Lecture 12

Physiologic Anatomy of the

Male Sexual Organs

The glandular structure of the testis consists of numerous lobules. Their number, in a single testis, is 250-400 lobules. Each lobule consists of from one to three, or more, minute convoluted tubes called seminiferous tubules. The total number of tubules is about 840-900, and the average length of each is 50-80 cm. Their diameter varies from 0.12 to 0.3 mm. Sperm are formed in these tubules. The sperm then empty into the epididymis, another coiled tube about 6 meters long. The epididymis leads into the vas deferens, which enlarges into the ampulla of the vas deferens immediately before the vas enters the body of the prostate gland.

Two seminal vesicles, one located on each side of the prostate, empty into the prostatic end of the ampulla, and the contents from both the ampulla and the seminal vesicles pass into an ejaculatory duct leading through the body of the prostate gland and then emptying into the internal urethra. Prostatic ducts, too, empty from the prostate gland into the prostatic urethra.Finally, the urethra is the last connecting link from the testis to the exterior.

The urethra is supplied with mucus derived from a large number of minute urethral glands located along its entire extent and even more so from bilateral bulbourethral glands (Cowper’s glands) located near the origin of the urethra.

Spermatogenesis

During formation of the embryo, the primordial germ cells migrate into thetestes and become immature germ cells called spermatogonia which lie in twoor three layers of the inner surfaces of the seminiferous tubules.The spermatogonia begin to undergo mitoticdivision, beginning at puberty, and continually proliferate and differentiatethrough definite stages of development to form sperm, as shown in Figure80–2B. The testes produce as many as 12 trillion sperm in a male's lifetime.

Steps of Spermatogenesis

Spermatogenesis occurs in the seminiferous tubules during active sexual life asthe result of stimulation by anterior pituitary gonadotropic hormones, beginningat an average age of 13 years and continuing throughout most of theremainder of life but decreasing markedly in old age.

In the first stage of spermatogenesis, the spermatogonia migrate amongSertoli cells toward the central lumen of the seminiferous tubule. The Sertolicells are very large with overflowing cytoplasmicenvelopes that surround the developing spermatogoniaall the way to the central lumen of the tubule.

Functions of Sertoli cells:

(1)The Sertoli cells provide nutrients to the differentiating sperm (which are isolated from the bloodstream).

(2)Sertoli cells form tight junctions with each other, creating a barrier between the testes and the bloodstream called the blood-testes barrier. The blood-testes barrier imparts a selective permeability, admitting "allowable" substances such as testosterone to cross but prohibiting noxious substances that might damage the developing sperm.

(3)Sertoli cells secrete an aqueous fluid into the lumen of the seminiferous tubules, which helps to transport sperm through the tubules into the epididymis.

Meiosis

Spermatogonia that cross the barrier into theSertoli cell layer become progressively modified andenlarged to form large primary spermatocytes (Figure80–3). Each of these, in turn, undergoes meioticdivision to form two secondary spermatocytes. Afteranother few days, these too divide to form spermatidsthat are eventually modified to become spermatozoa(sperm).

During the change from the spermatocyte stage tothe spermatid stage, the 46 chromosomes (23 pairs ofchromosomes) of the spermatocyte are divided, so that23 chromosomes go to one spermatid and the other 23to the second spermatid. This also divides the chromosomalgenes so that only one half of the geneticcharacteristics of the eventual fetus are provided bythe father, while the other half are derived from theoocyte provided by the mother.

The entire period of spermatogenesis, from spermatogoniato spermatozoa, takes about 74 days.

Sex Chromosomes.

In each spermatogonium, one of the23 pairs of chromosomes carries the genetic informationthat determines the sex of each eventual offspring.This pair is composed of one X chromosome, which iscalled the female chromosome, and one Y chromosome,the male chromosome. During meiotic division,the male Y chromosome goes to one spermatid thatthen becomes a male sperm, and the female X chromosomegoes to another spermatid that becomes afemale sperm. The sex of the eventual offspring isdetermined by which of these two types of spermfertilizes the ovum. This is discussed further inChapter 82.

