The Fine Structure of the Pre-Meiotic Stages of Sporogenesis in Onoclea sensibilis

Abstract

Ultrastructural features of the dividing meiocyte of Onoclea sensibilis L. were described by the author (1977), and those of the young spores arising from this cell by Marengo and Badalamente (1978). Since a light microscopic study of this species by the author (1949) showed an apparent decrease in size of cytoplasmic inclusions from the sporangium initial to the spore mother cell, it is of interest to elucidate these changes on the ultrastructural level. Individual sporangia dissected from young fertile fronds were routinely fixed in glutaraldehyde followed by osmium tetroxide and embedded in Epon (Spurr, 1969). Sections 0.5um thick were cut from individual sporangia and were examined by phase contrast microscopy without staining. From sporangia identified as containing pre-meiotic stages in sporogenesis, thin sections were cut with a diamond knife, stained with uranyl acetate and lead citrate, and examined with an Hitachi HU11A electron microscope. The sporangial initials (Fig. 1) appear to contain large, osmiophilic bodies (Fig. 1, t) which resemble closely the tannin accumulations described by Ledbetter and Porter (1970). The appearance of vacuoles (v) within these bodies is followed by their disappearance in the later stages of sporogenesis, including newly formed spores. Mitochondria (m) appear hardly larger than those in later stages and in newly formed spores. Their internal structure, however, differs from that of mitochondria in newly formed spores. Cristae appear not to be shelf-like, but rather mitochondrial material separated by internal cavities. This peculiar and distinctive internal structure persists through at least the early prophase of meiosis (Fig. 6,m), following which their cristae extend as plates, continuous across the mitochondrion (Marengo, 1977). The osmiophilic bodies are still present in early sporogenous cells (Fig. 2,t), with an occasional vacuole apparently replacing the tannin mass (Fig. 2, v). By the time the sporogenous cells approach the rounded-up meiocyte stage, the osmiophilic bodies have disappeared and lipid droplets appear in the cytoplasm (Fig. 3,L). In the dividing sporogenous cell in anaphase (Fig. 4), the mitochondria appear concentrated in the polar regions of the spindle. The distribution of mitochondria at mitotic anaphase appears to contrast markedly with that of the meiotic anaphase I, where the mitochondria are concentrated at the edge of the spindle preparatory to invading it, and form the mitochondrial plate characteristic of interphase following the first division as well as all phases of the second meiotic division (Marengo, 1949, 1977). In the early prophase of the first meiotic division (Fig. 5), spore mother cell polarity is reflected in a concentration of mitochondria at the base of a barely