Reproductive System

Introduction

The primary sex organs, or gonads, are the testes in the male and the ovaries in the female. The gonads produce gametes and secrete sex hormones. Other reproductive structures are accessory reproductive organs. The male manufactures sperm and delivers them to the female reproductive tract where fertilization may occur when the sperm fuses with an egg. The fertilized egg is the zygote which is the first cell of the new individual. The zygote begins the process of many rounds of mitosis and later differentiation will occur as body structures are formed.

Male reproductive physiology

The testes are housed in the scrotum which is located outside of the abdominopelvic cavity for reasons of sperm production. Sperm cannot be produced at core body temperature and the temperature in the scrotum is 3 degrees lower. The testes are divided into many wedge-shaped compartments called lobules which contain the seminiferous tubules. Sperm are produced in the seminiferous tubules. The tubules are surrounded by interstitial cells (Leydig cells) which produce testosterone.

Sperm leave the seminiferous tubules and travel to the epididymis. During the time in the epididymis the non-motile sperm gain the ability to swim. The sperm then pass into the ductus deferens or vas deferens which runs up through the spermatic cord and into the pelvic cavity. The end of this tubue expands and joins with the duct of the seminal vesicle to form the ejaculatory duct. This passes into the prostate gland and empties into the urethra.

Accessory glands assist in the production of semen. They include the seminal vesicles, the bulbourethral glands, and the prostate gland. The seminal vesicles produce an alkaline fluid containing fructose (provides energy to sperm), coagulating enzymes, ascorbic acid, and prostaglandins (decrease viscosity of mucus at the entry of the uterus and stimulate reverse peristalsis at the uterus). Sperm and seminal fluid mix in the ejaculatory duct. The prostate gland produces an acidic fluid which contains citrate (nutrient) and several enzymes which activate the sperm. The bulbourethral glands produce a thick, clear mucus which is secreted into the urethra before ejaculation to neutralize traces of acidic urine.

The urethra runs through the penis and is surrounded by a spongy network of vascular tissue. During an erection this tissue is flooded with blood which is temporarily unable to drain into the venous return. The influx of blood is stimulated by a parasympathetic reflex which promotes the release of nitric oxide locally. The NO causes the arterioles to dilate. Ejaculation of the sperm is accomplished by a sympathetic nervous system spinal reflex which causes three things: reproductive ducts and accessory glands contract and empty contents into the urethra, bladder sphincter muscle constricts preventing urine expulsion or reflux of semen, and smooth muscles of penis contract.

This process is influenced by hormones. The hypothalamus stimulates the anterior pituitary to make follicle-stimulating hormone and luteinizing hormone. FSH stimulates spermatogenesis by causing sperm-producing cells to be receptive to testosterone's effects. LH binds to interstitial cells and causes them to secrete testosterone which serves as final trigger for sperm formation. Testosterone also targets the accessory reproductive organs and causes them to grow to adult size and function. Testosterone also stimulates the male secondary sex characteristics of enhanced hair growth in pubic, axillary, and facial regions, deepening of the voice, bone and skeletal muscle growth, increased basal metabolic rate, and increased sex drive. Adrenal glands also secrete testosterone (sex drive in females).

Female reproductive physiology

The ovaries produce the eggs and also the female sex hormones, estrogen and progesterone. The accessory ducts (uterine tubes, uterus, and vagina) transport or otherwise serve the needs of the reproductive cells and a developing fetus. Ovaries are held in place by ligaments. Embedded in the ovarian tissue are saclike follicles each of which consists of an immature egg (oocyte) surrounded by other cells. Each month a follicle undergoes development to produce a viable egg cell.

When ovulation occurs the egg is cast into the peritoneal cavity. The uterine tubes distally branch out into fingerlike projections called fimbriae. They drape over the ovary and their cilia create currents in the peritoneal fluid which carries the oocyte toward the uterine tube. As the oocyte reaches the curve in the tube called the ampulla, fertilization may take place there if a sperm is present. The oocyte or the fertilized ovum (zygote) makes its way to the uterus. An unfertilized oocyte will eventually degenerate.

