Ultrastructure and Filtrating Function of the Ciliated Epithelial Cells of Foregut in Urechis unicinctus

개불(Urechis unicinctus) 전장 섬모 상피세포의 미세구조와 여과기능

It is suggested that Urechis unicinctus is a filler feeder as like many tide and watery invertebrates which filtrate food materials by ciliary movement. However, the structure of the filter is not yet known in U. unicinctus, nor the filtering mechanism is not well understood. This study reveals ciliated epithelial cells in the foregut and the features of the cilia are good accord with that of known filtrating apparatus of other tide animals. This may implies that the foregut is in function of filtration and the food materials are filtrated by the ciliary movement. With the observation of the filtrating apparatus in the foregut, the intestine of U. unicinctus can be functionally compartmented into 3 parts. These are already known midgut and hindgut in function of digestion and respiration respectively, and the foregut in function of filtrating apparatus for foods. The filtrating apparatus of U. unicinctus is composed of the pseudostratified columnar epithelial cells with numerous cilia. The cilia are well differentiated kinocilia with the typical microtubule pattern, kinetosome and cilia roots. There are two kinds of striated cilia roots, the main root and the accessory root. The main root is extended perpendicularly from the cell surface to basement membrane and the short accessory root is branched with an acute angle of about $80^{\circ}$ from the main root at level of basal plate of the kinetosome. The spacial approaches of the main root with the large fused form of mitochondria is one of the characteristic features which might be in structural consideration an intimate association between energy source and energy mass consuming cell organelles.