Intimate contact between human yolk sac endoderm and haemopoietic precursor cells

Although the localization of embryonic haemopoietic cells in the endodermal epithelium of the human yolk sac had been discovered several decades ago, the nature and significance of the localization were dubious, and it was supposed that light microscopic pictures represent technical artifacts. Our ultamicrographs now demonstrate that at 16--26 mm CR-length there is an intimate contact between yolk sac endoderm and free haemopoietic precursor cells: apart from interdigitations various types of intercellular contact could be discovered. These contacts, especially the gap-like ones with associated electron dense cytoplasmic areas speak for intercellular communication, i.e., the role of endoderm in early human embryonic haematopoiesis appears likely. The demonstrability of these contacts, however, does not mean that endodermal associations are indispensable for haemopoietic differentiation.