Fig. 7. Schematic representation of sex chromosome (X, Y) structure and behaviour throughout pachytene. The upper row depicts sex chromosomes as seen in spread spermatocytes, either by electron microscopy or by labelling with anti-SCP3 serum or MPM-2. AEs are initially short and thick and do not touch each other (A). In later stages, AEs become thicker and more loosely organised, and dense material starts to be deposited at their ends (B). Eventually, this material includes the ends of both chromosomes, and the AEs appear as faint threads (C,D). The lower row shows the sex chromosomal AEs as seen in electron microscopy sections and in squashes labelled with the anti-SCP3 serum or MPM-2 antibody. Initially X and Y AEs lie apart from each other, showing a stiff appearance and an irregular outline (A'). Subsequently, sex chromosomes approach and form the sex body, revealed as a dense chromatin mass, and their AEs touch (B'). Thereafter, AEs start to loose their stiffness, and the material composing the AEs is deposited on the nuclear envelope (NE) while the label on the AEs decreases (C'). Finally, AEs get thinner and looped, and the dense plate (DP) forms, extending from the attachment plates of sex chromosomal AEs to the nuclear envelope. The DP extends until it includes the ends of both sex chromosomes (D').