The terms "microsmatic" and "macrosmatic" are used to compare species with greater versus lesser olfactory capabilities, such as carnivores compared to certain primates. These categories have been morphologically defined based on the size of olfactory bulb and surface area of olfactory epithelium in the nasal fossa. The present study examines assumptions regarding the morphological relationship of bony elements to the olfactory mucosa, the utility of olfactory epithelial surface area as a comparative measurement, and the utility of the microsmatic concept. We examined the distribution of olfactory neuroepithelium (OE) across the anteroposterior length of the nasal fossa (from the first completely enclosed cross-section of the nasal fossa to the choanae) in the microsmatic marmoset (Callithrix jacchus) compared to four species of nocturnal strepsirrhines (Otolemur crassicaudatus, O. garnetti, Microcebus murinus, and Cheirogaleus medius). Adults of all species were examined and infant C. jacchus, O. crassicaudatus, M. murinus, and C. medius were also examined. All specimens were serially sectioned in the coronal plane and prepared for light microscopic study. Distribution of OE across all the turbinals, nasal septal surfaces, and accessory spaces of the nasal chamber was recorded for each specimen. The right nasal fossae of one adult C. jacchus and one neonatal M. murinus were also three-dimensionally reconstructed using Scion Image software to reveal OE distribution. Findings showed OE to be distributed relatively more anteriorly in adult C. jacchus compared to strepsirrhines. It was also distributed more anteriorly along the nasal septal walls and recesses in neonates than adults. Our findings also showed that OE surface area was not a reliable proxy for receptor neuron numbers due to differing OE thickness among species. Such results indicate that nasal cavity morphology must be carefully reconsidered regarding traditional functional roles (olfaction versus air conditioning) assigned to various nasal cavity structures. At present, the microsmatic concept itself lacks a basis in nasal chamber morphology, since OE may have varying patterns of distribution among different primates.
The vomeronasal organ (VNO) is a chemosensory structure that has morphological indications of functionality in strepsirhine and New World primates examined to date. In these species, it is thought to mediate certain socio-sexual behaviors. The functionality and even existence of the VNO in Old World primates has been debated. Most modern texts state that the VNO is absent in Old World monkeys, apes, and humans. A recent study on the VNO in the chimpanzee (Smith et al., 2001b) challenged this notion, demonstrating the need for further comparative studies of primates. In particular, there is a need to establish how the human/chimpanzee VNO differs from that of other primates and even nonhomologous mucosal ducts. Histochemical and microscopic morphological characteristics of the VNO and nasopalatine duct (NPD) were examined in 51 peri-and postnatal primates, including humans, chimpanzees, five species of New World monkeys, and seven strepsirhine species. The nasal septum was removed from each primate and histologically processed for coronal sectioning. Selected anteroposterior intervals of the VNO were variously stained with alcian blue (AB)-periodic acid-Schiff (PAS), PAS only, Gomori trichrome, or hematoxylin-eosin procedures. All strepsirhine species had well developed VNOs, with a prominent neuroepithelium and vomeronasal cartilages that nearly surrounded the VNO. New World monkeys had variable amounts of neuroepithelia, whereas Pan troglodytes and Homo sapiens had no recognizable neuroepithelium or vomeronasal nerves (VNNs). Certain unidentified cell types of the human/chimpanzee VNO require further examination (immunohistochemical and electron microscopic). The VNOs of P. troglodytes, H. sapiens, and New World monkeys exhibited different histochemistry of mucins compared to strepsirhine species. The nasopalatine region showed great variation among species.It is a blind-ended pit in P. troglodytes, a glandular recess in H. sapiens, a mucous-producing duct in Otolemur crassicaudatus, and a stratified squamous passageway in all other species. This study also revealed remarkable morphological/ histochemical variability in the VNO and nasopalatine regions among the primate species examined. The VNOs of humans and chimpanzees had some structural similarities to nonhomologous ciliated gland ducts seen in other primates. However, certain distinctions from the VNOs of other primates or nonhomologous epithelial structures characterize the human/ chimpanzee VNO: 1) bilateral epithelial tubes; 2) a superiorly displaced position in the same plane as the paraseptal cartilages; 3) a homogeneous, pseudostratified columnar morphology with ciliated regions; and 4) mucous-producing structures in the epithelium itself. Anat Rec 267: 166-176, 2002.
It is currently thought that New World monkeys, prosimians, and humans are the only primates to possess vomeronasal organs (VNOs) as adults. Recent studies of the human VNO suggest that previous investigations on Old World primates may have missed the VNO. We examined nasal septa from the chimpanzee (Pan troglodytes)grossly and histologically for comparison with nasal septa from humans, Old World monkeys (Macaca fascicularis, M. nemistrina) and prosimian primates (Microcebus murinus, Otolemur garnettii). Grossly, chimpanzees had depressions on the nasal septum similar to fossae reported anterior to the VNO openings in humans. Histologically, chimpanzees and humans had bilateral epithelial tubes which were above the superior margin of the paraseptal cartilages (vomeronasal cartilage homologue). The epithelial tubes had a homogeneous ciliated epithelium. These structures were thus positionally and structurally identical to the human VNO and unlike the well-developed prosimian VNOs which were surrounded by vomeronasal cartilage. Macaques had no structures which resembled the VNO of either the prosimians or humans. The results demonstrate that the VNO is present postnatally in the chimpanzee and is almost identical to the human VNO in its anatomical position and histological structure. This in turn suggests that the reported absence of the VNO in at least some adult Old World primates is artifactual, and that further study may provide evidence for its existence in other species.
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