Unusual cuticle-associated organs in the heterotardigrade, Echiniscus viridissimus.

Abstract

AbstractEchiniscus viridissimus possesses two organs that lie above the ventromedial body wall approximately at the level of the second and third pair of legs. Each organ is composed of one medial and two lateral cells and possesses a pair of convoluted ducts. The lateral cells produce and release profuse amounts of filamentous basement membrane‐like material into the body cavity. The proliferated material persists en masse adjacent to the lateral cells and is associated with disrupted droplets of pigmented body cavity material (BCM). The medial cell is larger than the lateral cells and possesses numerous tubular invaginations in which droplets of BCM are frequently found. Each duct originates as an infold that becomes a simple channel in the lateral cell and continues through the medial cell in a deep groove covered by a long process of the lateral cell. Although the ducts lie on the medial cell surface, they are “internalized” at the base of deep clefts and isolated from the body cavity by “tight” zonulae adherentes junctions. In the medial cell the ducts have a cuticle‐like lining and the associated plasma membrane is characterized by numerous saccular infolds containing flocculent material. The duct linings, which extend beyond the limits of the medial cell, project into the endocuticle and terminate on the lateral body wall. Because the lateral cells are producing and releasing profuse basement membrane and because material with an electron density similar to BCM is present in the saccules, the distal regions of the ducts and in the endocuticle, we propose (1) that the lateral cells are producing basement membrane to disassociate BCM and (2) that the medial cell is absorbing and via its ducts translocating that BCM to the endocuticle. Furthermore, because the medial cell exhibits characteristics of transporting epithelia, namely numerous tubular infolds of the outer surface, a layer of distended saccules surrounding the duct and a high density of mitochondria, we suggest that the medial cell also functions in ion or osmotic regulation. © 1992 Wiley‐Liss, Inc.