The joining together of neurites to form fascicles and the growth of axons along glial surfaces during early development suggest that neurone-neurone and neurone-glial adhesion interactions are of considerable importance for defining nerve tracts. In vitro studies have indicated that adhesion between neurones involves a glycoprotein that has been independently studied under the names of N-CAM (for neural cell adhesion molecule), D2-CAM and BSP-2 (refs 10, 11). As N-CAM/D2-CAM appears to be a homophilic ligand that binds to N-CAM/D2-CAM polypeptide on adjacent cells, this glycoprotein is potentially important in adhesion interactions between any two N-CAM/D2-CAM-expressing cells. While it has been suggested that neurone-glial adhesion involves molecules other than N-CAM/D2-CAM, it is known that N-CAM/D2-CAM antigenic determinants are expressed by glial cells in vivo and that injection of anti-N-CAM antibodies into the eye-cup of chick embryos disrupts normal patterns of neuritic apposition to glial endfeet in the developing optic stalk. Do the molecules expressed by glia share restricted antigenic determinants, or binding domains, with N-CAM/D2-CAM, or are N-CAM/D2-CAM polypeptides expressed by glia? Here we present immunocytochemical evidence which suggests that all classes of macroglia express N-CAM/D2-CAM antigenic determinants on their surfaces and immunochemical analyses which indicate that the molecules expressed by purified astrocytes are closely similar, or identical, to at least some forms of N-CAM/D2-CAM obtained from whole brain or purified neurones. However, our results also suggest that different N-CAM/D2-CAM polypeptides may be separately expressed by neurones and astrocytes.

SUMMARY1. Five major fibre types in chicken skeletal muscles are recognized, based upon their histochemical and morphological characteristics. A classification of these which is readily related to a commonly used classification of mammalian muscle fibre types is given.2. Seven muscles of the chicken were analysed in recognizing this range of fibre types. The proportions of the different types in each of these were determined. In some cases a gradient of fibre type composition exists across a single muscle.3. Measurements of muscle contraction were used in defining tonic muscles, which contain two fibre types. It was shown that in addition to the anterior latissimus dorsi (a.l.d.), previously well known to be a tonic muscle, two other muscles, the plantaris and the adductor profundus, are of the same class, but differ subtly from the ald. in certain features. Gross red colouration is not a useful diagnostic feature of slow muscles, since the tonic adductor profundus, for example, is white.4. Fibres similar histochemically to mammalian type I (slow-twitch) occur in some of the avian twitch muscles investigated. These are oxidative in character, and despite the fact that they are multiply innervated we suggest that these are avian slow-twitch fibres.5. The patterns of cholinesterases found in a skeletal muscle correspond to its fibre type composition, with regard to both the concentrations and the proportions of the multiple forms of enzyme present. The distinctive patterns of those forms of acetylcholinesterase in the different fibre types are described.6. The fibre type composition is changed by inherited muscular dystrophy in a characteristic manner. This change has so far been found (at the earlier stages of the disease) only in the muscles with a predominance of type II1B fibres in the normal chicken. Pathological changes within the fibres occur selectively in the type JIB fibres, but there are exceptions to this and the effect can be greatly modified by the type of neighbouring fibres.

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The biosynthesis of the neural cell adhesion molecule (N-CAM) was studied in primary cultures of rat cerebral glial cells, cerebellar granule neurons, and skeletal muscle cells. The three cell types produced different N-CAM polypeptide patterns. Glial cells synthesized a 135,000 Mr polypeptide B and a 115,000 Mr polypeptide C, whereas neurons expressed a 200,000 Mr polypeptide A as well as polypeptide B. Skeletal muscle cells produced polypeptide B. The polypeptides synthesized by the three cell types were immunochemically identical. The membrane association of polypeptide C was investigated with methods that distinguish peripheral and integral membrane proteins. Polypeptide C was found to be a peripheral membrane protein, whereas polypeptides A and B were integral membrane proteins with cytoplasmic domains of ~50,000 and ~25,000 Mr, respectively. The affinity of the membrane binding of polypeptide C increased during postnatal development. The posttranslational modifications of polypeptide C were investigated in glial cell cultures, and it was found to be N-linked glycosylated and sulfated.

Posttranslational modifications and intracellular transport of the D2-cell adhesion molecule (D2-CAM) were examined in cultured fetal rat neuronal cells. Developmental changes in biosynthesis were studied in rat forebrain explant cultures. Two D2-CAM polypeptides with Mr of 187,000-210,000 (A) and 131,000-158,000 (B) were synthesized using radiolabeled precursors in cultured neurons. A and B were found to contain only N-linked complex oligosaccharides, and both polypeptides appeared to be polysialated as determined by [14C]mannosamine incorporation and precipitation with anti-polysialic acid antibody. The two polypeptides were sulfated in the trans-Golgi compartment and phosphorylated at the plasma membrane. D2-CAM underwent rapid intracellular transport, appearing at the cell surface within 35 rain of synthesis. A and B were shown to be integral membrane proteins as seen by radioiodination by photoactivation employing a hydrophobic labeling reagent. In rat forebrain explant cultures, D2-CAM was synthesized as four polypeptides: A (195,000 Mr), B (137,000 Mr), C (115,000 Mr), and a group of polypeptides in the high molecular weight region (HMr) between 250,000 and 350,000. Peptide maps of the four polypeptides yielded similar patterns. Biosynthesis of C and HMr increased with age, relative to A and B. A and B were sulfated in embryonic brain, however, sulfation was not noticeable at postnatal ages. Phosphorylation, on the other hand, of A and B was observed at all ages examined. We suggest that D2-CAM function may be modified during development by changes in the relative synthesis of the different polypeptides, as well as by changes in their glycosylation and sulfation.