The role of the gastrointestinal epithelium as a possible pathway for the transfer of nutrients to the embryo's circulation.

The endodermal cells of the human yolk sac (YS) produce non-nucleated erythrocytes (NNEs) and numerous serum proteins that are transiently storage within the YS cavity. After their transfer via the vitelline duct to the embryo gastrointestinal lumen, the nutrients' final fate is unknown. With the aim of investigate how erythroid cells and nutrients are conveyed to embryo circulation, we studied, using a morphological and immunohistochemical approach, the embryo anatomy and the serum protein α-fetoprotein (AFP) presence, in 15 human embryos and their YS, collected from tubal pregnancies from 4 to 8 wpf. We observed at 5 wpf, a strong AFP staining in the endodermal cells of the YS, thereafter AFP was only present in the YS cavity and the gastrointestinal lumen. During 7 wpf, AFP expression declined and disappeared, concomitant with YS regression. Between 5 and 7 wpf, NNEs were observed in the gastrointestinal cavity, where they accumulate in the stomach. Here, the cells were attached to the endodermal epithelial cells or were free in the lumen. By scanning electron microscopy, we identified signs of NNEs phagocytized by endodermal cells. Those NNEs free in the lumen, after hemolysis, were probably removed by endocytosis (cell debris). Taking all together, we postulate that after reaching the endodermal epithelial cells of the stomach, nutrients are transferred to the embryo by a phagocytic/endocytic mechanism that is operative until the end of 6 wpf. After absorption, NNEs are probably degraded within phagosomes, nutrients delivered to the cell cytoplasm and then transported towards the embryonic circulation.

Two different pathways for the transport of primitive and definitive blood cells from the yolk sac to the embryo in humans.

During the early human embryonic period nutrients and blood cells are temporarily provided by the extraembryonic yolk sac (YS). The YS before week six is involved not only in primitive but also in definitive erythropoiesis. While the destiny of primitive erythroid cells that fill the blood vessels of the YS is well known, the final destination of erythrocytes present in the endodermal vesicular system is unknown. In the present study we have investigated, step by step, the destiny of the erythrocytes present in the endodermal vesicles during the embryonic period. Twelve human YSs and their corresponding yolk stalks were analyzed between weeks 4 and 7 of embryonic age by light and scanning electron microscopy. It is shown that erythrocytes (according to their size and morphological features) located within the endodermal vesicles of the YS wall are pulled out through endodermal pits into the YS cavity, from where they reach the lumen of the primitive gut of the embryo through the vitelline duct, a temporary pathway communicating both compartments. During the study period no erythrocytes were seen within the embryo's vascular network where only primitive erythroblasts were identified. Our results indicate that the vitelline duct plays an important transient role as a pathway for the transport of nutrients and blood cells between the YS and the embryo before week five of embryonic development that ends just at the time when YS-embryo circulation becomes established.

La formación quirúrgica de pregrado en Chile / The undergraduate surgical teaching in Chile

Para estudiar la formación quirúrgica de pregrado en las Escuelas de Medicina Chilenas se abordó el problema desde 2 puntos de vista diferentes: a)estudio descriptivo de los programas de enseñanza; b) constituye el aspecto principal del trabajo y consiste en un diseño experimental para conocer el grado de adecuación del sistema de formación a las necesidades de atención de la comunidad. Se reseñan los antecedentes históricos de la práctica de la cirurgía en Chile, se describe la costitución del sistema de salud y el rol que en el asume el médico recién egresado. A través de una encuesta dirigida a los jefes de Programa de enseñanza de la Cirugía de todas las Escuelas de Medicina del pais, se obtuvieron diversos parámetros sobre aspectos organizativos, de planificación, programación y criterios pedagógicos. La variable dependiente se determinó como la capacidad de atención de emergencias quirúrgicas por parte de los internos al término de su internado de cirugía. Son definidas 6 variables independientes y 1 variable interviniente. Son presentados los resultados obtenidos