Carrick F N, Hughes R L
Cell Tissue Res. 1982;222(1):127-41. doi: 10.1007/BF00218293.
The elongated spermatid nuclei of monotremes exhibit a circumferentially arranged spiral pattern of chromatin condensation, and ultimately form helical filiform sperm heads up to 50 microns long and either circular or slightly oval in transverse section. The acrosome is formed by the collapse of the proacrosomal vacuole onto the rostral surface of the elongating nucleus. However, genesis of acrosomal material occurs in the absence of a prominent proacrosomal granule. The flagellum becomes inserted into the distal extremity of the nucleus, the most proximal mitochondria of the midpiece directly abutting the nuclear membrane, so that a prominent neck region is absent. The axoneme is simple and, in the midpiece, small dense peripheral fibres are closely applied to the outer surface of each of the nine microtubule doublets. The cortical fibrous sheath of the principal piece is an anastomosing spiral that lacks lateral columnar elements. The spermatozoal cytoplasmic droplet undergoes migration and is lost during epididymal passage. Monotreme spermatozoa exhibit a montage of features, some of these being also found in marsupials and some in sauropsidan vertebrates, as well as a number of their own distinctive features. It is concluded that monotreme spermatozoa also have a close affinity with the unspecialised spermatozoa of some eutherian mammals.
单孔目动物细长的精子细胞核呈现出染色质浓缩的圆周排列螺旋模式，并最终形成长达50微米的螺旋丝状精子头部，其横切面呈圆形或略呈椭圆形。顶体是由前顶体泡塌陷到伸长细胞核的喙状表面而形成的。然而，顶体物质的发生在没有明显前顶体颗粒的情况下进行。鞭毛插入细胞核的远端，中段最靠近近端的线粒体直接邻接核膜，因此没有明显的颈部区域。轴丝简单，在中段，小而密集的外周纤维紧密贴附在九个微管二联体中每个的外表面。主段的皮质纤维鞘是一种缺乏侧向柱状元件的吻合螺旋。精子细胞质滴在附睾通过过程中发生迁移并丢失。单孔目动物的精子表现出一系列特征，其中一些也见于有袋动物，一些见于蜥形纲脊椎动物，还有一些是它们自己独特的特征。得出的结论是，单孔目动物的精子也与一些真兽类哺乳动物未特化的精子有密切的亲缘关系。
