Electron microscopic study of adipose tissue (fat organs) with special reference to the transport of lipids between blood and fat cells

In the present study we followed with the electron microscope the changes which the lipids undergo during their transport between the blood and the fat cells in both impletion and depletion of the fat depots. Impletion was studied in tissues from mice on the first or second day after birth, when the storage of fat begins, and from newborn mice and young rats that were refed after periods of starvation. Fat depots in the state of depletion were taken from the starving animals. During impletion the chylomicrons were found to be attached to the endothelium of the capillaries in the fat organs, where they seem to break up into smaller and less electron-dense particles. The absorption of the lipids by the capillary endothelial cells could not be observed, since no osmiophilic material was found within the capillary wall. No such material was present within the interstitial connective tissue between capillaries and fat cell. When the lipids enter the fat cell they again become observable as osmiophilic granules. These lipomicrons seem to represent the absorbed lipid material in transit between the surface of the fat cells and the fat droplets in the cells. Although dispersed throughout the cytoplasm, the lipomicrons were often accumulated at, and attached to, the surface of the growing fat droplets. They lost their identity when they were incorporated into the homogeneous material of the fat droplets. According to these observations, several steps were distinguished during impletion and were localized at the surface of the capillaries, at the plasma membrane of the fat cells, and at the surface of the fat droplet. The findings were discussed in the light of the physiological and biochemical literature concerning the clearance of the chylomicrons from the blood. The static pictures of the fat cells in the depleting state are very similar to those of the impleting cells. Lipomicrons are present, and they seem to emerge from the surface of the diminishing fat droplets. Also in these cells they seem to represent the lipids in transit, but in the reverse direction. At the cell border they disappear, and no osmiophilic material could be discovered within the interstitial tissue or within the capillary wall. Occasionally osmiophilic globules were found within the capillaries of the depleting fat organs, but they did not show the same relationship to the endothelium as the chylomicrons. We concluded from the similarity between the static pictures of depleting and impleting cells that the processes distinguished and localized during impletion are reversed when the cell releases fat. If this is correct, our pictures illustrate the lability of the fat organs, which respond to changing conditions by reversing the processes involved in their twofold activity of absorbing and releasing fat. The lipomicron pool in the fat cell seems to represent the lipids most immediately available either for storage or for release under ordinary nutritional conditions, where such drastic changes as in our experimental material do not occur. During both impletion and depletion the mitochondria were found to be profoundly changed. It is believed that these changes are mainly due to an infiltration of the mitochondria by lipomicrons. However, these observations do not permit us to draw conclusions as to the function of the mitochondria in the storage or release of fat.