This study describes the gross anatomical, histological and surface morphological features of the oropharynx and proximal oesophagus of the emu in order to address the scarcity of information on this region in this commercially important bird. Heads obtained from birds at slaughter (and a younger and older bird from emergency farm slaughter) were used for this study and described using basic gross anatomical and histological techniques, supplemented by scanning electron microscopy. The findings of the study were compared with the relevant literature. The oral and pharyngeal cavities could not be morphologically separated and formed a single cavity. This cavity was dorso-ventrally flattened and clearly divided, both on the floor and the roof, into rostral pigmented and caudal non-pigmented parts. The non-pigmented floor housed the tongue and laryngeal mound which had a wide glottis and no papillae. The choana was triangular-shaped, with a small caudo-lateral fold on either side, and was situated in the nonpigmented part of the roof. Caudal to the choana were two rounded pharyngeal folds with a pitted ventral surface. A small bilateral projection from the caudo-lateral edge consisted mainly of diffuse lymphoid tissue. The pharyngeal folds contained numerous large simple branched tubular mucus-secreting glands as well as large accumulations of lymphoid tissue. The pigmented regions of the roof and floor were aglandular and lined by a keratinised stratified squamous epithelium which, particularly in the roof, contained numerous Herbst corpuscles in the underlying connective tissue. SEM revealed the surface to be composed of sheets of desquamating flattened polygonal cells. The non-pigmented regions were glandular and lined by a non-keratinised stratified squamous epithelium. Surface cells displayed a pattern of microplicae or microvilli while individual surface cells were seen to desquamate. The connective tissue housed small, simple tubular and large, simple branched tubular mucus-secreting glands, Herbst corpuscles (only absent from the pharyngeal folds and proximal oesophagus), lymphoid tissue, blood vessels and nerves. The glands of the upper digestive tract were polystomatic and named as follows according to their location: Caudal intermandibular, lingual, crico-arytenoid, oral angular, caudal palatine, pharyngeal and oesophageal. The openings of the glands to the surface were seen on SEM as variably sized holes on the surface, some being obscured by mucus secretions from the underlying glands. Taste receptors were sparse and present only in the caudal non-pigmented oropharyngeal floor, tongue root and proximal oesophagus. Accumulations of lymphoid tissue were identified at the junction between the two regions of the roof, and in the non-pigmented roof, the non-pigmented floor, tongue ventrum, root and frenulum, proximal oesophagus and pharyngeal folds. The consistent dense accumulation of lymphoid tissue in the pharyngeal folds constituted pharyngeal tonsils (Lymphonoduli pharyngeales). The lymphoid tissue of the non-pigmented floor was visible macroscopically as round raised nodules. Specific, unnamed larger lymphoid tissue aggregations were located at the junction of the tongue ventrum and frenulum and in the small folds lateral to the choana. Surface morphology, as seen by SEM, revealed a pattern of microridges on the surface cells of the keratinised areas, whereas the surface cells of the non-keratinised areas displayed microplicae, microvilli and cilia. Microvilli and cilia were associated with the gland openings and ducts. The proximal oesophagus was a cylindrical tube with a longitudinally folded mucosa and displayed the typical tissue layers described in birds. The mucosa was formed by a nonkeratinised stratified epithelium which on SEM showed minimal surface desquamation. The lamina propria contained numerous simple tubular mucus-secreting glands which sometimes branched and occasional diffuse lymphoid tissue aggregations. The gland openings to the surface were seen on SEM as small and large dark holes. The muscularis mucosae was very prominent and was a longitudinal smooth muscle layer separating the mucosa from the submucosa. The tunica muscularis was composed of a thicker inner circular and a thinner outer longitudinal smooth muscle layer surrounded by the outer loose connective tissue forming the tunica adventitia. The emu tongue was divided into a body and a root. The body was triangular, dorso-ventrally flattened, pigmented and displayed caudally directed lingual papillae on both the lateral and caudal margins. The root, a more conspicuous structure in comparison to other ratites, was triangular, with a raised bulbous component folding over the rostral part of the laryngeal fissure. The lingual skeleton was formed by the triangular-shaped paraglossum (hyaline cartilage), forming the core of the tongue body, and the rostral projection of the basihyale, ventral to the paraglossum. Following the general trend in ratites, the emu tongue was greatly reduced in comparison to the bill length and specifically adapted for swallowing during the cranioinertial method of feeding employed by palaeognaths. The tongue was invested by a non-keratinised stratified squamous epithelium. The glands in the connective tissue formed the bulk of the parenchyma and were composed of both small simple tubular and large simple branched tubular mucus-secreting glands similar to those seen in the oropharynx. The lingual glands were grouped as follows: dorsal and rostro-ventral (large glands), caudo-ventral and radical (large and small glands) and frenular (small glands). The large glands were visible macroscopically as doughnut-shaped structures. Melanocytes were absent from the tongue ventrum and occasionally from the tongue root. Lymphoid tissue was absent from the tongue dorsum. Herbst