Germinal differentiation of the stem cell in Hydra fusca: a model system

Abstract

The consecutive stages in the differentiation of the stem cell to the spermatozoon were characterized based on time of appearance, cytology, volumes, and spatial distribution. In the first few days the stem cells (128 μ3) enlarged and entered a phase of intense mitosis. In these cells or primary spermatogonia (220–1140 μ3), the cytoplasm was intensely basophilic, the DNA dispersed as a fine meshwork, and a large nucleolus was present. In secondary spermatogonia (123 μ3), noted after 14 days, the DNA was organized in small granules and the nucleolus reduced in size. The primary spermatocyte (122 μ3) was seen by 16 days; its cytoplasm was weakly basophilic, the nuclear DNA appeared granular, and a nucleolus was absent. After meiosis I, the secondary spermatocyte (60 μ3) immediately entered meiosis II. The spermatids (31 μ3), noted by 18 days, lacked a nucleolus and had dense irregular masses of DNA scattered throughout the nucleus. Spermatozoa, seen after 20 days, consisted of a cone-shaped head, a middle piece containing four large mitochondria, and a long flagellum. All these cell types were distributed sequentially within the testis according to their chronological order of appearance. The use of spermatogenesis as a model system to study cell differentiation and control requires further methodological developments and biochemical information.