Regeneration of the pharynx in a freshwater planarian an electron microscopic study with special reference to the formation of the pharyngeal cavity and pharyngeal lumen

Asai, E.

Zoological Science 8(4): 775-784

1991


ISSN/ISBN: 0289-0003
Accession: 007731958

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Abstract
Formation of the pharynx, with special reference to the formation of the pharyngeal cavity and pharyngeal lumen, was studied by electron microscopy during regenration of head fragments of planarians transected at the prepharyngeal region. A slit occurs on the third-day at the proximal region of the blastema, which is then enlarged to become the pharyngeal cavity. Another slit is formed on the fourth day at the boundary between the proximal part of the pharyngeal rudiment and the closed end of the cut main-intestinal trunk, and this slit is later enlarged to penetrate into the pharyngeal rudiment, subsequently becoming the pharyngeal lumen. A common ultrastructural feature of cells that participate in the formation of these slits is the presence of rod-shaped bodies and microvilli. A few slits of small size in the tissue are often seen separate from the rudimental pharyngeal cavity. These slits become connected with one another and, consequently, the rudimental pharyngeal cavity develops into a larger cavity. Formation of phagosomes and disintegration of cells are seen as common morphological features of the processes of enlargement of the pharyngeal cavity and lumen, and it is deduced that such disintegration plays a significant role in the enlargement of the open space. The intestinal cells near the cut end of the intestine separate from the intestinal tissue and take part in the recruitment of epithelial cells facing the pharyngeal lumen. From these observation, transdifferentiation of separated intestinal cells is ascertained.