THE ELASMOBRANCH THYMUS - ANATOMICAL, HISTOLOGICAL, AND PRELIMINARY FUNCTIONAL-CHARACTERIZATION

Thymi from 22 species of elasmobranch fish (class Chondrichthyes) were examined. Information on the anatomical location, organ arrangement, and cellular composition of the thymi as a function of sexual maturation as well as preliminary characterization of thymocyte functional activities are reported. Most of the sharks, skates, rays, and guitarfish reported herein represent species from which thymus had not been previously documented. The elasmobranch thymus is bilateral and situated dorsomedial to both gill regions. Although its size and location relative to the surrounding musculature varied with sexual maturation, thymus was confirmed in every specimen examined. Separate cortical and medullary regions resembling the cortex and medulla of mammalian thymi were consistent features among all elasmobranch thymi examined. Tissue imprints confirmed the presence of thymocytes ranging in size from small, darkly staining cells to large, lightly staining blast-like cells. Large thymic cells, containing cell debris or intact thymocytes were often observed. In vitro functional assays including cytochemical visualization of lysosomal enzymes characteristic of stage-specific thymocyte maturation, and the presence of nuclear and cell surface biochemical markers associated with immature thymocytes suggest functional roles for elasmobranch thymi similar to those of the thymi of higher vertebrate classes. Proliferation of senescent nurse shark thymocytes in culture as determined by cell flow cytometry and tritiated thymidine incorporation was also similar to in vitro responses typical of higher vertebrate mitogen-independent thymocyte proliferation. (C) 1995 Wiley-Liss, Inc.