Distribution of hyaluronic acid and chondroitin sulfate proteoglycans in the presumptive aganglionic terminal bowel of ls/ls fetal Mice: An ultrastructural analysis

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Abstract
The terminal colon of the ls/ls mouse is aganglionic because an intrinsic defect prevents its colonization by cells migrating from the neural crest. Previous studies showed that laminin, type IV collagen, and glycosaminoglycans accumulate in the region of the presumptive aganglionic ls/ls bowel through which crest‐derived cells would be expected to migrate. It was suggested that crest‐derived cells might fail to enter the abnormal bowel because they receive inappropriate signals from a defective extracellular matrix. This hypothesis was evaluated by analyzing the ultrastructure of the extracellular matrix in mutant and control gut. Tissue was fixed in the presence of ruthenium red before or after selective enzymatic digestion. Heparan sulfate proteoglycan (diameter ≅ 15 nm) and chondroitin sulfate proteoglycan (diameter ≅ 20‐50 nm) granules were found in both control and presumptive aganglionic gut. The heparan sulfate proteoglycan granules were primarily located within formed basal laminae, while chondroitin sulfate proteoglycan granules decorated plasma membranes and 5 nm hyaluronic acid microfibrils that formed a network in the extracellular matrix. At day E11.5, the mutant gut differed from the control in the following: (1) Hyaluronic acid microfibrils were longer and more numerous. (2) There were larger numbers of chondroitin sulfate proteoglycan granules associated with cell membranes and with hyaluronic acid microfibrils. By day E13 the spaces between mesenchymal cells of the outer wall of the control bowel contained a regular lattice of hyaluronic acid microfibrils studded with chondroitin sulfate proteoglycan granules. Instead of this lattice, tangles of excessively long hyaluronic acid microfibrils, coated more heavily than in the control with chondroitin sulfate proteoglycan granules, were found in the presumptive aganglionic gut. These results confirm that the extracellular matrix is abnormal in the presumptive aganglionic bowel of the ls/ls mouse; moreover, they also indicate that the defect involves not one, but several components of the extracellular matrix, as well as their distribution. The defective extracellular matrix is apparent at a time when crest‐derived cells would be expected to be migrating in the terminal bowel and is located in their path. The observations thus support the idea that a localized abnormality of the extracellular matrix interferes with the colonization of the terminal bowel by crest‐derived cells in the ls/ls mouse.