A comparative study of Sertoli cell ectoplasmic specializations in selected non-mammalian vertebrates.

Ectoplasmic specializations (ES) facing spermatids were studied in species representative of four classes of non-mammalian vertebrates (Pisces--bluegill; Amphibia--bullfrog; Reptilia--red eared turtle; Aves--domestic chicken). ES was not seen in the bluegill but was present in all other species studied. In the frog, turtle, and chicken, ES did not resemble its mammalian counterpart and could only be characterized by the presence of 6 nm filaments (presumedly actin) within the somatic cell facing the head region of elongating spermatids. ES filaments were sparse in the frog and were sometimes associated with more deeply situated endoplasmic reticulum. Turtle ES filaments were abundant and encircled the acrosomal region of the spermatid head and were delimited by fenestrated saccules of endoplasmic reticulum. In the chicken, ES filaments were prominent but less abundant than in the turtle. Six nanometer filaments of the chicken ES appeared in a tangled mass and were not associated with clearly defined endoplasmic reticulum. In the three species where ES was found, it first developed as spermatids became entrenched within the surrounding somatic cell. Neither cell elongation, nuclear elongation, or movement of the nucleus to the cell surface was synchronized with the onset of ES development. That ES development was seen concomitant with spermatid entrenchment and spermatid orientation suggested a role for ES in these processes. This hypothesis was further strengthened by observations in the fish where ES was lacking and where spermatid entrenchment within the somatic cell, did not occur. The study also supported the hypothesis that ES acts as a cytoskeletal mantle to which other cytoskeletal elements within the cell interact to affect the position of elongate spermatids within the epithelium. The dissolution of ES prior to spermiation and concomitant loss of a close relationship between cells suggests that ES is also related to somatic cell-germ cell adhesion and therefore plays an important role in the spermiation process.