Formation of Sperm.

When the spermatids are firstformed, they still have the usual characteristics ofepithelioid cells, but soon they begin to differentiateand elongate into spermatozoa. As shown in Figure80–4, each spermatozoon is composed of a head and atail.

The head comprises the condensed nucleus of thecell with only a thin cytoplasmic and cell membranelayer around its surface. On the outside of the anteriortwo thirds of the head is a thick cap called the acrosomethat is formed mainly from the Golgi apparatus.This contains a number of enzymes similar to thosefound in lysosomes of the typical cell, includinghyaluronidase(which can digest proteoglycan filamentsof tissues) and powerful proteolytic enzymes(which can digest proteins). These enzymes playimportant roles in allowing the sperm to enter theovum and fertilize it.

The tail of the sperm, called the flagellum, has threemajor components:

(1)a central skeleton constructedof 11 microtubules, collectively called the axoneme—the structure of this is similar to that of cilia foundon the surfaces of other types of cells;

(2)a thin cell membrane covering theaxoneme; and

(3)a collection of mitochondria surroundingthe axoneme in the proximal portion of thetail (called the body of the tail or the middle piece).

Back-and-forth movement of the tail (flagellarmovement) provides motility for the sperm. Thismovement results from a rhythmical longitudinalsliding motion between the anterior and posteriortubules that make up the axoneme.The energy for thisprocess is supplied in the form of adenosine triphosphatethat is synthesized by the mitochondria in thebody of the tail.

Normal sperm move in a fluid medium at a velocityof 1 to 4 mm/min. This allows them to move throughthe female genital tract in quest of the ovum.

Hormonal Factors That StimulateSpermatogenesis

  1. Testosterone, secreted by the Leydig cells locatedin the interstitium of the testis, is essential forgrowth and division of the testicular germinalcells, which is the first stage in forming sperm.
  2. Luteinizing hormone, secreted by the anteriorpituitary gland, stimulates the Leydig cells tosecrete testosterone.
  3. Follicle-stimulating hormone, also secreted by theanterior pituitary gland, stimulates the Sertolicells; without this stimulation, the conversionof the spermatids to sperm (the process ofspermiogenesis) will not occur.
  4. Estrogens, formed from testosterone by theSertoli cells when they are stimulated by folliclestimulatinghormone, are probably also essentialfor spermiogenesis.
  5. Growth hormone (as well as most of the otherbody hormones) is necessary for controllingbackground metabolic functions of the testes.Growth hormone specifically promotes earlydivision of the spermatogonia themselves; in itsabsence, as in pituitary dwarfs, spermatogenesisis severely deficient or absent, thus causinginfertility.
  6. Thyroid hormones.

Maturation of Sperm in the Epididymis

After formation in the seminiferous tubules, the spermrequire several days to pass through the 6-meter-longtubule of the epididymis. Sperm removed from theseminiferous tubules and from the early portions ofthe epididymis are nonmotile, and they cannot fertilizean ovum (exept by ICSI). However, after the sperm have beenin the epididymis for some 18 to 24 hours, they developthe capability of motility, even though severalinhibitory proteins in the epididymal fluid still preventfinal motility until after ejaculation.

Storage of Sperm.

The two testes of the human adultform up to 120 million sperm each day.A small quantityof these can be stored in the epididymis, but mostare stored in the vas deferens.They can remain stored,maintaining their fertility, for at least a month. Duringthis time, they are kept in a deeply suppressed inactivestate by multiple inhibitory substances in thesecretionsof the ducts. Conversely, with a high level ofsexual activity and ejaculations, storage may be nolonger than a few days.

After ejaculation, the sperm become motile, andthey also become capable of fertilizing the ovum, aprocess called maturation.

