Notes on the Heterogeneity, Circulation, and Modification of Membranes, with Emphasis on Secretory Cells, Photoreceptors, and the Toad Bladder

Abstract

In considering the origin, intracellular circulation, and turnover of cellular membranes, microscopists, for obvious reasons, focus on the movement of membrane in bulk (Fig. 1) (cf. Bennett, 1969; Novikoff and Holtzman, 1976). They think largely in terms of such established or proposed mechanisms as exocytosis and endocytosis, the formation of secretory granules from sacs and vacuoles of the Golgi sacs, and the degradation of membrane within lysosomes (see legend to Fig. 1 for details and references). From morphologic evidence, and related cytochemical and autoradiographic findings, one can make a fairly persuasive case that membranes, such as those surrounding secretory structures, or even plasma membranes: (1) originate as membranes by growth or budding from the endoplasmic reticulum (ER); (2) circulate in the cell as morphologically intact entities (although, in microscopically or cytochemically detectable aspects, such as thickness, they may be modified by the Golgi apparatus or other organelles) (3) are degraded to small molecules by bulk processes such as incorporation in autophagic vacuoles or multivesicular bodies, perhaps after complex cycling among cellular compartments (see Fig. 1 and the recent studies on the involvement of the Golgi apparatus in membrane recycling by Herzog and Farquhar, 1977, and Farquhar, 1978). Such schemes account reasonably for many observations, especially when one adds to them the likelihood that membranes of certain cellular structures—notably the inner compartments of mitochondria and plastids—may be assembled locally, rather than imported from the ER.