TROPOELASTIN GENE-EXPRESSION IN THE DEVELOPING VASCULAR SYSTEM OF THECHICKEN - AN IN-SITU HYBRIDIZATION STUDY

Temporal and spatial patterns in the accumulation of Tropoelastin (TE) mRNA during development of the chick embryo were established by in si tu hybridization. Radiolabeled oligonucleotide probes of high specific activity were hybridized to serial sections of the cardiovascular sys tem from embryonic day 3.5 (ED 3.5) to ED 19. Tropoelastin mRNA was ob served as early as ED 3.5 in the dorsal part of the arterial trunk. Du ring septation varying levels of TE mRNA were seen in the pulmonary tr unk, the aorta and the aorticopulmonary septum. Thereafter TE mRNA lev els increased up to ED 12, and the appearance of message was distribut ed distally in the walls of developing arteries. From ED 4.5 on, we fo und a decreasing proximo-distal gradient of the hybridization signal a long the trunks and later along the main arteries (longitudinal gradie nt), and a radial gradient through the arterial vessel wall with the h ighest levels of TE mRNA in the outer layers of the media. Both gradie nts persisted in all major arterial vessels except in the proximal sys temic and pulmonary trunks, where the original radial gradient was inv erted or locally bimodal during the second half of development. The va lvular region of aortic and pulmonary trunks showed particularly strik ing patterns of TE mRNA distribution, notably a prominent label on the endothelial cell layer on aortic and pulmonary valves. Outside the ca rdiovascular system, TE mRNA was mainly present in prochondral or peri chondral cells in trachea and growing skeleton, and in the gap of grow ing joints. In kidney or nephric primordia, TE mRNA was only detectabl e in the wall of renal arteries. A hybridization signal was observed o n mesenchyme of pulmonary septae at ED 16. Our results suggest a compl ex regulation of elastin gene expression during development, particula rly within the proximal regions of the large arterial vessels.