IN-SITU LOCALIZATION OF THROMBOSPONDIN-1 AND THROMBOSPONDIN-3 TRANSCRIPTS IN THE AVIAN EMBRYO

Two novel cDNA probes to chicken thrombospondin-1 (TSP-1) and TSP-3 we re used to determine by in situ hybridization the origins of these ext racellular matrix proteins during embryogenesis. Both TSP-1 and TSP-3 are expressed in embryonic cartilage. TSP-1 expression is limited to e arly chondrocytes, in contrast to TSP-3 mRNAs that are found in older proliferative and hypertrophic chondrocytes. TSP-1 and TSP-3 are expre ssed consecutively during neurogenesis as well, with the TSP-1 probe h ybridizing in proliferating neuroblasts, and TSP-3 expressed by neuron s as they are actively extending processes. A TSP-1 hybridization sign al reappears in subpopulations of neurons in the spinal cord and brain after the periods of active neurite extension and programmed cell dea th are complete. TSP-1 is also expressed in the spinal cord floor plat e, in mesenchyme surrounding the developing paramesonephric duct, at t he tips of growing lung bronchioles, in lens, and in corneal endotheli um. These observations indicate that there is little overlap in the ex pression patterns of TSP-1 and TSP-3 and that they are expressed conse cutively during the development of cartilage and neurons. The pattern of TSP-1 expression in avascular tissues and in the floor plate is con sistent with possible roles for this glycoprotein in regulating angiog enesis and establishing morphogenetic gradients. (C) 1997 Wiley-Liss, Inc.