Ultrastructural morphology and relaxin immunolocalization in giant trophoblast cells of the golden hamster placenta

Abstract Relaxin immunoreactivity was previously demonstrated in three cell types within the hamster placenta; fetal primary and secondary giant trophoblast cells (GTCs) and maternal endometrial granulocytes. The objectives of the present research were to examine the ultrastructure of the GTCs and identify the intracellular relaxin storage site. Primary GTCs, first present on day 8 of gestation, were characterized by numerous polyribosomes and large heterogeneous cytoplasmic inclusions suggesting phagocytic activity. Primary and secondary GTCs from days 10, 14, and 15 of gestation contained numerous polyribosomes, mitochondria with tubular cristae, an extensive Golgi complex, and abundant rough endoplasmic reticulum, all characteristics of a cell actively involved in protein synthesis. Membranebound secretory granules were not present. Relaxin was immunolocalized within the Golgi complex of primary and secondary GTCs using the avidin‐biotin‐peroxidase method. Following differential centrifugation of hamster placental homogenates and radioimmunoassay (RIA) of subcellular fractions, the majority of relaxin immunoactivity was detected in the postmicrosomal fraction; however, the majority of relaxin immunoactivity from similarly treated pig corpora lutea was present in the mitochondrial/granule fraction. These data indicate that hamster placental relaxin is not stored in membrane‐bound secretory granules but is contained within the extensive Golgi complex of the GTC.