Intercellular relationships during cavitation of aggregates of extraembryonic endoderm cells from gastrulating chick embryos

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Abstract
Extraembryonic endoderm cells from gastrulating chick embryos undergo epiboly and change from a multilayered cell group to a single cell layer surrounding the yolk. Single cell suspensions from this cell layer can aggregate in vitro to form aggregates that cavitate. To study the stages of cavitation aggregates were harvested after different times in culture, and fixed and processed for light and electron microscopy. In aggregates harvested at 75 min of culture cell contact consisted of areas of parallel and close membrane apposition and interdigitation. Desmosomes were occasionally observed. Aggregates in the early stages of cavitation (24 h) contained numerous intercellular spaces bordered by irregularly shaped cells which appeared to be digesting their yolk and releasing material extracellularly. Long cytoplasmic projections were extended into these spaces. In addition to regions of parallel membrane apposition and interdigitation, desmosomes and adherens junctions were observed. Cells closer to the periphery of the aggregates displayed fewer cell projections and also showed signs of release of material extracellularly. After 48 h of culture, a single smooth-walled central cavity was present and cells still exhibited signs of extracellular release of material. These same cell shapes and intercellular junctions were also observed when area opaca tissue dissected from gastrulating embryos was examined. Aggregates of different sizes were created and cultured. The results suggest that a critical tissue mass may be important for cavitation.