Parietal eye

Abstract: This chapter discusses the data available on the innervation of the epiphysis and the accessory pineal organs in different vertebrate classes, such as fishes, amphibians, reptiles, and birds. In most submammalians, with a possible exception for the epiphysis of snakes, turtles, and birds, neurosensory cells are found in the organs of the pineal complex. Nerve cells have also been observed in both the pineal and accessory pineal organs of lower vertebrates. These structures, which vary widely in the shape, size, and degree of differentiation, have to be regarded in submammals as being primarily photosensory organs. They may function as such during shorter or longer periods of life. The epiphysis of mammals does not show sensory cells. Pineal nerve cells are exclusively present in primates. In general, fibers enter the epiphysis from both the habenular and the caudal commissure. In many mammalian species, the epiphysis contains a dense network of autonomic nerve fibers that enter the organ by two bilaterally symmetrical nervi conarii, vascular plexuses, or both. In subprimate mammals, these fibers are probably rather exclusively orthosympathetic postganglionics, their cells of origin lying within superior cervical ganglia. In primates, the nervi conarii may contain preganglionic parasympathetic fibers synapting with epiphyseal nerve cells, thereby constituting an intramural ganglion.

Abstract: Pax-6 genes have been identified from a broad range of invertebrate and vertebrate animals and shown to be always involved in early eye development. Therefore, it has been proposed that the various types of eyes evolved from a single eye prototype, by a Pax-6-dependent mechanism. Here we describe the characterization of a cephalochordate Pax-6 gene. The single amphioxus Pax-6 gene (AmphiPax-6) can produce several alternatively spliced transcripts, resulting in proteins with markedly different amino and carboxy termini. The amphioxus Pax-6 proteins are 92% identical to mammalian Pax-6 proteins in the paired domain and 100% identical in the homeodomain. Expression of AmphiPax-6 in the anterior epidermis of embryos may be related to development of an olfactory epithelium. Expression is also detectable in Hatschek's left diverticulum as it forms the preoral ciliated pit, part of which gives rise to the homolog of the vertebrate anterior pituitary. A zone of expression in the anterior neural plate of early embryos is carried into the cerebral vesicle (a probable diencephalic homolog) during neurulation. This zone includes cells that will differentiate into the lamellar body, a presumed homolog of the vertebrate pineal eye. In neurulae, AmphiPax-6 is also expressed in ventral cells at the anterior tip of the nerve cord; these cells are precursors of the photoreceptive neurons of the frontal eye, the presumed homolog of the vertebrate paired eyes. However, AmphiPax-6 expression was not detected in two additional types of photoreceptors, the Joseph cells or the organs of Hesse, which are evidently relatively recent adaptations (ganglionic photoreceptors) and appear to be rare exceptions to the general rule that animal photoreceptors develop from a genetic program triggered by Pax-6.

Abstract: From a phylogenetic aspect, few organs have undergone a change in form and cytological differentiation as has the pineal organ. Among the lower vertebrates, the pineal organ is a sense organ containing receptors and nerve cells; in some forms, it resembles an eye. From a comparative consideration of the pineal organ, the morphological crux of the problem lies in its transformation from a sense organ to a gland. Knowledge of the progressive and regressive processes in ontogeny and phylogeny is of more than theoretical interest because it may provide the basis for the avoidance of difficulties in the selection of experimental species. To understand the nature of an individual pineal form, cytological criteria are necessary. The presence of rudimentary structures and the multiplicity of forms urge caution in the interpretation of the pineal organ, even when the physiological information is encouraging in individual cases. Knowledge of the pineal organ, especially in lower vertebrates, is based on a relatively small number of species.

Abstract: The frontal eye field and parietal eye field are known to be involved during visually guided saccades. As the location of the human parietal eye field is not yet well known, functional MRI was used during such a saccade task to better localise this field. Besides activity in visual areas of the occipital cortex, bilateral activity was seen in the precentral sulcus, corresponding to the frontal eye field, and in the deep region of the intraparietal sulcus. It is suggested that this intraparietal area, bordering areas 39 and 40 of Brodmann, corresponds to the human parietal eye field.