Outer egg coats of the marsupial conceptus: secretion and protein composition

Little is known of the composition of the outer egg coats. We aimed to quantify secretion during embryonic development, identify precursor secreting cells and investigate protein composition. The study was based on 259 egg coats and 14 reproductive tracts of 104 T. vulpecula undergoing natural and induced cycles and 341 coats from 35 Sminthopsis macroura undergoing natural cycles. Following PAGE, Western blotting, and amino acid sequencing of egg coats, the short peptide sequences obtained from T. vulpecula and S. macroura coats were found to be dissimilar to each other and to any known protein. However, in T. vulpecula, S. macroura coat polyclonal antibody cross-reacted with coat precursors, suggesting similar epitopes, and showed mucoid precursors in secretory cells in oviduct epithelia and shell precursors in glands in the utero-tubal junction and uterus. Immuno-electron microscopy located shell coat precursors in various previously unidentified cell types, including pre-ovulatory apoptotic cells, early post-ovulatory holocrine cells, and milk-producing cells, found at blastocyst stages. Ultrastructural and quantitative volumetric analysis of the intact shell coat suggested a second wave of secretion at the blastocyst stages in T. vulpecula. Despite differences in protein composition, it was concluded that marsupial egg coats are homologous to each other because of similarities in ultrastructure and time and location of secretion.