Development of the notochord in the Japanese medaka, Oryzias latipes (Teleostei; Cyprinodontidae), with special reference to desmosomal connections and functional integration with adjacent tissues

Abstract

We describe development of the notochord from stage 28 (appearance of fin folds) to stage 35 (5 days after hatching). Beginning immediately after hatching (stage 32), the notochord, vertebral bone, and axial muscles develop as a structural unit. Integration of this unit is effected by (i) the appearance and maturation of desmosomes with cytoplasmic plaques and tonofilaments, (ii) the deposition of vertebral bone at stage 32, and (iii) integration of the periosteum with the connective tissue capsule of the axial muscles. Desmosomes connect notochordal cells with one another and with cells of the notochordal epithelium. Extracellular matrix integrates bone with inner notochordal epithelium, periosteal cells with one another, and periosteum with the muscle connective tissue capsules. Therefore, development of desmosomes and deposition of extracellular matrices, beginning at stage 28, provide the cellular basis for the structural integration of notochord, bone, and muscle. Onset of structural integration immediately precedes the first vigorous movement that occurs at stage 29. This developmental analysis has therefore identified the cellular basis of the structural and functional integration of the axial locomotory tissues. Such an approach provides a means of understanding the developmental basis of functional morphology.