Ultrastructure of the coxal gland of the horseshoe crab Limulus polyphemus: Evidence for ultrafiltration and osmoregulation.

Abstract

Electron microscopic examination of the paired coxal glands of the horseshoe crab Limulus polyphemus, focusing on urinary and vascular channels, shows six morphologically distinct regions. Each of four nephridial lobes consists of two cortical layers surrounding a medulla. The outer and inner cortexes contain blood vessels separated by a basement membrane from the urinary space lined by podocytes. Podocyte foot processes are applied to the basement membrane, interdigitate with those from other podocytes, and have a filtration slit diaphragm between them. Cortical morphology demonstrates ultrafiltration of blood, a previously undescribed function of the gland, as well as possible endocytic reabsorption of materials by the podocytes. The medulla drains into the stolon connecting the four lobes. These two areas have urinary tubules of cuboidal epithelium featuring microvillous-like apical projections, cytoplasmic vesicles and vacuoles, elaborate lateral interdigitations 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 medulla and stolon morphology are suggestive of both ion transport and water movement, in keeping with the gland's role in osmoregulation. The stolon empties into the end sac in the base of the most posterior lobe. It is lined by tall epithelium exhibiting apical overlap, blunt projections into the lumen, apparent endocytic vesicles, and basal plasma membrane infoldings with mitochondria. The end sac drains into the conducting nephric duct, the proximal end of which is lined by a cuticle. J. Morphol. 234:233-252, 1997. © 1997 Wiley-Liss, Inc.