Insulin-like growth factor I in the developing and mature rat testis: immunohistochemical aspects

The distribution of insulin-like growth factor I (IGF-I; somatomedin C) was mapped in testes of different aged rats by using immunohistochemical techniques. The antiserum used, K 624, has been demonstrated to be specific for human IGF-I, as defined by several criteria. Antibodies to the M1 subunit of ribonucleotide reductase, a key enzyme in DNA synthesis, were used to visualize meiotic and mitotic cells. Cytoplasmic IGF-I-like immunoreactivity as demonstrable during the first two postnatal weeks in spermatogenic cells, in Sertoli cells, and in Leydig cells. The IGF-I-like immunoreactivity decreased in the Sertoli and Leydig cells during the third and fourth postnatal weeks, and in adult rats, only spermatogenic cells showed IGF-I-like immunoreactivity. In mature rat testes, the spermatocytes were strongly immunoreactive. During puberty and adulthood, the spermatogonia expressed subunit M1 ribonucleotide reductase immunoreactivity, whereas no IGF-I-like immunoreactivity could be detected. No extracellular immunoreactivity was observed. We propose that IGF-I and/or IGF-I-like substances, possibly formed by primary spermatocytes, are likely to be involved in differentiation processes, but not in the initiation of cell proliferation in adult testes. The autocrine and/or paracrine action of IGF-I and/or IGF-I-like substances may thus have different action in developing testes than in adult testes. Our results do, however, not allow firm statements about whether IGF-I and related substances exert their actions on Sertoli cells or spermatogenic cells.