corpuscles were present in the tongue body and root and generally closely associated with the large mucus-secreting glands. The surface morphology varied in the different regions of the tongue. The dorsal and rostro-ventral tongue body showed individual desquamating cells and large gland openings only, the caudo-lateral ventrum showed less desquamation and both large and small openings. The mid-ventral aspect had an undulating uneven appearance with round raised cells on the surface which were densely packed with microvilli. Very large, large and small openings were present in this region and ciliated cells occurred in the vicinity of gland openings. This study presented various unique findings regarding the morphology of the emu oropharynx compared to other ratites. Although the sense of taste has been confirmed in many avian species, this study presented the first evidence of taste in the emu and ratites in general and suggests the possibility of taste being previously overlooked in the other birds studied (ostrich and greater rhea). The tongue root of the emu was clearly defined and is unique in structure and possible function amongst the ratites and other birds. Previously unmentioned functions of the emu tongue revealed by this study include: touch (Herbst corpuscles), taste (taste bud), lubrication and mechanical protection (mucus-secreting glands), immunological (lymphoid tissue) and digestive (swallowing). It was also noted that the various structures and organs of the oropharynx revealed important and often interesting differences between the emu and the other ratites documented. The prominent serrations of the rostral mandibular tomia of the emu also appear to be unique amongst ratites. The presence and wide distribution of Herbst corpuscles within the emu oropharynx and tongue show these areas to be highly sensitive to touch. The caudo-lateral projections of the pharyngeal folds effectively formed pharyngeal tonsils, a feature not apparent in other ratites. Despite the differences noted between the emu and other ratites it was possible to discern a common pattern of structures and features, with their modifications, both within and forming the oropharynx in this group of birds. / Dissertation (MSc)--University of Pretoria, 2009. / Anatomy and Physiology / unrestricted

This study describes, on a comparative basis, the morphology of mechanical (the linguo-laryngeal apparatus) and sensory (Herbst corpuscles and taste buds) specialisations in the upper digestive tract (bill and oropharynx) of the ostrich and emu, with a view to a better understanding of the functional significance of these structures. The ostrich and emu are commercial entities that constitute important niche industries and are farmed intensively throughout South Africa. A lack of information on the mechanical and sensory specialisations of the upper digestive tract in these two birds hampers a sound understanding of food selection and intake. A total of 48 adult (12-14 months) ostrich heads and 48 adult emu (12-14 months) heads obtained from birds at slaughter at commercial abattoirs and farms, as well as 5 ostrich chick (2-4 weeks) heads and 1 emu chick (8 weeks) head, obtained from previous research projects, were used for this study. Morphological features were described using basic gross anatomical (dissection and stereomicroscopy) and histological techniques (H&E staining), supplemented by differential staining for cartilage and bone, transmission electron microscopy and immunohistochemistry. The findings of the study were compared with the relevant literature and hypotheses for functional significance were formed. The avian glottis channels air from the oropharynx to the trachea and is situated on an elevated structure, the laryngeal mound. It is imperative that the glottis be protected and closed during swallowing, which in mammals is achieved by covering the glottis with the epiglottis, as well as by adduction of the arytenoid cartilages. An epiglottis, however, is reportedly absent in birds. Ratites such as the ostrich and emu possess a very wide glottis in comparison to other birds. The question therefore arises as to how these large birds avoid inhalation of food particles through a wide glottis, with apparently little protection, particularly as their feeding method involves throwing the food over the glottis to land in the proximal esophagus. In the ostrich, when the glottis was closed and the tongue body retracted, the smooth tongue root became highly folded and the rostral portion of the laryngeal mound was encased by the pocket in the base of the ∩-shaped tongue body. In this position the lingual papillae also hooked over the most rostral laryngeal projections. However, in the emu, retraction of the tongue body over the closed glottis resulted in the prominent, triangular tongue root sliding over the rostral portion of the laryngeal mound. In both the ostrich and emu these actions resulted in the rostral portion of the laryngeal mound and weakest point of the adducted glottis being enclosed and stabilised. Only after conducting a comparative study between these two birds using fresh specimens did it become clear how specific morphological peculiarities were perfectly specialised to assist in the closure and protection of the wide glottis. A unique anatomical mechanism in ratites was identified, described and proposed, which may functionally replace an epiglottis; the linguo-laryngeal apparatus. The oropharynx of the ostrich and emu is richly supplied with Herbst corpuscles. This widespread distribution of these mechanoreceptors has not previously been reported in birds. Specific concentrations of Herbst corpuscles within the oropharynx, which differ between the ostrich and emu, assist in the accurate positioning of the tongue and laryngeal mound for cleaning the choana (internal