Histological and ultrastructural reconstruction of ventral epidermal glands of Spio (Polychaeta, Spionidae, Annelida)

摘要

With present study, we confirm that the ventral structures on anterior and middle chaetigers of two species of Spio are special kind of intraepidermal multicellular glands. The general anatomy and coherence morphology of these ventral epidermal glands (VEGs) were 3D-reconstructed based on semithin section series. Specific anatomical characters were explored using electron microscopy, such as scanning electron microscopy for characterization of gland pores and transmission electron microscopy for investigation into cytoanatomical details. The VEGs are tubular or acinar; their pores are spherical or lens-shaped. The glands consist of three cell types: secretory cells, sheath cells and canal cells, forming and strengthening the short gland duct and the pore region. The secretory cells are tightly packed into a single-layered, curved glandular epithelium. Secretory cells are separated from each other by thin projections of interstitial sheath cells. The apices of the secretory and sheath cells surround a tubular or vase-shaped extracellular space, the reservoir. The reservoir is traversed by microvilli of the secretory and sheath cells. Two sorts of microvilli characterize the apex of secretory cells: slender, elongated ones located at the periphery (outer microvilli), and shorter but thicker ones, arranged in a distinct collar in the centre (inner microvilli). Secretion, encased in regular secretory granules and discharged only at the apex' centre, is guided through the space enclosed by the collar of inner microvilli. Though also consisting of numerous cells, the VEGs of Spio spp. are not comparable to parapodial glands described in other spionid taxa since they remain in a strictly intraepidermal position, are distant from parapodia and contain sheath cells. Nonetheless, the VEGs are very helpful for taxonomic work on representatives of the genus Spio and closely related genera.