The Sertoli cells and theepithelium of the epididymis secrete a special nutrientfluid that is ejaculated along with the sperm.This fluidcontains hormones (including both testosterone andestrogens), enzymes, and special nutrients (carnitine) that areessential for sperm maturation.

Physiology of the Mature Sperm.

  • The normal motile, fertilesperm are capable of flagellated movement though thefluid medium at velocities of 1 to 4 mm/min.
  • Theactivity of sperm is greatly enhanced in a neutraland slightly alkaline medium, as exists in the ejaculatedsemen, but it is greatly depressed in a mildlyacidic medium. A strong acidic medium can causerapid death of sperm.
  • The activity of sperm increases markedly with increasingtemperature, but so does the rate of metabolism,causing the life of the sperm to be considerablyshortened.
  • Although sperm can live for many weeks inthe suppressed state in the genital ducts of the testes,life expectancy of ejaculated sperm in the femalegenital tract is only 1 to 2 days. (found motile after 208 hours in cervical mucous).

Function of the Seminal Vesicles

Each seminal vesicle is a tortuous, loculated tube linedwith a secretory epithelium that secretes a mucoidmaterial containing an abundance of fructose,citric acid, and other nutrient substances, as wellas large quantities of prostaglandins and fibrinogen.

During the process of emission and ejaculation, eachseminal vesicle empties its contents into the ejaculatoryduct shortly after the vas deferens empties thesperm.

This adds greatly to the bulk of the ejaculatedsemen, and the fructose and other substances in theseminal fluid are of considerable nutrient value forthe ejaculated sperm until one of the sperm fertilizesthe ovum.

Prostaglandins are believed to aid fertilization intwo ways:

(1)by reacting with the female cervicalmucus to make it more receptive to sperm movementand

(2)by possibly causing backward, reverse peristalticcontractions in the uterus and fallopian tubes tomove the ejaculated sperm toward the ovaries (a fewsperm reach the upper ends of the fallopian tubeswithin 5 minutes).

Function of the Prostate Gland

The prostate gland secretes a thin, milky fluid that containscalcium, citrate ion, phosphate ion, a clottingenzyme, and a profibrinolysin.

During emission, thecapsule of the prostate gland contracts simultaneouslywith the contractions of the vas deferens so that thethin, milky fluid of the prostate gland surrounds the emitted from the vas deference and adds further tothe bulk of the semen.

A slightly alkaline characteristicof the prostatic fluid may be quite important forsuccessful fertilization of the ovum, because the fluidof the vas deferens is relatively acidic owing to thepresence of citric acid and metabolic end products ofthe sperm and, consequently, helps to inhibit spermfertility.Also, the vaginal secretions of the female areacidic (pH of 3.5 to 4.0).

Sperm do not become optimallymotile until the pH of the surrounding fluidsrises to about 6.0 to 6.5. Consequently, it is probablethat the slightly alkaline prostatic fluid helps to neutralizethe acidity of the other seminal fluids duringejaculation, and thus enhances the motility and fertilityof the sperm.

Semen

Semen, which is ejaculated during the male sexualact, is composed of

(1)the fluid and sperm from the vasdeferens (about 10 per cent of the total),

(2)fluid from the prostate gland (about 30 per cent), and

(3)fluid from theseminal vesicles (almost 60 per cent),

(4)small amountsfrom the mucous glands, especially the bulbourethralglands.

Thus, the bulk of the semen is seminal vesiclefluid, which is the last to be ejaculated and servesto wash the sperm through the ejaculatory duct andurethra.

The average pH of the combined semen is about 7.5,the alkaline prostatic fluid having more than neutralizedthe mild acidity of the other portions of thesemen.

The prostatic fluid gives the semen a milkyappearance, and

fluid from the seminal vesicles andmucous glands gives the semen a mucoid consistency.

