Yolk precursor transport in the laying hen.

The fully grown chicken oocyte, better known as the egg yolk, is a giant single cell that contains, besides bona fide cytoplasm, endocytosed, serum-derived lipoproteins and minor components that are essential for normal embryo development. The transport of bulk lipoproteins, micronutrients and morphogens to oocytes, in parallel with the maintenance of somatic homeostasis, is achieved by the cell-specific expression of receptors and subtle differences in the ligand structure that determine their target sites. Lipoprotein metabolism is the paradigm for these regulatory principles. In this review, the laying hen is described as a model system for intercellular transport that has uncovered new aspects of the biological rationale for the simultaneous expression of closely related genes in a single organism.