium
– Continued development of the follicle into an endocrine corpus leutem
– Without fertilization, the sloughing of the endometrium and the involution of the corpus leutem
– 28 days
- begins with menstruation (1-5)
- day 5- the hypothalmus releases gonadotropin-releasing hormone (GnRH) which stimulates to pituitary gland to increase secretions of two gonadotropins—-follicle stimulating hormone (FSH) and luteinizing hormone (LH)
- prior to the inc. secretions of the gonadotropins the primordial follicles thicken from squamous to cuboidal and are then termed primary follicles.
- The follicle cells and the oocyte together secrete a small amount of glycoprotein onto the surface of the oocyte which forms the zona pellucida. The epithelium of 5 to 12 of these primary follicles then proliferates to form a multi-layered capsule around the oocytes and are now termed growing follicles. Some continue to grow in response to the increased FSH, while others degenerate.
- Those that continue to grow take up fluid and form a central fluid-filled cavity called the antrum. These follicles are now termed antral or vesicular follicles.
- At the same time the connective tissue of the ovarian stroma surrounding each of these vesicular or antral follicles differentiates into two layers, the inner layer is called the theca interna and the outer layer is called the theca externa. These two layers become vascularized and the follicle cells do not.
- A SINGLE FOLLICLE CELL BECOMES DOMINANT AND THE REST DEGENERATE
- The dominant follicle cell continues to absorb fluid.
- Cumulus oopherous- the small mass of follicle cells surrounding the oocyte
- large swollen vesicle is now termed the mature vesicular follicle or mature graafian follicle; still has not resumed meiosis.
- Resumption of meiosis and ovulation are stimulated by an ovulatory surge in the levels of FSH and LH
- Day 13 or 14- the levels of LH and FSH rise very sharply (ovulatory surge)
– stimulates the primary oocyte to resume meiosis
– the cumulus oopherous expands in response to the ovulatory surge in LH and FSH
– depends on the breakdown of the follicle wall
– similar to an inflammation response
– initiated by the secretion of histamine and prostaglandins
– within a few hour of the ovulatory surge of LH and FSH, the follicle becomes more vascularized and becomes increasingly pink.
– The follicle is then displace to the surface of the ovary, where it forms a bulge.
– As ovulation approaches, the projecting wall of the follicle begins to thin, resulting in the formation of a small, nipple-like protrusion called the stigma.
– FINALLY, a combination of tension plus the release of collagen-degrading enzymes and other factors by fibroblasts in the region causes the follicle to rupture; NON-EXPLOSIVE.
– The oocyte, accompanied by a large number of investing cumulus cells bound in the hyaluronic acid matrix and by some follicular fluid, is SLOWLY extruded onto the surface of the ovary.
– Ovulation occurs about 38 hours after the ovulatory surge of LH and FSH.
Endocrine corpus leutem- the ruptured follicle forms this structure
– the corpus leutem is an endocrine structure that secretes steroid hormones that maintain the uterine endometrium in a conditioned state.
– If no implantation occurs within 14 days, it converts into a scarlike structure termed a corpus albicans.
Estrogens and Progesterone secreted by the follicle control the uterine events of the menstrual cycle.