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The effect of lengthened incubation periods on embryonic development, especially vestigial structures, is poorly understood. An example of which is the avesuchian vomeronasal organ (VNO), a nasal chemosensory organ found in many tetrapods but absent in adult avesuchians (crocodilians and birds) in whom it is presumed to be a transitory fetal structure. The Laysan Albatross (Phoebastria immutabilis) has an incubation period of their eggs of about 65 days. This incubation period is twice that of domestic fowl, wherein a putative VNO has been documented as an epithelial thickening. The purpose of this study is to document the development of a putative VNO in the albatross. Serial histological sections of nine albatross embryonic heads, across 6 stages (representing days 19 to 32: stages 31-39), were examined. A paired putative VNO was present as a short, tubular structure in the anterodorsal aspect on either side of the nasal septum from stage 32 onwards, getting steadily longer in later specimens. At the earliest stages, the epithelial walls of the tube resemble a neuroepithelium, but then becomes thinner and simpler in morphology. Based on our available age range, it is unclear whether it persists as a rudimentary structure (like that of the human) or if it is a transitory structure (like in chickens) in these mid embryonic stages. Though future studies must determine the fate of the Laysan albatross VNO (e.g., is it retained postnatally?), the role of incubation period length on embryonic development is a bigger question to be explored.

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An avian uropygial gland is located on the mid-dorsum of the tail, and is the only external gland found in birds. Most studies have focused on the function, gross anatomy and chemical nature of this gland, with little research on its ontogeny. The purpose of this study was to examine the development of this gland in a series of Laysan Albatross (Phoebastria immutabilis) embryos. Specimens were examined anatomically and histologically. It was found that grooves preceded glandular development by many stages. The embryogenesis of the uropygial gland was divided into 6 phases: preinception, groove inception, mesodermal separation, migrating mesodermal cells, oval shaped "depressions", constriction and finally glandular inception. No other gland is known to develop similarly, though there may be parallels with femoral gland development. In comparison to other bird species, the length of the development period in the Albatross, as well as other compounding factors, make it difficult to determine the significance of these observations. The development of a mesodermal band, soon to be a connective tissue capsule, is more complex than originally described in ducks. Thus, the unique nature of this gland is established, but the significance of the observations required further studies into uropygial gland development. Anat Rec, 300:1420-1428, 2017. © 2017 Wiley Periodicals, Inc.

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Rabbits have been proposed as a model organism for the human lacrimal apparatus (LA), including the nasolacrimal duct (NLD), based principally on comparative studies of adult morphology; however, little is known about its development. The NLD first appears as an incomplete primordium in the subcutaneous region of the primordial eyelid and subsequently elongates to reach the naris. One posterior and three anterior orbital glands are present fetally although one of the anterior glands is soon lost. The NLD follows a tortuous path and passes through a bony canal consisting of lacrimal, maxilla, and maxilloturbinal bones at different regions. Although early developmental similarities exist to haplorhine primates, the narial opening of the NLD resembles strepsirrhines. This distinction, along with the ductal and glandular differences at the orbital end of the NLD, indicates that rabbits may be a poor model for LA drainage in primates, specifically humans.

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The anterior orbital glands of tetrapods, which include the Harderian and nictitans glands, can usually be differentiated either anatomically (nictitans gland is more anterior) or histochemically (Harderian gland secretes lipids). However, conflicting statements exist in the literature about the presence and identity of these glands. Two previous studies on Sminthopsis crassicaudata (Dasyuridae: Marsupiala) either failed to note any anterior ocular glands or used no histochemical analyses. This study reexamined the structure of the anterior orbital glands of S. crassicaudata. Histological, histochemical, and ultrastructural examination revealed three glandular units: two of which are located superficially in the nictitating membrane, the third lying deeper in the connective tissue. The ducts of these three glandular units all open onto the corneal aspect of the nictitating membrane. These cells contain mainly serous granules with sparse intracellular lipid droplets. The nomenclature of these structures depends upon the definition used. According to the anatomical definition, S. crassicaudata has two glands: anteriorly the nictitans and posteriorly the Harderian gland. In contrast, if the histochemical definition is used, there is only one gland, but its precise identity cannot be confirmed until the role of the lipid droplets is established. Moreover, the histochemical definition poses additional problems with respect to the mechanism of secretion, multiple secretions, and glandular plasticity. Finally, the unitary definition identifies one deeply subdivided gland with an anterior and a posterior lobe in S. crassicaudata. This last definition is broad enough to accommodate a wide level of anatomical variation in the anterior ocular glands of tetrapods.

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The nictitans and Harderian glands are enigmatic glands situated in the anterior aspect of the orbit. Traditionally, the nictitans and Harderian glands of mammals have been considered to be two fundamentally distinct glands. However, a consistent, unambiguous distinction between these two glands has remained elusive due to conflicting anatomical and histochemical definitions. The Harderian gland was originally described, and first distinguished from the nictitans gland, in adult deer. We examined the organogenesis and histochemistry of the anterior orbital glandular mass in two species of deer (Muntiacus reevesi and Dama dama) to determine whether it comprises two distinct glands or one bilobed gland. The anterior orbital regions of 30 fetal specimens of both species, along with some adult material, were examined histologically. Four stages of glandular organogenesis were observed. Most notably, both glandular portions developed from the same inception point, but the deep lobe developed faster than the superficial lobe. The common inception point and the relationship of the collecting ducts clearly shows that this is a single glandular mass that differentiates into two lobes rather than two distinct glands. Moreover, although the histochemical profiles of the two lobes differ slightly, both lobes produce lipids, which is further indication that these are not profoundly different glands but part of a single, heterogeneously developed gland. Thus, it is proposed that the terms nictitans and Harderian glands, as separate entities, be discontinued and that the entire gland be referred to as the anterior orbital gland (glandula orbitalis anterior), with superficial and deep lobes (pars superficialis and pars profundus, respectively).

