ULTRASTRUCTURE OF THE COXAL GLAND OF THE HORSESHOE-CRAB LIMULUS-POLYPHEMUS - EVIDENCE FOR ULTRAFILTRATION AND OSMOREGULATION

Electron microscopic examination of the paired coral glands of the hor seshoe crab Limulus polyphemus, focusing on urinary and vascular chann els, shows six morphologically distinct regions. Each of four nephridi al lobes consists of two cortical layers surrounding a medulla. The ou ter and inner cortexes contain blood Vessels separated by a basement m embrane from the urinary space lined by podocytes. Podocyte foot proce sses are applied to the basement membrane, interdigitate with those fr om other podocytes, and have a filtration slit diaphragm between them. Cortical morphology demonstrates ultrafiltration of blood, a previous ly undescribed function of the gland, as well as possible endocytic re absorption of materials by the podocytes. The medulla drains into the stolen connecting the four lobes. These two areas have urinary tubules of cuboidal epithelium featuring microvillous-like apical projections , cytoplasmic vesicles and vacuoles, elaborate lateral interdigitation s with septate junctions, and basal invaginations containing numerous mitochondria. These tubules are closely surrounded by blood channels, lined by a basement membrane containing embedded support cells. The me dulla and stolen morphology are suggestive of both ion transport and w ater movement, in keeping with the gland's role in osmoregulation. The stolen empties into the end sac in the base of the most posterior lob e. It is lined by tall epithelium exhibiting apical overlap, blunt pro jections into the lumen, apparent endocytic vesicles, and basal plasma membrane infoldings with mitochondria. The end sac drains into the co nducting nephric duct, the proximal end of which is lined by a cuticle . (C) 1997 Wiley-Liss, Inc.