Overview of the Reproductive System

The Female Reproductive System

The menstrual cycle is defined as the duration from the first day of the menstrual flow to the commencement of the following flow, usually lasting approximately 24 to 35 days. During the menstrual cycle many changes occur in the reproductive system of the woman. The uterus prepares for pregnancy under the influence of the ovarian derived hormones, estrogen and progesterone. The lining grows rich and thick in anticipation of the arrival of the preimplantation embryo. In the absence of implantation the uterine lining is shed in the first days of the cycle during the menstrual flow.

The menstrual cycle is divided into two portions. The first half is called the follicular phase, during which the oocyte matures inside of the ovary in a small encasement, called the follicle. The follicle is composed of the oocyte, its surrounding support cells, called the theca and granulosa cells, and the follicular fluid contained within. The follicle grows in response to the pituitary derived hormones, follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Estrogen is produced by the follicle in response to this stimulation and plays a dominant role in the continued growth of the follicle and oocyte. The follicular fluid accumulates progressively during the follicular phase. The follicle secretes a large amount of estrogen into the blood stream, which stimulates uterine lining development preparing for implantation. Therefore, the follicular phase of the menstrual cycle is geared toward follicle/ovarian growth and maturation, and preparation of the uterine environment for implantation.

The second phase of the menstrual cycle begins after ovulation, the release of the oocyte from the follicle, which occurs approximately two weeks after the beginning of the cycle. The process is not eruptive but follows a slow degradation of the wall of the follicle, allowing release of the fluid and the oocyte. With ovulation comes the final stages of oocyte maturation and subsequently the oocyte is developmentally ready for insemination by the sperm. The cells of the ruptured follicle differentiate to form the corpus luteum, although still capable of producing estrogen, the differentiated cells mainly produce high concentrations of progesterone. Progesterone's effect on the uterus is to maintain an adequate environment to nurture the growing embryo, thus complimenting the work begun by estrogen. The synchronization of hormone synthesis during the menstrual cycle involves a very complex self-regulating system involving the brain, pituitary and the ovary itself.

The internal structures composing the reproductive organs include the two fallopian tubes, which are attached to the upper part of the uterus, and the ovaries. The ends of the fallopian tubes are structured with hair-like extensions, which facilitate capture of the mature oocyte once released by the ovary. Thus, the tubes serve as a passageway for the oocyte to travel from the ovary into the uterus and actually facilitate this transport. Additionally the fallopian tube is the site of fertilization and functions to provide nourishment to the oocyte, sperm and early embryo during transport.

A woman is born with all of the oocytes she will have during her lifetime, which continually decreases with time. This is in contrast to the male who produces sperm continuously throughout his lifetime. At birth the ovary contains approximately 1.2 million oocytes. This number falls to 600,000 at the time of menarche during the first period. Following menopause (around the age 51 years) there are no oocytes remaining in the ovaries.

The Male Reproductive System

Testosterone stimulates the production of sperm within the seminiferous tubules of the testicles. The testicles are positioned outside of the abdominal cavity because sperm production occurs best at temperatures cooler than normal body core. Spermatogenesis is initiated at the time of puberty and will continue for the rest of life, although reports now suggest that sperm quality may be compromised at later ages. Sperm move from the seminiferous epithelium and tubules of the testes into the epididymis, which acts as a storehouse until ejaculation. The epididymis, which is positioned at the top of the testes like a cap, is a very long narrow tubule, which runs back and forth in convolutions forming a tiny, compact structure with a head, body and tail. The epididymis also functions to concentrate the sperm, removing much of the fluid fraction, and also provides specialized fluid to assist in the final maturation of the sperm. The tail of the epididymis leads to the vas deferens, a tube through which the sperm travels through during ejaculation. Mature sperm take about 70 +/- 4 days to develop. Production occurs in waves, which is why there can be significant variation in the results of semen analyses.

During ejaculation the muscles surrounding the epididymis and the vas deferens contract to push the sperm into the urethra. During the ejaculation process the sperm is joined with products secreted from the seminal vesicles and prostate gland, which contribute nutritional material to the sperm. Semen never mixes with urine despite the common final passage for both because the bladder sphincter muscle contracts during sexual stimulation, closing the passage from the bladder to the urethra. In some remote cases such as with diabetes or following extensive surgery, this mechanism fails and sperm enter the bladder. Sperm can still be recovered from the urine following the appropriate laboratory protocol.

The volume of the ejaculate is approximately two to five milliliters, or about one teaspoon. The semen is a milky white color, with a consistency of an egg white. Sperm only composes 2-3% of the ejaculate with the remainder being mostly secretions from the seminal vesicles and the prostrate gland.

Testosterone is the main male sex hormone produced by the testicles starting at puberty. Testosterone is not produced by the same cells that support sperm production but by the Lydia cells. These cells are located external to the Sertoli cell compartment where sperm production occurs. Testosterone also triggers growth of facial hair, the deepening of a young man's voice and the developments of muscles. It is also an important hormone to create the desire for sex, which is called libido.

The Sperm/Oocyte Interaction

Millions of motile sperm are released inside of the vagina during intercourse. The mere dilution of the seminal fluid within the female reproductive tract stimulates capacitation of the sperm. During these visible maturational changes, sperm acquire the capacity to inseminate the oocyte. Sperm become very active as a result of these changes which is necessary for transport and penetration through the outer oocyte investments.

While a few hundred sperm will reach the oocyte, normally, only one will successfully penetrate it. Initially the sperm attaches to the zona pellucida, the gel-like covering surrounding the oocyte, and then purportedly breaches the covering with self-contained chemicals called enzymes. After traveling through the zona, the sperm will come in contact with the outer surface of the oocyte. The sperm will bind to this membrane and initiate a cascade of events designed to create an impenetrable barrier, preventing entry by other sperm. The genetic material (chromosomes) of the sperm and the genetic material of the oocyte then fuse to form the genetically new embryo. From this point on the embryo will start dividing into two cells, four cells, eight cells, etc as it travels towards the uterus. This journey will take approximately 3 to 5 days. In preparation for implantation, the embryo must be released from the zona covering, which has provided protection until this point. Once the embryo "hatches" from the zona it is free to associate with the lining of the uterus, which is the initial process of implantation.