2. CHITIN AND MUCOSUBSTANCES
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merit is required on the unusual stability of tunicin, which in crystalline
structure resembles bacterial and Valonia celluloses (190). Mann and
Marrinan (191) have found it to be highly crystalline and in its unit
cell structure to resemble the cellulose of higher plants. The enhanced
stability is attributed to the higher degree of macromolecular orientation
rather than to the presence of different types of bonds.
The species where cellulose is present appear to be largely confined
to the tunicates; elsewhere, however, there is much greater diversification. In the fungi, cellulose and chitin appear to be generally alternative
though a few cases are reported where both chitin and cellulose occur
together (192, 193). In fungi and cellulose-synthesizing bacteria, the
cellulose is frequently obtained in a pure form, i.e., unassociated with
protein, and is synthesized in a primarily oriented form in contrast to
invertebrate chitin where a secondary orienting process follows biosynthesis. Whereas invertebrate chitin is formed in a protein matrix,
development of plant cellulose fibers takes place in association with
hemicelluloses.
2. Other Carbohydrate
Complexes
Chitin represents a distinctive integumental constituent of invertebrates, structurally identical or closely similar to that found in fungi,
produced by both ectodermal and endodermal cells. Where cellulose is
present, it occurs both outside the epidermis as well as interior to it.
In biochemical terms, however, the wide incidence of chitin in so many
invertebrate species can be regarded as indicating a dominant position
of amino sugar metabolism, for structural as well as for reserve purposes.
In this context, chitin is only one of many related mucopolysaccharides
in which amino sugars participate. Mucopolysaccharides in invertebrates
are found as epithelial mucins performing biological functions of lubrication and protection similar to those in the vertebrates.
In vertebrates, development of calcified bone is preceded by the
synthesis of a mucopolysaccharide matrix in conjunction with which
collagen microfibers are formed. Calcification then takes the form of
crystal deposition of apatite-type minerals, initiated in the molecular
spacings of the collagen macrostructure. As in the crustacean integument, mammalian bone is a metabolic reserve as well as a structural
element and is equipped with enzymatic apparatus for resorption of both
its organic and mineral constituents. In connective tissue similarly, collagen formation increases with age and, by adding to the ground substance, results in a corresponding decrease in the cellularity of this tissue.
The principal mucosubstances characterized in connective tissues and
bone include two forms of chondroitin sulfate (A and C), derman sulfate
125
merit is required on the unusual stability of tunicin, which in crystalline
structure resembles bacterial and Valonia celluloses (190). Mann and
Marrinan (191) have found it to be highly crystalline and in its unit
cell structure to resemble the cellulose of higher plants. The enhanced
stability is attributed to the higher degree of macromolecular orientation
rather than to the presence of different types of bonds.
The species where cellulose is present appear to be largely confined
to the tunicates; elsewhere, however, there is much greater diversification. In the fungi, cellulose and chitin appear to be generally alternative
though a few cases are reported where both chitin and cellulose occur
together (192, 193). In fungi and cellulose-synthesizing bacteria, the
cellulose is frequently obtained in a pure form, i.e., unassociated with
protein, and is synthesized in a primarily oriented form in contrast to
invertebrate chitin where a secondary orienting process follows biosynthesis. Whereas invertebrate chitin is formed in a protein matrix,
development of plant cellulose fibers takes place in association with
hemicelluloses.
2. Other Carbohydrate
Complexes
Chitin represents a distinctive integumental constituent of invertebrates, structurally identical or closely similar to that found in fungi,
produced by both ectodermal and endodermal cells. Where cellulose is
present, it occurs both outside the epidermis as well as interior to it.
In biochemical terms, however, the wide incidence of chitin in so many
invertebrate species can be regarded as indicating a dominant position
of amino sugar metabolism, for structural as well as for reserve purposes.
In this context, chitin is only one of many related mucopolysaccharides
in which amino sugars participate. Mucopolysaccharides in invertebrates
are found as epithelial mucins performing biological functions of lubrication and protection similar to those in the vertebrates.
In vertebrates, development of calcified bone is preceded by the
synthesis of a mucopolysaccharide matrix in conjunction with which
collagen microfibers are formed. Calcification then takes the form of
crystal deposition of apatite-type minerals, initiated in the molecular
spacings of the collagen macrostructure. As in the crustacean integument, mammalian bone is a metabolic reserve as well as a structural
element and is equipped with enzymatic apparatus for resorption of both
its organic and mineral constituents. In connective tissue similarly, collagen formation increases with age and, by adding to the ground substance, results in a corresponding decrease in the cellularity of this tissue.
The principal mucosubstances characterized in connective tissues and
bone include two forms of chondroitin sulfate (A and C), derman sulfate