The male reproductive system consists of two testes responsible for sperm and hormone production, a network of ducts to store and transport sperms to the exterior, and a number of glands that produce semen and empty into the ducts, and a penis. The ducts, glands and penis are collectively known as the male accessory reproductive organs.
The testes are suspended in the scrotum outside the abdomen where the temperature is a few degrees lower than that of the internal body temperature. This is important because the otherwise high temperature will destroy the sperms. This is achieved by air circulating around the scrotum and a heat-exchange mechanism in the blood vessels in the testes.
Sperms are produced in the seminiferous tubules forming a complex network in the testes. Each tubule is filed with fluid containing sperm and lined with Sertoli cells. On the other hand, testosterone is secreted by the Leydig cells located in the tiny connective tissue spaces between the tubules. Seminiferous tubules from different parts of the testis finally join together as a network of interconnected tubes, known as rete testis, and form the efferent ducts. Efferent ducts then leave the rete testis and then leave the testis as a single duct in the epididymis. The epididymis is loosely attached to the outside of each testis and drains into a vas deferens, a large thick-walled tube, out of each testis. The vas deferens , blood vessels and nerves supplying the testis are all bound into a spermatic cord which leads to the abdomen.
The two vas deferens become the ejaculatory ducts in the abdomen and coil back at the base of the urinary bladder. Two large glands called seminal vesicles then drain into the vas deferens. The prostate gland located in the upper urethra also secretes its contents in the ejaculatory ducts through numerous openings in the urethra, followed by the paired Cowper's glands underlying the prostate. The secretions from the seminal vesicles and prostate glands, together with the semen, contribute to the semen. The glandular secretions also contain nutrients, buffer against the acidic vaginal secretions to nourish the sperms. Finally, the ejaculatory ducts join with the urethra and enter the penis.
Sperms, the male gametes, are produced by the Leydig cells of the seminiferous tubules in the testes. The very first stage of sperm is called spermatogonium which is an undifferentiated germ cell. It starts to undergo mitotic divisions at puberty. After a number of mitosis, differentiation begins to appear and the resultant daughter cell is called a primary spermatocyst which is ready to undergo meiosis. Each spermatogonium can give a number of spermatocysts after a specific number of division cycles. These mitotic divisions take place in the basal compartments of the Sertoli cells, between the tight junctions of the Sertoli cellos and the basement membrane of the seminiferous tubules. However, not all the spermatogonia are converted to primary spermtocysts by mitosis. One clone from each original spermatogonium escapes from the mitosis-differentiation cycle and reverts to a primitive spermatogonium which is capable to carry out its own cycle of mitosis later. As a result, there will be no threat of shortage of spermatogonium.
The primary spermatocysts gradually increase in size as they move through the tight junction to the central compartment of the Sertoli cells where meiosis takes place. Two secondary spermocysts are formed from the first meiotic division of each primary spermatocyst. Each of which then undergo second meiotic division to give rise to another two spermatids. As a result, each primary spermatocyst, with 46 chromosomes, leads to four spermatids, having the chromosome number of 23.
The spermatids stop to divide, but differentiation occurs instead. They elongate into three parts, namely head, midpiece and flagellum (tail). The resultant cell is now a spermatozoa, or a sperm. Almost its entire head is the nucleus containing the DNA of the sperm. The nucleus is surrounded by the acrosome which contains essential enzymes for the sperm to penetrate in to an ovum. The midpiece houses a large number of mitochondria to provide energy for the sperm's movement. The tail composes of a bundle of contractile filaments that is capable of generating whiplike movement to propel the sperm. The sperms then move out of the Sertoli cells and enter lumen of the seminiferous tubules.
Sperms produced in the testes must proceed to the external, through a number of ducts, in order to fertilize an ovum. Fluid pressure generated by the Sertoli cells in the seminiferous tubules, together with peristalsis of the tubules, carry the sperms from the testes to the epididymis. The sperms are nonmotile themselves. When the sperms pass through the epididymis, the lumen of the epididymis absorbs fluid bathing the sperms and causes the sperm concentration to increase drastically. The peristaltic contractions of the smooth muscle in the epididymis and the vas deferens force this compact mass of sperms to proceed. However, when the sperms reach the vas deferens, they are stored there until sexual arousal leading to ejaculation.
The penis is normally flaccid as the small arteries supplying blood to the three vascular compartments of the penis are constricted. If someone is sexually aroused, the parasympathetic nerves to the small arteries of the penis are stimulated and the arteries become dilated. At the same time, the veins leading to the penis are compressed as the vascular compartments expand. The penis is engorged with blood at high pressure and become rigid. This process is termed erection and usually lasts for 5 to 10 seconds.
