Intermediate trophoblast—A distinctive, unique and often unrecognized population of trophoblastic cells

The trophoblast forms an outer layer of the blastocyst in the developing placenta and fetal membrane chorion. It is composed of different types of cells. Two main cell types are cytotrophoblasts and syncytiotrophoblasts. The third type of trophoblastic cells, often "forgotten" in most of histological and embryological textbooks, is morphologically and functionally between the first and second one, therefore, it is called the intermediate trophoblast. There is no mention of it in the internationally accepted Terminologia Embryologica. This term is not universally used by pathologists as some of them prefer the name extravillous trophoblast. This review provides an overview of morphology, localization, function and immunohistochemistry of different types of intermediate trophoblast cells. An indisputable reason for categorizing these cells as a distinct group is the fact that they are a source of various forms of gestational trophoblastic disease.