EEB267H1 Lecture Notes - Pharyngeal Slit, Pharyngeal Arch, Fish Fin
1 Mar 2013
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Behold the gnathostomata: gnathos (jaw) + stoma (mouth) Myxiniformes petromyzontiformes chondrichthyes osteichthyes jaws skull proto-vertebrae, true gills, two eyes, olfactory lobes many molecular characters dorsal hollow nerve cord, notochord, post-anal tail. Before we get to jaws, we need to know something about hard tissues. The first type of hard tissue to evolve was cartilage. It is composed of: a matrix of collagen (a type of protein) fibers + proteoglycans (proteins coupled with long chains of sugars [e. g. , chondroitin]) + Specialized cells (called chondrocytes) produce the matrix and lay it down in thin layers. lymphocytes have unique antigen receptors. In the basal chordata, the chondrocytes are not part of the matrix so this type of cartilage is acellular and is therefore nonliving . In the ancestor of the cristozoa, the chondrocytes deposit the matrix around themselves. Because these cells are now part of the matrix, cartilage is living tissue in these animals.
