Phagocytosis: Mechanism of uptaking the nutrition of the embryonic cells.

Objective : To demonstrate the mechanisms of the embryonic cells, the epiblast, the mesoblast and the hypoblast of the chick embryo uptaking their nutrition by transmission electron microscopic (TEM) point of view. Materials and Methods : The chick embryos of about 18-27 hours incubation were used, the primitive streak and the 7 somites stages of the embryo were processed for routinely transmission electron microscopic study at the region of primitive streak, where the mesoblast originates from the epiblast. These stages correspond to the 15 days and 20 days of the human embryos, respectively. Results : The ultrastructural features of the epiblast, mesoblast and the hypoblast were observed. The epiblast cells arrange themselves as stratified columnar epithelium. The apical cells are columnar while the deeper cells are bottle-shaped and the deepest cells are round and separated to become the mesoblast. The superficial surfaces of the apical cells show many pseudopodia some of which curve to join with the adjacent. The clear materials are completely surrounded when the encompassing plasma membranes fuse and the membrane surround the engulfed material forms a vacuole, known as the phagosome, which detaches from the plasma membrane to float freely within the cytoplasm. The phagosome is then in some way recognized by one or more lysosomes which fuse with the phagosome to form the secondary lysosome. When the digestion is completed, the lysosomal membrane ruptures, discharging its contents into the cytoplasm. Undigested material remains within the membrane-bound vesicles called residual bodies, the contents of which later discharge at the cell surface by exocytosis. These processes also occur at the external surfaces of the hypoblast cells which arrange themselves as the simple squamous epithelium. The ventral plasma membrane shows many pseudopodia and also the series of phagocytotic process. The numerous clear vacuoles distribute in the cytoplasm of the epiblast and the hypoblast. These vacuoles are the nutritional uptake of the cell from the outside environment, the epiblast gets its nutrition from the albumin while the hypoblast from the egg yolk. Concluison: TEM evident shows clearly that numerous clear and unclear vacuoles distribute throughout the cytoplasm of the epiblast and hypoblast. These vacuoles are the phagosomes of several stages of phagocytosis, and serve as the nutritions for the cells, as the cells are still young and also have no definitive organelles to produce their own materials. This findings also give another excellent model of explaining the series of the phagocytotic process.

The origin of endoderm: Transmission electron microscopic point of view.

Objective: To show the transmission electron microscopic (TEM) evidence to confirm that the endoderm originates from the epiblast of the primitive streak or from other sources. Methods: 60 fertilized Leghorn hen’s eggs were used in this study by incubating the eggs for about 18-27 hours at 38oC, then the chick embryos of the primitive streak stage to 7-somite stage were further processed for routine TEM study at the region of the primitive streak. Results: The epiblast proliferates and accumulates to form the primitive streak at the midcaudal of the embryonic disc from 18-27 hours incubation which corresponds with the early third week of the human embryo. TEM evidence shows that the epiblast at the primitive streak is the stratified columnar type of epithelium while the hypoblast is the simple squamous and the mesoderm cells are irregular in shape. The process of gastrulation begins with the formation of the filopodia of the epiblast by numerous protrusions of the plasma membrane from lateral side of the cell. These structures initiate the separation of the contacted cells. The deepest epiblast cells separate first while the superficial epiblast cells exhibit the desmosome between the adjacent cells. The separated epiblast cells are bottle-shaped with numerous filopodia and gradually change the shape into round or oval cells which migrate in the space between the epiblast and hypoblast. Some of these migrate to the hypoblast and contact with the hypoblast, the mesoblasts lose the filopodia and gain more close contact to the hypoblasts which become a very thin sheet of cells. The facing cell membrane later gradually disappears and the mesoblast then occupies the region of pre-existing hypoblast. There is no evidence that the mesoblast displaces the pre-existing hypoblast laterally to form the extraembryonic endoderm. Conclusion: These are TEM evidences that the epiblast of the primitive streak separates and migrates to form the mesoblast and some contact with the hypoblast. The later process appeared to reveal that the mesoblast compresses the hypoblast until the facing plasma membrane disappears and occupies the region of the pre-existing hypoblast.