Testis: Microscopic Anatomy
External Structure of Testis
Shape and Size: The testis is an oval-shaped organ found in pairs in the male reproductive system. In humans, each testis measures about 4.5–5 cm in length, 2–3 cm in width, and 2–2.5 cm in thickness. The weight of each testis is 10–15 grams, and its normal volume is 15–25 ml.
Location and Suspension: The testes are suspended within the scrotum, a pouch-like structure located outside the abdominal cavity. Each testis is held in position by the spermatic cord, which contains nerves, blood vessels, and ducts. The inguinal canals connect the scrotum with the abdominal cavity and allow for the passage of the spermatic cords.
Descent of Testes: During fetal development, the testes form in the abdominal cavity. By the seventh month of gestation, they descend through the inguinal canals into the scrotum. This descent is guided by the gubernaculum, a fibromuscular band. Failure of the testes to descend results in cryptorchidism, which can cause sterility.
Protective Coverings (Tunicae of Testis):
The testis is surrounded by three protective layers:
Tunica vaginalis – The outermost protective serous membrane.
Tunica albuginea – A tough, white fibrous capsule that gives shape and strength to the testis.
Tunica vasculosa – The innermost layer, consisting of capillaries and connective tissues.
Internal Structure of Testis
Internally, the testis is divided into compartments and contains specialized structures involved in sperm production.
1. Testicular Lobules: The tunica albuginea extends inward, forming septa that divide the testis into about 250 testicular lobules. Each lobule contains 1 to 4 seminiferous tubules, which are highly coiled structures where spermatogenesis occurs.
2. Seminiferous Tubules: Each lobule contains coiled tubules (1-4 per lobule) known as seminiferous tubules. The tubules are lined by germinal epithelium, which consists of:
Spermatogenic cells – These undergo meiosis to form sperm.
Sertoli cells (Sustentacular cells) – Sertoli cells are found between the germinal epithelial cells of the seminiferous tubules. Structure: They are elongated and present as single cells within the seminiferous epithelium.
Function: Nourishment: Sertoli cells provide nutrients and structural support to developing spermatozoa. Secretion of Proteins: Androgen-Binding Protein (ABP): Helps in concentrating testosterone in the tubules, which is critical for spermatogenesis. Inhibin: Regulates the production of follicle-stimulating hormone (FSH) by providing negative feedback to the pituitary gland.
3. Tubuli Recti:
These are short, straight tubules that form when the ends of seminiferous tubules converge and join. The tubuli recti act as a connection between the seminiferous tubules and the rete testis.
4. Rete Testis
It is a network of interconnected tubules within the mediastinum of the testis. Its primary function is to transport sperm from the seminiferous tubules (where sperm is produced) to the vasa efferentia (which connect to the epididymis).
5. Vasa Efferentia: The vasa efferentia are a series of small ducts that connect the rete testis to the epididymis. They play a crucial role in transporting sperm from the testis to the epididymis for storage and maturation.
6. Gubernaculum: The gubernaculum is a short fibromuscular band that connects the testes to the wall of the scrotum. It helps guide the descent of the testes during fetal development.
Function: It ensures the testes move from the abdominal cavity into the scrotal sacs, a process necessary for normal reproductive function.
7. Leydig Cells (Interstitial Cells)
Leydig cells, also called interstitial cells, are in the interstitial spaces outside the seminiferous tubules. These spaces also contain small blood vessels, which facilitate hormone transport.
Function: Leydig cells secrete male sex hormones, known as androgens, primarily testosterone. Testosterone is essential for: Development of secondary sexual characteristics in males, such as body hair, deeper voice, and increased muscle mass. Supporting the process of spermatogenesis within seminiferous tubules.
Regulation: The activity of Leydig cells is regulated by the luteinizing hormone (LH), which stimulates testosterone production.
Importance: Testosterone secreted by Leydig cells plays a crucial role in: Maintaining male reproductive health. Enhancing libido and maintaining fertility.
8. Blood-Testis Barrier
Tight junctions between Sertoli cells form a protective barrier known as the blood-testis barrier.
It prevents immune cells from attacking developing sperm, as sperm cells contain unique antigens.
Functions of Testis
The human testis performs two primary functions:
Spermatogenesis (Sperm Production): The seminiferous tubules inside the testis are responsible for producing sperm cells, which are the male gametes. This process is essential for male fertility and reproduction.
Hormone Secretion (Testosterone Production): The Leydig cells in the testis produce the hormone testosterone. Testosterone plays a crucial role in: Development and maintenance of male secondary sexual characteristics (such as facial hair, deeper voice, and muscle mass). Regulating libido (sexual drive). Supporting overall male reproductive health.