Human Development

Male Reproductive System

The main structures of the male reproductive system illustrated below are:
1. Testes
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:

  1. Production of male gametes by the process of spermatogenisis and spermiogenesis.
  2. Secretion of male sex hormones.

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:

  1. Head
  2. Midpiece
  3. Tail

The head contains the condensed nucleus and is capped by an apical vesicle (acrosome) filled with hydrolytic enzymes.
The acrosome plays an important role in fertilisation.
The midpiece contains large helical mitochondria that generate the energy for swimming.
The tail contains microtubules.

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.
Changes in the acrosome take place that permit release of the hydrolytic enzymes.
These enzymes allow the sperm to penetrate the zona pellucida, which surrounds the oocyte.
Capacitation takes place in the female genital tract and requires contact with secretions of the oviduct.
Spermatozoa used for in vitro fertilisation are artificially capacitated.

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)

structure of the sperm

Photomicrograph 1, Seminiferous tubules
image 2 Photomicrograph 2, Seminiferous tubule
Photomicrograph 3
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:

  1. Ovaries
  2. Fallopian tubes
  3. Uterus
  4. Vagina
  5. Breasts

The ovary serves two important functions:

  1. Production of female gametes by the process of oogenesis.
  2. Secretion of hormones.
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.


Ovarian Cycle

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:.

  1. Follicular phase
    From many primordial follicles one Graafian follicle develops.
  2. Ovulatory phase
    The graafian follicle is released.
  3. Luteal phase
    The corpus luteum and corpus albicans are produced.

The ovarian cycle (Figure 4)

1. Follicular Phase

Follicular development from the primordial follicles is as follows (Figure 4):

  1. Primary follicle stage:
    Follicular epithelium surrounding the oocyte thickens to form cuboidal granulosa cells, Zona pellucida forms between the oocyte and granulosa cells.
  2. Growing follicle stage:
    Granulosa cells proliferate forming a multi layered capsule around the oocyte. Most growing follicles regress and die.
  3. Antral (or secondary follicle) stage:
    Remaining follicles take up fluid and develop a central cavity (antrum). Connective tissue surrounding the follicles forms 2 layers, an inner theca interna and an outer theca externa. Cells of the theca interna secrete oestrogen. Usually, only one antral follicle becomes dominant and enlarges. The rest degenerate (atresia).
  4. Graafian follicle stage:
    The first meiotic division is completed just before ovulation. The oocyte is now known as the secondary oocyte and commences its 2nd meiosis. The antrum enlarges. The granulosa cells around the secondary oocyte settle at the edge of the antrum as the cumulus oopherus. The follicle is now 1.5 - 2.5 cm in diameter.
2. Ovulatory Phase

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.

Hormonal Control of the Ovarian Cycle (Figure 5)

During the ovarian cycle, growth and development of the follicle is driven by two gonadotrophic hormones:

  1. Follicle Stimulating Hormone (FSH)
  2. Luteinising hormone (LH)

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.

menstrual cycle
The menstrual cycle and its relation to the ovarian cycle (Figure 6)

Menstrual Cycle

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 hormones.
Three phases of the menstrual cycle are recognised. These correlate with the ovarian cycle such that the endometrium is most receptive to implantation of the fertilised oocyte 7 days after ovulation.

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.
If fertilisation does not occur, the corpus luteum degenerates, progesterone levels fall and the endometrium degenerates. The cycle starts again with the first day of menstrual flow.

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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