A morphological analysis of the transition between the embryonic primitive intestine and yolk sac in bovine embryos and fetuses

Abstract

The yolk sac (YS) is the main source of embryonic nutrition during the period when the placenta has not yet formed. It is also responsible for hematopoiesis because the blood cells develop from it as part of the primitive embryonic circulation. The objective of this study was to characterize the transitional area between the YS and primitive gut using the techniques of light microscopy, transmission electron microscopy, and immunohistochemistry to detect populations of pluripotent cells by labeling with Oct4 antibody. In all investigated embryos, serial sections were made to permit the identification of this small, restricted area. We identified the YS connection with the primitive intestine and found that it is composed of many blood islands, which correspond to the vessels covered by vitelline and mesenchymal cells. We identified large numbers of hemangioblasts inside the vessels. The mesenchymal layer was thin and composed of elongated cells, and the vitelline endodermal membrane was composed of large, mono- or binucleated cells. The epithelium of the primitive intestine comprised stratified columnar cells and undifferentiated mesenchymal cells. The transitional area between the YS and the primitive intestine was very thin and composed of cells with irregular shapes, which formed a delicate lumen containing hemangioblasts. In the mesenchyme of the transitional area, there were a considerable number of small vessels containing hemangioblasts. Using Oct4 as a primary antibody, we identified positive cells in the metanephros, primordial gonad, and hepatic parenchyma as well as in YS cells, suggesting that these regions contain populations of pluripotent cells.

Isolation and characterization of mesenchymal stem cells from the yolk sacs of bovine embryos

“…The YS of the bovine embryo appears at 18 to 23 days of gestation as a trilaminate cylindrical disc consisting of trophoblast, endoderm, and mesoderm [4][5][6]19,27,28] and is enclosed by a unicellular endodermal layer [20].…”

“…However, vestiges of the central part of this sac can still be observed during this time [2]. The YS is located in the ventral region of the embryo and is connected to the medium intestine by the vitelline duct [5,8].…”

“…Microscopically, the YSs of species, such as canines [3,4], bovines [2,5], bubalines [6], and goats [7], are extrafetal membranes that are morphologically composed of three layers: the endoderm, an intermediate layer called the mesoderm, and a single layer called the mesothelium [8,9].…”

Bovine yolk sac: from morphology to metabolomic and proteomic profiles

“…It is suggested that central hemangioblasts present in these vessels are related to the development of the first hematopoietic stem cells, while the peripheral hemangioblasts differentiate into angioblasts and precursors of vascular endothelium (Zago and Covas, 2006). Analysis by light and transmission electron microscopy revealed vascular islets in Group II (33-42 days) embryos, mainly in the mesenchymal region of the membrane, as described by Mançanares et al (2013). In the course of embryonic development, islets cluster and merge with each other, forming endothelial tubes.…”

“…Furthermore, we note that the yolk sac of bovine embryos, in the course of embryonic development, changes its anatomical insertion position. First, the yolk sac is located in the ventral region of the embryo formation, which maintains a connection to the primitive gut, which is extremely important for the transfer of nutrients (Mançanares et al, 2013).…”