are housed within the scrotum
. Their primary responsibility is to produce ample sperm and androgens. Androgens are responsible for the regulation of spermatogenesis on top of determining development and function relating to secondary sex organs. Each testis is approximately 4 centimeters long and about 2.5 centimeters in diameter. Their appearance is similar to a white egg, ovular and white, but smaller. Each individual testis weighs in between 10 to 14 grams. The testes are encased in two distinctive layers of tissue known as tunics
The outer tunic is a thin layer known as the tunica vaginalis. When the testicles descend, a layer of thin serous peritoneum drops as well, creating this outer layer. The inner layer is tougher, made from a fibrous membrane. The testes are encased within the sac created by the inner layer, which is referred to as the tunica albuginae. The tunica albuginae protrudes into the testes in small fiber extensions which creates tiny partitions of the testes. Each testis is actually subdivided into anywhere from 250 to 300 tiny lobes via these fibrous divisions. Within each one of these separate divisions, there are tightly convoluted tubules that would reach about 70 centimeter when laid out flat. These seminiferous tubules are considered the functional elements of the testes. It is within these tubules that sperm is created. This process is known as spermatogenesis. Sperm is created at a phenomenal speed, the testes can create over a million per day, which rounds out to thousands of sperm per second. This number is average for a healthy male during his sexually mature years.
The individual segments of the seminiferous tubules create various stages of meiosis, the process of cell division necessary for sexual reproduction. The initiation of this vital process begins with spermatogonia, which are specialized germinal cells. The spermatogonia undergo the process of cell division, or meiosis, in order to produce the primary spermatocytes. From there, the process of meiosis matures into the next stage known as the secondary spermatocytes. The final stage produces spermatids.
Nurse cells, also known as sustentacular cells or Sertoli cells, are responsible for creating the walls of the siminiferous tubules. These cells also are capable of secreting nutrients which nourish the spermatozoa
which is developing in between the nurse cells. While the spermatozoa are completely formed when they finally reach the lumen of the seminiferous tubules, they have not reached a stage of complete maturity. Specialized endocrine cells known as interstitial cells (the cells of Leydig) are responsible for the production and secretion of the necessary male sex hormones
. Because of this, the testes are considered both endocrine glands as well as exocrine glands. Their ability to produce both androgens and spermatozoa is very unique and qualifies them for both systems.
The spermatozoa then continue on their journey toward maturation via a specialized tubular network known as the rete testis. A small percentage of the rete testis cells are ciliated, which encourages the spermatozoa along and into the network. Once the spermatozoa have completed their process in the rete testis, they are then transported into the epididymis
through a simple series of efferent ductules.
The process of maturation, from spermatogonium to spermatozoon takes as long as 8 to 10 weeks. Mature sperm live in a state of suspended animation for several months, maintaining their ability to fertilize an egg. Once their life span has passed without ejaculation, they simply are able to disintegrate and are absorbed into the bloodstream as waste product and appropriately filtered and excreted.
The testicular arteries
are the main blood
supply source for the testes. Just below the origin of the renal arteries, the abdominal aorta then gives rise to the testicular arteries. Blood flow is released from the testes via the testicular veins
. The right testicular vein flows directly to the inferior vena cava while the left testicular vein drains the blood into the left renal vein.
Both motor and sensory neurons
are necessary for proper function of the testes. Testicular nerves
are able to innervate the testes with both these requirements. The motor and sensory neurons initiate at the tenth thoracic portion of the spinal column. The testicular nerves innervate the testes with primarily sympathetic neurons. A small percentage of innervation comes from parasympathetic neurons.
Sexually mature testes produce primarily a higher concentration of testosterone. This androgen, which is also known as an anabolic steroid as it increases muscle growth during male maturation, is responsible for the sexual maturation process in young males. The increase in testosterone production is responsible for the onset of the secondary sex organs, the seminal vesicles
and the prostate
. In the event that castration occurs, the loss of the secretion of androgens causes secondary sex organ atrophy.
Androgens also stimulate other organs, such as the growth of the larynx
which causes a deeper vocal range. It can stimulate the growth of bone
, resulting in thicker bone mass than females as well as a higher hemoglobin levels resulting from the stimulation of hemoglobin synthesis. While androgens are responsible for bone growth, their result is not what one would typically associate with the term “growth.” Rather, androgens stimulate the conversion of cartilage to bone which in effect seals the epiphyseal plates and prevents further growth thereafter.