| Abstract: |
The present study described the olfactory epithelium of postnatal rats using both light and electron microscope either transmission or scanning electron microscopy, also the study described the olfactory bulb and tract of the rat using light microscope.This study showed that the olfactory epithelium was formed of cells arranged into superficial, middle and deep layers and underlying lamina propria. The superficial layer was formed of the dendrites of olfactory receptor cells and the supranuclear parts of the supporting cells, while the middle layer included the nuclear parts of olfactory receptor cells; the deep layer was formed of the basal cells. The lamina propria includes the olfactory nerve bundles, bowman’s glands and connective tissue cells.There was marked increase in the thickness of olfactory epithelium form the age of newborn till the age of six months with maximal increase at the age of seven days. The arrangement of the nuclei of the supporting cells and olfactory receptor cells was obvious in the development till the adult age, and then nuclei were intermingled with each other in the old age. Also there was marked increase in thickness of lamina propria mainly due to increased diameter of olfactory axons.The laminar pattern of the olfactory bulb was not well developed in the newborn age and was only formed of olfactory nerve layer, the external plexiform layer, the mitral cell layer, the inner plexiform layer, the granule cell layer and subependymal layer. With the progress of development, the laminar pattern of the bulb started to be arranged. At the age of 7days the glomerular layer appeared to be well formed with marked demarcation between the mitral and the inner plexiform layer. At the sixteen days age and adult age, there is marked increase in thickness of the mitral, glomerular, outer plexiform and granule layers. The mitral layer was the most rabidly developed one in relation to other layers. In old age there was marked decrease in the thickness of the epithelium with marked deterioration of the laminar pattern especially among the mitral and inner plexiform layers.The olfactory receptor cells were bipolar cells with peripheral and central processes. The peripheral one extended outwards towards the surface of the cell and projecting as olfactory knobs. On the other hand, the central processes extended into the deep layers of the epithelium towards the basal layer. The cytoplasm of the cell contained numerous elongated mitochondria and heterochromatic nuclei surrounded by scanty cytoplasm and few cell organelles. The olfactory knobs showed many cilia projecting over its surface. These cilia were thick, short and occasionally horizontal, having basal bodies, small vacuoles and microtubules. Few basal ciliary dilatations were detected at the bases of the cilia. The cilia extended in different directions over the surface of the knobs. The olfactory receptor cells in newborn rats showed signs of immaturity as small knob and large irregular heterochromatic nucleus and pale cytoplasm. There was marked increase in the length of olfactory cilia from newborn age to seven day rats, and it almost doubled its length and became thin. In old aged rats, the olfactory receptor cells showed thinning of the dendrites, the knobs were smaller with fewer cilia having irregular cell membrane, The cytoplasm of the knobs contained minimal basal bodies and dense lysosomal bodies, while that of the dendrites contained few small rounded mitochondria and scattered microtubules, also the cytoplasm around the nucleus contained aggregates of smooth endoplasmic reticulum arranged in a whorly appearance. Also respiratory epithelium was found intermingled with the olfactory mucosa in old age rats.The luminal surface of the supporting cells showed microvilli projecting and forming a network between the cilia of the olfactory knobs. The number and length of supporting cell microvilli was increased and became more parallel to each other in seven days old rats. In sixty days rats the number and the length of microvilli extensively increased, but they became irregularly intermingled in a network surrounding the olfactory knobs and this became more evident in six month old rats. The nuclei of the cells appeared euchromatic, with their supranuclear and infranuclear cytoplasm was characterized by being filled with different lysosomal bodies. These findings were almost seen in all ages of development but in newborn rats the cells contain large secretory vacuoles, some of them evacuate their contents by exocytosis through the cell membrane in the intercellular spaces. These vacuoles decreased markedly in size and almost disappeared at 16 days and older ages from both the cytoplasm and the intercellular spaces. These vacuoles being prominent in younger ages supported the secretory function of supporting cells especially in the absence of bowman’s glands in newborn rats.Tight junctions were demonstrated in this study between the olfactory receptor cells and supporting cells.The bowman’s glands were found to develop in rats after the age of 7 days they were formed of large cells with basal nuclei; the supranuclear part of the cytoplasm was filled with numerous secretory granules with different electron densities.The mature electron lucent granules were found to evacuate by exocytosis (merocrine) into the lumen
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