Testosterone is secreted by Leydig cells.

Testosterone is the primary male sex hormone that is responsible for the development and maintenance of male reproductive tissues and characteristics. It plays a crucial role in the growth and development of the male reproductive organs, including the testes, prostate, and seminal vesicles, as well as secondary sexual characteristics such as muscle mass, bone density, and body hair distribution.

Leydig cells:
- Leydig cells, also known as interstitial cells of Leydig, are located in the interstitial tissue of the testes.
- These cells are responsible for the production and secretion of testosterone.
- Leydig cells are stimulated by luteinizing hormone (LH), which is released by the anterior pituitary gland in response to gonadotropin-releasing hormone (GnRH) from the hypothalamus.
- LH binds to receptors on Leydig cells and triggers the production and release of testosterone.
- The production of testosterone by Leydig cells is regulated by a negative feedback loop, where high levels of testosterone inhibit the release of GnRH and LH, thereby reducing testosterone production.

Other cell types:
- Sertoli cells are located within the seminiferous tubules of the testes and play a crucial role in supporting the development and maturation of sperm cells. They provide nutrients and physical support to developing sperm cells but do not secrete testosterone.
- Kupffer cells are a type of macrophage found in the liver that are involved in the immune response and the breakdown of old red blood cells. They do not secrete testosterone.
- Mast cells are a type of immune cell involved in allergic reactions and the release of inflammatory mediators. They do not secrete testosterone.

In conclusion, testosterone is primarily secreted by Leydig cells in the testes in response to luteinizing hormone (LH) stimulation. These cells play a crucial role in male reproductive development and function.