The Immunohistochemical Localization of α2‐Macroglobulin in Rat Testes Is Consistent with Its Role in Germ Cell Movement and Spermiation

Abstract

ABSTRACT:α2‐Macroglobulin (α2‐MG) is a nonspecific protease inhibitor and binding protein for peptide hormones that was recently isolated from Sertoli cell‐enriched culture medium and shown to be the same protein as α2‐MG in serum. The present study was conducted to determine the localization of α2‐MG in the seminiferous epithelium in order to gain insight into its possible site(s) of action. Immunostainable α2‐MG was present in the lumen of the tubules consistent with its proposed role as a protease inhibitor needed to inactivate the protease released from defective spermatozoa in the male reproductive tract. Immunoreactive α2‐MG was also localized adjacent to the heads of elongated spermatids, the most mobile cells in the seminiferous epithelium; immunostainable α2‐MG was not observed adjacent to round spermatids and spermatocytes, which are relatively less mobile. The intensity of the staining around the elongated spermatids was dependent on the stage of the spermatogenic cycle. Stainable α2‐MG was present adjacent to the spermatids in stage XI soon after the elongation process began. Immunoreactive product was in stages XI‐XIV but only faintly visible. The most intense staining reaction for α2‐MG was in stages XI‐XIV; it was reduced in stage VII; and virtually no α2‐MG was detectable in stages VIII‐ X at and just after spermiation. The postnatal changes of α2‐MG in the testis was also examined. During the first 2 weeks after birth, α2‐MG was not detected in the seminiferous epithelium. Positive α2‐MG staining was detected in Sertoli cells by 21 days of age, which progressively increased. By day 42, α2‐MG was mainly in the perinuclear region of the Sertoli cells and in the longer cytoplasmic processes extending to the heads of the elongate spermatids. Conclusions: (1) The localization of α2‐MG adjacent to etongated spermatids suggests that this molecule is secreted to limit the action of proteases that are required for migration of these germ cells in the epithelium; and (2) the marked reduction of α2‐MG just prior to spermiation allows for maximal activity of proteases that facilitate this process.