Ultrastructural and biochemical alterations in the livers from chick embryos maintained in shell-less culture

Biogenesis and function of lipolysosomes in developing chick hepatocytes

Proliferation of lipolysosomes is one of the characteristic aspects of embryonic chick hepatocytes. Formation of lipolysosomes is observed in the well-developed trans-Golgi network, with the highest frequency occurring from 11 to 14 days of incubation. The lipolysosomes usually contain a small and electron-dense lipid inclusion; however, during development, they gradually enlarge with an accompanying reduction in the electron density of the inclusion. Lipolysosomes isolated from neonatal chick liver homogenates were mainly composed of esterified cholesterol and showed considerably high activity of lysosomal enzymes. Moreover, the lipolysosome fraction is clearly shown to be a function of intralysosomal lipolysis via acid lipase. This accumulation of esterified cholesterol within lipolysosomes might be attributed to an excessive uptake and conversion of plasma lipoproteins to lipolysosomes. This concept is supported by the appearance of an abundance of coated pits and both "early" and "late" endosomes. The major components of plasma lipoprotein are low density lipoprotein (LDL) and high density lipoprotein (HDL), the cholesterol-rich lipoproteins, whose cholesterol content increases during the last week of incubation when the lipolysosomes quickly enlarge. Plasma lipoprotein particles are produced in the yolk sac epithelium from yolk very low density lipoprotein (VLDL) and transferred via the vitelline circulation to the chick liver. After hatching, when the supply of nutrients from the yolk sac is terminated, lipolysosomes immediately decrease in size and number. The cholesterol and fatty acids released are useful as an energy source and lipid metabolism in general, especially after hatching. Food intake induces the use of and accelerates the disappearance of lipolysosomes. Instead of lipolysosomes, lipid droplets appear and increase in number and size with concomitant increases of triglyceride concentrations in the liver homogenates, suggesting that lipogenesis has begun in the chick hepatocyte.

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.

Effects of calcium deficiency on chondrocyte hypertrophy and type X collagen expression in chick embryonic sternum

Maintenance of chick embryos in long-term culture without their calcareous egg-shell is a useful method for studying the relationship between calcium homeostasis and cell differentiation during skeletogenesis. Previously, we have shown that in shell-less (SL) embryos, calcium deficiency induces a cartilage-like phenotype in osteogenic tissues, such as calvaria (Jacenko and Tuan [1986] Dev. Biol. 115:215). In this investigation, we have studied the relationship between cartilage calcification and hypertrophy, and the expression of type X collagen, a specific product of hypertrophic chondrocytes. For this study, the cephalic (calcifying) and caudal (permanently cartilaginous) regions of sterna from day 18 and day 20 normal (NL) and SL embryos were metabolically labeled with [14C]-proline. Analysis of the biosynthetic products revealed significant differences in type X collagen expression in the cephalic region of sternal cartilage. In NL tissues, type X collagen production increased from 13.1% of total collagen at day 18 to 43.7% at day 20. In contrast, in SL embryos, type X collagen was not detectable until day 20, when it represented only 1% of total collagen. Comparison of the NL and SL embryos with respect to their serum calcium level and sternal calcium content and histology revealed a direct relationship between low systemic calcium and limited cartilage hypertrophy, undermineralization, and decreased type X collagen production in the sternal cephalic cartilage. Supplementation of CaCO3 to SL embryos increased their serum and sternal calcium, and restored cartilage hypertrophy, mineralization, and type X collagen synthesis in the cephalic portion of the sterna. These findings confirm that a critical relationship exists between calcium homeostasis, chondrocyte hypertrophy, mineralization, and type X collagen synthesis in the cephalic region of sternal cartilage. These results further demonstrate the importance of calcium in the morphogenetic events of endochondral ossification, in particular the transition from hyaline cartilage to hypertrophic cartilage, and eventually to bone.

Failure to turn eggs during incubation: effects on embryo weight, development of the chorioallantois and absorption of albumen

Turning eggs during incubation is essential for good hatchability. In the present paper additional effects on the development of the chorioallantois, absorption of albumen and growth of the embryo are recorded. The ability of an unturned egg to hatch was not affected by egg weight, egg shell porosity or water loss during incubation. The ability of the chorioallantois to spread around the inner surface of the inner shell membrane and the degree of absorption of the residual albumen affected the growth of the embryo and its ability to hatch. Unturned eggs hatched later than eggs which were turned throughout incubation.