Barrier cells in gills of the alligator gar

Studies of barrier cells in fishes are few and have been restricted primari ly to salmonids. Moreover, these studies revealed the presence of barrier c ells only in hematopoietic and lymphoid organs. The alligator gar, Leplsost eus spatula, possesses similar tissues within the gills which are readily a ccessible; therefore, they were the organs of choice. An electron microscop ical analysis of the gills was undertaken to determine if barrier cells exi st and if they possess an ultrastructural design comparable to their counte rparts in the brain and lungs of higher vertebrates and hematopoietic/lymph oid tissues in fishes. The present study revealed that barrier cells were f ound only within non-hematopoietic/lymphoid areas. Barrier cells surrounded endocrine components, nerves, and sinusoids rather than capillary endothel ium or hematopoietic/lymphoid tissues. Barrier cells in the alligator gar d isplayed a complex envelopment more similar to those found in the blood-cer ebrospinal fluid, blood-brain, and blood-gas barriers in higher vertebrates than in salmonids. The barrier in the alligator gar consisted of: (1) an e ndothelium whose cells displayed tight junctions; (2) a basement membrane; and (3) an outer adventitia composed of fibrocytic cells in syncytium. (C) 1999 Harcourt Publishers Ltd.