Although erection is usually a reflex action during sexual intercourse, the higher brain centers also can play a part in it. Mechanical stimuli, such as thoughts, sight, smell and emotions, can also contribute to erection by passing the stimuli to the small arteries of the penis.
Sperms are discharged from the penis into the vagina under spinal reflex action. The action of sexual intercourse stimulates the discharge of semen which is automatic and sequential. Firstly, semen is emptied into the urethra under the contractions of the smooth muscles of the epididymis, vas deferens, ejaculatory ducts, prostate and seminal vesicles as a result of sympathetic stimulation. Then, intense contractions of smooth muscle of the urethra and skeletal muscle at the base of the penis follow, forcing the semen out of the urethra. This is accompanied by the closure of the sphincter at the base of urethra to prevent sperms from entering the urinary bladder and no urine is expelled from it.
During ejaculation, many rhythmical and intense skeletal muscular contractions occur at other parts of the body and the event is termed an orgasm. Heart rate and blood pressure also increase significantly. All of these phenomenon are associated with intense pleasure and systemic physiological changes throughout the body. Once the ejaculation stops, it is impossible to have another ejaculation right after. This period is known as the latent period which may last for a few minutes to hours, depending on individuals.
The female reproductive structure comprises two fallopian tubes, a uterus and a vagina, collectively known as the female reproductive tract, and two ovaries. These structures are also called the female internal genitalia which is completely separated from the urinary ducts. The ovaries are located in the upper pelvic cavity, one on each side of the uterus. The fallopian tubes, also known as the oviducts, are not directly connected to the ovaries. Instead, their ends are like funnels and are surrounded by finger-like projections called fimbriae. Inside the tubes are cilia which sweeps in the direction of the uterus. The other ends of the fallopian tubes lead directly to the cavity of the uterus. The uterus is lined with a thick muscular wall and is suspended in the pelvic cavity by ligaments. The lower part of the uterus is termed the cervix which is a narrow opening extending to the vagina.
Outside the vagina is the female external genitalia which is a combination of the mons pubis, clitoris, labia minora (inner lips), labia majora (outer lips) and vestibule. All these structures can be collectively termed vulva.
All potential ova are present in the female ovaries of each individual at birth, which account for 2 to 4 million in number. However, only about 400 to 500 of which will be released from their follicles in the life of a female, from puberty to menopause. Others are degenerated during their development.
The very first stage of an ovum is an oogonium. It undergoes numerous mitosis to generate a huge amount of oogonia as early as the first three months of a fetus and no such mitosis occurs hereafter. All the oogonia then develop into primary oocysts in the fetus. By then, the primary oocysts are capable of undergoing their first meiotic division. However, there is no complete division and so each primary oocyst contains 46 chromosomes at birth. Such kind of meiosis continues until puberty. The first meiotic division will only be completed for those primary oocysts which are to be ovulated as it completes just before ovulation. The resultant daughter cells are of two forms, the secondary oocyst and the first polar body. Each with 23 chromosomes and each chromosome bearing 2 chromatids. The secondary oocyts retains nearly all the cytoplasm, together with the nutrients, whereas the first polar body is very tiny and adheres to the secondary oocyst.
If the secondary oocyst is fertilized by a sperm, it will undergo its second meiosis in the fallopian tube. The resultant cells are the ovum and the second polar body, each bearing 23 chromosomes and one chromatid per chromosome. At the same time, the first polar body also perform meiosis to produce two second polar bodies. However, all cytoplasm goes to the ovum and the second polar bodies are so small and non-functional which will be degenerated eventually. As a result, each primary oocyst can only give one ovum finally.
An oocyst is not freely exposed in the ovary, but is surrounded by tissues and forms into a follicle. The most primitive form of a follicle is a primary oocyst enclosed by one layer of granulosa cells capable of hormone secretion. This is called a primordial follicle. The oocyst continues to grow in size, and the granulosa cells undergo mitosis and multiply into several layers. The oocyst and the granulosa cells then become separated by a layer of zona pellucida. However, materials are allowed to exchange between the two. The connective-tissue cells on the outer surface of the granulosa cells also differentiate into a layer called theca. The granulosa cells secrete a fluid into a cavity termed antrum, surrounding the oocyst and zona pellicida. By then, the primary oocyst has already attained its full size and the follicle is termed a primary follicle. Only one primary follicle is selected to mature into a Graafian follicle in every menstrual cycle and the rest all undergo atresia, a process which the ova in the under-developed follicles die. But sometimes, it happens that more than one follicles mature simultaneously such that more than one ova are ovulated. If they are fertilized, multiple births will occur. However, the twins are not identical as they have different genetic informations.
