Control of pigment synthesis in culture.

Chicken embryo retinal pigment cells provide an excellent opportunity for studying a number of aspects of cell growth and synthesis of a differentiative cellular product. These cells can be removed from the eyes of chicken embryos virtually uncontaminated by other cell types, satisfactorily maintained in cell culture and produce a highly specific, morphologically and biochemically identifiable macro-molecular product, the pigment melanin (1-3). Under conditions which allow proliferation , they usually stop manufacturing pigment, and previously made pigment is diluted by ensuing cell divisions (2-4). When the cultures become confluent and cell growth slows, the cells resume melanin synthesis. Whittaker attributed this to the cessation and resumption of tyrosinase synthesis (3). Attempts to elucidate the nature and origin of the melanin granules that characterize vertebrate pigment cells relied heavily upon the electron microscope. Such studies have intimated that the Golgi complex in pigment cells of neural crest origin (skin, hair, and melanomas) are responsible for the elaboration of this organelle (5-7). An "intermediate vesicle" appears which hypothetically acquires the internal periodic structure characteristic of melanosomes. Tyrosinase is believed to constitute a major part of this protein matrix and probably becomes inactive due to the deposition upon itself of the oxidation products of L-tyrosine. Moyer thought that retinal epithelium cells have no well-developed Golgi, and may construct melanosomes directly from intracisternal dilations of the endoplasmic reticulum (8). More recently, smoothed surfaced endoplasmic reticulum closely associated with Golgi membranes of melanoma cells have been implicated in the production of melanosomes (9, 10). The present investigation is concerned with: (a) the relationship of growth and differentiation in cultured chicken embryo retinal pigment epithelium cells as reflected both histologically and in patterns of DNA and melanin synthesis; (b) translational level controls on protein synthesis at various stages of growth and differentiation, i.e., the stability of pulse-labeled RNA, protein synthesis and melanogenesis in the presence of actinomycin D; (c) the kinetics of melanogenesis; and (d) the origin of melanosomes. All rights of reproduction in any form reserved.