The uptake, intracellular transport, and secretion of protein by guinea pig wound fibroblasts was studied by electron microscope radioautography using L-proline-3,4-H3 as a tracer. Experiments were performed to determine the curve of concentration of free amino acid in the blood after intraperitoneal administration of the labeled proline. Radioautographs were quantitatively analyzed and the concentration of isotope, in grains per unit area, was determined for the following cellular and extracellular compartments: ergastoplasm, Golgi complex, peripheral cytoplasmic structures, and collagen. The concentration of label, expressed as number of grains per unit area of each subcellular system, reveals the period during which each cellular compartment is maximally labeled, and presents a clearer picture of the passage of the label through each of these compartments. The data demonstrate appearance of the label at maximum concentration in the ergastoplasm 15 minutes after injection, and this compartment remains maximally labeled for 2 hours. In the Golgi complex, concentration is not maximal until 60 minutes after injection of isotope, and appears to decrease before or at about the same rate as that of the ergastoplasm. The present experiment is consistent with previous light microscope radioautographic studies, and no storage phase was found in the fibroblasts. The findings are not simply consistent with a direct precursor-product relationship between the contents of the ergastoplasm and those of the Golgi complex. Morphologic observations of regions in the fibroblast interpretable as possible sites of communication between the ergastoplasm and the extracellular space, together with the kinetic studies, permit the suggestion of an alternate pathway of passage of at least some of the synthesized protein directly from the ergastoplasmic cisternae to the cell exterior.

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