|Woodchuck or groundhog (Marmota monax)|
CC Wikimedia Commons
|Interhaemal region in the labyrinth of a woodchuck placenta|
Marmota monax Courtesy of Dr. Allen C. Enders
|Early Development of the placenta in the Colorado chipmunk|
Tamias quadrivittatus Courtesy of Dr. Allen C. Enders
Later the fetal mesoderm grows into the trophoblast bringing with it the fetal capillaries. To start with the outgrowths are fingerlike (villi) as can be seen in a recent publication on the woodchuck (here). Nearer term, however, the labyrinth occupies most of the depth of the placenta. The spongy zone is then very thin and occupied by syncytiotrophoblast with clumps of nuclei as well as mononucleate giant cells (Dr. Allen C. Enders, personal communication).
Syncytins are endogenous retrovirus envelope genes (previous post). Two occur in murid rodents and another in South American hystricomorphs. Now a search of the genome of the thirteen-lined ground squirrel has turned up several candidates and further work in the woodchuck has shown one of them to be a bona fide syncytin. By in situ hybridization the gene was not expressed in the labyrinth but rather in the part of the spongy zone that had yet to be reached by the fetal vessels.