Germ cell aplasia does not represent a diagnosis but it is rather a characteristic histopathologic finding first described by Del Castillo et al. in 1947 which is sometimes referred to by his name. In 1979, Levin described germ cell aplasia with focal spermatogenesis where a variable percentage of seminiferous tubules contain germ cells. It is important to discriminate between both (by repeated testicular biopsies if the patient desires so) in view of ICSI (IntraCytoplasmic Sperm Injection) / IMSI (Intracytoplasmic Morphologically selected Sperm Injection).
No germ cells are present in the seminiferous tubules but Sertoli cells are present (hence the name Sertoli-cell-only syndrome / Johnsen Score 2: no germ cells - Sertoli cells only). The Sertoli cell nucleus is typically triangular or ovoid in shape with a prominent nucleolus. Sertoli cells with their apical aspects displaced to one side “wind swept tree tops” are seen. Tunica propria and basement membranes are not appreciably thickened, and the seminiferous tubules are of normal or slightly decreased diameter.
The testes of men with Del Castillo's syndrome could be classified into three groups, according to the Sertoli cell type present: mature, dysgenetic (with immature nucleus) and involuting cells (with irregularly outlined nucleus). It is unknown whether this classification has a correlation with Inhibin B levels. In Del Castillo’s syndrome serum FSH is most often elevated.
Inhibin B is a marker of spermatogenesis and Sertoli cell function and is produced by the Sertoli cells. It is the major negative feedback regulator of FSH release. Inhibin B correlates positively with sperm concentration and testicular volume. Inhibin B correlates negatively with the degree of testicular damage.
Mostly germ cell aplasia is congenital (primary) and is associated with primary infertility. For some reason the germ cells do not migrate into or survive in the seminiferous tubule. Microdeletions of the Y chromosome are an important genetic cause of congenital germ cell aplasia. Germ cell aplasia can be acquired (secondary) through endogenous (e.g. maldescended testes) and exogenous (e.g. radiotherapy) damage of sufficient magnitude to destroy the germ cells without destroying the Sertoli cells.
It should be noted that when bilateral testicular biopsies show germ cell aplasia the patient can have oligozoosermia and not necessarily azoospermia as shown in a retrospective review*, hence the importance of clinicopathologic correlation.
*Bettocchi C, Parkinson MC, Ralph DJ, Pryor JP. Clinical aspects associated with Sertoli-cell-only histology. Br J Urol. 1998; 82(4): 534-7.