Cell differentiation in microsporangia of Pinus sylvestris. III. Late pachytene

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Abstract
Cycles of hyperactivity were observed in tapetal and microspore mother cells of Pinus sylvestris L. during the pachytene stage of meiosis in microspore mother cells. Hyperactive periods were characterized by dilated rough ER, hypersecretory dictyosomes, autophagic vesicles having one to two sequestration envelopes, and maze‐like whor‐les of the endomembrane system. The extent and sequence of differentiation differed between the two cell types. During distinct phases of development there was either fi‐brillar flocculent or lipoidal material or both within dilations and at cell surfaces. Tapetal cell transfer of material involved endocytotic and exocytotic vesicles and channels opening directly to the cell surface. Dilations of the nuclear envelope of microspore mother cells in late pachytene intruded into the nucleus and, in conjunction with dilated ER, dominated cell profiles. Cellular morphogenesis in a microspo‐rangium was seldom synchronous except for intervals of dedifferentiation when plas‐modesma‐like connections were formed between tapetal cells. Cycles of differentiation and dedifferentiation were correlated each spring season for five years with the progressive change of pachytene chromosomes, suggesting control by a genetic program rather than annual variations in the environment.