Points of Attachment of Pachytene Chromosomes to the Nuclear Membrane in Mouse Spermatocytes

Abstract

AT the present time the only chromosomal element which is readily identifiable on electron microscopic examination is the “synaptinemal complex”¹ of the male primary spermatocyte. In the course of a study of the mode of attachment of this structure to the nuclear membrane in a variety of mammals, we noted that, in the mouse, two distinct types of endings were seen near the point of attachment. In one type the bivalent appeared to be condensed around the synaptinemal complex to form a heterochromatic “basal knob”² while, at the other type of ending, the complex was not surrounded by condensed material. The appearances are shown in the electron micrograph (Fig. 1) in which, by a fortunate chance, the entire length of the synaptinemal complex of a bivalent is seen in one section. Since it is now generally agreed that the mitotic chromosomes of the mouse are telocentric with truly terminal centromeres^3,4 and the heterochromatic regions of mouse spermatogenial autosomes are thought to be located adjacent to the loci of the kinetochores⁵, there seems to be justification for identifying the attachment labelled A in Fig. 1 with the centromeric end of the pachytene chromosome and that labelled B with the distal end.