Testicular Macrophages

This chapter discusses the structure and function of testicular macrophages, with an emphasis on their hormonal receptiveness and paracrine involvement with Leydig cells. A small number of macrophages of unknown origin are present in the testis during early stages of testicular development and then increase in number by proliferation during postnatal life to populate the testis. Testicular macrophages are generally spherical and contain an indented nucleus, as most macrophages in other tissues do. The heterochromatin is commonly clumped at the periphery of the nucleus. Testicular macrophages have lysosomes that are highly varied in morphology, presumably representing various stages of phagocytosis. Two types of specialized membrane associations termed, “digitations” and “subplasmalemmal linear densities” occur where macrophages are located next to Leydig cells. It is speculated that these digitations play an important role in the communication between macrophages and Leydig cells. Macrophages of adult animals have the potential to be involved in two separate phenomena. The first role is traditional: participation in immunological processes. The second phenomenon is tissue specific. It relates to responsiveness to follicle-stimulating hormone (FSH) and participation in paracrine and juxtacrine interactions with Leydig cells. There are several markers that can be used to identify testicular macrophages, depending on the species such as ED-1, ED-2, OX3, HLA-DR, and OX6.