AssessmentsAnatomy and physiology of the male reproductive system
Anatomy and physiology of the male reproductive system
USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A 50-year-old man comes to the emergency department because of injuries after falling off a ladder. He describes falling about 5 feet off the ground and suffering blunt trauma to his perineum and penis, resulting in immediate pain lasting 5 minutes. Following the injury, the patient experienced burning with urination and noted large amounts of hematuria. He denies pain with ambulation and does not recall any trauma to the pelvis during the fall. Genitourinary examination reveals blood at the urethral meatus. Which of the following is the most appropriate initial step in management?
Content Reviewers:Rishi Desai, MD, MPH
The male reproductive system includes internal and external organs and structures that help with reproduction.
Now, the testes are two organs the size of small plums, that are located in a skin and muscle pouch called the scrotum.
When it’s cold outside, the scrotal skin wrinkles and the scrotum elevates to bring the testes closer to the body to warm up.
When it’s warm outside, the scrotal skin loosens up and the scrotum lowers the testes away from the body, and heat is released through sweating.
The testes themselves are covered on the outside by the tunica albuginea - a white, fibrous layer.
If we slice a testis open and look inside - sorry for the cringe moment, guys - we can see that the tunica albuginea, sends fibrous projections called septa towards the center of the testis.
These septa partition each testis into about 250 lobules, and each lobule contains at least one and up to four seminiferous tubules, which is where sperm is synthesized.
The seminiferous tubules come together and form a single straight tubule that exits the lobule, and enters a small network of tubules called the rete testis.
The rete testis are a small network of ducts that split up and come together again - like catacombs, and these are located in the center of the testes - a region called the mediastinum testis.
Once the sperm make it through the rete testes, they go through the efferent ducts to the epididymis.
Now let’s zoom in on a single seminiferous tubule - which is the “sperm factory”.
A seminiferous tubule has a thick wall of epithelial cells that surround a fluid-filled lumen - a bit like a garden hose.
The wall of the tubule is made up of three kinds of cells: at the periphery, there’s the spermatogonia - which are the primordial sperm cells that begin dividing over and over in puberty, and give rise to male gametes.
Next, there are the spermatocytes - which migrate towards the lumen as they differentiate into sperm.
Finally, there are Sertoli cells - which are large cells that extend from the margin all the way to the lumen of the tubule.
Sertoli cells provide nutrients to developing sperm cells, and contribute to the blood-testis barrier by only allowing certain molecules, like testosterone, in the seminiferous tubule.
That GnRH travels to the nearby pituitary, which secretes two hormones of its own - luteinizing hormone, or LH, and follicle stimulating hormone, or FSH, that reach the gonads.
Before puberty, GnRH and, in turn, FSH and LH are secreted in low, constant amounts.
At puberty, secretion of these hormones intensifies and becomes pulsatile - sometimes more of the hormone is released, and sometimes less of the hormone is released.
LH binds to Leydig cells, and stimulates the production of testosterone, whereas FSH binds to Sertoli cells, making them produce androgen binding protein - or ABP, for short - which allows more testosterone to cross the blood-testis barrier and enter the seminiferous tubule.
So, it’s the increased concentrations of FSH, LH, testosterone that really gets spermatogenesis going.
Spermatogenesis starts with the spermatogonia, which are diploid cells - so they have 46 chromosomes, two of which are sex chromosomes, an X and a Y.
Following mitosis, a spermatogonium gives rise to two 46-chromosome daughter cells: one of them eventually becomes a primary spermatocyte, and the other one becomes a spermatogonium - this way the population of spermatogonia stays constant.
The primary spermatocyte slowly moves towards the lumen of the seminiferous tubule, passing between two Sertoli cells that nourish it.
The primary spermatocyte enters meiosis I, and emerges with 23 chromosomes each, one of which is either an X or a Y chromosome.
Secondary spermatocytes enter meiosis 2, which also have 23 chromosomes, but only one chromatid - so the right number of chromosomes and the proper amount of DNA.
Spermatids enter the lumen of the seminiferous tubule and undergo spermiogenesis - which is when they acquire a tail, and turn into sperm.
It takes roughly 2 months for spermatogonia to develop into sperm, and this process is regulated by various hormones.
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