The Reproductive System

Male and Female

General Reproduction Principles- sexual reproduction involves two types of gametes:

- motile sperm cells (male)- immotile (stationary) eggs (ova) (female)

• Both were produced through the process of meiosis in the male and female, respectively.

• The sperm cell contributes 23 chromosomes and the egg cell (ovum) contributes 23 chromosomes

• The ovum is much larger than the sperm cell – the ovum contributes all of the cytoplasm to the fertilized egg (aka zygote).

• The zygote possesses 23 + 23 = 46 chromosomes and develops into an embryo, and eventually a fetus.

• The reproductive systems of males and females are generally described as two organ sets:

- the internal reproductive organs (testes, ovaries, (glands that produce hormones and gametes), uterus, fluid-producing structures, and a set of ducts connecting everything).

- the external genitalia (scrotum, penis, vagina, clitoris).

- Male: fig. 21.1 p. 420.

- Female: fig. 21.5 p. 424.

Male Reproductive System• Refer to fig. 21.1 p. 420 and table 21.1 p. 423.• The male gonads are paired testes (sing. testis), which are

suspended ‘outside’ of the body cavity within the scrotum (this, to keep them cooler).

• The testes are a pair of tightly-coiled tubes surrounded by several layers of connective tissue.

• The testes actually develop inside the abdominal cavity and descend into the scrotum during the last two months of fetal development.

• Later, we’ll examine the process by which the testes produce sperm, but for now, we’ll mention that sperm production is most efficient, and sperm health (viability) is more prominent when the temperature of the testes is lower than body temperature (~ 20C lower).

Parts of the Testes (fig. 21.3 p. 422):

i. Seminiferous Tubules – make up the ‘tubular’ portion of the testes (uncoiled = ~250 m); responsible for producing sperm cells (process known as spermatogenesis); packed with cells called spermatogonia at their outer tubule walls; these spermatogonia eventually mature to become sperm cells (discussed later).

ii. Sertoli Cells – support, nourish, and regulate cells destined to become sperm cells; also produce and release the hormone inhibin as sperm cells are produced in order to regulate their numbers (form of negative feedback).

iii. Interstitial Cells – scattered between the seminiferous tubules produce testosterone (the male sex hormone).

• The sperm that are produced by the seminiferous tubules (of the testes) are stored in the epididymis (pl. epididymides) – here, they actually complete their maturation and gain their ability to swim (ie. each of their flagella complete their developments).

• Each epididymis lies just outside each testis, and each is a tightly-coiled tubule.

• Each epididymis connects to a vas deferens (pl. vasa deferentia), a tube where sperm may be stored (once they have matured).

• The vasa deferentia are lined with smooth muscle.

• Upon ejaculation, the sperm are propelled through the vas deferens by peristaltic contractions.

• Each of these tubes ascends into the abdominal cavity, loops around and behind the urinary bladder, and drain into the urethra via the ejaculatory duct (urinary and reproductive systems merge in males; not in females).

• During orgasm/ejaculation, the bladder’s connection to the urethra is closed off to disallow urine from entering the semen (sperm + fluid).

• The penis is the male organ of sexual intercourse; it has erectile tissue with blood spaces. During arousal, these spaces fill with blood – this squeezes off the veins that carry blood away from the penis – creates an erection that is necessary for intercourse. An erection changes the penis’ angle in order to allow semen/sperm to actually enter the urethra (fig. 21.2 p. 421).

Spermatogenesis (begins at Puberty)• Occurs within seminiferous tubules of testes.• Spermatogonia (46 chromosomes) exist near outer wall of

seminiferous tubules; undergo mitosis often in order to maintain numbers for future sperm prod’n.

• FYI, within a mature male, ~3 million spermatogonia mature per day into primary spermatocytes (spermatogonia replicate their chromosomes to get 92).

• Primary spermatocytes move away from wall of tubules and undergo meiosis I, which produces two secondary spermatocytes (46 chromosomes each) per one primary.

• Secondary spermatocytes undergo meiosis II to produce four spermatids (23 chromosomes each) (sperm without ability to swim).

• Sertoli cells nourish spermatids, which gradually move to center of seminiferous tubules for transference to epididymis for final maturation – ability to move.

• Spermatids, upon growing a flagellum, become known as motile sperm cells (spermatozoa).

• Each ejaculation contains ~400 million sperm cells.

• Spermatogenesis, as a whole, takes 65-75 days; that said, the process is continuous and prolific.

