Sponge dermal membrane morphology: Histology of cell-mediated particle transport during skeletal growth.

Abstract

The keratose demosponge, Dysidea etheria, secretes a skeletal network of interconnecting spongin fibers and incorporates sand grains and other foreign particles into the fibers. Previous studies showed that foreign particles enhanced fiber growth and increased the stiffness of skeletal fibers. The present study uses light and electron microscopy to study the mechanism of particle incorporation. These histological observations show that particles become engulfed into thickened regions of the mesohyl of the dermal membrane, whereupon ameboid cells contact particles and secrete an electron dense layer of extracellular material onto the particle surface. Within several days, particles are moved towards primary skeletal fibers located at cone-shaped protuberances on the sponge surface called conules. Particles accumulate at conules and are consolidated by secretion of spongin at the growing fiber apices. The observations suggest that coordinated migration of groups of mesohyl cells control particle transport to conules and that patterns of cell migration are associated with the structural organization of the dermal membrane. Particle transport may be homologous and analogous to spicule transport in other demosponges.