SEDLMAYR, J; LEGENZOFF, E; REHOREK, S; School of Medicine, Univ of Ca, Los Angeles; Slippery Rock Univ; Slippery Rock Univ: Anatomy of the Circadian Clock in Archosaurs

The Harderian-pineal-retinal-hypothalamic axis (HaH) and its relation to circadian rhythms (cyclical behavioral patterns regulated by indoles) is a critical component of amniote biology. Little is known of this system in Crocodylia, a clade that purportedly lacks a pineal gland but demonstrates a rhythmic melatonin cycle. In tetrapods ancestrally the Harderian gland is functionally linked with the vomeronasal organ and through secretion of melatonin with the circadian system. However, extant avesuchian archosaurs (crocodilians and birds) are derived in the absence of the vomeronasal organ. To further elucidate the evolutionary history of the HaH in amniotes, a comparative anatomical and experimental study was made of this system in several tetrapod clades, with special attention to extant archosaurs. Although there are some structural similarities in the Harderian gland of crocodilians and lepidosaurs which are not shared with birds, there are more, far-reaching similarities within the archosaurs. This includes the level of vascularity in the three components (retina, Harderian gland and �pineal� organs), the secretory and immunological nature of the Harderian gland, and the presence of a follicular �pineal� gland. The follicular nature of the crocodilian �pineal� does bear some resemblance to that of basal birds. Additionally, avesuchians are characterized by distinct neural and vascular paths and a Harderian gland that rhymically secretes melatonin, implying a role of the Harderian gland as an intrinsic component of the circadian endocrinological network.