Ultrastructural evidence of the excretory function in the asteroid axial organ (Asteroidea, Echinodermata).

The ultrastructure of the axial organ of Asterias amurensis has been studied The organ is a network of canals of the axial coelom separated by haemocoelic spaces. The axial coelom is lined with two types of monociliary cells: podocytes and musculo-epithelial cells. Podocytes form numerous basal processes adjacent to the basal lamina on the coelomic side. Musculo-epithelial cells form processes running along the basal lamina. Some bundles of these processes wrapped in the basal lamina pass through haemocoelic spaces between neighboring coelomic canals. It is hypothesized that the axial organ serves for filtration of fluid from haemocoelic spaces into the axial coelom cavity, from which urine is excreted through the madreporite to the exterior.

[Ultrastructure of coelomic epithelium and coelomocytes of intact and wounded starfish Asterias rubens L].

Samples of coelomic epithelium and coelomocytes suspension of intact and wounded starfishes Asterias rubens L. were analyzed by electron microcopy. It has been demonstrated that coelomic epithelium is composed of three types of cells: flagellar (approx. 60%), secretory (approx. 3%) and myoepithelial (approx. 37%). Flagellar and secretory cells form the apical surface of coelomic epithelium. Secretory cells are represented by two subtypes: granular and mucous secretory cells. Myoepithelial cells are located in the basal zone of the epithelium. Adjacent flagellar cells are separated by intercellular gaps of various size in 4-5% of cases. These gaps are apparently the lacunae left by the flagellar cells after their departure to the coelomic cavity. The morphological pattern of transition of coelomic epithelium flagellar cells to the coelomocytes has been characterized. No significant structural alterations in organization of the coelomic epithelium were revealed after moderate wounding used in the present study. Small round-shaped young coelomycytes (approx. 3%) and bigger mature coelomocytes (approx. 97%) were found in coelomocytes suspension. A flagellum was revealed on the surface of one of the young coelomocytes. Surface of the mature coelomocytes forms the processes of various size and structure; their cytoplasm contains lysosomes and fagocytic vacuoles of different size. After wounding, activation of coelomocytes was noted finding expression in the sharp rise in the number and the length of their surface filopodia, and in the multicellular aggregates formation. By the sum of the ultrastructural data, histogenesis of coelomocytes from the flagellar cells of the coelomic epithelium is supposed to be a process of cellular transdifferentiation.

Adhesive papillae on the brachiolar arms of brachiolaria larvae in two starfishes, Asterina pectinifera and Asterias amurensis, are sensors for metamorphic inducing factor(s).

It has been hypothesized by Barker that starfish brachiolaria larvae initiate metamorphosis by sensing of metamorphic inducing factor(s) with neural cells within the adhesive papillae on their brachiolar arms. We present evidence supporting Barker's hypothesis using brachiolaria larvae of the two species, Asterina pectinifera and Asterias amurensis. Brachiolaria larvae of these two species underwent metamorphosis in response to pebbles from aquaria in which adults were kept. Time-lapse analysis of A. pectinifera indicated that the pebbles were explored with adhesive papillae prior to establishment of a stable attachment for metamorphosis. Microsurgical dissections, which removed adhesive papillae, resulted in failure of the brachiolaria larvae to respond to the pebbles, but other organs such as the lateral ganglia, the oral ganglion, the adhesive disk or the adult rudiment were not required. Immunohistochemical analysis with a neuron-specific monoclonal antibody and transmission electron microscopy revealed that the adhesive papillae contained neural cells that project their processes towards the external surface of the adhesive papillae and they therefore qualify as sensory neural cells.