placentaltype

Placentaltype is a term used in reproductive biology to describe the pattern and structure of placentation in a species, i.e., how the fetal membranes connect with and interact with the maternal uterus. This classification reflects differences in how the placenta forms, attaches, and interfaces with maternal tissues, and it has implications for nutrient transfer, gestation, and immune tolerance.

The most widely recognized placental types by morphology are:

- Diffuse: chorionic villi are distributed over nearly the entire surface of the placenta. This type is found in horses and pigs.

- Cotyledonary: the placenta comprises multiple localized sites called cotyledons that attach to corresponding caruncles on the maternal side, typical of ruminants such as cows, sheep, and goats.

- Zonary: a band or zone of placental tissue encircles the chorion, seen in many carnivores like dogs and cats.

- Discoidal: a single or few disc-shaped areas of placental contact are formed, as in humans, primates, and many rodents.

In addition to these morphological patterns, placentation is often described by the degree of invasion into maternal tissues, using terms such as epitheliochorial, endotheliochorial, and hemochorial. These terms describe how many maternal tissue layers separate fetal blood from maternal blood and vary across species, influencing nutrient transfer and immune interactions.

Placentaltype thus encompasses both the gross placental architecture (diffuse, cotyledonary, zonary, discoidal) and the histological degree of maternal–fetal tissue contact. It is used to compare reproductive strategies across mammals and to understand species-specific pregnancy biology.