Ultrastructure of serotonin-containing cells in the pineal organ of Lampetra planeri (Petromyzontidae)

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Abstract
The ultrastructure of the “cells containing residual bodies” (Collin, 1969) was investigated in the pineal organ of Lampetra planeri. These cells are characterized by their indoleamine metabolism (Meiniel, 1978; Meiniel and Hartwig, 1980). Morphologically, they belong mainly to two types: (1) a photoreceptor cell type, and (2) a pinealocyte cell type. The first type is present in the pineal sensory epithelium and in the atrium, while the second is observed in the deep part of the atrium. Intermediate cell types are rare. All these cells are characterized by the presence of voluminous dense bodies, the 5-HT-storing structures, in their cytoplasm. The elongated cone-type photoreceptor cells show a segmental organization and well-developed outer segments consisting of short disks (2–3 μm), while their basal pedicles form synapses with the dendritic processes of neurons. The pinealocytes are spherical or oval in shape, their receptor poles being regressed to cilia of the 9+0 type. In these cells, no synaptic ribbons have to date been observed. In both cell types a Golgi apparatus is present producing dense granules 130 nm in diameter and a polymorphous dense material. The photoreceptor cells most probably respond to light and transmit a sensory (i.e., nervous) message. In addition, they produce and metabolize indoleamines, probably including, melatonin (Meiniel, 1978; Meiniel and Hartwig, 1980). The pinealocytes, in spite of their loss of direct photosensitivity, retain their capacity to metabolize indoleamines (Meiniel, 1978; Meiniel and Hartwig, 1980). The presence, in the same pineal organ, of another photoreceptor cell type (cf. Collin, 1969–1971) differing morphologically as well as biochemically (no detectable indoleamine metabolism) from the photoreceptor cell type described in the present investigation, points to the existence of two different sensory cell lines: (1) a “pure” photoreceptor line, and (2) a photoneuroendocrine line. The phylogenetic evolution of these two cell lines is discussed in terms of functional analogy.