Title: The Biological bulletin
Identifier: biologicalbullet188mari (find matches)
Year: [1] (s)
Authors: Marine Biological Laboratory (Woods Hole, Mass. ); Marine Biological Laboratory (Woods Hole, Mass. ). Annual report 1907/08-1952; Lillie, Frank Rattray, 1870-1947; Moore, Carl Richard, 1892-; Redfield, Alfred Clarence, 1890-1983
Subjects: Biology; Zoology; Biology; Marine Biology
Publisher: Woods Hole, Mass. : Marine Biological Laboratory
Contributing Library: MBLWHOI Library
Digitizing Sponsor: MBLWHOI Library

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URN CELL COMPLEX OF SIPUNCULUS NUDL'S 273 .Tfjgp^r- '* * -
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Figure 5. Vesicle cell of the urn cell complex (UCC). (A) Note external surface (arrows) and central space (asterisk). Partial collapse of cell is due to fixation, n. nucleus. (B) Filamentous material in dispersed units (asterisk) or in aggregates (arrow) is present within the UCC central space. Note also lipid droplets (1) and mitochondria (m) within the vesicle cell. Both scale bars: 0.5 ^m. ited abundant extracellular material in this region. The material had a nbrillar-granular structure and was con- tinuous with fibrillar strands originating from ectoplasmic areas of the inner aspect of ciliated cells (Fig. 9D) and from distended bleb-like structures (Fig. 9E). Electron- dense granules were also observed among extracellular fibrils and immediately above the cell membrane (Fig. 9D). Tail-on views of UCCs in transmission and scanning electron micrographs showed that the fibrillar material encircled the inner circumference of the ciliated cell (Fig. 9F-G). This cell contained many empty vacuoles and lipid droplets, little glycogen, and few of the electron-dense ag- gregates observed in UCCs at rest around intracellular vacuoles and below the cell's plasma membrane. Promi- nence of intracytoplasmic vacuoles was also obvious in UCCs examined 20 min after exposure to serum (Fig. 10A). When seen in appropriate longitudinal sections, the released tail contained a fibrillar material and occasional third-type cells (Fig. 10B). The electron density and structure of the tail were similar to those of the fibrillar material present within the bubble-like cavity of the UCC (Fig. 10A). Discussion The present study analyzed the cytology of free- swimming urn cell complexes of the benthic coelomate Sipunculns nudiis. The two large cells, ciliated and ves- icle, of the UCC are complexed by marginal desmo- somes. A smaller and variable population of cells (third- type cells) is frequently, but not invariably, found in close juxtaposition to the ciliated cell. Exposure of a UCC to serum causes a prompt and massive release of a fibrillar-granular material that appears first in the concave side of the ciliated cell and elongates rapidly to form a characteristic tail. During and after release of this tail, increasing cytoplasmic vacuolization occurs in the ciliated cell, together with an apparent trans- membranous discharge of fibrillar material and some loss of glycogen. When observed, third-type cells exhibit phagocytic vacuoles and are displaced distalward by the elongating fibrillary tail. Exocytosis cannot be dem- onstrated in either ciliated or third-type cells. To this date, the cytology of the UCC of S. nudiis has been investigated almost exclusively by light microscopy.

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