MORPHOLOGY, HISTOCHEMISTRY, AND DIFFERENTIATION OF THE CATS EPIGLOTTIC CARTILAGE - A SUPPORTING ORGAN COMPOSED OF ELASTIC CARTILAGE, FIBROUS CARTILAGE, MYXOID TISSUE, AND FAT TISSUE

Background: In carnivores, the supporting organ of the epiglottis is u sually called ''epiglottic cartilage'' (EC) although it is composed of elastic cartilage and unilocular fat storing cells. We studied the ca t's EC in order to decide whether these fat storing cells are true adi pocytes or fat storing (dedifferentiated) chondrocytes. Methods: ECs w ere studied in cat embryos at gestation days 40 and 60, in newborn, po stnatal, and adult cats. We used classical staining methods, immunohis tochemistry, and transmission electron microscopy to identify the diff erent kinds of tissues contributing to the EC and to follow their diff erentiation. Results: The cat's EC was defined by a layer of coarse co llagen fibers representing a tunica albuginea. This tunica covered irr egularly formed and irregularly sized areas of elastic cartilage, fibr ous cartilage, myxoid tissue, and lobules of unilocular fat cells. All these tissues showed regular morphology. Adipocytes were provided wit h continuous basal laminae and fat lobules were well supplied with cap illaries. Alcianophilia of ground substance was observed in all tissue components but was strongest in elastic cartilage. Most islets of ela stic cartilage adhered to the tunica albuginea of the EC at one surfac e and were connected to the opposite surface by coarse strands of conn ective tissue traversing the organ. Intercalated areas of fibrous cart ilage contained fuchsinophilic collagen bundles. Myxoid tissue was cha racterized by stellate cells in alcianophilic ground substance with in termingled fuchsinophilic bundles. All kinds of supporting tissues com bined with each other without clear demarcation. Immunohistochemistry revealed strong reactivity for S-100 of chondrocytes, myxoid cells, an d fat cells. Chondrocytes and myxoid cells also stained for glial fibr illary acidic protein, neurofilament protein 200, and neuron specific enolase. During development, condensation of mesenchymal cells indicat ed the blastema of the EC at gestation day 40. At day 60, delicate col lagen fibrils indicated the future tunica albuginea, faint alcianophil ia was noted in the ground substance, and multilocular fat cells were scattered throughout the blastema. At birth, alcianophilia was moderat e and multilocular fat cells were numerous. Three weeks after birth, s ingle and grouped unilocular fat cells were seen, alcianophilia of gro und substance was prominent, and former blastema cells presented as ra mified myxoid cells. Eight weeks after birth, the EC primarily consist ed of myxoid tissue, but the first islets of cartilage were seen in th e center of myxoid areas. Unilocular fat cells already formed lobules. Conclusions: These results show that in the cat EC a) differentiation of adipocytes precedes differentiation of all the other tissue compon ents, and b) differentiation of myxoid tissue precedes differentiation of cartilage. It is concluded that myxoid tissue may serve as a precu rsor of fibrous and elastic cartilage. (C) 1995 Wiley-Liss, Inc.