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Placentation

Although placental diversity in mammals received growing attention in the 1600s through the early 1800s, placentation was not documented in reptiles until the mid-19th century.

In his classic 1855 study on a viviparous lizard, Cesare Studiati (University of Pisa) described a structural/functional arrangement of fetal and maternal tissues that meets contemporary criteria for recognition of placentation. Through the fortuitous selection of a highly placentotrophic species, Chalcides chalcides, Studiati recognized the functional role of placental tissues in provision of oxygen as well as nutrients.

Although Studiati worked in a pre-evolutionary milieu and without the benefits of histological techniques, his findings revealed that viviparous reptiles could exhibit placental specializations that paralleled those of mammals.

Accordingly, his classic paper initiated a highly productive body of research that has continued to the present and highlighted specializations of a species that has figured importantly in placental research. PMID: 26474917

By the 1890s, placental arrangements had been documented macroscopically in lizards and fishes, but placental studies on such species lagged far behind research on mammals. In 1891, the biologist Ercole Giacomini (at the University of Siena, Italy) published the first histological analysis of a reptile placenta. Focusing on a placentotrophic lizard (Chalcides chalcides) with a morphologically complex placenta, Giacomini documented the histological and cellular bases for placental nutrient transfer and gas exchange. In conjunction with a follow-up study in 1906, he demonstrated that placental structure is correlated with function and can vary dramatically between related species. Giacomini's work was highly influential in showing that placentation in lizards had converged evolutionarily on that of mammals, while establishing reptile placentology as a highly promising area for future research.