The testes or testicles are found and housed within the scrotum. Their main use is to produce sperm and androgens. Androgens are the ones responsible for the regulation of spermatogena as well as for determining the development and function relating to the secondary sex organs. Each testicle is roughly 4 centimetres long and around 2.5 centimetres in diameter. The testes are encased within two distinct layers of tissue known as tunics.
The outer tunic is a thin layer which is known as the tunica vaginalis. Whenever the testes descend, a layer of fine serous peritoneum drops along with it create the outer layer. The inner layer, known as the tunica albuginae, is much stronger and made from fibrous membrane. The tunica albuginae protrudes into the testes in tiny fiber extensions which produces partitions of the testes. Each testicle is amazingly subdivided into 250 to 300 small lobes through these fibrous divisions. In each of these divisions, there are tightly-convoluted tubules that, if laid out flatly, would be about 70 centimetres long. These seminiferous tubules are known as functional elements of the testes. Sperm is created within these tubules and the process is known as spermatogenesis. Sperm is generated at an incredible rate; it is known that the testes can make over a million sperms in one day, which would mean it makes thousands of sperm every second.
Each individual segment of the seminiferous tubules leads to many stages of meiosis, which is the process of cell division that is required for sexual reproduction. This vital process is started with spermatogonia, which are specialized germinal cells. The spermatogonia go through a process of meiosis, or cell division, and will lead to the production of spermatocytes. After this, the process will mature into the next stage known as the secondary spermatocytes. It is in the last stage that spermatids are produced.
Nurse cells known as sertoli cells are responsible for creating the walls of these seminiferous tubules. These cells secrete nutrients which will provide sustenance for the spermatozoa which are developing within the nurse cells. When they are completely formed, the spermatozoa will go to the lumen of the seminiferous tubules, but they are still not considered mature. Endocrine cells known as interstitial cells are in charge of the production and secretion of the needed male sex hormones. It is because of this that the testes are considered endocrine glands as well as exocrine glands. They are capable of producing androgens and spermatozoa which qualifies them for both systems.
It is both motor and sensory neurons which are necessary for the proper functions of the testes. It is the testicular nerves which are able to innervate the testes with both of these requirements. Innervation is slightly assisted by the parasympathetic neurons. Sexually-mature testes will produce a much higher concentration of testosterone. It is this androgen which is also known as an anabolic steroid because it increases muscle growth during male maturation that is responsible for the onset of secondary sex organs such as the prostate and seminal vesicles.
Androgens are responsible for stimulating other organs such as the larynx, which causes a deep voice range in men. It also stimulates the growth of bone which results in thicker bone mass than females as well as higher hemoglobin levels. Androgens stimulate the conversion of cartilage.