• Males can ejaculate daily with no loss of fertilizing capacity.

Structure of Sperm Cells

- Comprised of a head, midpiece, and tail.

- The tail is a flagellum, the midpiece consists of a concentration of mitochondria, which provide ATP energy to the flagellum for ‘swimming’.

• The head contains the nucleus (remember, 23 chromosomes), which is behind (protected by) the acrosome – a specialized lysosome containing enzymes required to penetrate the ovum (egg) so that DNA may be injected into the egg (fertilization).

• Only one sperm normally penetrates and injects its DNA into an egg.

Seminal Fluid• Upon ejaculation, sperm cells exit the penis in a fluid

known as semen (or seminal fluid).• Three sets of glands add secretions to the semen:

i. Seminal Vesicles – paired glands that lie below and behind the bladder and empty into the ejaculatory duct (which connects to the vas deferens); contribute ~60% of seminal fluid; fluid is thick and clear, contains mucus, amino acids, and large amounts of fructose, which provides the sperm with simple sugar energy for their mitochondria in their midpieces; the seminal vesicles also secrete prostaglandins (a steroid) that promotes uterine contractions in the female in order to help propel semen up the uterus to the oviducts; the amino acids act to help coagulate semen once it enters the female so that it is more easily conducted by the uterine contractions.

ii. Prostate Gland – the largest accessory gland (it is singular); surrounds the initial segment of the urethra and secretes its ‘stuff’ directly into the urethra through several small ducts; fluid secreted is thin, milky, and basic (HCO3

-); the alkalinity of the fluid acts to balance the acidity of any urine still existing in the urethra, and to balance the acidity of the vagina; as well, sperm cells are more viable (active) in fluid possessing a basic pH.

iii. Bulbourethral (Cowper’s) Glands – paired, small glands along the urethra and below the prostate; secrete a viscous fluid as pre-ejaculate to serve as lubrication for the penis in the vagina; pre-ejaculate can carry some sperm (withdrawal method of contraception has low success rate).

Hormonal Control (Male Sex Hormones)• See fig. 21.4 p. 423.• Hypothalamus possesses the ultimate control over the

testes with respect to their sexual functioning (ie. spermatogenesis and testosterone production) by secreting gonadotropin-releasing hormone (GnRH), which stimulates the anterior pituitary gland to release the gonadotropic hormones follicle-stimulating hormone (FSH) and luteinizing hormone (LH) (this actually occurs in females as well).

• FSH has two effects:1. Promotes spermatogenesis in the seminiferous tubules of the

testes;2. Promotes production of hormone inhibin in the Sertoli cells of

the testes.

• FSH stimulates spermatogonia to become primary spermatocytes, whereas inhibin feeds back (negatively) on the hypothalamus and the ant. pituitary to slow/stop GnRH/FSH release so that epididymis is not overloaded with sperm.

• LH targets the interstitial cells of the testes and stimulates them to produce testosterone, the main sex hormone in males (aka an androgen).

• Testosterone is directly responsible for the primary and secondary sexual characteristics of males.

• The primary characteristics include those associated with the reproductive system – development of the vasa deferentia and other ducts, the penis, scrotum, and testes (associated with pre-puberty testosterone).

• Testosterone is also responsible for sex drives and erections – both associated with the reproductive system.

• At puberty, GnRH secretions increase, which leads to increased FSH, LH, spermatogenesis, and testosterone.

• In an interesting hormonal cross-over, testosterone plays a role in spermatogenesis FSH actually stimulates developing sperm cells to take up testosterone, which promotes further maturation and activity (ie. Without testosterone, sperm would not be mature or viable).

• ‘Puberty’ testosterone promotes secondary sexual characteristics, including:

- increased facial/body hair;- growth of larynx (deeper voice, larger Adam’s Apple);- greater muscular development;- increased oil/sweat gland secretions (acne);- receding hairline;- increased height, broader shoulders, longer legs relative to trunk length.

*Increased testosterone at puberty also contributes to further maturation of the primary characteristics (ie. leads to final maturation of penis and testes).

*Testosterone levels in blood feed back (negatively) upon hypothalamus and anterior pituitary to slow/stop GnRH/LH release so as to regulate testosterone.

• Thus, GnRH, FSH, LH, and testosterone are all regulated by negative feedback loops.

• Regulation performed by inhibin (FSH) and testosterone (LH).