The Graafian follicle undergoes further enlargement mainly due to antrum expansion and the oocyst is suspended in the fluid by a structure called cumulus oophorous. When the follicle is about to ovulate, the oocyst completes its first meiosis to become a secondary oocyst. During ovulation, the follicle wall ruptures and the ovum, still enclosed by zona pellucida, is carried by antral fluid current onto the ovarian surface.
After ovulation, the remaining of the follicle, which composes mainly of granulosa cells, transforms into a structure called corpus luteum. As it is yellow in colour, it is also known as the yellow body. It is capable of releasing estrogen and progesterone. However, if no pregnancy occurs, the corpus luteum will degenerate 10 days after ovulation.
As a girl enters her puberty, she will notice that her fertility becomes cyclic and involved with the menstrual cycle which is about 28 days each time, depending on individual . She starts to experience menstruation which is regulated by estrogen and progestreone, hormones secreted in ovary which are enhanced by the presence of gonadotropin releasing hormone (GnRH). The first day of each menstruation marks the first day of each menstrual cycle. Each menstruation usually lasts for about 5 days and the whole period is known as a menstrual phase. At these days, the plasma estrogen and progesterone levels are too low to maintain the uterus lining because the previous corpus luteum becomes regressed. As a result, the endometrium lining the uterus degenerates and flow out of the vagina as thick bloody mucus. At the same time, the pituitary glands produces slightly more follicle-stimulating hormone (FSH) and lutenizing hormone (LH) as the plasma estrogen level is low. These two hormones stimulate the development of 10 to 25 primary follicles at the same time. By the end of the menstrual phase, only one dominant follicle can become mature, which is the Graafian follicle and it is left to be ovulated.
After menstruation, the estrogen secretion rises again due to the presence of the Graafian follicle. As a result, the endometrium starts to thicken again. This period is called the proliferative phase which lasts for around 10 days, before ovulation. During this period, the cervical mucus becomes thinner and clearer gradually. Also, FSH secretion is suppressed by estrogen and LH secretion is stimulated, resulting in LH surge 2 days before ovulation. The LH surge induces the oocyst to complete its first meiosis and the rupture of follicle. An ovum is released from the Graafian follicle which is ready to be fertilized by sperm. This process is called ovulation which occurs at the 14th day of the menstrual cycle. On this day, the cervical mucus is very thin and clear which can help the movement of sperms and increase the chance of fertilization. The menstrual phase and the proliferative phase in the uterus correspond to the follicular phase in the ovary when follicles develop.
When ovulation completes, the high level of LH induces the remaining follicles to become corpus luteum. The corpus luteum secrets estrogen and progesterone to maintain the thick endometrium lining of the uterus in the next 10 days so as to prepare the uterus for implantation if pregnancy results. Therefore, this period is also known as secretory phase in the uterus when new cells and glands responsible for secreting nourishing substances for the embryo are developed. The cervical mucus is thickened to prevent entry of any other sperm. However, the high levels of the two ovarian hormones inhibit the secretion of LH so that no new follicles develop. With a low level of LH, the corpus luteum gradually degenerate from the 25th day of the menstrual cycle. If there is no pregnancy, the degeneration of corpus luteum leads to the declined level of plasma estrogen and progesterone until they finally cease. As a result, the endometrium cannot be maintained and starts to deteriorate again. This period is also called the luteal phase in ovary, starting from ovulation until the corpus luteum totally degenerates. The low level of estrogen and progesterone trigger off the production of GnRH so that the entire cycle can start over.
Female experiences marked vasocongestion, that is the filling of the erectile tissue with blood, due to artery dilation and vein constriction, in the clitoris, ovary, female labia minora and breasts. This is accompanied by myotonia, increased muscle tension, leading to erection of nipples. A number of skeletal and smooth muscles throughout the whole body also undergo rhythmic contractions. The uterus becomes elevated, preparing to receive the sperms. The vaginal epithelium at the same time secretes thick mucus which acts as a lubricant.
When the female reaches orgasm, the uterus and the outer vagina undergo sudden, rapid and rhythmic contraction. The heart and breathing rates rise significantly. Female, unlike her male counterpart, can have multiple orgasm in one sexual intercourse.