The Origin of Endoderm: Transmission Electron Microscopic Point of View

Abstract

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 38°C, 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.