Ultrastructure and Differentiation in Chara (Fibrosa) IV. Spermatogenesis

Abstract

Spermatogenesis in Ohara is described at the ultrastructural level. A large nwnber of mitoses form spermatogenous threads, derived from the capitula inside the antheridiwn. Centrioles appear in the spermatogenous cells early as indistinct organelles that quickly become conspicuous. There is some evidence for the existenc of a procentriole. Centrioles are subsequently associated in a normal fashion with the mitotic spindle. Intracellular differentiation commences after mitosis ceases. The nucleus moves to one side of the cell; a flat band of manchette microtubules is soon formed near it. The microtubules, which increase in nwnber and elongate considerably, are inserted at one end into (and possibly extruded from) a densely staining, homogeneous inclusion, termed the manchette adjunct, which appears close to the centrioles. The centrioles, connected together by a spindle-shaped ciliary rootlet structure, move to the edge of the cell and start extruding flagella, which are covered in scales; another organelle, termed the vesicular adjunct and of unknown significance, appears near these centrioles. The manchette grows in length, and so asswnes a spiral course in the cell; plastids then line up along the microtubules next to the nucleus. While still interconnecting cells, the cytoplasm shrinks steadily. With further elongation of the manchette, the flagella apparatus moves away from the nucleus, and mitochondria also line up along the manchette tubules between them. Lipid (?) bodies move near the plastids, which steadily accumulate starch. Golgi bodies show marked structural changes during differentiation; they are initially associated with a profusion of various vesicles, and later lose their identity, as does the endoplasmic reticulwn which earlier interconnected cells through plasmadesmata. The nucleolus disappears, and later chromatin condensation gives the elongating nucleus an increasingly lamellate structure; finally these lamellae fuse to form a dense homogeneous nucleus. The cytoplasm continues to shrink, eliminating almost all cell organelles. The mature spermatozoid, tightly coiled in the cell, finally contains plastids and (lipoid) inclusions at one end, next to the dense, elongate nucleus, with linearly arranged mitochondria at the other end, and flagella inserted above the mitochondria. Manchette microtubules run the length of the organism as a flat band opposed to the nucleus and plastids, and finally as a tubular sheath partly enclosing the mitochondria. Four other tubules, possibly derived from a ciliary root structure, are also close to the mitochondria. The flagella are quite long by this stage. The observations are discussed in terms of the functions of cell organelles. In particular, it is suggested that centriolar movement in mitosis may be only one example of everal morphogenic movements associated with microtubule organization; their unction is in flagella formation, and not in synthesis of spindle, manchette, or other ytoplasmic microtubules.