Human meiosis V. Substages of pachytene in human spermatogenesis

Abstract

The pachytene stage of human spermatogenesis has been investigated by serial sectioning of spermatocytes and three dimensional reconstruction of nuclei from electron micrographs. The analysis of 73 completely reconstructed normal nuclei from late zygotene to early diplotene revealed a continuous sequence of changes in the fine structure and morphology. The developmental sequence permits the recognition of 7 substages on the basis of morphological changes of autosomal chromatin, centromeric, chromatin, the XY bivalent, the secondary constriction on bivalents 1,9 and 16, the nucleolus and the centrioles. Of particular interest is the observation that the disappearance of the synaptonemal complex segment between the X and Y chromosomes at mid pachytene is only temporary, a distinct complex segment being present in nearly all analysed XY bivalents by the end of pachytene. The number of recombination nodules and recombination bars in nuclei of the different substages is recorded. The total number of nodules decreases from 75 at the beginning of pachytene to 1 or none at the end of this state. The bars appear at early pachytene, reach a number of 35 at mid pachytene and disappear following the nodules towards the end of pachytene. The morphology of pachytene nuclei at various stages of degeneration as well as the structural characteristics of two tetraploid spermatocytes are described.