ELASTIC EXTRACELLULAR-MATRIX OF THE EMBRYONIC CHICK HEART - AN IMMUNOHISTOLOGICAL STUDY USING LASER CONFOCAL MICROSCOPY

The ''elastic matrix'' constitutes a specialized component of the extr acellular matrix which confers resiliency to tissues and organs subjec ted to repeated deformations. The role of the elastic matrix in living organisms appears to be of key importance since diseases characterize d by expression of defective inherited genes which encode components o f the elastic matrix lead to premature death. While the elastic matrix of adult organs has received a great deal of attention, little is kno wn about when it first appears in embryonic tissues or its possible ro le in developing organs. In the present study we have performed an imm unohistochemical study of the distribution of elastin and three additi onal components often associated with elastic matrices in adult tissue s (i,e., fibrillin, emilin, and type VI collagen) during the developme nt of the chicken embryonic heart. The three-dimensional arrangement o f these components was established through the observation of wholemou nt specimens with scanning laser confocal microscopy. Our results reve aled three different periods of heart development regarding the compos ition of the elastic matrix. Prior to stage 21 the embryonic heart lac ks elastin but exhibits a matrix scaffold of fibrillin and emilin asso ciated with the endocardium and the developing cardiac jelly. Between stages 22 and 29 the heart shows a transient elastic scaffold in the o utflow tract which contains elastin, fibrillin, and emilin. Elastin-po sitive fibrillar material is also observed during these stages in the base of the atrioventricular cushion adjacent to the myocardial wall. In addition, emilin-positive material appears to be associated with th e zones of formation of ventricular trabeculae. Collagen type VI was n ot detected during these early stages. From stage 30 to stage 40 a pro gressive modification of the pattern of distribution of elastin, fibri llin, emilin, and collagen type VI is observed in association with the formation of the definitive four-chambered heart. The distribution of the elastic scaffold in the outflow tract appears to be rearranged an d becomes restricted to the roots of the main arteries. Each of the co mponents studied here is also deposited at increasing levels in the de veloping valvular apparatus including the valve leaflets and the chord ae tendinea. The components are also present in the subendocardial spa ce where they form aligned fibrillar tracts, an arrangement suggestive of a role in ventricular contractile function. The epicardium constit utes an additional region of elastic matrix deposition during these la ter stages and contains elastic, fibrillin, and collagen type VI. Fina lly, during the later stages the intramyocardial matrix (''myocardial interstitium'') is formed and characterized by an abundance of collage n type VI, emilin, and fibrillin but lacks elastin-positive material. This study suggests that during cardiac development there is not a fix ed composition of the so-called ''elastic matrix.'' Rather, combinatio ns of the different components of the elastic matrices appear to chara cterize the matrix associated with specific regions of the embryonic h eart and may reflect the different tensile properties required in thes e regions during development. Possible roles for these specific elasti c matrices during heart morphogenesis are discussed. (C) 1994 Wiley-Li ss, Inc.