Divergence of smooth muscle target and sympathetic pathway cell phenotypesin the orbit of the developing rat

The periorbital sheath serves as a major pathway for sympathetic nerves tra veling to distal orbital targets in the rat. This tissue accommodates sympa thetic fiber sprouting in the neonate but becomes impassable by postnatal d ay 30 (PND 30). In contrast, smooth muscle target remains receptive to symp athetic ingrowth. To determine the attributes of receptive and nonreceptive tissues, we compared periorbital pathway and target tissue phenotypes prio r to (PND 5 and PND 15) and after (PND 30 and PND 60) the period when pathw ay receptivity is lost. Both pathway cells and superior tarsal smooth muscl e cells expressed oc-smooth muscle actin and smooth muscle myosin heavy cha in throughout development. At PND 5-15, both tissues also expressed vimenti n, collagen IV, laminin 1 and laminin beta 2, whereas fibronectin was detec ted only in pathway tissue. At PND 30, vimentin, collagen IV, and fibronect in were absent in tarsal muscle but were robust in pathway tissue. Laminin 1 and laminin beta 2 expression was maintained in muscle; however, in pathw ay cells, laminin 1 declined modestly, and laminin beta 2 decreased precipi tously to barely detectable levels. Quantitative competitive polymerase cha in reaction showed that nerve growth factor mRNA was present in the pathway throughout development at levels that were greater than both surrounding c onnective tissue and tarsal muscle. We conclude that the loss of pathway re ceptivity to sympathetic nerve ingrowth is associated with a transition fro m a phenotype similar to fetal smooth muscle cells to one that is more cons istent with myofibroblast-like cells. (C) 1999 Wiley-Liss, Inc.