Formation and accumulation of lipolysosomes in developing chick hepatocytes

Formation and accumulation of lipolysosomes in developing chick hepatocytes were investigated by means of electron microscopy in combination with biochemical analyses of the lipid composition in liver homogenates. The lipolysosomes occurred with highest frequency from days 11 to 14 of incubation. They were usually small and electron-dense, but during development they gradually enlarged with an accompanying reduction in electron density. Coinciding with this enlargement was an accumulation of esterified cholesterol in the liver homogenates. After hatching, an immediate decrease in the size and number of lipolysosomes occurred along with a reduction in the concentration of esterified cholesterol, of which only a very small amount remained by 9 days of age. Instead of cholesterol, triglycerides subsequently increased in concentration and accounted for the major lipid content of the liver homogenates. In keeping with the ultrastructural changes, the total volume of cytoplasmic lipid droplets rapidly increased with increasing age. This transient accumulation of esterified cholesterol within lipolysosomes may be attributed to an excessive uptake and processing of plasma lipoprotein particles, probably derived from the egg yolk. This concept is supported by an abundance of coated pits, endosomes and multivesicular bodies in the embryonic hepatocytes.