FSH and testosterone receptors are present on Sertoli cells; therefore, their secretory function is regulated by both FSH and testosterone (Fig. 22.18). Sertoli cells secrete several endocrine substances, such as inhibin, a 32-kilodalton glycoprotein hormone involved in the feedback loop that inhibits follicle-stimulating hormone (FSH) release from the anterior pituitary gland. In addition, Sertoli cells synthesize plasminogen activator, which converts plasminogen to the active proteolytic hormone plasmin, transferrin (an iron-transporting protein), and ceruloplasmin (a copper-transporting protein). Furthermore, the Sertoli cells secrete other glycoproteins that function as growth factors or paracrine factors, such as the Mlerian-inhibiting factor (MIF), stem cell factor (SCF), and glial cell line-derived neurotrophic factor (GDNF).
At the end of each seminiferous tubule there is an abrupt transition to the straight tubules, or tubuli recti. This short terminal section of the seminiferous tubule is lined only by Sertoli cells (Plate 87, page 820). Near their termination, the straight tubules narrow, and their lining changes to a simple cuboidal epithelium.
The straight tubules empty into the rete testis, a complex series of interconnecting channels within the highly vascular connective tissue of the mediastinum (Fig. 22.19). A simple cuboidal or low columnar epithelium lines the channels of the rete testis. These cells have a single apical cilium and relatively few short apical microvilli.
logic importance of the blood–testis barrier:  Spermatozoa and spermatogenic cells possess molecules that are unique to these cells and are recognized as “foreign” (not self) by the immune system.  Spermatozoa are first produced at puberty, long after the individual has become immunocompetent (i.e., capa-ble of recognizing foreign molecules and producing an-tibodies against them). Failure of the spermatogenic cells and spermatozoa to remain isolated results in the production of sperm-specific seen after vasectomy and in some cases of infertility. After vasectomy, sperm-specific antibodies are pro-duced as the cells of the immune system are exposed to the spermatozoa that may leak from the severed ductus deferens. Thus, sperm no longer remain isolated from the immune system within the reproductive tract. In some cases of infertility, sperm-specific antibodies have been found in the semen. These antibodies cause the sperm to agglutinate, preventing movement and interaction with the ovum.
 FOLDER 22.3 Clinical Correlation: Sperm-Specific Antigens and the Immune Response
Two basic facts are well established about the immuno-antibodies. Such an immune response is sometimes
The excurrent duct system develops from the mesonephric (Wolffian) duct and mesonephric tubules.
The initial development of Leydig cells and initiation of testosterone secretion stimulate the mesonephric (Wolffian) duct to differentiate into the excretory duct system for the developing testis (Fig. 22.20). The portion of the mesonephric duct adjacent to the developing testis becomes convoluted and differentiates into the duct of the epididymis. In addition, a number (about 20) of the remaining mesonephric tubules in this region make contact with the developing seminiferous cords and  finally develop into the efferent ductules (Fig. 22.21 and Plate 88, page 822). They connect the developing rete testis with the duct of the epididymis. The distal part of the mesonephric duct acquires a thick, smooth muscle coat and becomes the ductus deferens. The end of the distal mesonephric duct gives rise to the ejaculatory duct and seminal vesicles.