Comparative ultrastructure of the cellular components of the unconstricted notochord in the sturgeon and the lungfish.

Unconstricted notochords are found in only a few extant vertebrates. Within the bony fishes the notochord is present throughout life in the sturgeons and the lungfishes. To better understand the function of this organ, the cellular ultrastructure of the notochord was examined in the shortnose sturgeon, Acipenser brevirostratus, and in the African lungfish, Protopterus annectens. In both species, the medulla of the notochord consists of vacuolated cells connected by desmosomes. The vacuoles are surrounded by a dense cytoplasmic network of intermediate filaments. In Acipenser, the cuboidal to columnar basal cells are avacuolate and contain an extensive network of rough endoplasmic reticulum. In Protopterus, the cytoplasm of the basal cells lacks rough endoplasmic reticulum and often contains a large central vacuole surrounded by a network of intermediate filaments. Cells resembling fibroblasts are found in the fibrous sheath of the notochord of the lungfish, but the fibrous sheath of the sturgeon is acellular. On the basis of the structures observed in the two species, the medulla of the notochord has many of the characteristics of a keratinized epithelium, and the fibrous sheath may be derived from different cellular sources. J. Morphol. 236:75-104, 1998. © 1998 Wiley-Liss, Inc.