The testes and epididymis are paired structures, located within the scrotum. The testes are the site of sperm production and hormone synthesis, while the epididymis has a role in the storage of sperm. Although this location of the testes, outside the abdominal cavity, may seem to make them vulnerable to injury, it provides a temperature about 3° C below normal body temperature. This lower temperature is necessary for the production of viable sperm.
The testes are located within the scrotum, with the epididymis situated on the posterolateral aspect of each testicle. Commonly, the left testicle lies lower than the right. They are suspended from the abdomen by the spermatic cord – collection of vessels, nerves and ducts that supply the testes.
Originally, the testes are located on the posterior abdominal wall. During embryonic development they descend down the abdomen, and through the inguinal canal to reach the scrotum. They carry their neurovascular and lymphatic supply with them.
The testes have an ellipsoid shape. They consist of a series of lobules, each containing seminiferous tubules supported by interstitial tissue. The seminiferous tubules are lined by Sertoli cells that aid the maturation process of the spermatozoa. In the interstitial tissue lie the Leydig cells that are responsible for testosterone production.
Spermatozoa are produced in the seminiferous tubules. The developing sperm travels through the tubules, collecting in the rete testes. Ducts known as efferent tubules transport the sperm from the rete testes to the epididymis for storage and maturation.
Inside the scrotum, the testes are covered almost entirely by the tunica vaginalis, a closed sac of parietal peritoneal origin that contains a small amount of viscous fluid. This sac covers the anterior surface and sides of each testicle and works much like the peritoneal sac, lubricating the surfaces of the testes and allowing for friction-free movement.
The testicular parenchyma is protected by the tunica albuginea, a fibrous capsule that encloses the testes. It penetrates into the parenchyma of each testicle with diaphragms, dividing it into lobules.
The epididymis consists of a single heavily coiled duct. It can be divided into three parts; head, body and tail.
- Head – The most proximal part of the epididymis. It is formed by the efferent tubules of the testes, which transport sperm from the testes to the epididymis.
- Body – Formed by the heavily coiled duct of the epididymis.
- Tail – The most distal part of the epididymis. It marks the origin of the vas deferens, which transports sperm to the prostatic portion of the urethra for ejaculation.
The main arterial supply to the testes and epididymis is via the paired testicular arteries, which arise directly from the abdominal aorta. They descend down the abdomen, and pass into the scrotum via the inguinal canal, contained within the spermatic cord.
However, the testes are also supplied by branches of the cremasteric artery (from the inferior epigastric artery) and the artery of the vas deferens (from the inferior vesical artery). These branches give anastomoses to the main testicular artery.
Venous drainage is achieved via the paired testicular veins. They are formed from the pampiniform plexus in the scrotum – a network of veins wrapped around the testicular artery. In the retroperitoneal space of the abdomen, the left testicular vein drains into the left renal vein, while the right testicular vein drains directly into the inferior vena cava.
Since the testes are originally retroperitoneal organs, the lymphatic drainage is to the lumbar and para-aortic nodes, along the lumbar vertebrae. This is in contrast to the scrotum, which drains into the nearby superficial inguinal nodes.
Sperm are produced by spermatogenesis within the seminiferous tubules. A transverse section of a seminiferous tubule shows that it is packed with cells in various stages of development. Interspersed with these cells, there are large cells that extend from the periphery of the tubule to the lumen. These large cells are the supporting, or sustentacular cells (Sertoli’s cells), which support and nourish the other cells.
Early in embryonic development, primordial germ cells enter the testes and differentiate into spermatogonia, immature cells that remain dormant until puberty. Spermatogonia are diploid cells, each with 46 chromosomes (23 pairs) located around the periphery of the seminiferous tubules. At puberty, hormones stimulate these cells to begin dividing by mitosis. Some of the daughter cells produced by mitosis remain at the periphery as spermatogonia. Others are pushed toward the lumen, undergo some changes, and become primary spermatocytes. Because they are produced by mitosis, primary spermatocytes, like spermatogonia, are diploid and have 46 chromosomes.
Each primary spermatocytes goes through the first meiotic division, meiosis I, to produce two secondary spermatocytes, each with 23 chromosomes (haploid). Just prior to this division, the genetic material is replicated so that each chromosome consists of two strands, called chromatids, that are joined by a centromere. During meiosis I, one chromosome, consisting of two chromatids, goes to each secondary spermatocyte. In the second meiotic division, meiosis II, each secondary spermatocyte divides to produce two spermatids. There is no replication of genetic material in this division, but the centromere divides so that a single-stranded chromatid goes to each cell. As a result of the two meiotic divisions, each primary spermatocyte produces four spermatids. During spermatogenesis there are two cellular divisions, but only one replication of DNA so that each spermatid has 23 chromosomes (haploid), one from each pair in the original primary spermatocyte. Each successive stage in spermatogenesis is pushed toward the center of the tubule so that the more immature cells are at the periphery and the more differentiated cells are nearer the center.
Spermatogenesis (and oogenesis in the female) differs from mitosis because the resulting cells have only half the number of chromosomes as the original cell. When the sperm cell nucleus unites with an egg cell nucleus, the full number of chromosomes is restored. If sperm and egg cells were produced by mitosis, then each successive generation would have twice the number of chromosomes as the preceding one.
The final step in the development of sperm is called spermiogenesis. In this process, the spermatids formed from spermatogenesis become mature spermatozoa, or sperm. The mature sperm cell has a head, midpiece, and tail. The head, also called the nuclear region, contains the 23 chromosomes surrounded by a nuclear membrane. The tip of the head is covered by an acrosome, which contains enzymes that help the sperm penetrate the female gamete. The midpiece, metabolic region, contains mitochondria that provide adenosine triphosphate (ATP). The tail or locomotor region, uses a typical flagellum for locomotion. The sperm are released into the lumen of the seminiferous tubule and leave the testes. They then enter the epididymis where they undergo their final maturation and become capable of fertilizing a female gamete.
Sperm production begins at puberty and continues throughout the life of a male. The entire process, beginning with a primary spermatocyte, takes about 74 days. After ejaculation, the sperm can live for about 48 hours in the female reproductive tract.