STUDIES IN CRANIAL SUTURE BIOLOGY .1. INCREASED IMMUNOREACTIVITY FOR TGF-BETA ISOFORMS (BETA-1, BETA-2, AND BETA-3) DURING RAT CRANIAL SUTURE FUSION

The mechanisms involved in normal cranial suture development and fusio n as well as the pathophysiology of craniosynostosis, a premature fusi on of the cranial sutures, are not well understood, Transforming growt h factor-p isoforms (TGF-beta 1, beta 2, and beta 3) are abundant in b one and stimulate calvarial bone formation when injected locally in vi vo, To gain insight into the role of these factors in normal growth an d development of cranial sutures and the possible etiology of prematur e cranial suture fusion, we examined the temporal and spatial expressi on of TGF-beta isoforms during normal cranial suture development in th e rat, In the Sprague-Dawley rat, only the posterior frontal cranial s uture undergoes fusion between 12 and 22 days of age, while all other cranial sutures remain patent, Therefore, immunohistochemical analysis of the fusing posterior frontal suture was compared,vith the patent s agittal suture at multiple time points from the fetus through adult, W hereas the intensity of immunostaining was the same in the posterior f rontal and sagittal sutures in the fetal rat, there was increased immu noreactivity for TGF-beta isoforms in the actively fusing posterior fr ontal suture compared with the patent sagittal suture starting 2 days after birth and continuing until approximately 20 days, There were int ensely immunoreactive osteoblasts present during fusion of the posteri or frontal suture. In contrast, the patent sagittal suture was only sl ightly immunoreactive, A differential immunostaining pattern was obser ved among the TGF-beta isoforms; TGF-beta 2 was the most immunoreactiv e isoform and was also most strongly associated with osteoblasts adjac ent to the dura and the margin of the fusing suture, Since the increas ed expression of TGF-beta 2 during suture fusion suggested a possible regulatory role, recombinant TGF-beta 2 was added directly to the post erior frontal and sagittal sutures in vivo to determine if suture fusi on could be initiated, Exogenously added TGF-beta 2 stimulated fusion of the ectocranial surface of the posterior frontal suture, These data provide evidence for a regulatory role for these growth factors in cr anial suture development and fusion, Additionally, the intense immunos taining for TGF-beta 2 in the dura mater underlying the fusing suture supports a role for the dura mater in suture fusion, It is possible th at premature or excessive expression of these factors may be involved in the etiopathogenesis of craniosynostosis and that modulation of the growth factor profile at the suture site may have potential therapeut ic value.