Extracellular Connective Tissue Fibrils: A Single Line of Descent

Abstract

The fine structure of developing extracellular connective tissue fibrils, is demonstrable in normal chick embryos. Before incubation begins primary fibrils are present in the intermittent boundary membrane (basement membrane, basal lamina) of the basal surface of the epiblast. These give rise to microfibrils which first become free in the tissue space at about 24 hours incubation (Fig. 1). The first noticeable locus of microfibrillar concentration appears between the head process (early notochord) and the dorsal ectoderm. By 40 to 45 hours a heavy tangle of microfibrils occupies the acellular area surrounding the notochord and remains conspicuous during the third day (Fig. 2). At 60 to 72 hours microfibrils begin to appear in close association with free mesenchymal cells (Fig. 3). Periodicity slowly develops in the larger microfibrils and the resultant unit collagen fibrils will aggregate to form the reticular and collagen fibers of light microscopy. During the second and third weeks of incubation microfibrils and unit collagen fibrils contribute to the formation of elastic fibers.