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 maturati on as well as preliminary characterization of thymocyte functional act ivities are reported. Most of the sharks, skates, rays, and guitarfish reported herein represent species from which thymus had not been prev iously documented. The elasmobranch thymus is bilateral and situated d orsomedial to both gill regions. Although its size and location relati ve to the surrounding musculature varied with sexual maturation, thymu s was confirmed in every specimen examined. Separate cortical and medu llary regions resembling the cortex and medulla of mammalian thymi wer e consistent features among all elasmobranch thymi examined. Tissue im prints confirmed the presence of thymocytes ranging in size from small , darkly staining cells to large, lightly staining blast-like cells. L arge thymic cells, containing cell debris or intact thymocytes were of ten observed. In vitro functional assays including cytochemical visual ization of lysosomal enzymes characteristic of stage-specific thymocyt e 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 vertebr ate classes. Proliferation of senescent nurse shark thymocytes in cult ure as determined by cell flow cytometry and tritiated thymidine incor poration was also similar to in vitro responses typical of higher vert ebrate mitogen-independent thymocyte proliferation. (C) 1995 Wiley-Lis s, Inc.