The ultrastructural differentiation of the endoplasmic reticulum in choroidal epithelial cells of the chick embryo

Abstract

During the development of the choroidal epithelium in the chick embryo, a substantial concentration of granular endoplasmic reticulum differentiates in the subnuclear cytoplasm of the epithelial cells. The formation of the membranous components of this organelle is preceded by the appearance of a dense, localized population of small, free polyribosomes. Subsequently, numerous membrane-bound vesicles appear in the perinuclear cytoplasm. These primordial ER vesicles measure from 0.1 μ to 0.5 μ or more and they originate from evaginations of the outer nuclear membrane. These vesicles commonly occur in successive rows situated around the margin of the nucleus, and they expand and/or interconnect to form incipient ER tubules. Most vesicles and early tubules are smooth to nearly smooth in appearance. With continued development nuclear evaginations cease, and ER tubules expand in Situ to form an elaborate, laminated system of 7–12 ‘bag-like’ cisternae. Throughout this period of expansive growth, small polyribosomes attach to the developing ER cisternae. As the ER cisternae progressively attain their granular appearance, the number of small, free polyribosomes diminishes. During later stages of development larger polyribosomes appear in association with the subnuclear concentration of ER, and the first accumulations of electron-dense material develop within cisternal spaces.