Placentation and fetal membrane development in the South American coati, Nasua nasua (Mammalia, Carnivora, Procyonidae)

Background Placental research in carnivores has concentrated on domestic species, which have zonary, labyrinthine placentas with an endotheliochorial barrier. Although the coati, Nasua nasua, is a widely distributed species in South America, data on the development of the placenta and the fetal membranes in this species are very sparse. Findings Four placentas from mid-gestation to near term were collected from wild individuals and were investigated based on gross morphology, histology, immunohistochemistry and electron microscopy. The available data support the concept that the ancestral condition of placentation in carnivores is phylogenetically characterized by a zonary and labyrinthine placental type with an endotheliochorial fetomaternal barrier, comprising extended epitheliochorial and haemochorial zones, such as hemophagous organs for iron supply and histiotrophe uptake and a yolk sac placenta. Conclusions Because of the foundational mechanisms that lead to the considerable complexity of fetomaternal contact zones in carnivores have not been studied, carnivores are interesting animal models for interhaemal barrier differentiation.


Background
Carnivores are regarded as having zonary, labyrinthine placentas with an endotheliochorial barrier. In addition, specialized areas are common, such as hematomas or hemophagous organs for the uptake of uterine secretion or the phagocytosis of extravasated maternal blood components, serving as mode of iron supply [1][2][3]. However, placental research in carnivores has mainly concentrated on domestic dogs and cats [4][5][6][7][8][9][10][11], rather than other species (e.g., [12][13][14]). Within Musteloidea, the mink, ferret and sea otter have been investigated [15][16][17][18], in addition to the raccoon and certain other species, including the coatimundi Nasua narica [19][20][21], ( Figure 1A). Because more information, especially on wildlife species, is essential to better understanding the evolution of placentation in carnivores, we herein describe this system in the South American coati Nasua nasua (Linnaeus, 1766), which is native to southern South America, for the first time [22]. Coatis are omnivorous, medium-sized animals with a body mass of approximately 6 kg [21,22]. These animals live in groups organized within a complex social system consisting of up to 70 individuals. Two different groups are usually present; one composed entirely of adult males and a second group consisting of an alpha female (dominant), adult females, and young [23,24]. Females reach sexual maturity at 2 years of age, and males do so at approximately 3 years. The mating season usually occurs in late spring, from September to November. This is the only time when adult reproductive males meet with female groups [22][23][24]. After 10-11 weeks of gestation, 2-6 very altricial young with a body weight of 100 to 180 g are born [21,22]. Conclusions about the evolutionary course of the characters have been drawn from comparative analyses and analyzed within the relationships of carnivoran groups ( Figure 1A), as indicated by recent morphological and molecular phylogenetics [25,26]. We use the terminology of phylogenetic systematics [27] in differentiating between derived character conditions and ancestral ones, with special reference to ancient conditions or the stem species patterns of Carnivora. In total, four placentas from mid-gestation (n = 2) to near term (n = 2) were collected from 2 wildlife individuals captured in Mangabeiras Park in Belo Horizonte-MG, Brazil. Hemi-ovariohysterectomy was performed following surgical and anesthetic protocols used for domestic carnivores [28]. Measurements of the occipital-sacral distance of the fetuses' heads were performed with a stainless-steel caliper, using the nuchal crest at one end and the last sacral vertebra at the opposite end as references (crown-rump, CR), following the methodology proposed by Evans and Sack [29]. The CR distances and the external features of the fetuses were used to estimate the age of each individual. The weight (g) was determined using a digital scale (0.0001 g; BEL Engineering).

Macroscopy of the fetuses
The two Nasua nasua offspring displayed CR lengths of 6.3 cm and 6.5 cm at mid-gestation and weighed 21.37 g and 23.88 g. Thus, their estimated age was 37-40 days of pregnancy. Near term (estimated age of 50-53 days of pregnancy), the offspring displayed CR lengths of 8.9 cm and 9.5 cm and weighed 33.61 g and 35.28 g. In both stages, the individuals had a body with distinct cranial, thoracic, and abdominal regions. An elongated and curved tail was also a feature observed in these stages of pregnancy. The forelimbs and hindlimbs were completely formed and presented keratinized claws on the fingers ( Figure 1B). On the face, well-formed upper and lower lips were identified. The eyes displayed pigmented retinas. The nasal region was elongated, with several sensorial hair follicles, and the external ear was short but evident ( Figure 1C). In addition, the entire body of the fetuses was covered by hair that was short, with dark pigmentation in certain regions, including the face, forelimbs and hindlimbs, and tail ( Figure 1B,C).

