Table of Contents
- What are the Seminiferous Tubules?
- Seminiferous Tubules Function
- Seminiferous Tubules Diagram & Structures
- Medical Implications
- Lesson Summary
The male reproductive system is vital for the process of sexual reproduction, which involves the fusion of the male gamete (sperm) with the female gamete (egg) to produce an embryo that would be born as offspring. The male reproductive system is composed of both internal and external structures. Internal structures store and maintain the integrity of sperm by producing fluid to nurture it as it releases into the female reproductive system; these structures include the prostate gland, the epididymis, and the bulbourethral gland. External structures of the male reproductive system include the penis, the testicles (testes), and the scrotum. Testicles are responsible for sperm production and male hormones, and they hang below the penis as they need to be kept at temperatures slightly lower than body temperature. If the temperature gets too cold or too hot, sperm production will stop. Within the testicles, there are highly convoluted structures named the seminiferous tubules. Seminiferous tubules are the specific site of sperm production, also known as spermatogenesis, and provides the necessary microenvironment to support this process.
An important term in the title 'seminiferous tubule' is the word seminiferous, which means bearing a seed or producing sperm in the context of biology. This meaning gives a hint on what seminiferous functions entail. Seminiferous tubules are the site of spermatogenesis (sperm production).
Round immature sperm cells are present within the seminiferous tubule, and they undergo cell division to form into mature sperm. Primary spermatocytes found in the seminiferous tubules undergo meiotic division to produce secondary spermatocytes and then round spermatids. At that stage, the round spermatids are closer to the lumen of the seminiferous tubules. The seminiferous tubule and its surrounding tissues provide the microenvironment necessary for spermatogenesis by producing nutrients and specific proteins that control this process.
As mentioned earlier, the seminiferous tubules are tightly coiled tubular structures found in the testes. The seminiferous tubules compose stratified epithelial cells; the seminiferous tubules can be divided into two distinct regions: Sertoli cells and the blood-testis barrier.
Somatic cells present in the seminiferous tubules are referred to as Sertoli cells. Sertoli cells respond to the hormone follicle-stimulating hormone (FSH) released from the pituitary gland in the brain. FSH promotes Sertoli cell proliferation; Sertoli cells are necessary for spermatogenesis, as they regulate this process via direct contact and control the microenvironment vital for sperm production. Sertoli cells release what is known as Sertoli tubule fluid, which includes nutrients and hormones such as inhibin B and activins that nourish the process of spermatogenesis. Furthermore, Sertoli cells release anti-mullerian hormone (AMH) during male embryonic development to develop the male reproductive system.
Tight junctions between Sertoli cells form a blood-testis barrier, which creates a physical barrier between the blood vessels in the testicles and the seminiferous tubules. The blood-testis barrier is also known as interstitial cells. The blood-testis barrier is crucial for the process of spermatogenesis by preventing toxic substances from the blood present outside the testes from entering and interfering in the process of spermatogenesis. For this reason, the fluid present in the lumen of seminiferous is very different than the one present in plasma, as it is mainly composed of hormones such as androgens and estrogen necessary for the progression of spermatogenesis.
Somatic cells called Leydig cells reside adjacent to the seminiferous tubule; Leydig cells have a large prominent nucleus and respond to pulses of luteinizing hormones released from the pituitary gland. Leydig cells are responsible for the production and release of androgens such as testosterone that are crucial for the progression of spermatogenesis.
The seminiferous tubules can also vary in size and shape; two prominent shapes are straight or convoluted. Convoluted seminiferous tubules are twisted and coiled, and they are where spermatogenesis primarily occurs.
On the other hand, straight tubules are not coiled, and they are continuous with the rate testis. The rate testis is where produced sperm cells go to be stored in the epididymis, ready for ejaculation.
Since the seminiferous tubules are the site of sperm production, they are a core element contributing to an individual's fertility potential. Some genetic abnormalities can result in the compromised structure of the seminiferous tubules. For instance, Klinefelter's syndrome, a phenomenon in which a male is born with an extra X chromosome (47, XXY), leads to various medical complications such as enlarged breasts and small testicles. Klinefelter's syndrome also leads to the degeneration of the seminiferous tubules leading to the absence of sperm in the ejaculate. Another example regarding genetic abnormalities would be mutations in the FSH-Beta gene, leading to impaired FSH production in the pituitary gland leading to narrowed seminiferous tubules and abnormalities in the structure of Leydig and Sertoli cells.
Congenital hypogonadotropic hypogonadism is a rare disorder that causes the impaired release of gonadotropin-releasing hormone (GnRH) from the hypothalamus; GnRH is the hormone that controls the release of FSH and LH from the pituitary gland. This hormone impairment usually overlaps with a deficiency in AMH production, which leads to the failure to form the tubular structure of the seminiferous tubules during embryonic development. Testicular atrophy, when the testicles decrease in size and volume, manifests due to various reasons such as age, trauma, and hormone imbalances. Testicular atrophy causes the shrinkage of the seminiferous tubules, replaced by a thick fibrous wall. These phenomena are detrimental to male fertility and can be treated with hormone replacement therapy.
The seminiferous tubules are crucial to the male reproductive system and male fertility. The seminiferous tubules are located in the testicles (or the testes) and are long, coiled, tubular structures that carry out the function of spermatogenesis (sperm production). Cells in the seminiferous tubules, called the primary spermatocytes, divide by meiosis to produce sperm. The seminiferous tubules contain Sertoli cells, which are nurse cells that provide support to the process of spermatogenesis by producing substances to maintain the microenvironment necessary for the progression of sperm productions. Tight junctions between Sertoli cells are called the blood-testis barrier and are vital to isolate the testes from the outside environment that could be toxic to sperm. Other structures outside the seminiferous tubules that support spermatogenesis are Leydig cells that produce androgens necessary for spermatogenesis. Disorders of the seminiferous tubules arising from hormone imbalances or genetic abnormalities can cause degenerative or narrowed structures of the seminiferous tubules and can cause infertility.
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The hormone produced by cells in the seminiferous tubules are activins and inhibin B. The lumen of the seminiferous tubule also contains androgens and estrogen necessary for the process of spermatogenesis.
Interstitial cells, also known as the blood-testis barrier, form a physical separator between the seminiferous tubule and the blood to prevent the entry of toxic substances into the blood that can be harmful to spermatogenesis.
The seminiferous tubules are the site of sperm production, or also known as spermatogenesis. The seminiferous tubules provide the necessary microenvironment for spermatogenesis progression.
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