The pineal gland is an evagination from the roof of the diencephalon that is organized and shaped differently across species [1]. The pineal gland of fish and frogs is a vesicle connected to the roof of the diencephalon by a slender stalk with a cerebrospinal fluid-filled lumen that is opened to the third ventricle [2, 3]. In lizards and avian, the pineal gland shape becomes follicular. Vollrath (1981) described the pineal gland of mammals as formed in a gland shape and located in the epithalamus, near the center of the brain, between the two hemispheres [4, 5]. The avian pineal gland is composed of a very complex internal structure with intricate functional organization, which may be due to its intermediate evolutionary place between lower vertebrates and mammals [6].
Three types of pineal glands can be distinguished based on the histological structure of avian pineal glands: saccular, tubule-follicular, and solid (Collin, Calas, & Juillard, 1976; Vollrath, 1981). Tubule-follicular pineal glands are present in pigeon (Columba livia), Japanese quail (Coturnix japonica), Muscovy duck (Cairina moschata), turkey (Meleagris), and young chicken (Gallus Gallus) [5, 7–9]. In chicken, the tubule-follicular-type pineal gland consists of many follicles and tubules with cavities. During development, the chicken pineal gland gradually loses its follicular features and takes on parenchymal structures, similar to those observed in most mammalian pineal glands [10](. The morphological change in the chicken pineal gland represents the dramatic transformation of its cell types [6].
The pineal gland produces melatonin, which helps to maintain circadian rhythm and regulate reproductive hormones [1]. In some of the lower vertebrates, the pineal gland has a sensory function to detect light. This kind of pineal gland is called the “parietal eye” because it has photoreceptors and is superficially situated in the brain [11]. The pineal gland is photoreceptive and endocrine in avian, but only endocrine in mammals [6, 12]. Although the pineal gland of postnatal mammals no longer has photoreceptors, there are other nerve conduction pathways that indirectly affect photosensory responses of the pineal gland [13–15].
The avian pineal gland contains three main types of cell populations: photoreceptor-like cells, pinealocytes, and supporting cells. Immunocytochemical characterization of the chicken and pigeon pineal glands has shown that pineal photoreceptor-like cells contain molecules that are very closely related to those expressed in retinal photoreceptors [16] indicating that they are modified or rudimentary photoreceptors. The photoreceptor-like cells organize radially around the lumen of each follicle and their outer segments are regressed and are less regular than the true photoreceptor cells [12]. The pinealocytes of the avian pineal gland are located among supporting cells near the luminal surface of follicles.
Based on the ultrastructure and the presence of outer segments, avian pinealocytes can be further characterized into three types: receptor pinealocytes, rudimentary-receptor pinealocytes, and secretory pinealocytes [5, 7]. In chickens, rudimentary-receptor pinealocytes are characterized by the presence of apical protrusions that lack membranous whorls [17]. Further, rudimentary-receptor pinealocytes are the predominate cells of the photoreceptor line, although receptor pinealocytes and secretory pinealocytes may also be present in young chicken [8, 17]. The supporting cells of the avian pineal gland mainly consist of ependymal- and astrocyte-like cells. Supporting cells occupy the pineal parenchyma and isolate the other cell types from the blood vessels surrounding the organ. Previous studies have shown that the supporting cells in domestic turkey contain numerous intermediate filaments (IFs) that fill the basal part of ependymal-like cells and most of the cytoplasm of astrocyte-like cells [18, 19].
Neuronal IFs are important cytoskeletal filaments expressed in neurons that can be characterized into five categories: low, middle, and high molecular mass neurofilament (NF) triplet proteins; α-internexin; and peripherin [20]. Neuronal IFs serve supporting and scaffolding roles in axon and dendrite outgrowth, stabilization, and function [21]. The IF protein vimentin is also found in neurons, especially during early development and injury-induced axonal degeneration [22, 23]. The 66 kDa protein α-internexin was purified with IFs from the rat spinal cord and optic nerve [24]. α-Internexin is expressed in most neurons as they begin to differentiate, and its expression precedes NF triplet protein expression [25–27]. Unlike the NF triplet proteins, which are obligate heteropolymers, α-internexin can form homopolymers or assemble with NF proteins or vimentin [26–29]. Previous studies from our laboratory have molecularly cloned the mRNA sequence encoding the chicken α-internexin (chkINA) protein. chkINA expression was detected during the early stage of brain development and was the major IF protein in the parallel processes of the cerebellar granule neurons[30]. Moreover, chkINA was expressed in all neuronal lineages of the developing chicken retina, including photoreceptors [31].
Previous work has also shown that some cells may differentiate into neuron-like cells in the postnatal mouse pineal gland, and that these cells possess dual properties of neurons from the central and peripheral nervous systems (CNS and PNS, respectively). Further, the neuron-like cells may act as interneurons to convey signals to the pinealocytes [32]. Some pinealocytes in non-mammalian vertebrates also have similar functions to the retina photoreceptor in many non-mammalian vertebrates [33]. However, little is known about the molecular biology of pinealocytes and photoreceptor-like cells in non-mammalian vertebrates, leaving an evolutionary gap regarding the understanding of the pineal gland between mammals and non-mammalian vertebrates. The chicken is an essential model organism in developmental biology, but the role of chkINA in the developing and adult chicken pineal gland remains unknown. Hence, the purpose of this study was to analyze the spatial and temporal distribution patterns of chkINA in photoreceptor-like cells during chicken pineal gland development.