Normal function of testis is dependent upon hormones acting through endocrine and paracrine pathways. The  endocrine function of the testis resides primarily in the Leydig cell pop-ulation that synthesizes and secretes the principal circulating androgen, testosterone. Nearly all of the testosterone is produced by the testis; less than 5% is produced by the adrenal glands. It is estimated in humans that the total Leydig cell population produces about 7 mg of testosterone per day. As testosterone leaves the Leydig cells, it passes into blood and lymphatic capillaries and across the peritubular tissue to reach the seminiferous  epithelium. High local levels of testosterone within the testis (esti-mated to be as much as 200 times the circulating levels) are necessary for the proliferation and differentiation of sper-matogenic cells. This high testicular level of testosterone can be significantly decreased by negative feedback from exogenous testosterone. Intensive research in this area is being directed into development of a prototype of testos-terone-based contraceptive drugs for men. In early clinical studies, these drugs have been shown to cause a significant decrease in the testicular testosterone concentration and inhibition of spermatogenesis. Recovery of spermatogene-sis occurs after discontinuation of contraceptive use. How-ever, in some individuals, this type of contraceptive is not efficacious and does not cause spermatogenic suppression. Peripheral testosterone levels have the following effects:  Differentiation of the central nervous system (CNS) and the genital apparatus and genital excurrent duct system  Growth and maintenance of secondary sexual charac-teristics (such as the beard, male distribution of pubic hair, and low-pitched voice)  Growth and maintenance of the accessory sex glands (seminal vesicles and prostate and bulbourethral glands), genital excurrent duct system, and the external genitalia (mainly byproducts of testosterone conversion to DHT)  Anabolic and general metabolic processes, including skeletal growth, skeletal muscle growth, distribution of subcutaneous fat, and kidney function  Behavior, including libido The steroidogenic and spermatogenic activities of the testis are regulated by hormonal interaction among the hy-pothalamus, anterior lobe of the pituitary gland, and gonadal cells (i.e., Sertoli, spermatogenic, and Leydig cells). The anterior lobe of the pituitary gland produces three hormones involved in this process: luteinizing hormone (LH), which in the male is sometimes referred to as interstitial cell–stimulating hormone (ICSH); follicle-stimulating hormone (FSH); and prolactin (PRL). In response to LH release by the pituitary, Leydig cells pro-duce increasing amounts of testosterone. PRL acts in combination with LH to increase the steroidogenic activity of Leydig cells. Because FSH and testosterone receptors are found in Sertoli cells, these cells are the primary regu-lators of spermatogenesis.
muscles causes the scrotum to wrinkle when cold to help regulate heat loss (Folder 22.2).
Structure of the Testis
The testes have an unusually thick connective tissue capsule, the tunica albuginea.
An unusually thick, dense connective tissue capsule, the tunica albuginea, covers each testis (Fig. 22.4). The inner part of this capsule, the tunica vasculosa, is a loose connective tissue layer that contains blood vessels. Each testis is divided into approximately 250 lobules by incomplete connective tissue septa that project from the capsule. Along the posterior surface of the testis, the tunica albuginea thickens and projects inward as the mediastinum testis. Blood vessels, lymphatic vessels, and the genital excurrent ducts pass through the mediastinum as they enter or leave the testis.
Each lobule consists of several highly convoluted seminiferous tubules.