Comparative morphology and morphometry of the olfactory mucosa in the domestic Dog and Sheep
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The qualitative and quantitative features of the olfactory mucosa of the domestic dog and sheep were examined at light, scanning and transmission electron microscopy. Samples were obtained from adult and young (suckling) male dogs and sheep, each represented by groups of five animals comprising of dogs aged 15 - 24 months, puppies 4 - 6 weeks, sheep 12 - 19 months and lambs 3 - 6 weeks. The dogs and puppies were crosses between German shepherd and Rottweiler breeds while the sheep and lambs were dorpers. Variations in morphometric parameters between groups were analyzed using the Student's t-test, considered statistically significant at p < 0.05 and expressed as mean ± standard deviation (SO) The olfactory mucosa occupied the caudal roof of the nasal cavity where it lined the ethmoturbinates, which were relatively longer and with prominent folds of lamellae in the dogs and puppies. The mucosa comprised of an epithelium and a lamina propria. The epithelium was pseudostratified columnar and had the olfactory, supporting and basal cells as its principal cell types. It comprised of three histological zones named from the surface inwards as the free, non-nuclear and nuclear zones. Within the nuclear zone, each cell type established its own layer of nuclei identified as an upper layer of supporting cells, a middle layer of olfactory cells and a basal layer of progenitor (basal) cells. The thickness of the olfactory epithelium varied among the species, being significantly thicker in the dogs (72.5 ± 2.9 urn) than in sheep (56.8 ± 3.1 urn) and in puppies (64.7 ± 1.7 urn) than in lambs (47.8 ± 3.8 urn). In both species, the thickness of the epithelium increased with age (between suckling and adult age), with the olfactory cell nuclear layer contributing more to its thickening in the dog (25.9 %) than in sheep (13.1 %). Further, cell counts (per square millimeter) showed a significantly higher number of olfactory cells in the epithelium of the dog (81,667 ± 7,177 mm") as compared with that of the sheep (46,667 ± 4,924 mm'"), and an insignificant difference in the puppies (66,667 ± 4,924 mrn'") and lambs (63,333 ± 4,924 rnrn"). Within the non-nuclear zone of the olfactory epithelium, the dendrites of the olfactory cells were identified by their electron-Iuscent appearance and were joined to the apical segments of the supporting cells by occluding junctions. On the surface of the epithelium, the dendrites terminated by forming cilia-bearing knobs, whose width was significantly greater in the dogs (2.9 ± 0.1 urn) than in sheep (1.9 ± 0.2 urn), though not significantly different in the puppies (1.8 ± 0.3 urn) and lambs (1.7 ± 0.1 urn). Microvilli arising from the apical surfaces of the supporting cells intermingled with the dendritic cilia to form a dense tangle of the free zone of the epithelium, which was significantly thicker in the dogs (4.6 ± 0.3 IJm) than in sheep (3.6 ± 0.3 IJm) and in the puppies (3.3 ± 0.4 IJm) than in lambs (2.6 ± 0.2 IJm). Marked interspecies variations were noted with regard to the number, length and diameter of the cilia sprouting from the dendritic knobs. In the dog, a thick mesh of cilia whose number averaged 19 in dogs and 18 in puppies radiated from the entire circumference of the knob while in the sheep, a bunch with a mean cilia number of 7 in sheep and 9 in lambs projected from the apical end of the knob. The cilia were significantly longer in the dog (1.9 ± 0.2 IJm) than in sheep (1.6 ± 0.2 IJm) and in the puppies (1.7 ± 0.1 IJm) as compared with lambs (1.4 ± 0.1 IJm). Similarly, the cross-sectional diameters of the cilia were significantly larger in the dogs (0.29 ± 0.01 IJm) than in sheep (0.23 ± 0.01 IJm) and in the puppies (0.25 ± 0.01 IJm) than in lambs (0.20 ± 0.01 IJm). The lamina propria of the olfactory mucosa contained many Bowman's glands and bundles of olfactory nerve axons. The glands were tubuloacinar and were mainly located in the superficial region of the propria. Their acini comprised of pyramid-shaped secretory cells, which emptied their serous secretion on the surface of the epithelium through narrow vertical ducts. As depicted by the product of their lengths and breadths, the acini of the glands were significantly bigger in the dogs (1,416.8 ± 70.8 urn") than in sheep (1,243.0 ± 45.9 ~m2) and in the puppies (1,015.8 ± 42.3 urn") than in lambs (718.7 ± 23.5 urn"). On the other hand, the olfactory bundles were confined to the deep part of the propria where they were separated by collagen containing clefts and enveloped by a sheath of fibroblastic processes. Within each bundle, the axons were packed in hundreds of fascicules, which were in turn ensheathed by sheets of Schwann cell processes. The cross-sectional diameters of the bundles were significantly larger in the dogs (73.3 ± 10.3 urn) than in sheep (50.6 ± 6.8 urn) and in the puppies (68.2 ± 9.9 urn) than in lambs (43.8 ± 5.7 urn), Unlike in the sheep and lambs, one to three blood capillaries were contained within each nerve bundle in the dogs and puppies. In conclusion, the morphologic and morphometric results of this study showed that the mucosa of the dog is structurally more specialized than that of the sheep. This may partly be explained by the differences in their feeding and reproductive habits, where the olfactory sense is essential for both feeding (hunting) and reproduction (mating) in the dog while in the sheep, olfaction is of main importance only in the control of reproductive events such as mating, estrus and maternal offspring recognition.