The Immortalized Mouse Germ Cell Lines GC-1spg and GC-2spd as a Model for Mitochondrial Differentiation during Meiosis

Abstract

It is well recognized that mitochondria of germ cells modify their morphological organization, number and location during spermatogenesis. Mitochondria gradually transform from the usual cristae-rich (“orthodox”) type found in the various types of spermatogonia via an intermediate form located in leptotene and zygotene spermatocytes into a so-called “condensed-type” found in pachytene spermatocytes and early spermatids. In pachytene spermatocytes, the mitochondria are grouped together in small clusters. The intramembranous space is greatly enlarged, with the membrane pushed to the periphery leaving almost no cristae. The matrix is condensed to a thin ring-like structure underneath the outer membrane leaving the interior of these mitochondria looking “empty”. In early round spermatids, the mitochondria retain the “empty” central regions, but later in spermatogenesis, more cristae become evident. Very little is known about the factors that regulate or trigger the various steps of mitochondrial differentiation during sperm production. Recent studies from our group (Meinhardt et al., 1995; Seitz et al.,1995) indicate the involvement of proteinaceous factor(s), termed paracrine mitochondrial maturation factors (PMMF), which are secreted by Sertoli cells and act in a paracrine fashion on primary spermatocytes. However, the study of factors that induce the differentiation of mitochondria in isolated leptotene and zygotene spermatocytes is hampered by low numbers of this cell type in rat testis. Mitochondrial differentiation is accompanied by the step-wise occurrence of three marker proteins in the matrix, each typical for a distinct organelle form: heat shock protein 60 (hsp60) for orthodox mitochondria mainly in spermatogonia, LON-protease in leptotene and zygotene spermatocytes and sulfhydryl oxidase (SOx) for the condensed type in pachytene spermatocytes and early round spermatids (Seitz et al, 1995). The immortalized germ cell line GC- lspg has been characterized at a stage between B spermatogonia and an early primary spermatocyte, while the GC-2spd line encompasses germ cells from mid to late meiosis (Hofmann et al, 1992; 1994).