The placenta and fetal membranes
The chorioallantoic placenta was fully zonary and surrounded each fetus in the abdominal region ( Figure 1D).
Outside the girdle, the trophoblast of the chorioallantoic membrane was apposed to the vascularized, gland-rich uterus, which had an intact, cubic epithelium ( Figure 1E).
The placental girdle in Nasua nasua had a prominent, lamellar labyrinth that was endotheliochorial in nature. Each lobe was supplied by central maternal and fetal vessels in a crosscurrent arrangement (Figure 2A,B). The mesenchyme of the fetal vessels was strongly reactive for vimentin, and it was possible to delimit the maternal and fetal blood systems in the labyrinthine zone ( Figure 2C). In contrast, the trophoblast near the maternal vessels was positive for cytokeratin ( Figure 2D). The trophoblastic cells close to the fetal blood vessels were especially proliferative, including in the branches of the chorioallantoic membrane near the placental girdle in mid-gestation ( Figure 2E,F). Both cellular and syncytiotrophoblasts were frequent in mid-gestation ( Figure 2G,H). Near term, the barrier was mainly syncytial and thin, but clustered cytotrophoblasts were present near the fetal vessels ( Figure 2B). At that point, the endothelium of the maternal vessels was more hypertrophied than in mid-gestation ( Figure 2H). In addition to the main placenta, the coati had a prominent, sac-like, orange area, vascularized by vessels from the umbilical cord that persisted until term ( Figure 3A,B). Based on a centrally situated area of destruction allowing leakage of maternal blood, the area emanated into the allantoic cavity ( Figure 3B) and was distinct from the labyrinth ( Figure 3C). Iron deposits ( Figure 3D) indicated a haemophagous nature. Inside, branched, vascularized villi were present, bathed in extravasated maternal blood ( Figure 3E). The trophoblast was columnar, with large nuclei, apical vacuoles, and liquid droplets that stored ingested erythrocytes ( Figure 3D,E).
The amnion in the coati included large amounts of yellow-pigmented liquids ( Figures 1D, 3G) and was avascular in mid-gestation but vascular near term due to fusion with the allantois ( Figure 3H). The yolk sac was surrounded by the amniotic membrane ( Figure 3G) and had a cubic epithelium composed of the endoderm, with iron deposits (Figure 3H,I). Numerous, large vitelline vessels were present, with blood cells forming vascular islets ( Figure 3H,I).
Regarding the fine structure of the fetomaternal interface, the following characters are widespread among carnivores and seemed to belong to their stem species pattern or ancestral pattern: an endotheliochorial barrier in the zonary or circumferential placenta, in addition to a yolk sac and a large allantoic sac that persist until term. In contrast, the coati and other procyonids were derived from the ancient carnivore condition, maintaining the cellular trophoblast to term [19][20][21].

Conclusions
The data on the coati supported previous views on the ancestral placental characters of carnivores; the maintenance of cellular trophoblasts in the barrier and the large, central hemophagous organ that persisted until near term were confirmed. The ancestral pattern of carnivores includes not only an endotheliochorial, labyrinthine placental girdle, but also extended epitheliochorial and hemochorial zones and placental function for the yolk sac. This considerable complexity of fetomaternal contact zones must have evolved before the radiation of carnivores, approximately 65 million years ago [25,26]. It is currently unclear what types of tissue-specific inflammation patterns and associated molecular mechanisms are involved in establishing specific fetomaternal contact in various regions. Thus, carnivores are interesting animal models for studying the full range of interhaemal barrier differentiation and function within an individual species. The coati may be of special interest to further study persisting hemophagous organs to better understand the nature and functional significance of substance transfer in the placental areas.