Synthesis, Migration, and Release of Precursor Collagen by Odontoblasts as Visualized by Radioautography After [3h]proline Administration

Abstract

The elaboration of dentin collagen precursors by the odontoblasts in the incisor teeth of 30-40-g rats was investigated by electron microscopy, histochemistry, and radioautography after intravenous injection of tritium-labeled proline .At 2 min after injection, when the labeling of blood proline was high, radioactivity was restricted to the rough endoplasmic reticulum, indicating that it is the site of synthesis of the polypeptide precursors of collagen, the pro-alpha chains .At 10 min, when the labeling of blood proline had already declined, radioactivity was observed in spherical portions of Golgi saccules containing entangled threads, and, at 20 min, radioactivity appeared in cylindrical portions containing aggregates of parallel threads . The parallel threads measured 280-350 nm in length and stained with the low pHphosphotungstic acid technique for carbohydrate and with the silver methenamine technique for aldehydes (as did extracellular collagen fibrils) . The passage of label from spherical to cylindrical Golgi portions is associated with the reorganization of entangled into parallel threads, which is interpreted as the packing of procollagen molecules .Between 20 and 30 min, prosecretory and secretory granules respectively became labeled . These results indicate that the cylindrical portions of Golgi saccules transform into prosecretory and subsequently into secretory granules . Within these granules, the parallel threads, believed to be procollagen molecules, are transported to the odontoblast process .At 90 min and 4 h after injection, label was present in predentin, indicating that the labeled content of secretory granules had been released into predentin . This occurred by exocytosis as evidenced by the presence of secretory granules in fusion with the plasmalemma of the odontoblast process .It is proposed that pro-alpha chains give rise to procollagen molecules which assemble into parallel aggregates in the Golgi apparatus . Procollagen molecules are then transported within secretory granules to the odontoblast process and released by exocytosis . In predentin procollagen molecules would give rise to tropocollagen molecules, which would then polymerize into collagen fibrils .

In vivo experimentation on rat incisor enamel organs through a surgical window

“…Labeled amine acids are essentially c1eared from the b100d plasma by 1 ho ur after intravenous injection (Nakagami et al, 1971). Weinstock and Leblond (1974) found that the amount of free labe1ed proline in the b100d plasma dropped sharp1y during the first 10 minutes after injection and concluded that a pulse of high raclioactivity was achieved. However, a 10w but significant level of radiol activity pers isted between 30 minutes and 30 hours after injection.…”

“…In bone and dent ln, some of the secreted proteins become part of a collageno~s uatrix which 18 transformed into mineralized products of the cells (Carneiro and Leb1ond.' 1959;Young and Greulich, 1963;Weinstock and Leblond, 1974).…”

Location of procollagen in chick corneal and tendon fibroblasts with ferritin-conjugated antibodies.

“…However, quantitative electron microscope autoradiographic studies gave conflicting results as to the amount of proline-rich protein passing from endoplasmic reticulum into the Golgi vacuoles of THE JOURNAL OF CELL BIOLOGY • VOLUME 65, 1975 • pages 75-87several cell types (12,27,28,34) and suggested that two intracellular pathways might exist for collagen transport (27,28).…”

“…Procollagen is unusual insofar as it is secreted by several distinct cell types. In fact, morphological differences in the extent and character of the Golgi apparatus among collagen-secreting cells may be part of the basis for the different interpretations regarding the mode(s) of intracellular transport and secretion of this protein (12,34).…”

“…The intracellular transport of the secretory protein, procollagen, and the role of the Golgi apparatus in this function have been the subject of much discussion and study (3,5,11,12,24,(26)(27)(28)34). Procollagen is unusual insofar as it is secreted by several distinct cell types.…”