On the fine structure of differentiating mucilage papillae of Marchantia

Abstract

In Marchantia, the mucilage papillae are initiated by differential divisions occurring in marginal cells of scales at early stages of their development. Establishment of a new polarity axis was observed in mucilage papilla mother cells as well as in their daughter cells, which were destined to develop into mucilage papillae. During polarization the marginal cells synthesize cytoplasm and free ribosomes; subsequently, the cells grow outwards. Concomitantly, organelles migrate to the apical region, where some rough endoplasmic reticulum (ER) membranes and dictyosomes are polarly placed. The subplasmalemmal microtubules become reoriented during polarization.The spindle-shaped preprophase and prophase nucleus of the mucilage papilla mother cell is surrounded by two distinct microtubular systems: the preprophase band of microtubules and an extranuclear sheath of microtubules; the latter is aligned along the spindle axis, close to the nuclear membrane and convergent at polar areas which contain ER membranes.Among the first structural signs of mucilage papillae differentiation are the following: increase of cytoplasm, free ribosomes, rough ER membranes, and dictyosomes; preferential association of plastids with ER membranes and mitochondria, as well as the transverse orientation of subplasmalemmal microtubules. As differentiation ensues the cells acquire more cytoplasm and their organelles proliferate markedly, while new specific organelle relationships become evident.The increase of rough ER membranes predominates and keeps pace with dictyosome proliferation. Among the organelles the ER membranes display a key role in diverse mucilage papilla differentiation and an intermediary one in their secretory activity.The particularly active secretory phase of mucilage papillae is marked by an hypersecretory activity of dictyosomes which produce abundant vesicles. Histochemical staining revealed polysaccharides in the larger vesicles, the space between plasmalemma and cell wall, and within the wall.Ultimately, the mucilage papillae either undergo a partial degradation of protoplasm or even degenerate. In the former case they undergo an intense vacuolation and finally appear structurally similar to other cells of scales.