Quantification of endocytosis-derived membrane traffic.

The main data covered by this article have been summarized in Table I. A fairly uniform picture is obtained for endocytosis-derived membrane transfer and compartmentation. This may be due to the limited amount of information and the resulting low resolution. Data on mainly three cell types are presented: macrophages, fibroblasts and amoebae. The data vary as much for one cell type as between different cells. Therefore, no possible differences related to cell function emerge. More detailed data, for more cell types, may change the picture. The values for cell surface area, although significantly different in absolute terms (column S in Table I), are rather similar when related to cell diameter, all being about 3-fold in excess of the surface area of the smooth sphere of comparable volume (column xi in Table I). The rate of plasma membrane internalization for macrophages and amoebae both professional phagocytes, is about 2 cell surface area equivalents per h or more. This may be somewhat higher than for fibroblasts (column PM/h in Table I). The average residence time for membrane on the cell surface, therefore, is about 30 min. A most interesting finding seems to be the rather uniform values obtained for the average size (volume weighted) of primary pinosomes, being about 0.3 micron in diameter (column phi-Internalization in Table I). Due to their rapid increase in size as a result of fusion (cf. Fig. 2), it has not been feasible to directly measure the size of primary pinosomes by morphometric means. The values in Table I, give no information on the size distributions of primary pinosomes and on whether these consist of one or more size classes. The steady-state average diameter of pinosomes is noticeably larger than that of primary pinosomes (column phi-pinosomes in Table I; cf. Table II for Acanthamoebae). The corresponding decrease in surface-to-volume ratio can make about 50% of pinosomal membrane available for recycling directly from this membrane compartment. Membrane recycling from the pinosomal compartment occurs after an average residence time of about 3 min for macrophages and 4-6 min for fibroblasts (column tau-pinosomes in Table I). The relative pool size of intracellular membranes participating in shuttling to and from the cell surface is significantly different for animal cells and amoebae (column rho in Table I). For macrophages, fibroblasts, CHO cells, and mast cells, this intracellular membrane pool amounts to about 10-20% the plasma membrane area, compared to 150-200% in the case of amoebae.(ABSTRACT TRUNCATED AT 400 WORDS)