Ultrastructure of outer tangential cell wall thickenings and adjacent cuticle and protoplast of epidermal cells.
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TEM images of material postfixed in osmium tetroxide, embedded in SPI 812, and viewed in transverse section. (A) Outer tangential cell wall showing cuticle (cu) and slightly different electron-dense layers demarcated as the remains of the original primary cell wall (cw), thickening of Igersheim (ti), and an electron-transparent zone (et) (enlarged in C) between the wall and the plasmalemma (pl). The boundary between the primary wall and the thickening of Igersheim is not clear because the two layers merge gradually. Note extracellular deposits, probably including sticky subtances, on the cuticle. Visible in the cytoplasm are, among others, mitochondria (mi) and chloroplasts (ch) with starch grains (st). (B) Cuticle pervaded by a branched system of electron-dense fibrillar material, most probably representing microchannels. (C) Electron-transparent layer (compare A) between the plasmalemma and thickening of Igersheim showing several secretory vesicles. These vesicles are often seen to be fused to the plasmalemma and are evidently derived from the cytoplasm through exocytosis. It is suggested that these vesicles contain materials used in the formation of the cell wall thickenings; most probably also sticky secretions (compare Figure 2C, D) exuded from the cuticle surface. In young epidermal cells, and before the formation of wall thickenings (not illustrated), the cytoplasm next to the plasmalemma contains an abundance of rough endoplasmic reticulum; some of the ribosomes are still visible in the image; labels as in A.