Urology - Hormonal control of testicular function Flashcards
When does spermatogenesis start?
Spermatogenesis is the production of male gametes, or spermatozoa. It occurs in the testes around the onset of puberty. In males, testosterone production and spermatogenesis occur continuosly.
These 2 processes are controlled by the posterior pituitary glycoprotein gonadotrophins, follicle stimulating hormone (FSH) and leutinising hormone (LH).
When should the testes descend? If they fail to do so why does this cause infertility?
The testes should descend a few weeks before birth. They pass out of the abdominal cavity and into the scrotal sac. Failure of the testes to descend is called cryptorchidism. It causes infertility because spermatogenesis is disrupted, as it depends on a temperature of about 4 degrees below body temperature.
This lower temperature is maintained by a counter current heat exchange between the testicular artery and the papiniform plexus of veins that are filled with blood at a lower temperature.
Where in the testes does spermatogenesis take place?
Spermatozoa are the male gametes and these are produced (by the process of speramtogenesis) in the seminiferous tubules. These tubules are lined with germ cells and Sertoli cells. The Sertoli cells surround the developing germ cells and provide them with nutrients and stimulating factors (they are sometimes referred to as “nurse cells”). They also secrete a number of hormones such as androgen binding protein and inhibin.
What triggers spermatogenesis?
At puberty, GnRH secretion from the hypothalamus becomes pulsatile and stimulates gonadotrophin (LH and FSH) release from the anterior pituitary. These hormones initiate spermatogenesis which occurs continuously thereafter.
What cells are spermatozoa produced from?
Primordial germ cells that enter the foetal testes divide and become spermatogonia - the stem cells of the testes that divide by mitosis and meiosis. There are 3 phases of spermatogenesis:
1) proliferation
2) division
3) differentiation
What happens during the proliferation phase of spermatogenesis?
This happens during puberty. Primordial germ cells are reactivated and undergo mitosis in the basal compartment of the tubule.
They form As spermatogonia (spermatogonial stem cells) which are a reservoir of self renewing stem cells. These go on to form A spermatogonia which are the start of spermatogenesis.
Each A spermatogonium undergoes mitosis to form type B spermatogonium which divide again to form primary spermatocytes.
What happens during the division phase of spermatogenesis?
This is where primary spermatocytes undergo a meiotic division. Primary spermatocytes move into the adluminal compartment by passing through the blood testes barrier (BTB). They undergo a first meiotic division to form 2 secondary spermatocytes, and these in turn undergo a second meiotic division to form haploid round spermatids.
Spermiogenesis
This is another term for the third phase of spermatogenesis or differentiation. Round spermatids elongate to form elongated spermatids and finally mature spermatozoa.
How long does spermatogenesis take?
Approximately 70 days.
Are spermatozoa capable of fertilising an ovum straight away?
No. When spermatozoa are released into the seminiferous tubule they are non-motile and incapable of fertilising an ovum. From the seminiferous tubule, spermatozoa pass into the epididymis where they mature and are stored until ejaculation.
What happens to sperm that are not ejaculated?
The production of spermatozoa is continuous. Spermatozoa that are not ejaculated sometimes pass into the urine (spermaturia) or eventually deterioate and are reabsorbed within the epididymis.
Describe the features of a mature sperm
Mature sperm consist of a head, middle peice, principal piece and end piece. The head is composed mainly of the nucleus and is covered by a cap known as the acrosome. This is a large vesicular structure containing lytic enzymes that help penetrate the zona pellucida of the ovum. The middle piece consists of a helical sheath of mitochondria surrounding a core of contractile microtubules called axoneme. This extends to the tail.
What is the blood testes barrier (BTB)?
The BTB is located between the basal and adluminal compartments of the seminiferous tubule. Adherens and gap junctions link adjacent Sertoli cells to each other to form the BTB. It develops during puberty prior to the onset of spermatogenesis.
It has 2 functions:
1) separates sperm from the immune system preventing an immune response
2) selective transport of ions and small molecules
What is capacitation?
Capaciation is not to be confused with sperm maturation that occurs in the epididymis. Capacitation is a physiological change that sperm undergo before they can penetrate the female egg. It involves hyperactivation of the tail and the acrosome reaction in the head. It occurs in the female tract.
What type of hormone is testosterone?
Testosterone is the principal androgenic hormone produced by the testes. It is a C19 steroid hornome synthesized from cholesterol. It is secreted by the interstitial cells of Leydig which are found between the seminiferous tubules. Most of the androgens are synthesised here, but a small amount is also produced by the adrenal cortex.
