A subplacenta da paca (Agouti paca, Linnaeus 1766) / The subplacenta of the paca (Agouti paca, Linnaeus 1766)




The rodents of suborder hystricomorph, in which is classified the Agouti paca, present hemochorial type placentation and develops an unique structure known as subplacenta. In spite of many report of its presence in several species, the possible functions of this structure related to placenta and pregnancy remains speculative, mainly due to the diversity of form, size, localization and lack of experimental studies focusing such a placental structure. In the present study were used eight placentas collected from paca at midle and end stage of gestation in the aim to evaluate their tissue components and organization, anatomical relation of the organ at light and electron microscopy level, associated with cytochemical and immunocytochemical techniques and perfusion of vascular bed. The results showed the subplacenta was localized at apical portion of chorioallantoic placenta of paca, separated from this by mesechimal tissue and inserted in the uterine wall in intimate contact with maternal tissue. The subplacenta consisted of lamellar structures with mesenchimal axis of fetal origin on which, cytotrophoblast and syncytiotrophoblasts layers were organized as epithelial sheets. The interfaces at peripheral portion of subplacenta in contact with maternal tissue were also found populations of multinucleated giant trophoblast cells. These giant cells and syncytiotrophoblast were cytokeratin positives and were found far from the limits of subplacenta, deeply invading the completely damaged endometrium, as did the maternal vessels showing presence of trophoblast cells in their walls devoid of endothelial cells attested by vimentin and cytokeratin immunostaining. These findings suggest the invasive capability of trophoblast cells either through the maternal vascular bed and interstitium of endometrial stroma. However, it was seen a well defined border line of helthy decidualized endometrium near the miometrium that seems to retain the invasive progression of trophoblast cells. In the interlamellar space of subplacenta formed by syncytiotrophoblast, it was found amorphous materials originated from degradation of endometrial tissue retained during the progression and growth of placenta into the uterine wall. In these interlamellar spaces were not found blood vessels, but in the mesenchimal axes were found vimentin positive small vessels, which suggest absence of maternal vascularization inside the subplacenta. The strategical localization of subplacenta upside of chorioallantoic placenta of paca and the ultra structure of cytotrophoblast cells and their strong reaction to PCNA (proliferating cell nuclear antigen) supports these subplacental cells could be the source of trophoblast cells, namely the giant and syncytiotrophoblast. The vascularization of subplacenta evaluated by stained latex perfusion and by microvascular casting seen at scaning electron microscopy clearly showed the absence of maternal vascularization in the subplacenta. The blood supply of subplacenta seems to be exclusively from fetal artery whieems after capilarization in the subplacenta flows to the lobules of main placenta. Such a blood flow remind the portal type circulation, being the second capilarization of venous type vessels coming from the subplacenta inside the lobulules of the main placenta, where the fetal blood could make the metabolic changes to return with enough oxygenation level before join to the fetal vein in the umbilical cord.


placenta subplacenta morphophysiology microvascularização agouti paca agouti paca placenta subplacenta microvascularization morfofisiologia

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