Quantitative Ultrastructural Data of Secretory Duct Epithelial Cells in Rhus toxicodendron

Abstract

An analysis of the epithelial cell ultrastructure of secretory ducts in the primary phloem of Rhus toxicodendron was performed quantitatively using stereological techniques. The quantitative parameters were volume densities and absolute volumes per cell of nine epithelial cell components. The linear dimensions, individual volumes, and total numbers of plastids, mitochondria, peroxisomes, and Golgi zones were also calculated. In addition, the surface densities of membranes were calculated for plastids, mitochondria, and rough ER (RER) and smooth ER (SER). Parameters of the Golgi apparatus and frequencies of cytoplasmic, plastid, and mitochondrial ribosomes, as well as the frequencies of plasmodesmata, were also determined. Four stages of epithelial cell development were recognized, i.e., procambial, RER, SER, and postsecretory stages. Epithelial cells begin secretion at the procambial stage, together with the differentiation of the first elements of the protophloem and protoxylem. At this stage they contain well‐developed SER and RER and a hypersecretory Golgi apparatus. The RER stage is marked by the proliferation of the RER cisternae and an increase in the number of dictyosomes and Golgi vesicles. Tubular SER becomes a dominant cell component at the next stage. The surface area of its membranes per cell is 6.5 times larger than that of membranes of plastids and mitochondria combined and similar to that of steroidogenic animal cells. There is also a proliferation of peroxisomes at this stage. The leucoplasts of epithelial cells appear to be blocked in their development. At the postsecretory stage, the epithelial cells become highly vacuolated and lose any signs of secretory activity. I conclude that the RER and Golgi apparatus are involved in glycoprotein secretion, which occurs by means of exocytosis of large granular vesicles. The SER is the main component responsible for terpene synthesis and intracellular transport. Peroxisomes are proposed to be involved in the regulation of terpene synthesis, which is most active in the third stage. Plastids do not appear to participate actively in the secretory processes. Before segregation within the canal, secretory glycoproteins and terpenoids are accumulated in the extraplasmic phase of the cytoplasm, i.e., in the intermembraneous spaces of the Golgi apparatus, ER, mitochondria, and plastids, rather than in the protoplasmic phase (cytosol and organelle’s matrix). The synthesis of glycoproteins and terpenes occurs at all three stages of secretion, though the rate of glycoprotein production is highest at the second stage, while terpene synthesis is maximal at the third stage.