FORMATION AND ACCUMULATION OF LIPOLYSOSOMES IN DEVELOPING CHICK HEPATOCYTES

Formation and accumulation of lipolysosomes in developing chick hepato cytes were investigated by means of electron microscopy in combination with biochemical analyses of the lipid composition in liver homogenat es. The lipolysosomes occurred with highest frequency from days 11 to 14 of incubation. They were usually small and electron-dense, but duri ng development they gradually enlarged with an accompanying reduction in electron density. Coinciding with this enlargement was an accumulat ion of esterified cholesterol in the liver homogenates. After hatching , an immediate decrease in the size and number of lipolysosomes occurr ed along with a reduction in the concentration of esterified cholester ol, of which only a very small amount remained by 9 days of age. Inste ad of cholesterol, triglycerides subsequently increased in concentrati on and accounted for the major lipid content of the liver homogenates. In keeping with the ultrastructural changes, the total volume of cyto plasmic lipid droplets rapidly increased with increasing age. This tra nsient accumulation of esterified cholesterol within lipolysosomes may be attributed to an excessive uptake and processing of plasma lipopro tein particles, probably derived from the egg yolk. This concept is su pported by an abundance of coated pits, endosomes and multivesicular b odies in the embryonic hepatocytes.