GROWTH AND DIFFERENTIATION OF CYTOPLASMIC MEMBRANES IN THE COURSE OF LIPOPROTEIN GRANULE SYNTHESIS IN THE HEPATIC CELL

The synthesis of "very low" density lipoprotein in liver cells is characterized by the fact that the synthesized products, mostly triglycerides, are processed in the form of discrete, size-limited granules or globules, about 400 A in diameter. The present investigation has been made possible in part by the use of a fixative (OsO4 in bidistilled H2O at pH 6.0, in the absence of electrolytes) particularly effective in preserving cytoplasmic membranes and lipids, and giving them high stainability and differential contrast. Under these technical conditions, the lipoprotein granules retain their morphology and high density to electrons practically unaltered, and may serve as tracers in determining their route of transport from the sites of synthesis, starting at the rough-smooth ER junctions, to the lumen of Golgi concentrating vesicles. From the observations, it may be deduced that, along with lipoprotein granule synthesis and transport, there are also production and transfer of new membranes in the form of tubular extensions of smooth ER network which, by progressive fusion and coalescence, participate in the elaboration of fenestrated plates and solid Golgi sacs. In contradistinction to the entire process of liver lipoprotein granule synthesis, transport, and segregation, as reported in the present paper, appears to constitute a developmental sequence which includes the following communicating compartments, in consecutive order: cisternae of rough ER where proteins and possibly phospholipids are synthesized, smooth ER network where triglycerides are synthesized and transported in the form of dense granules, fusion of smooth ER tubular extensions into Golgi fenestrated plates, and further coalescence into solid Golgi sacs, ending in the segregation of the granules in appended concentrating vesicles, or detached "secretory vesicles." It seems that it is this progressive evolution in growth and configuration of membranes which is reflected in the so called polarity, from forming to mature faces, of the Golgi apparatus.

LIPOPROTEIN GRANULES IN THE CORTICAL COLLECTING TUBULES OF MOUSE KIDNEY

The light and, to a lesser extent, the dark cells of the cortical collecting tubules in mouse kidney contain a great number of granules which according to histochemical tests are composed of phospholipids and proteins. These granules are bounded by a triple-layered membrane measuring approximately 75 A across, and contain one or several crystals with a hexagonal or square lattice. These crystals are built up of rod-shaped units, which appear dense after osmium fixation, measure about 48 A in diameter, and are separated by a light interspace of similar dimensions. The mean center-to-center distance of the rods is about 96 A. The structure is explained as a lipoprotein crystallized within a membrane-bounded vacuole. No relationship between these granules and mitochondria was found. The physiological significance of the granules remains unknown.

The intracellular localization of adenosinetriphosphate metabolism in skeletal muscle

The metabolism of adenosinetriphosphate (ATP) in skeletal muscle can be considered from two aspects. On the one hand there are anaerobic and aerobic systems concerned with the production of the high-energy phosphate bonds of ATP, whereas on the other hand this chemical energy is converted into mechanical work in the highly specialized contractile system. These phases in ATP metabolism show precise intracellular localization. Anaerobically the ATP is produced by soluble enzyme systems in the sarcoplasm, whereas the main aerobic source of supply is to be found in the mitochondria, the large granular components of skeletal muscle (Chappell & Perry 1952, 1953). The mechanism of aerobic ATP production in muscle follows the general pattern which has already been demonstrated in other tissues such as liver, but the utilization of ATP by the myofibrils is a process which is characteristic of muscle. It is generally conceded that the energy required for contraction is derived from the splitting of ATP. Within the skeletal muscle cell there are two main sites of adenosinetriphosphatase (ATPase) activity, namely, the myofibrils (Perry 1951) and the lipoprotein granules of the sarcoplasm (Perry 1952 a ). The ATPase of the granules is dominantly magnesium activated, and although the role of this enzyme in intracellular function is not at all clear, that associated with the mitochondria may be in some way concerned with the phosphorylating function of these particles. Certainly the ATPase activity of granules with oxidative activity which can be isolated from pigeon breast muscle (Chappell & Perry 1953) is very similar to that of mitochondria obtained from liver.