Accessory Placental Structures—A Review

Abstract

Many studies have reported on the different types of placenta in different species, but accessory or paraplacental structures are mentioned only in species that have such a nutrient-exchanging mechanism. Hystricomorph rodents possess a subplacenta, swine and equines have the placental areola, equines have the chorionic girdle, and carnivores have hemophagous organs that are equivalent to the placental hematoma in ruminants. These accessory structures are specialized tissues for specific nutrient exchanges in different species, and they are adaptive modifications exhibited by domestic and wild species to remedy the nutritional deficiencies that are related to permeability of the main placenta to important nutrients for the growth and development of the fetus during the gestation.

Trophoblast of domestic and companion nimals: basic and applied clinical perspectives

“…Along the uterine gland openings, simple structures called areolae develop in association with each conceptus, which increase during gestation until approximately day 70 (Olio et al, 2014). Areolae are characteristic of epitheliochorial placentas and in pigs large numbers of these are present.…”

Fertilization and Embryology

“…In a later review Minot noted, "the chorion is separated by a dense forest of villi from the decidua, … the ends of some of the villi touch and are imbedded in the decidual tissue … the decidua is plainly divided into two strata … the section passes through a wide tube, … which opens directly into the interior of the placenta and contains blood … this opening is … a vein …" (Minot 1903). He also was the fi rst to use the term "subplacenta," (in this case for the rabbit placenta [Minot 1889]) to refer to the zone of the chorion between the placental disc and the decidua basalis (Olio et al 2014). These observations did much to explain the nature of the fetal-maternal (i.e., placental) barrier.…”