House mouse sperm head. (a) SEM image of the sperm head indicating the position of landmarks and semilandmarks. (b) Localization of the main regions of the sperm head: AA, anterior acrosome; ES, equatorial segment; SAR, sub-acrosomic ring; PAS, post-acrosomal sheath; VS, ventral spur. The acrosomal (APM) and post-acrosomal (PAPM) plasma membranes are shown in light and dark shading respectively. (c-e) Subsets of landmarks for hypotheses H1-H3.