The uterus functions to receive, retain, and nourish a fertilized ovum. The unfertilized ovum will degenerate. A fertilized ovum burrows into the endometrium (inner most layer of uterus). The middle layer, myometrium, contains smooth muscle cells, the outer perimetrium is the visceral peritoneum. If implantation does not occur part of the endometrium is shed during the monthly menstrual cycle.

The cervix is the inferior opening of the uterus between uterus and vagina. Cervical glands produce mucus to protect the uterus from bacterial infections and also keep sperm out unless it is in midcycle, when it becomes less viscous. After implantation of a zygote more mucus is produced to keep other sperm out of the uterus.

The vagina is inferior to the cervix and opens to the exterior of the body between the urethra and rectum. The epithelial cells secrete glycogen which is metabolized to lactic acid, making the vagina normally have an acidic pH. This keeps the vagina free from infection but is also hostile to sperm. However, the sperm contain substances that allow them to resist the acidity of the vagina and to neutralize some of the mucus at the cervical opening in order for them to swim through the uterus, into the uterine tube and there potentially fertilize an oocyte.

Males produce sperm throughout life beginning in puberty. A female's total supply of eggs is already in place when she is born. Each egg is suspended in the first part of meiosis (the series of divisions to make a gamete). At puberty each month one of these eggs is stimulated to finish the process and be released by ovulation.

As estrogen levels rise during puberty it promotes oogenesis and follicle growth and development of the female reproductive tract. It causes a temporary growth spurt and development of secondary female sex characteristics such as breast growth, increase in subcutaneous fat, widening and lightening of the pelvis, growth of axillary and pubic hair, and other metabolic effects.

The Ovarian Cycle

Here is the progression of follicle development:

Follicular Phase - During this phase the hypothalamus stimulates the anterior pituitary to secrete FSH and LH. These stimulate follicle growth and maturation and estrogen secretion. As estrogen rises it shuts down the release of FSH and LH from the pituitary but FSH and LH are still being produced and stored.

· primordial follicle - single layer of surrounding cells

· primary follicle - multiple surrounding granulosa cells

· secondary follicle - fluid-filled spaces coalesce to form a central fluid-filled cavity called the antrum

· vesicular (Graafian) follicle - bulges from surface of ovary

At the beginning of the follicular phase, the apical surface of the endometrium is being shed during the menstrual cycle from the end of the previous month's ovarian cycle. As estrogen levels rise during this phase the endometrium is generates a new functional layer in preparation for possible pregnancy.

Ovulation - When estrogen levels get really high, the anterior pituitary is stimulated to release a burst of LH. This causes ovulation around day 14.

· ovulation - one of the developing follicles completes develoment and releases the oocyte from the ovary

Luteal Phase - After ovulation estrogen levels decline. The previous LH surge also transforms the ruptured follicle into the corpus luteum and causes it to begin to secrete hormones.

· remnant of follicle becomes corpeus luteum

· corpus luteum begins to secrete progesterone and some estrogen - this shuts down LH and FSH from the pituitary so no more follicles develop at this time.

· if pregnancy occurs - corpus luteum persists because the embryo produces an LH-like hormone; eventually the placenta assumes the corpus luteum hormone-producing duties (i.e. producing progesterone and estrogen)

· if pregnancy does not occur - declining levels of LH cause the corpus luteum to begin degenerating in ten days; as corpus luteum degenerates estrogen and progesterone levels fall; as estrogen and progesterone levels fall, the blockade of FSH and LH ends and they are released in a new cycle

During the luteal phase the endometrium prepares for implantation of an embryo. Rising levels of progesterone causes more blood vessel development and nutrient secretion and the cervical plug of mucus to seal off the other opening to the uterus. If pregnancy does not occur the corpus luteum declines from lack of LH. This causes progesterone to fall and that depriceves the endometrium of hormonal support, causing the arteries to kink, spasm, cells to rupture, and menstruation occurs. And the cycle begins again.