Pineal organ of the Muscovy duck

Abstract

Summary The avian pineal is a photosensory organ taking part in the organization of circadian and seasonal rhythms and playing an important role in the regulation of many behavior and physiological phenomena. The morphology of the pineal organ shows an enormous diversity. The aim of study was to investigate the histology and ultrastructure of the pineal organ in the Muscovy duck (Cairina moschata). The study was performed on 14-week-old females kept under natural lighting. The investigated pineals consisted of a wide distal part and narrow middle and proximal parts. The proximal part was attached to the diencephalon via the choroid plexus. The light microscopy showed a mixed structural form of the organ, which comprised both tubulo-follicular and solid parts. The stroma contained the lymphatic tissue, which occurred in a diffused form and as lymphoid nodules. Electron microscopy revealed the huge complexity of the parenchyma architecture, caused by the presence of several lumen-containing structures. According to their size and shape, they were classified into classical follicles, canals (diameter > 1 m) and canaliculi (diameter < 1 m). Pinealocytes were characterized by high variability of their shapes and sizes, frequently occurring partial or total reduction of the apical protrusions and presence of extremely numerous microtubules. The supporting cells constituted a prominent part of the pineal parenchyma and were represented by ependymal-like cells and astrocyte-like cells. Ependymal-like cells limited the lumen of follicles, canals and canaliculi.