The main structures of the male reproductive system illustrated below are:
2. Reproductive ducts (epididymis, vas deferens, ejaculatory duct, urethra)
3. Accessory glands (seminal vesicles, prostate gland, bulbourethral glands)
4. Supporting structures (scrotum, penis, spermatic cords)
The testes have two main functions:
Spermatogenesis and spermiogenesis begin at puberty and continue throughout adult life. At the onset of puberty the testes secrete large amounts of testosterone. Testosterone is a steroid hormone which stimulates growth of the testis and initiates maturation of the seminiferous tubules.
Maturation of the Seminiferous Tubules.
The primitive "sex chords" in the embryonic testis contain primordial
germ cells and Sertoli cells. At puberty as the testes descend into the scrotum,
the sex chords develop a central lumen and differentiate into the seminiferous
tubules. Within the seminiferous tubules the primordial germ cells divide several
times by Mitosis differentiating into spermatogonia (Figure 1).
Spermatogenesis (Figure 1)
The development of male gametes (spermatids) from spermatogonia is called spermatogenesis.
Spermatogonia are located beneath the basement membrane, which surrounds each seminiferous tubule. Each spermatogonium is attached to an adjacent Sertoli cell by specialised membrane junctions.
Spermatogonia undergo mitosis to form primary Spermatocytes. Each primary spermatocyte undergoes the first meiosis to form two secondary spermatocytes. Each secondary spermatocyte undergoes a second meiosis to form two spermatids (Figure 1).
As each division takes place, the daughter cell migrates closer to the lumen of the seminiferous tubule, so that spermatids are immediately adjacent to the lumen. This is the process that converts spermatids into mature sperm (spermatozoa).
Spermatozoa (Figure 2) consist of:
The head contains the condensed nucleus and is capped by an apical vesicle
(acrosome) filled with hydrolytic enzymes.
Spermatozoa are then released into the lumen of the seminiferous tubule (spermiation) and transported to the epididymis where they are stored. During ejaculation the sperm are propelled through the vas deferens and urethra and are mixed with secretions from the seminal vesicles, prostate and bulbourethral glands.
This is the final step of sperm maturation.
Leydig and Sertoli Cells
Leydig cells are the principal cell type found in the interstitial supporting tissue between the seminiferous tubules. They synthesise and secrete the male sex hormones, mainly testosterone. They occur as single cells or in clumps embedded in the blood and lymph capillaries surrounding the seminiferous tubules.
Sertoli cells support the cells of the speratogenic series during development. They rest on the basement membrane of the seminiferous tubule. The cytoplasm is large, highly irregular and constantly changing to permit the movement of developing spermatozoa towards the luminal surface. Their functions include secretion of factors that regulate spermatogenesis and spermiogenesis; secretion of factors that regulate the function of Leydig cells; secretion of inhibin that regulates hormone production; secretion of tubular fluid and phagocytosis of discarded spermatid cytoplasm. Sertoli cells secrete an androgen-binding protein that transports testosterone to the lumen of seminiferous tubules.
|Structure of the sperm (Figure 2)
|Photomicrograph 1, Seminiferous tubules|
|Photomicrograph 2, Seminiferous tubule|
|Photomicrograph 4, Epididymis|
|Photomicrograph 5, vas deferens|
|Photomicrograph 6, low power of Seminal Vesicle (self study)|
|Photomicrograph 7, Prostate Gland (self study)|
|Photomicrograph 8, high power of Prostate Gland (self study)|
|Photomicrograph 9, Prostatic Urethra (self study)|
|Photomicrograph 10, low power cross-section of Penis (self study)|
|The Female Reproductive System
The main structures of the female reproductive system are:
The ovary serves two important functions:
Oogenesis (Figure 3)
The embryonic gonads contain primordial germ cells which undergo mitosis to form oogonia. By the 5th month of fetal development, some of the oogonia enlarge and divide, becoming primary oocytes. Primary oocytes begin their first meiosis at this stage. By the 7th month of fetal development, the primary oocytes become surrounded by a layer of flattened cells and become known as primordial follicles. There are about half a million primordial follicles in the ovary at birth. The process of encapsulation of the primary oocyte blocks the first meiotic division of the primordial follicle in the prophase. No further development of the follicle occurs until puberty. Thus, in contrast to the male, all the female germ cells are present at birth but meiosis is only completed during follicular maturation in the lead up to ovulation. This suspension in prophase may last up to 40 years!
