Embryogenesis of the Nasolacrimal Apparatus in the Laysan Albatross (Phoebastria immutabilis: Procellariiformes)
Authors: Rehorek Susan Joyce , Cioccio Joseph , Hillenius Willem Jaap , Smith Timothy D 
Published: November 2023

The Nasolacrimal Apparatus (NLA) is a feature common to many tetrapods, consisting of an orbital gland (Deep Anterior Orbital gland: DAOG aka Harderian gland), a nasal structure (Vomeronasal organ: VNO) and a connecting structure (Nasolacrimal Duct: NLD). There is variation in presence, relative development and ontogeny of each of these components between species. For example, in birds, the DAOG and NLD are well‐developed in many species, but the VNO has not been documented in any adult bird. The embryogenesis of the NLA has only been described in chickens, a group of birds with a short incubation period. The objectives of this study were to describe the embryogenesis of the NLA in a bird with one of the longest incubation periods in an effort to determine whether 1) length of incubation period influences sequence and/or timing of NLA development and 2) whether the chicken is a suitable model for bird development. A series of embryos of the Laysan Albatross, a migratory sea bird whose incubation period averages 65 days, were processed for paraffin histology, serially sectioned and examined with special reference to the NLA. The embryogenesis of the NLA can be subdivided into three broad phases: 1) Origin (Stage 27–28); which includes inception of NLD as an epidermal, near vertical structure residing within the lacrimal lake of the lateral nasal wall. 2) Displacement (Stage 29–33): NLD detaches from the epidermal wall and shifts more deeply within the mesenchyme in the lateral nasal wall. This structure steadily elongates, approaching but not strictly attaching to the orbital and nasal regions. Lacrimal canaliculi form with the DAOG anlage opening just below the upper lacrimal canaliculus. 3) Development of ancillary features (Stages 34–39): The NLD continues to elongate and migrate laterally approximating its final position. Two features develop during this phase: 1) a VNO‐like structure. This is a pair of small medial diverticula in the nasal septum, which, unlike a true VNO, this is not found in the floor of the nasal cavity and do not appear to have any sensory neurons. 2) Bony support for the NLA. NLD passes medial to the nasal/premaxilla bones (lateral to the nasal capsule), traverses the antorbital fenestra and passes lateral to the lacrimal bone, thereby connecting the orbital to the nasal region. This sequence of events in the Laysan Albatross is similar to those described in both the American Alligator and in humans, but unlike that described in the chicken. Though adult birds lack a VNO, the presence of a VNO‐like structure in the developing Laysan Albatross, as in the developing crocodile, supports prior suggestions that it is a primitive characteristic which appears as a transitional embryonic structure. This would imply that a longer incubation period allows for the development of features, even if they are transitory, that are otherwise lost in species with shortened incubation period. Thus the chicken and the Laysan albatross, with their different incubation periods, show that there is a spectrum of bird embryology, and that a single bird (i.e., the chicken) should not be used as the sole model for Avian development.