Participation of endodermal epithelial cells on the synthesis of plasma LDL and HDL in the chick yolk sac

Abstract

We ultrastructually examined the chick yolk sac endodermal epithelium and evaluated our findings in combination with the biochemical analysis of serum and yolk lipoproteins. Twenty‐five to 30 nm‐sized particles were demonstrated to be a principal element of the extracellular yolk mass and these were determined to be yolk very low density lipoprotein (VLDL). The particles were shown to be taken up by the epithelial cells via coated pits and engulfed by plasma membrane invaginations together with yolk subdroplets, another element of the yolk mass. Through apical vacuoles, the two yolk elements were incorporated into yolk drops, which were identified to be one of the lysosomal structures by a cytochemical procedure using acid phosphatase (AcP)ase activity. During the last week of incubation, which is the final third of the incubation period, the digestion seemed to progress rapidly in the yolk drops, which came to resemble lipolysosomes; lipoprotein production became active as expressed by an enlarged Golgi apparatus. The newly produced lipoprotein particles were electron‐lucent and irregular in size (50–120 nm). They were sequestered in secretory vacuoles and secreted from the vascular surface of the epithelial cells. Finally, the particles were thought to be taken into the vitelline circulation as plasma lipoproteins. The major component of lipoprotein in serum was determined to be low density lipoprotein (LDL) and high density lipoprotein (HDL), while cholesterol content was found to increase during incubation. We concluded that endodermal epithelial cells participate the synthesis of plama LDL and HDL. For this synthesis the cells probably apply lipids and apo‐protein generated from yolk VLDL degradation. © 1996 Wiley‐Liss, Inc.

The Developing Chicken Yolk Sac Acquires Nutrient Transport Competence by an Orchestrated Differentiation Process of Its Endodermal Epithelial Cells

“…lipoproteins) for targeting to the embryo. Although a participation of the YS EECs in the synthesis of lipoproteins has been implied previously (35), proteins required for lipoprotein production…”

“…Furthermore, in functional terms, this finding complements data from electron microscopy studies showing that the EECs from the area vasculosa, but not from the area vitellina, contain numerous coated pits and canaliculi at their apical plasma membrane (8). Recently, based on gene expression profiling (9) and ultrastructural studies (35), secretion of serum proteins and lipoproteins by the chicken YS endoderm has been proposed but was not analyzed by biochemical means. Studies by Farese et al (47), using embryonic-lethal apoB knock-out mice, concluded that apoB plays an essential role in the transport of lipids via secretion of apoB-containing lipoproteins; however, again, lipoprotein synthesis and secretion were not documented by direct evidence.…”

Lipoprotein Receptors in Extraembryonic Tissues of the Chicken

“…The yolk sac has previously been implicated as the major source of lipoproteins and, in particular, the site of cholesterol esterification, in the embryonic circulation (4, 33, 47). However, studies on the biosynthetic capacity of EECs have not addressed the composition and nature of the protein moieties of secreted lipoproteins (27,48). Previous studies have reported the presence of apoB mRNA in yolk sac (49).…”

Acyltransferase activities in the yolk sac membrane of the chick embryo

“…The results described in the present study may help to define the biochemical processes that link the absorptive events at the apical surface with the subsequent secretory events at the basal surface of the endodermal cells. There is evidence that, as a result of fusion with lysosomes, the large lipid-rich vacuoles formed by the phagocytotic process are converted to socalled lipolysosomes in which the hydrolysis of the vacuolar contents proceeds at a high rate, catalyzed by lysosomal lipases and proteases (32). We propose that the products of this hydrolysis (free fatty acids, partial glycerides, glycerol, free cholesterol, lyso-PL, and the like) diffuse out of the large vacuoles and are transported to the endoplasmic reticulum where reesterification to synthesize TAG, PL, and CE is rapidly achieved by the very high activities of MGAT, GPAT, DGAT, acyl-CoA:acyltransferase, and presumably of the other enzymes required for complex lipid synthesis.…”