Smooth muscle development and cell proliferation patterns contribute to the development of the spiral valve intestine of Leucoraja ericacea

The spiral intestine of sharks and skates is a morphologically complex organ, and the origin of the elongated looping intestine seen in terrestrial animals today. The looping of the intestine in terrestrial animals has been shown to be a result of many factors including left right signaling, orientation of muscle formation, cell proliferation patterns, and gene expression. In contrast, the intestine of sharks and skates is a helical spiral contained inside an encased tube. Since the spiral intestine is structurally different from the elongated coiled intestines of other vertebrates, I wanted to see if factors affecting looping also controlled spiraling in the skate. Using phalloidin staining, I labeled smooth muscle fibers at different stages of embryonic development to determine how muscle differentiation may play a role in forming the developing spiral. Muscle differentiation initiates prior to spiraling at stage 24, as circumferential muscle fibers start to form around the gut tube. By stage 27, there is longitudinal as well as circumferential muscle. In addition, cell proliferation was studied by labeling dividing cells with Histone H3 phospho antibody. Findings show unique patterns of proliferation in relation to the spiral intestine at different embryonic stages of development. Understanding the biomechanics of spiral intestine formation will give great insight in an evolutionary context of how the digestive tract has evolved starting from ancient fish with a rudimentary spiral intestine, to the spiral intestine seen in the skate, to the long coiled intestine mammals possess today