Male Reproductive Anatomy

arteries and vaginal arteries. The vagina is innervated by the pelvic splanchnic nerves. Somatic innervation of the vagina comes from the pudendal nerve.

MALE REPRODUCTIVE ANATOMY

In this section, we will review the anatomy of the male reproductive tract. Figure 6 shows the major structures of the male reproductive tract:

Figure 06.

The testes are the gonads of the male reproductive system. The testes are located within the scrotum. The teste produce the male gamete, called sperm, spermatozoon, or sperm cell. The testes have both an exocrine and endocrine function. The exocrine function of the testes includes producing sperm and seminal fluid to support sperm cells. The endocrine function of the testes is to synthesize and secrete steroid hormones, such as testosterone and dihydrotestosterone. The endocrine function of the testes will be

discussed in length in chapter two. Blood to the testes is supplied by the testicular arteries. The testicular arteries arise from the abdominal aorta.

Each testis is about 4 to 5 centimeters in length, surrounded by two separate layers of connective tissue. The outermost layer is called the tunica vaginalis, which itself has a parietal and visceral layer. Inside the tunica vaginalis is the tunica albuginea, which is tough, fibrous, and white in color. This inner layer forms multiple septa that divides each testis into about 300 to 400 lobules. Within the lobules are the seminiferous tubules. Figure 7 shows the structure of the testis:

Figure 07.

The seminiferous tubules are tightly coiled and make up most of the testis. They are configured with ducts that have several layers each that have a central lumen. Spermatogenesis starts and ends in these tubules. We will talk about spermatogenesis in a minute. As the sperm cell is created, it enters the strait tubules or rete testes. There are a few of these ducts that have coalesced from minor ducts. These are called efferent ductules, which exit the confines of the tunica albuginea.

Within the seminiferous tubules are cells of six different types. There are supporting cells known as sustentacular cells and five separate types of germ cells that start with the most immature cells at the outer basement membrane and end with the most mature but haploid cells nearest the lumen.

The Sertoli cells are the sustentacular cells that secrete molecules to promote the production of sperm. They also determine if a given germ cell will survive or not. These are particularly important because they have tight junctions between them that form the blood-testes barrier. Men need this barrier to protect the developing sperm cells from outside influences and protect the body’s immune system from seeing these sperm cells as foreign, making autoantibodies against them. Should this happen, fertility can be impaired.

The germ cells go from least mature to most mature in the seminiferous tubules. The least mature cells are the spermatogonia, which are just inside the basement membrane. These are essentially stem cells. They divide into primary and secondary spermatocytes, evolve into spermatids, and mature further into sperm cells. This is the process of spermatogenesis, which does not start until puberty. It takes 64 days to go from a stem cell to a formed sperm cell, with new cycles beginning every sixteen days.

Spermatogonia are diploid cells having 46 chromosomes, while mature sperm cells have 23 chromosomes and are therefore haploid. The process of meiosis performs this change with a great deal of DNA crossover happening so that each sperm cell is unique. One spermatogonia creates four unique haploid sperm cells. Figure 8 shows the process of spermatogenesis:

Figure 08.

The scrotum is an external cutaneous sac. The scrotum is located externally because spermatogenesis – or sperm production – takes place at temperatures about 2 to 4 degrees Celsius below body temperature. The scrotum has its own layers beneath the skin. There is a dartos muscle that is just beneath the skin. It extends up to make the scrotal septum, dividing the two testes. The abdominal internal oblique muscle descends to create the cremaster muscles that cover the testes. These muscles contract in cold environments in order to bring the testes closer to the core of the body. They also relax in warm weather in order to move them away from the body so as to lower their temperature. They also change the surface area of the testes to retain heat if necessary. The midline area of the scrotum on the skin is called the raphe.

Mature sperm cells are very small with very little cytoplasm. About 100 to 300 million sperm cells are made each day. Each sperm cell has a distinct head, a midpiece, and a tail. The head contains the genome of the cell. It is covered with an acrosome or cap that contains enzymes to help in egg penetration. The midpiece has packed mitochondria that help provide energy for the tail to be motile. The flagellum has an

axial filament that participates in its movement. Figure 9 shows the structure of a mature sperm cell:

Figure 09.

Once the sperm cell is made in the seminiferous tubules, they enter the epididymis for further maturation of the sperm. The epididymis is a coiled tube that would be 20 feet in length is it is straightened out and not coiled. It takes twelve days to have a sperm cell migrate through the epididymis. It is propelled through smooth muscle contractions in this structure. Eventually, these sperm cells will acquire the ability to move under their own power. They are stored in the epididymis until the time of ejaculation.

Even though sperm are mature in the epididymis, they must undergo processing and must travel through a duct system in order to exit the body. The ductus deferens or vas deferens is a muscular tube that, together with nerves and blood vessels, forms the spermatic cord. This is the part of the male reproductive system that can be cut in a vasectomy to prevent a pregnancy. Sperm itself make up just five percent of semen. This means that there is more to semen, gotten from different glands and structures in the male reproductive system.

The first place these sperm cells go is the seminal vesicles. This is where volume is added, along with a great deal of fructose. Fructose is a sugar used specifically by the mitochondria in order to provide fuel to them as the cells proceed through the male and female reproductive tract. This fluid then travels through the ejaculatory ducts to enter into the prostate gland.

There is just one prostate gland that sits at the base of the bladder and surrounds the male urethra. This secretes an alkaline substance to the semen in order to allow sperm to coagulate and then decoagulate after ejaculation. Sperm must first coagulate in order to stay within the vagina and then must decoagulate in order to pass through the cervix.

The bulbourethral glands is the last place where the semen travels. These are also called Cowper’s glands. There is a thick, salty substance secreted by the glands in order to lubricate and cleanse the semen as they pass through the urethra. Pre-ejaculate is the substance released prior to ejaculation as part of sexual responsiveness. While not containing much sperm, pre-ejaculate can still cause a pregnancy.

The penis consists of three distinct sections – the root, the body, and the glans. The root of the penis is the most proximal portion and is located internally within the superficial perineal pouch. The body of the penis is the external portion connecting the root to the glans. The body of the penis contains three columns of erectile tissue – the corpus spongiosum and two corpus cavernosa. The corpus spongiosum is a small capillary bed that surrounds the urethra. The corpus cavernosa is a larger capillary bed that fills with blood during an erection. The glans is the most distal portion of the penis. The glans of the penis contains the external urethral opening surrounded by corpus spongiosum. Figure 10 shows the structure of the male penis:

Figure 10.

Blood is supplied to the penis by the dorsal penile arteries, the deep penile arteries, and the bulbourethral arteries. The dorsal penile arteries, the deep penile arteries, and the bulbourethral arteries all arise from the internal pudendal artery. The penis receives sympathetic, parasympathetic, and sensory innervation. Innervation to the penis arises primarily from the S2 through S4 spinal regions. Parasympathetic input controls erection, whereas sympathetic input controls ejaculation. The mnemonic “point and shoot” is often used to help medical students remember the innervation of the penis. Point equals parasympathetic and shoot equals sympathetic. Parasympathetic input controls “pointing” or erection, while sympathetic input controls “shooting” or ejaculation.