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The Harderian gland is a poorly understood anterior ocular gland that occurs in most terrestrial vertebrates. Numerous extraorbital functions have been ascribed to the Harderian gland, principally based on its association with the nasolacrimal duct. Few studies have centered on archosaurs and the majority of those available focused solely on the Harderian gland of birds. Little is known about the lacrimal apparatus of the crocodilians. We examined the lacrimal apparatus of several specimens of Alligator mississippiensis anatomically, histologically, and histochemically and studied the embryogenesis of this system. The nasolacrimal duct possesses a distal secretory area, which is more convoluted than that of typical mammals or lepidosaurs. The alligator Harderian gland possesses a unique combination of characteristics found in lepidosaurs, birds, and mammals. Like that of both mammals and lepidosaurs, it is a large, tuboloacinar gland that appears to secrete both mucoprotein and lipids. However, the presence of blood vessels and immune cells is reminiscent of that of the avian Harderian gland. The immunogenesis of the alligator Harderian gland appears to be tied to the development of the vascular system. The presence of a distinct palpebral gland in the anterior aspect of the ventral eyelid is a feature unique to alligators. Based on position, this gland does not appear to be homologous to the anterior lacrimal gland of lepidosaurs. Lymphatic aggregations were also found in the palpebral gland. The presence of interstitial immune cells in the orbital glands of alligators suggests that the alligator lacrimal apparatus, like that of birds, may play a role in the head-associated lymphatic tissue system.

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The luminal surface of the chemosensory epithelia of the main olfactory organ of terrestrial vertebrates is covered by a layer of fluid. The source of this fluid layer varies among vertebrates. Little is known regarding the relative development of the sources of fluid (sustentacular cells and Bowman's glands) in reptiles, especially in gekkotan lizards (despite recent assertions of olfactory speciality). This study examined the extent and morphology of the main olfactory organ in several Australian squamate reptiles, including three species of gekkotans, two species of skinks and one snake species. The olfactory mucosa of two gekkotan species (Christinus marmoratus and Strophurus intermedius) is spread over a large area of the nasal cavity. Additionally, the sustentacular cells of all three gekkotan species contained a comparatively reduced number of secretory granules, in relation to the skinks or snake examined. These observations imply that the gekkotan olfactory system may function differently from that of either skinks or snakes. Similar variation in secretory granule abundance was previously noted between mammalian and non-mammalian olfactory sustentacular cells. The observations in gekkotans suggests that the secretory capacity of the non-mammalian olfactory sustentacular cells show far more variation than initially thought.

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The vomeronasal organ is a poorly understood accessory olfactory organ, present in many tetrapods. In mammals, amphibians and lepidosaurian reptiles, it is an encapsulated structure with a central, fluid-filled lumen. The morphology of the lubricatory system of the vomeronasal organ (the source of this fluid) varies among classes, being either intrinsic (mammalian and caecilian amphibian vomeronasal glands) or extrinsic (anuran and urodele nasal glands). In the few squamate reptiles thus far examined, there are no submucosal vomeronasal glands. In this study, we examined the vomeronasal organs of several species of Australian squamates using histological, histochemical and ultrastructural techniques, with the goal of determining the morphology of the lubricatory system in the vomeronasal organ. Histochemically, the fluid within the vomeronasal organ of all squamates is mucoserous, though it is uncertain whether mucous and serous constituents constitute separate components. The vomeronasal organ produces few secretory granules intrinsically, implying an extrinsic source for the luminal fluid. Of three possible candidates, the Harderian gland is the most likely extrinsic source of this secretion.

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BACKGROUND: The Harderian gland is an orbital feature found in most terrestrial vertebrates. Although there have been several reports on the structure of the squamate Harderian gland, there has been little recent discussion as to its potential function. This article reviews both the recent morphological observations and their implications on the potential functions of the squamate Harderian gland. METHODS: Literature on the gross structure, histochemistry, and ultrastructure of the squamate Harderian gland and associated structures was reviewed. These observations were then used to assess morphologically the likelihood of the proposed functions. RESULTS: A high level of morphological variation was found in the squamate Harderian gland. Three functional hypotheses, including roles in orbital lubrication, digestion, and vomerolfaction, were considered. Both morphology of the squamate Harderian gland and the presence of alternate secretory sources suggest that it is unlikely to function in orbital lubrication. There is little evidence to suggest a function in digestion. Both the presence of the connecting lacrimal apparatus and the reduced intrinsic secretory capacity of the vomeronasal organ suggest that the Harderian gland may function in vomerolfaction. CONCLUSIONS: The most likely role of the squamate Harderian gland seems to be in vomerolfaction. Morphological variations observed in the Harderian gland may mirror the different degrees and mechanisms of vomerolfaction. Further studies, including comparative morphological, experimental, and microchemical analyses, are required to test this hypothesis.

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