Also, a clotting enzyme from the prostatic fluid causesthe fibrinogen of the seminal vesicle fluid to form aweak fibrin coagulum that holds the semen in thedeeper regions of the vagina where the uterine cervixlies.The coagulum then dissolves during the next 15 to30 minutes because of lysis by fibrinolysin formedfrom the prostatic profibrinolysin. In the early minutesafter ejaculation, the sperm remain relatively immobile,possibly because of the viscosity of the coagulum.As the coagulum dissolves, the sperm simultaneouslybecome highly motile.

Although sperm can live for many weeks in the malegenital ducts, once they are ejaculated in the semen,their maximal life span is only 24 to 48 hours atbody temperature. At lowered temperatures, however, semen can be stored for several weeks, and whenfrozen at temperatures below -100°C, sperm havebeen preserved for years.

Effect of Temperature on Spermatogenesis. Increasing thetemperature of the testes can prevent spermatogenesisbycausing degeneration of most cells of the seminiferoustubules besides the spermatogonia. It has oftenbeen stated that the reason the testes are located inthe dangling scrotum is to maintain the temperatureof these glands below the internal temperature of thebody, although usually only about 2°C below the internaltemperature. On cold days, scrotal reflexes causethe musculature of the scrotum to contract, pulling thetestes close to the body to maintain this 2° differential.Thus, the scrotum theoretically acts as a cooling mechanismfor the testes, withoutwhich spermatogenesis might be deficient during hotweather.

Cryptorchidism

Cryptorchidism means failure of a testis to descendfrom the abdomen into the scrotum at or near the timeof birth of a fetus. During development of the malefetus, the testes are derived from the genital ridges inthe abdomen. However, at about 3 weeks to 1 monthbefore birth of the baby, the testes normally descendthrough the inguinal canals into the scrotum. Occasionallythis descent does not occur or occurs incompletely,so that one or both testes remain in the abdomen, in theinguinal canal, or elsewhere along the route of descent.

A testis that remains throughout life in the abdominalcavity is incapable of forming sperm. The tubularepithelium becomes degenerate, leaving only theinterstitial structures of the testis. It has been claimedthat even the few degrees’ higher temperature in theabdomen than in the scrotum is sufficient to causethis degeneration of the tubular epithelium and, consequently,to cause sterility, although this is not certain.Nevertheless, for this reason, operations to relocate thecryptorchide testes from the abdominal cavity into thescrotum before the beginning of adult sexual life arefrequently performed on boys who have undescendedtestes.

Testosterone secretion by the fetal testes themselvesis the normal stimulus that causes the testes to descendinto the scrotum from the abdomen. Therefore, many, ifnot most, instances of cryptorchidism are caused byabnormally formed testes that are unable to secreteenough testosterone. The surgical operation for cryptorchidismin these patients is unlikely to be successful.

Effect of Sperm Count on Fertility. The usual quantity ofsemen ejaculated during each coitus averages about 3.5milliliters, and in each milliliter of semen is an averageof about 120 million sperm, although even in “normal”males this can vary from 35 million to 200 million. Thismeans an average total of 400 million sperm are usuallypresent in the several milliliters of each ejaculate.Whenthe number of sperm in each milliliter falls below about20 million, the person is likely to be infertile.Thus, eventhough only a single sperm is necessary to fertilize theovum, for reasons not understood, the ejaculate usuallymust contain a tremendous number of sperm for onlyone sperm to fertilize the ovum.

Effect of Sperm Morphology and Motility on Fertility. Occasionallya man has a normal number of sperm but is stillinfertile. When this occurs, sometimes as many as onehalf the sperm are found to be abnormal physically,having two heads, abnormally shaped heads, or abnormaltails, as shown in Figure 80–5. At other times, thesperm appear to be structurally normal, but for reasonsnot understood, they are either entirely nonmotileor relatively nonmotile. Whenever the majority of thesperm are morphologically abnormal or are nonmotile,the person is likely to be infertile, even though theremainder of the sperm appear to be normal.