Ultrastructure of mucocartilage in the larval anadromous sea lamprey,Petromyzon marinus L.

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Abstract
The fine structure of mucocartilage, a tissue unique to larval lampreys, was examined in Petromyzon marinus L. This tissue is surrounded by a perichondrium of vascularized, dense connective tissue composed of fibroblasts, collagen fibrils, and elastic-like microfibrils, but it is avascular itself and consists of elastic-like microfibrils, ground substance, and a few diffusely scattered fibroblasts. Fibroblasts possess rough endoplasmic reticulum, many free ribosomes, a well-developed Golgi apparatus, a tubulo-vesicular network, and a number of secondary lysosomes containing crystalline material. The appearance of the organelles suggests the involvement of the cell in the synthesis and secretion of the ground substance and microfibrils. Tubular microfibrils, 11–13 nm in diameter, comprise the major portion of the matrix, and they are similar to those described in developing mammalian elastic tissue (Ross and Bornstein, 1969). The retention of the microfibrils may represent either a primitive form of elastic fiber in this “primitive” vertebrate or reflect the larval condition of the lampreys under examination. Scattered spherical to polyhedral-shaped matrix granules and inter-granular filaments make up the remainder of the matrix. It was concluded that mucocartilage in larval lampreys is not a conventional type of vertebrate connective tissue.