Testis Hormones — The Chemical Signals That Make a Male
Think of the testes as two factories. One production line makes sperm — the male gametes. The other production line makes hormones — chemical messengers that travel through the blood to tell the rest of the body, "You are male, and here is what that means."
The second factory is what we are talking about.
The Factory Workers: Leydig Cells
Scattered in the connective tissue between the sperm-producing tubules of the testis are clusters of cells called Leydig cells (or interstitial cells). These are the hormone factories. They detect signals from the pituitary gland (LH, or luteinizing hormone) and, in response, start manufacturing a class of hormones called androgens.
The primary and most potent androgen is testosterone. When exam questions say "testis hormones" or "androgens," they almost always mean testosterone.
What Testosterone Does — The Two Big Jobs
Testosterone is not just one thing. It has two distinct sets of effects, and you must keep them separate in your mind.
1. Development and maintenance of male secondary sexual characters. At puberty, testosterone surges and causes:
- Growth of facial, axillary, and pubic hair
- Deepening of the voice (larynx enlargement)
- Increased muscle mass and bone density
- Broadening of shoulders
- Enlargement of the penis and scrotum
- Increased libido (sex drive)
These are the visible, external signs of maleness. They are called secondary sexual characters because they appear at puberty, not at birth.
2. Spermatogenesis — the making of sperm. Testosterone is absolutely required for the process of sperm formation inside the seminiferous tubules. Without adequate testosterone levels, sperm production stops. This is a direct, local effect — the Leydig cells sit right next to the tubules, so testosterone reaches its target at high concentration.
Spermatogenesis also needs FSH from the pituitary. Testosterone and FSH work together — testosterone acts on the Sertoli cells (the "nurse" cells inside the tubules) to support the developing sperm, while FSH stimulates the Sertoli cells directly.
The Feedback Loop — How the Body Keeps It in Check
The brain (hypothalamus) and pituitary gland control testosterone levels through a negative feedback loop.
- Hypothalamus releases GnRH → tells pituitary to release LH and FSH
- LH → stimulates Leydig cells → produce testosterone …