Molecular Analysis of Gastrointestinal Smooth Muscle Development

Gastrointestinal development and function is a complex process that requires the precise interaction of distinct cell types derived from all three embryonic germ layers: epithelial cells from the endoderm, smooth muscle cells and fibroblasts from the mesoderm, and intrinsic neurons from the ectoderm. In addition, development of the gastrointestinal tract requires the proper cephalocaudal and lateral folding of the embryo, as well as a series of distinct spatial reorderings. Miscues in any of these individual processes can provide the basis for many of the birth defects, genetic disorders, and diseases associated with the gastrointestinal tract. Morphologic and biochemical studies have characterized many of the cellular interactions associated with normal gastrointestinal development and function. Even so, little is currently known regarding the molecular basis of these processes. The recent identification of a variety of molecular markers specific for many gastrointestinal cell types has begun to expand our current understanding of the molecular events controlling normal gastrointestinal development and function (1-7). In general, these studies have demonstrated a cellular complexity in the developing and mature gastrointestinal tract well beyond that identified by prior techniques. This review focuses on recent molecular studies examining normal gastrointestinal smooth muscle development. These studies are presented in the context of prior morphologic and biochemical studies that have examined normal and abnormal gastrointestinal smooth muscle development. This review is to provide the practicing clinician with some insight into: 1) the processes involved in the development of gastrointestinal smooth muscle, 2) the current state of knowledge and research tools being used to further our understanding of this process, and 3) what role, if any, this process has in gastrointestinal pathogenesis. No attempt is made to summarize similar data for either the gastrointestinal mucosa or enteric nervous system except as they directly relate to smooth muscle development. ANATOMIC ORGANIZATION OF GASTROINTESTINAL SMOOTH MUSCLE The adult gastrointestinal tract is composed of two primary smooth muscle layers: 1) the muscularis mucosae and 2) the muscularis propria (Fig. 1). The muscularis mucosae is a two- to three-cell thick smooth muscle layer that is located between the submucosa and lamina propria. The function of the muscularis mucosae involves mucosal movement, which is important in both gastrointestinal mixing and absorption. The muscularis propria is a multilayered smooth muscle sheet located between the submucosa and serosa (visceral peritoneum). In general, the muscularis propria is divided into two distinct smooth muscle layers: 1) an inner circular smooth muscle layer and 2) an outer longitudinal smooth muscle layer. Several notable exceptions to this organization occur. First, the muscularis propria of the stomach contains a third, middle oblique smooth muscle layer. Second, the circular smooth muscle layer of the small intestine is subdivided into a morphologically distinct inner (interstitial cells of Cajal) and outer smooth muscle layers. Finally, the outer longitudinal smooth layer of the large intestine is structurally organized into a distinct series of smooth muscle bands called taenia coli. The primary function of the muscularis propria involves gastrointestinal mixing and motility. Specialized regions of sphincteric smooth muscle (presumably derived from the circular smooth muscle layer of the muscularis propria) are found at the junctions of various gastrointestinal organs. These include the lower esophageal sphincter, pyloric sphincter, ileocecocolic sphincter, and internal anal sphincter. The primary function of these smooth muscle sphincters is to partition the gastrointestinal tract into discrete functional units. Additional individual clusters of smooth muscle cells are also found directly associated with the lamina propria and villus core of the small intestine. The function of these mucosal smooth muscle cells appears to involve villus and mucosal movement. ONTOGENY OF GASTROINTESTINAL SMOOTH MUSCLE Gastrointestinal smooth muscle develops along a cranial to caudal gradient from a continuous layer of amorphous mesenchyme positioned between a primitive epithelium (presumptive mucosa) on the luminal side and primitive mesothelium (presumptive visceral peritoneum) on the serosal side. Studies by Kedinger et al. (8-10) have demonstrated that the reciprocal interaction of fetal gut mesenchyme and primitive endoderm (i.e., epithelial:mesenchymal induction) is necessary for the normal differentiation and maturation of both gastrointestinal smooth muscle and epithelium. These distinct cellular interactions represent a classic example of epithelial to mesenchymal induction, a process that appears critical to the normal morphogenesis of the gastrointestinal tract. Although the precise signals involved in these important cellular interactions remain to be determined, a variety of extracellular matrix molecules and growth factors have been shown to be involved in this process (11-14). Two morphologically distinct smooth muscle cell phenotypes have been identified during smooth muscle development in vitro (15). These include the classically defined: 1) synthetic smooth muscle cell and 2) mature smooth muscle cell. The synthetic smooth muscle cell is a noncontractile, highly proliferative, migratory, and synthetic cellular phenotype capable of modifying its local microenvironment by producing a wide range of molecules, including extracellular matrix and growth factors. By comparison, the mature smooth muscle cell is a highly contractile, less proliferative, less synthetic, and nonmigratory cellular phenotype that is represented by the classic spindle-shaped smooth muscle cell found in most mature smooth muscle tissues. The anatomic organization and ontogeny of gastrointestinal smooth muscle clearly demonstrate the complexity involved in normal gastrointestinal smooth muscle development. At present, a variety of fundamental, but important, questions regarding gastrointestinal smooth muscle development remain unanswered. Are distinct cellular phenotypes observed during normal gastrointestinal smooth muscle development? What is the precise temporal and spatial timing of gastrointestinal smooth muscle development? How do the various smooth muscle layers of the mature gastrointestinal tract develop? Are there organ-specific and segment-specific patterns of development? Does gastrointestinal smooth muscle develop in a manner similar to other smooth muscle tissues (i.e., vascular, and What are the factors controlling normal gastrointestinal smooth muscle development? 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