Abstract
Studies of receptor-mediated lipoprotein metabolic pathways in avian species have revealed that physiological intricacies of specific cell types are highly analogous to those in mammals. A prime example for the power of comparative studies across different animal kingdoms, elucidated in the chicken, is that the expression of different lipoprotein receptors in somatic cells and oocytes are the key to oocyte growth. In avian species, yolk precursor transport from the hen's liver to rapidly growing oocytes and the subsequent transfer of yolk nutrients via the yolk sac to the developing embryo are highly efficient processes. Oocytes grow from a diameter of 5 mm to 2.5-3 cm in only 7 days, and the yolk sac transfers nutrients from the yolk stored in the mature oocyte to the embryo within just 2 weeks. The underlying key transport mechanism is receptor-mediated endocytosis of macromolecules, i.e., of hepatically synthesized yolk precursors for oocyte growth, and of mature yolk components for embryo nutrition, respectively. Recently, the receptors involved, as well as the role of lipoprotein synthesis in the yolk sac have been identified. As outlined here, lipoprotein degradation/resynthesis cycles and the expression of lipoprotein receptors are not only coordinated with the establishment of the follicular architecture embedding the oocyte, but also with the generation of the yolk sac vasculature essential for nutrient transfer to the embryo.
Highlights
The most astounding events in the reproduction of oviparous species are the growth and develop ment of the female germ cell
Subsequent to uptake and degradation of lipoproteins and other macromolecules from the yolk, the epithelial cells (EECs) synthesize lipoproteins de-novo that differ in composition from those in yolk and secrete them for targeting to the embryonic circulation
We have previously reported that the yolk sac (YS)’s EECs express LR8, which is capable of internalizing ApoB-containing lipoproteins such as very low density lipoprotein (VLDL), as well as VTG and clusterin[31], expanding the list of receptors involved in the uptake of yolk precursor macromolecules into the EECs
Summary
The most astounding events in the reproduction of oviparous (egg-laying) species are the growth and develop ment of the female germ cell. Triglyceride-rich particles, the likely physiological substrate for mammalian VLDLRs, could be transported into metabolically active tissues (such as muscle, where receptors are abundant), while in avian oocytes the uptake of VTG, VLDL, and other hepatically synthesized yolk precursors by LR8 provides nutrients and energy for the developing embryo.
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