An ultrastructural study of the differentiation of the spermatozoid of Equisetum

Abstract

The ultrastructure of spermatogenesis in Equisetum is described with particular reference to the origin and development of the multilayered structure (MLS) and nuclear metamorphosis. Simultaneously with the formation of centrioles, by the fragmentation of the blepharoplast, in young spermatids, the MLS appears in their vicinity. This comprises 4 layers recalling the Vierergruppe of bryophyte spermatids. The outer layer, or microtubular band, consists of juxtaposed microtubules. The three inner lamellar strata, which lie along the anterior edge of the microtubular band, are composed of parallel plates oriented at 35-45° to the axes of the microtubules. Keels are present on the microtubules where these overlie the lamellar layers. A mitochondrion lies subjacent to the lamellar layers and on the outer surface of the anterior edge of the microtubular band is a crest of osmiophilic material. The position of the osmiophilic crest suggests that it may have a role in microtubule synthesis. However, its persistence in the mature gametes after microtubular elongation has ceased, and its banded substructure, reminiscent of flagellar roots, perhaps indicate that its function is mainly mechanical in holding the microtubular band together. Approximately oval in shape and overlain by less than 50 short microtubules initially, the lamellar strata and subjacent mitochondrion rapidly increase in length. Eventually they form a strip 15-20 µm in length overlain by over 300 microtubules. This extensive microtubular band in Equisetum is more likely related to the final shape of the nucleus in the mature gamete than to the presence of numerous flagella. The entire MLS now becomes associated with the nucleus. The microtubular band is closely adpressed to the nuclear envelope and acts as a cytoskeletal framework along which the nucleus undergoes elongation and coiling. Initially the lamellar strip and mitochondrion run along the nuclear envelope with one of their edges touching it and the other projecting into the cytoplasm. However, continuous elongation of the microtubules throughout nuclear metamorphosis results in the gradual separation of the strip and mitochondrion beyond the anterior tip of the nucleus. Simultaneously, the posterior parts of the nucleus become ensheathed by rearward extension of the microtubular band. The centrioles arrange themselves in a single layer on the outer surface of the microtubular band and during the early stages of nuclear metamorphosis give rise to flagella from their distal ends, concomitantly undergoing differentiation into basal bodies. Intense Golgi activity during early and mid-spermatid stages is thought to be related to the accumulation of mucopolysaccharides between the cell wall and cell membrane. In the mid-spermatids rough endoplasmic reticulum is closely associated with the plastids which later accumulate starch, a characteristic feature of spermatogenesis in archegoniate plants.