ABSTRACT
The yolk sac is the first definitive fetal membrane of vertebrates. During embryonic development, hematopoiesis begins first in the yolk sac. This study was carried out to investigate the ultrastructural characteristics of the early rat extraembryonic visceral yolk sac membrane and hematopoietic cells localized in the yolk sac vitelline vessels. Pregnant albino rats were killed at gestational days 10-12. Yolk sac specimens were processed for light, transmission, and scanning electron microscopic examination. The study demonstrated the microscopic structure of the yolk sac membrane and the different hematopoietic cells in yolk sac vitelline vessels at this stage of gestation. The endoderm cells revealed a cytoplasm rich in cell organelles characteristic of endocytosis, phagocytosis, and active protein synthesis, as well as cell junctions between adjacent cells. The apices of the endoderm cells revealed numerous microvilli of two types and a whitish coat on top of the cells. In yolk sac vitelline vessels, a large number of blood cells were seen. Many cells were at different stages of mitosis. Ultrastructurally, blast cells including immature differentiating erythroblasts were recognized, together with a few mature erythrocytes. Furthermore, mature lymphocytes were also seen in vitelline vessels on gestational day 12. Yolk sac lymphocytes revealed a morphology typical of peripheral blood lymphocytes. This study demonstrated the importance of the yolk sac for adequate embryonic nourishment, as well as different blood elements necessary for potent immune response. The location of mature lymphocytes inside vitelline vessels suggests a possible emergence from the yolk sac membrane. However, future studies are needed to explore the exact origin of these cells
Subject(s)
Female , Animals, Laboratory , Yolk Sac/ultrastructure , Microscopy, Electron , Pregnancy , Rats , Female , Hematopoietic System/physiologyABSTRACT
Este trabalho visou caracterizar macro e microscopicamente o saco vitelino em pacas (Agouti paca) e cutias (Dasyprocta aguti) no início de gestação. Três embriões/fetos de pacas e três de cutias foram utilizados para a análise do saco vitelino, durante as fases iniciais de gestação. Fragmentos do saco vitelino foram removidos do embrião/feto e rotineiramente processados para inclusão em parafina (técnica histológica rotineira) e em resina Spurr (análise ultra-estrutural). Macroscopicamente, a placenta vitelínica em ambas as espécies inseria-se na superfície da placenta principal, com suas margens projetando-se completamente sobre o embrião/fetos. Na microscopia de luz, a placenta vitelínica apresentava-se constituída pelo epitélio endodérmico e um mesenquima com inúmeros vasos vitelínicos. Ultraestruturalmente, a placenta vitelínica visceral da paca era formada por células endodérmicas com núcleos na região mediana e da cutia por núcleos dispostos apicalmente; outra característica foi o grande número de mitocôndrias, vesículas de conteúdo eletrodenso e com microvilosidades. Com base nos resultados concluímos, que (1) a placenta vitelínica das duas espécies apresenta inserção na superfície da placenta principal; (2) a placenta vitelínica de paca se apóia na membrana de Reichert, diferentemente da cutia, que não possui tal membrana; (3) o cório e alantóide apresentam-se fusionados, formando a placenta corioalantoídea; e (4) o saco vitelino em ambas as espécies é invertido e vascularizado.
The study aimed to characterize gross and microscopic features of the yolk sac in paca (Agouti paca) and agouti (Dasyprocta aguti) in early gestation. Fragments of the yolk sac of 3 paca and 3 agouti fetuses at early gestation were taken and processed for histological and ultrastructural analyses. Gross features of the vitelline placenta in both species showed its insertion over the main placenta surface and projections to the embryos/fetuses. Microscopically, the vitelline placenta was constituted by endoderm epithelium and mesenchyme, in which vitelline vessels are abundant. The ultrastructure of the samples showed that the visceral yolk sac of the paca was formed by endodermic cells with nuclei in the median region, and that the visceral yolk sac of the agouti was formed by nuclei arranged apically; other characteristic was the large number of mitochondrias, eletrodense vesicles with microvilosities We conclude that (1) the vitelline placenta of the two species presents insertion in the surface of the main placenta; (2) the vitelline placenta of paca rests on the Reichert's membrane, whereas the agouti vitelline placenta does not have this membrane; (3) the chorion and allantoic are fusioned; and (4) the chorioallantoic placenta and the yolk sac in both species are reversed and vascularized.
Subject(s)
Animals , Anatomy, Veterinary , Rodentia/anatomy & histology , Yolk Sac/anatomy & histology , Yolk Sac/ultrastructureABSTRACT
Con el objeto de conocer las características de las superficies endodérmica y mesotelial del saco vitelino (SV) humano y los cambios que experimenta en función del tiempo, se estudió con microscopía electrónica de barrido (SEM) en combinación con microscopía electrónica de transmisión (TEM) las superficies endodérmica y mesotelial de SV sub s provenientes de embriones humanos de 30-33-42 y 49 días de edad gestacional. Las muestras se procesaron para TEM y SEM y los resultados mostraron: 1. La existencia de densas poblaciones de microvellosidades cubriendo ambas superficies celulares. 2. Que las microvellosidades mesoteliales eran más numerosas, finas y largas que las endodérmicas y, 3. Que no existían cambios significativos en el patrón de distribución de microvellosidades en función del tiempo. Junto con describir por primera vez el SV humano con SEM se discute el posible rol que las superficies endodérmicas y mesotelial puedan jugar en la función del SV