A re-examination and re-evaluation of salamander orbital glands

The amphibian integument contains numerous multicellular glands. Although two of these, the nasolabial and orbital glands and the associated nasolacrimal duct (NLD), have historically received considerable attention, interpretation of the original observations can be problematic in the context of current literature. Salamanders, in particular, are frequently regarded as at least indicative of aspects of the morphology of the common ancestor to all extant tetrapods; hence, an understanding of these glands in salamanders might prove to be informative about their evolution. For this study, the orbitonasal region of salamanders from three families was histologically examined. Three themes emerged: (1) examination of the effect of phylogeny on the nasolabial gland and NLD revealed a combination of features that may be unique to plethodontid salamanders, and may be correlated to their nose-tapping behavior by which substances are moved into the vomeronasal organ; (2) ecology appears to impact the relative development of the orbital glands, but not necessarily the nasolabial gland, with smaller glands being present in the aquatic species; (3) the nomenclature of the salamander orbital gland remains problematic, especially in light of comparative studies, as several alternate possibilities are viable. From this nomenclatural conundrum, however, it could be concluded that there may be a global pattern in the location of tetrapod orbital gland development. Molecular questions in terms of ontogeny and genetic homology affect the nature of the debate on orbital gland nomenclature. These observations suggest that rather than reflecting an ancestral condition, salamanders may instead represent a case of specialized, convergent evolution.

The orbitofacial glands of bats: an investigation of the potential correlation of gland structure with social organization

The facial glands of bats are modified skin glands, whereas there are up to three different orbital glands: Harderian, lacrimal, and Meibomian glands. Scattered studies have described the lacrimal and Meibomian glands in a handful of bat species, but there is as yet no description of a Harderian gland in bats. In this study we examined serial sections of orbitofacial glands in eight families of bats. Much variation amongst species was observed, with few phylogenetic patterns emerging. Enlarged facial glands, either sudoriparous (five genera) or sebaceous (vespertilionids only) were observed. Meibomian and lacrimal glands were present in most species examined (except Antrozous), though the relative level of development varied. Two types of anterior orbital glands were distinguished: the Harderian gland (tubulo-acinar: observed in Rousettus, Atribeus, Desmodus and Miniopterus) and caruncular (sebaceous: observed in Eptesicus and Dieamus). The relative development of the nasolacrimal duct and the vomeronasal organ did not appear to be correlated with the development of any of the exocrine glands examined. There does, however, appear to be a correlation between the presence of at least one well developed exocrine gland and the level of communality and known olfactory acuity, best documented in Artibeus, Desmodus, and Miniopterus.

Testing the orbital lubrication hypothesis: the Harderian glands in burrowing skinks (Reptilia: Squamata)

The Harderian gland is an orbital gland thought to be a source of corneal lubricant, supplementary to the other orbital glands. This study investigated the possible role of skink Harderian glands in corneal lubrication. It was hypothesized that if these glands play a role in corneal lubrication, then the structure of these glands would be affected by structural orbital modifications. We examined the Harderian and lacrimal glands of five species of Australian skinks (Lygosominae), two of which possessed orbital modifications in the form of a transparent immoveable eyelid (spectacle) and skull reduction. All species possessed well-developed posterior lacrimal glands, but no anterior lacrimal glands. Anatomically, the Harderian glands were smaller in the burrowing species relative to the nonburrowing species. No other obvious species-specific differences were observed. The absence of any differentiation at the microscopic level suggests that although there is some change in the relative amount of secretant produced, the nature of the secretion studied by classical histochemistry remains essentially unchanged. However, at higher taxonomic levels, the size and structure of the Harderian gland may be taxon-specific and unrelated to the orbital environment. Thus, orbital lubrication may not necessarily be the sole function of the Harderian gland.

"A new lachrymal gland with an excretory duct in red and fallow deer" by Johann jacob Harder (1694): English translation and historical perspective

The Harderian gland is an enigmatic orbital gland that has been described for many tetrapods, although a consistent definition of this structure has remained elusive. In particular, an unambiguous distinction between the Harderian gland and the nictitans gland, which may both occur in the anterior aspect of the orbit of mammals, remains problematic. These glands were first distinguished in 1694 by Johann Jacob Harder, a Swiss physician and anatomist. To facilitate a renewed examination of the anatomical and developmental relationships of the anterior orbital glands, we review the historical context of Harder's discovery, and provide Harder's original Latin text as well as an English translation.