Styletogenesis in nemertean worms: The ultrastructure of organelles involved in intracellular calcification

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Abstract
The ultrastructure of cellular organelles involved in stylet formation is examined in six species of nemertean worms by transmission electron microscopy (TEM). Stylets are nail‐shaped structures containing calcium phosphate that are assembled intracellularly in large uninucleate cells, called styletocytes. Each stylet develops within a membrane‐bound vacuole in the styletocyte cytoplasm. Well developed arrays of Golgi bodies are typically found in the vicinity of developing stylet vacuoles, and fully formed vacuoles are filled with PAS+ material that appears to be derived from the smooth endoplasmic reticulum. At the onset of styletogenesis, a conical sliver of organic material differentiates on the inner surface of the vacuolar membrane. This material displays a species‐specific banding pattern in decalcified sections, and apparently acts as a template during calcification of the stylet shaft. After the organic core of the shaft is formed, mitochondria aggregate around the stylet vacuole and presumably help accumulate the calcium used in mineralization of the stylet. A knob‐shaped proximal piece is subsequently assembled on the base of the shaft. The proximal piece contains a nonbanded matrix and has electron‐dense material at its surface that may help in correctly orienting this region toward the basis during replacement of the central stylet.