Structural and functional characterization of bladder smooth muscle in fetal rats with retinoic acid-induced myelomeningocele.

Myelomeningocele (MMC) is the most common cause of neurogenic bladder dysfunction (NBD). We recently developed a novel retinoic acid (RA)-induced MMC model in fetal rats. The objective of this study was to use this model to assess functional and structural characteristics of the detrusor muscle in MMC-associated NBD. Time-dated pregnant Sprague-Dawley rats were gavage fed 60 mg/kg RA dissolved in olive oil or olive oil alone [embryonic day 10 (E10)]. Bladder specimens from olive oil-exposed fetuses (OIL; n = 71), MMC (n = 79), and RA-exposed-no MMC (RA, n = 62) were randomly assigned for functional and histopathological evaluation and protein analysis. Contractility responses to field and agonist-mediated stimulation (KCl and bethanecol) were analyzed. The expression patterns of alpha-smooth muscle actin, myosin, desmin, vimentin, and collagen III and I were analyzed by immunohistochemistry and Western blotting. Spatial and temporal distribution of nerve fibers within the detrusor muscle was monitored by neurotubulin-beta-III throughout gestation. Neither OIL, MMC, nor RA detrusor responded to field stimulation. MMC bladder strips showed a significant decrease in contractility after KCl and bethanechol stimulation compared with OIL and RA bladders. Bladder detrusor morphology and expression patterns of smooth muscle markers were similar between groups. Detrusor muscles in OIL and RA fetuses were densely innervated, possessing abundant intramural ganglia and nerve trunks that branch to supply smooth muscle bundles. In MMC bladders, neurotubulin-beta-III-positive nerve fibers were markedly decreased with advancing gestational age and were almost completely absent at term (E22). We conclude that the biomechanical properties of fetal rat MMC bladders are analogous to that seen in humans with MMC-associated NBD. Decreased nerve density indicates loss of peripheral neural innervation throughout gestation. The early observation of decreased innervation and decreased contractility in the absence of morphologic abnormalities in muscle structure or extracellular matrix supports a pathophysiological hypothesis that denervation is the primary insult preceding the observed alterations in bladder muscle structure and function.