The assembly of the collagenous extracellular matrix during tendon development was studied to determine the mechanisms involved in collagen fibril growth. Developing avian metatarsal tendons were studied using structural, immunochemical and biochemical approaches. Tendon fibroblasts were shown to establish a hierarchy of extracellular compartments associated with fibrils, bundles, and macroaggregates during development. These distinct domains provide a mechanism for the fibroblast to influence the extracellular steps in matrix assembly. A discontinuous fibrillar matrix was assembled and fibril segments approximately 10-20 microns long were deposited into bundles by the 14 d embryo fibroblasts. The fibril segment is a normal assembly intermediate, permitting orderly linear, lateral and intercalatory development and growth. A lateral and/or linear fusion of segments may be responsible for the formation of mature continuous fibrils. Fibril segments were isolated from 12-18 d chick embryo metatarsal tendons. Homogenisation almost completely disrupted the 12-15 d tendons. Transmission electron microscopy demonstrated intact segments. Between d 12 and 15 of development, mean segment length increased from approximately 22 to 33 microns. The incremental increase in length with development indicates a limited linear fusion of segments which is supported by morphological examination. At 16 d, there was a significant decrease in segment extractability and by 17-18 d, intact segments were unextractable. Mean segment lengths were 37 microns and > 70 microns for 16 and 17 d tendons respectively. During this period, fibril diameter also increased.(ABSTRACT TRUNCATED AT 250 WORDS)