Cytomembranes in first cleavage xenopus embryos

Summary The ultrastructure and interrelationships of the Golgi body, endoplasmic reticulum and lipid droplets have been studied in the first cleavage Xenopus embryos. Lipid droplets, usually spherical or sometimes multilobed, did not have a discernible limiting membrane, although some had an incomplete electron dense partition. The Golgi bodies and endoplasmic reticulum were seen continuous with lipid droplets and the profiles indicated a probable formation of these membranes from lipid droplet material. Rough endoplasmic reticulum (ER) mainly consisted of paired tubular cisternae and vesicles containing filamentous material that gave a fringed appearance. The relationships of paired cisternae with the Golgi body suggested a transformation of ER membranes into the Golgi body membranes. In addition, paired ER cisternae showed a close apposition with the limiting membrane of the yolk platelet. Lone ER cisternae that contained moderately electron dense material instead of filaments were also present and showed numerous associated vesicles near the Golgi body. The Golgi body showed several morphological forms including a single fenestrated cisterna, two to four flat or cup-shaped cisternae, or up to seven cisternae, some of which were dilated and similar to fringed ER in appearance. These forms could be different developmental stages of the organelle. Coated vesicles were seen continuous with the cisternae of the Golgi body. A probable route for the assembly of the cell surface material has been proposed.This work was supported by a grant from the Medical Research Council of Canada to one of us (E.J.S.).