THE PRONEPHROS OF THE EARLY AMMOCOETE LARVA OF LAMPREYS (CYCLOSTOMATA, PETROMYZONTES) - FINE-STRUCTURE OF THE RENAL TUBULES

The renal tubules of the paired pronephros in early larvae (ammocoetes) of two lamprey species, Lampetra fluviatilis and Petromyzon marinus, were studied by use of light-, scanning- and transmission electron microscopy. They consist of (1) a variable number of pronephric tubules (3 to 6), and (2) an excretory duct. By fine-structural criteria, the renal tubules can be divided into 6 segments. Each pronephric tubule is divided into (1) the nephrostome and (2) the proximal tubule, the excretory duct consisting of (3) a common proximal tubule followed by (4) a short intermediate segment, and then by a pronephric duct composed of (5) a cranial and (6) a caudal section. The epithelium of the nephrostome displays bundles of cilia. The cells of the proximal tubule possess a brush border, many endocytotic organelles and a system of canalicule (tubular invaginations of the basolateral plasmalemma). The same characteristics are encountered in the epithelium of the common proximal tubule; however, the number of these specific organelles decreases along the course of this segment in a posterior direction. In the intermediate segment, the epithelium appears structurally nonspecialized. The cells of the cranial pronephric duct lack a brush border; they have an extensive system of canaliculi and numberous mitochondria. The caudal pronephric duct is lined by an epithelium composed of light and dark cells; the latter are filled with mitochondria and the former contain mucus granules beneath the luminal plasmalemma. The tubular segments found in the pronephros are the same in structure and sequence as in the lamprey opisthonephroi. However, only the nephrostomes and proximal tubules occur serially in the pronephros, while the common proximal tubule, the intermediate segment and the cranial pronephric duct form portions of a single excretory duct.