The functional properties of tendon require an extracellular matrix (ECM) rich in elongated collagen fibrils in parallel register. We sought to understand how embryonic fibroblasts elaborate this exquisite arrangement of fibrils. We show that procollagen processing and collagen fibrillogenesis are initiated in Golgi to plasma membrane carriers (GPCs). These carriers and their cargo of 28-nm-diam fibrils are targeted to previously unidentified plasma membrane (PM) protrusions (here designated “fibripositors”) that are parallel to the tendon axis and project into parallel channels between cells. The base of the fibripositor lumen (buried several microns within the cell) is a nucleation site of collagen fibrillogenesis. The tip of the fibripositor is the site of fibril deposition to the ECM. Fibripositors are absent at postnatal stages when fibrils increase in diameter by accretion of extracellular collagen, thereby maintaining parallelism of the tendon. Thus, we show that the parallelism of tendon is determined by the late secretory pathway and interaction of adjacent PMs to form extracellular channels.
Coalignment of plasma membrane channels and protrusions (fibripositors) specifies the parallelism of tendon
The online version of this article contains supplemental material.
Abbreviations used in this paper: 3-D, three-dimensional; ADAMTS, a disintegrin and metalloprotease (reprolysin type) with thrombospondin motifs; BMP, bone morphogenetic protein; dpc, days post coital; GPC, Golgi to plasma membrane carrier; GPC+cf, GPC containing one or more 67-nm periodic collagen fibrils; pCcollagen, a naturally occurring intermediate in the cleavage of procollagen to collagen that contains the C-propeptides and not the N-propeptides; PM, plasma membrane; pNcollagen, a naturally occurring intermediate in the cleavage of procollagen to collagen that contains the N-propeptides and not the C-propeptides.
Elizabeth G. Canty, Yinhui Lu, Roger S. Meadows, Michael K. Shaw, David F. Holmes, Karl E. Kadler; Coalignment of plasma membrane channels and protrusions (fibripositors) specifies the parallelism of tendon . J Cell Biol 24 May 2004; 165 (4): 553–563. doi: https://doi.org/10.1083/jcb.200312071
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