Structure of anchoring villi and the trophoblastic shell in the human, baboon and macaque placenta

Anchoring villi of first trimester placentae of the macaque, baboon and hum an were examined by light and electron microscopy. The anchoring villi of t he baboon and macaque are similar in having mole elongated cell columns tha n those of the human and in having more extracellular matrix between cytotr ophoblast cells. These species also have a thicker and more uniform trophob lastic shell. The generative region of cytotrophoblast cells adjacent to th e villous mesenchyme is similar in all three species, with the aspect of th e core abutting this area being lined bq a thickened basal lamina. Similarl y, migratory cytotrophoblast cells form extracellular matrix in all three s pecies, but matrix-rich regions of the anchoring villi and shell nle more e xtensive in the non-human primates. The extracellular matrix and especially the material resembling fibrillin may serve to strengthen the villi, parti cularly the elongated villi of the non-human primate. and also may prevent maternal cells migrating into the trophoblastic shell, The baboon and macaq ue cytotrophoblast cells that form this matrix tend to he linked by gap and desmosomal junctions and are in contiguous arrays, whereas those in the hu man that are blocked from reaching normal decidua form abundant extracellul ar matrix but have no gap junctions. Whether the lack of extensive invasion of the endometrium by baboon and macaque cytotrophoblast cells is related to the increased amount of extracellular matrix. their greater distance fro m the mesenchymal cole, or their intercellular linkages is not known. The i nvestigation of isolated villi from the macaque or baboon, as has been exte nsively carried out in the human, might help to determine whether the cytot rophoblast cells are intrinsically. different or are responding (C) 2001 Ha rcourt Publishers Ltd.