الفهرس 
I. INTRODUCTIONOnly 14 pages are availabe for public view
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Abstract
The domestic rabbit when compared with other livestock animals is characterized by early sexual maturity, relatively short gestation length, short generation interval, rapid growth, low cost per breeding female and by its profitability for small-scale system of production and in backyards. The gustatory system allows animals to distinguish between safe and harmful food and affects the consumption of the food, hence the health and growth of animals and subsequently, increase the production of meat, furring, leather and offspring. As in many other mammals, rabbits have three different types of gustatory papillae; namely fungiform (FFP), vallate (VLP), and foliate papillae (FLP), on the dorsal or lateral surface of the tongue.
Samples were taken from the tongues of 48 Chinchilla rabbit (Oryctolagus cuniculus f. domestica) 3 for each age of both sexes at E16, E18, E20, E22, E24, E26, E28 and E30 and from rabbits at days 1, 2, 5, one week, two weeks, one month, three months, six months after birth. Specimens for light microscope (LM) were stained with Hematoxylin and eosin (H & E), Van Gieson, Masson trichrome, Gomeri and Periodic Acid Schiff (PAS) in combination with Alcian blue (AB) stains. Specimens of VLP, FLP and FFP were taken for scanning electron microscope (SEM).
In the present study, the earliest VLP primordia appeared at E16 as a pair of small pits of the surface oral epithelium at the middle of the root of tongue. By E18, the rostral part was remained as a thick mass of epithelial cells dipped into the lingual surface. Meanwhile, the caudal part was prominent with a mainly acellular loose mesenchymal tissue core. At E20-24, the papillary oral epithelium (OE) became well-defined, 2-3 cells-thick, and the side epithelium (SE) was invaginated into the underlying connective tissue on each side of the papilla forming the primitive annular groove with beginning the primordial of serous and mucous glands. At E26-30, there were some signs of beginning of separation of cells of annular groove from above. Primordial of the taste buds were firstly observed at E26 in the papillary surface epithelium. They were formed of small ovoid aggregation of cells. Its cells had ovoid to elongated lightly stained nuclei, lighter cytoplasm than the neighbouring cells and were separated from the free surface by about 4-5 layers of polyhedral shaped cells and flattened cells. Taste pores of apparently mature taste buds were firstly observed at E28, where the surface epithelium was covered by a developing keratin layer. By E30, there were well developed working mucous glands. At birth, the surface epithelium was formed of thick keratinized stratified squamous epithelium with presence of well developed taste buds and a remarkable degree of dermal interdigitation into the connective tissue. Ebner’s glands were found in the deeper parts of the tongue, and the cells of secretory end pieces were completed with active secretion but lacked the lumen. Well developed trench appeared at this age with no taste buds inside it. At 5-8 days after birth, the taste buds were noticed at the two sides of annular groove; in addition to they were found at the surface epithelium. Well functioning Ebner’s glands appeared at this stage. By 14-30 days, taste buds disappeared on the majority of surface epithelium and found occasionally on the surface epithelium in addition to annular groove.
In the present study, the earliest primordia of FFP were observed at E18 as small thickenings of variable sizes on the dorsal surfaces of the apex and body of tongue and was formed of two or three-cell thick layer protruding from the lingual surface, and had a small mesenchymal core. At E20-24, the papillae were easily discernible from the nascent filiform primordia. The FFP had a remarkable connective tissue core, and the epithelial covering was composed of two layers of cells; basal layer formed by columnar cells with large oval nuclei, and superficial layer formed of flat cells with flattened nuclei. At E26-30, primordia of the taste buds were firstly observed at E26 in the papillary surface epithelium. They were formed of small ovoid aggregation of cells and they had larger and ovoid nuclei, lighter cytoplasm than the neighbouring cells and were separated from the free surface by few layers of flattened cells. Taste pores of apparently mature taste buds were firstly observed at E28, where the surface epithelium was covered by a developing keratin layer. At birth, the fungiform papillae had mushroom – like shape with upper convex surface which was slightly keratinized. At upper surface, there were (1-3) taste buds that opened to the upper surface with taste pores.
In the present study, we observed the earliest primordial of the FLP at E22-24 and formed of by primary epithelial cords of almost equal sizes and shapes and covered with 2-3 layers of squamous cells. Growing capillaries associated with abundant mesenchymal cells invaded into the connective tissue between each cell cord. At E26-28, the primary epithelial cell cords penetrated much deeper into the underlying connective tissue and the secondary epithelial cell cords aroused from the proximal portion of the primary epithelial cells. Primordia of the taste buds were found at the connection of primary and secondary cords. At E30, the secondary cords were extensive and separated from the primary cords. Taste buds with taste pore were found at the connection of primary and secondary cords. At birth, fetal taste buds of foliate papillae began to disappear after birth. At 2nd day taste buds began to appear again within the primary cords with connection of excretory ducts of Ebner’s glands within the cords to form gutter. Not matured enough taste buds with taste pore appeared by 5th day at proximal end of primary cords. At 8th day, some gutters formed in some primary cords with mature taste buds at both sides of the formed gutters and proximal unsplitted primary cords, in addition to immature taste buds within unsplitted primary cords. By 15-30 day, complete splitting of all primary cords with disappearance of taste buds at the dorsal surfaces and precence only on lateral surfaces.
In conclusion, the present study showed that the development of gustatory papillae took place at the different stages of rabbit pre and postnatal lives in which VLP were the first followed by FFP and FLP were the last to develop. There were also different patterns of development of taste buds in each of the gustatory papillae. These findings provide basic data for future studies of the differentiation of cells in the gustatory epithelium and the distribution of various growth factors and their receptors inside it.