Compare this to male gamete development. Meiosis I occurs only after puberty and the whole process of spermatogenesis and spermiogenesis takes about 70 days. It is also important to note from Figure 3, that oogenesis is unequal i.e. each oogonia gives rise to 1 oocyte and 3 polar bodies. In contrast, during spermatogenesis 1 spermatogonium gives rise to 4 spermatozoa.
During each ovarian cycle, up to 20 primordial follicles are activated to begin the maturation process, but usually only one reaches full maturity, the remainder regress (Figure 4).
The ovarian cycle can be divided into 3 phases:.
The ovarian cycle (Figure 4)
1. Follicular Phase
Follicular development from the primordial follicles is as follows (Figure 4):
Once fully developed the graafian follicle bulges from the external wall of the ovary. Ovulation occurs when the ovarian wall ruptures and expels the secondary oocyte into the peritoneal cavity. The second meiotic division of the oocyte is completed only after penetration of the oocyte (known as an ovum) by a spermatozoon.
3. Luteal Phase
After ovulation, the granulosa cells of the graffian follicle proliferate and form the corpus luteum which is highly vascularised and secretes progesterone and estrogens.
Progesterone, from the corpus luteum prepares the endometrium for implantation (see menstrual cycle). If fertilisation does not occur and an ovum does not implant into the uterine wall, the corpus luteum degenerates and forms the corpus albicans.
If implantation does occur, the developing placenta secretes human chorionic gonadotrophin (hCG) which prevents degeneration of the corpus luteum and prolongs secretion of progesterone. After 5-6 weeks the placenta is sufficiently developed and takes over as the main organ of progesterone secretion.
During the ovarian cycle, growth and development of the follicle is driven by two gonadotrophic hormones:
Both FSH and LH are secreted by the anterior pituitary and are under the control of gonadotrophin releasing hormone (GnRH) secreted by the hypothalamus.
FSH and LH stimulate follicle growth. As the follicle grows, thecal cells secrete estrogens.
Rising levels of estrogen in the plasma have a negative feedback effect on the anterior pituitary inhibiting output of FSH and LH.
However, this negative feedback is only transient and as levels of estrogen increase they begin to have a positive effect on the hypothalamic-pituitary axis resulting in a burst of LH and, to a lesser extent, FSH.
This sudden burst of LH and FSH stimulates completion of meiosis I in the primary oocyte and is also believed to be involved in stimulating synthesis of enzymes involved in bulging of the ovarian wall.
After ovulation, LH promotes the transformation of the ruptured graafian follicle into the corpus luteum. LH stimulates the corpus luteum to secrete progesterone and estrogen.
Progesterone and estrogen from the corpus luteum have a negative feedback effect on the anterior pituitary and inhibit FSH and LH production. This prevents development of new follicles.
As LH levels fall, the corpus luteum begins to degenerate. Levels of progesterone and estrogen fall. FSH and LH are again produced by the anterior pituitary and a new cycle begins.
The uterine or menstrual cycle is a series of cyclic changes in the endometrium
that occur on a monthly basis in response to changes in levels of ovarian
Menstrual phase (1-5 days)
The functional layer of the endometrium becomes detached from the uterine wall and this results in bleeding. Blood loss is usually between 50-150ml. Detached tissues and blood pass through the vagina as the menstrual flow.
Proliferative phase (days 6-14)
As levels of estrogen increase the endometrium begins to proliferate and thicken, tubular glands and spiral arteries form. Estrogen also stimulates the synthesis of progesterone receptors in endometrial cells. Ovulation occurs at the end of this phase.
Secretory phase (days 15-28)
Rising levels of progesterone produced by the corpus luteum act on the
endometrium stimulating the enlargement of glands which begin secreting
mucus and glycogen in preparation for implantation of the fertilised ovum.
|Photomicrograph 11, Ovary|
|Photomicrograph 12, Growing Follicle|
|Photomicrograph 13, High power of Graafian Follicle|
|Photomicrograph 14, Ovary|
|Photomicrograph 15, Fallopian Tube|
|Photomicrograph 16, Endometrium in proliferative phase|
|Photomicrograph 17, High power of endometrium in proliferative phase|
|Photomicrograph 18, Endometrium in secretory phase|
|Photomicrograph 19, Uterus|
|Photomicrograph 20, Vaginal mucosa|
|Photomicrograph 21, Cervix|
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