nares). The Herbst corpuscles are strategically located to aid in the handling and transport of food and the median palatine and ventral ridges in the ostrich display a concentration of Herbst corpuscles which denote these structures as sensory organs, namely the palatal and interramal organs. Three specific arrangements of Herbst corpuscles were noted in the oropharynx. The first arrangement consisted of groups of corpuscles located peripherally around a myelinated nerve and was present in the bill tip. The second arrangement, possibly linked to the first, was that of individual or groups of corpuscles without an obvious associated nerve and was present throughout the remaining regions of the oropharynx. The third arrangement was that of corpuscles associated with large, simple branched tubular mucus-secreting glands. The basic structure of Herbst corpuscles in the ostrich and emu, observed by light and transmission electron microscopy, of a capsule (with cellular and acellular lamella), an outer zone (collagen fibrils, fibroblasts and a fluid matrix), an inner core (formed by bilaterally symmetrical specialised Schwann cells) and a receptor axon, is similar to that noted for other avian species. However, unlike in other birds, the capsule of the Herbst corpuscle in the ostrich and emu is formed by myofibroblasts which indicates contractile properties for this component of the corpuscle in ratites. Sensory cilia were noted in the myofibroblasts of the capsule and fibroblasts of the outer zone of the ostrich Herbst corpuscle which may assist in regulating the tension of the capsule. These features have not been reported in other avian species. Although the structure of the palaeognathous palate has been widely studied, relatively little information is available on the morphology of the ratite bill. The kiwi possesses a bill tip organ and the present study confirmed the existence of this somatosensory organ in the ostrich and emu. Examination of the rhamphotheca of these two birds demonstrated numerous specialisations. In the emu, rhamphothecal serrations with intervening keratinised pegs on the rostral mandibular tomia resembled a form of pseudo-teeth. These structures may share a similar embryological origin to teeth; however, they would appear to function by channelling and enhancing vibratory stimuli to Herbst corpuscles in nearby bony pits. In the ostrich, epidermal troughs were present in the regions overlying the bill tip organ and functioned to enhance vibratory stimuli to the underlying Herbst corpuscles. Additionally, in the ostrich only, and not related to the structure or functioning of the bill tip organ, the rostral tomia and maxillary and mandibular nails were composed of typical tubular and inter-tubular horn. This may represent a unique feature in birds. The structure of the mandible and premaxilla was similar to that described previously for these birds. However, the persistence of Meckel’s cartilage through to the adult bird in the ostrich and emu is a novel avian feature not previously reported. The bony bill tips were adorned with numerous sensory (bony) pits which displayed similar distribution patterns in the ostrich and emu and indicated the presence, macroscopically, of a bill tip organ. The total number of pits in the bill tip of the ostrich and emu did not differ significantly, although regional differences did occur. The sub-divisions of the trigeminal nerve (N. opthalmicus R. medialis and N. intramandibularis) innervating the bill tip were well developed in both birds and displayed extensive branching. The emu displayed more myelinated nerve fibres in both nerves than in the ostrich. As myelinated nerve fibres supply Herbst corpuscles, the number of nerve fibres is correlated to the number of corpuscles. No correlation could be made between the number of pits in a particular region and the number of nerve fibres or with the relative percentage of Herbst corpuscles in that region. The bill tip organ in both species was basically similar except for the epidermal specialisations noted above. Two parts of the bill tip organ were recognised; the bony bill tip organ (Herbst corpuscles stacked in bony cavities and pits) and the peripheral bill tip organ (Herbst corpuscles in sheets or chains in the connective tissue between the epithelium and bone). The morphology of the bill tip organ in the ostrich and emu indicates that it is an organ that functions by direct touch. These two ratite species appear to possess the most elaborate bill tip organ of any pecking bird. The existence of a bill tip organ in the ostrich and emu is an enigma and points to the possibility that a bill tip organ is a basal structure in all palaeognathous birds (living and extinct). Furthermore, it is evident by observing the exploratory behaviour of the ostrich and emu, that they use their bill tip organ extensively as a tool for exploring and interpreting their environment as well as for discriminating food. The sense of taste in birds is an important motivator for feeding as well as initial food selection. The existence of this sense in ratites has remained largely speculative. In the present study taste buds were only identified in the emu and were predominantly located in the caudal region of the non-pigmented oropharyngeal roof and sparsely located on the oropharyngeal floor. The taste buds extended the full width of the epithelium in which they were located and were ovoid structures. The taste bud was composed of centrally located, vertically oriented light and dark cells (representing both receptor cells and supporting elements) and peripherally situated follicular cells which were continuous with the surrounding Str. germinativum of the stratified squamous epithelium. Positive IHC labelling for neurofilament demonstrated numerous fine nerve fibres (Neurofibra gustatoria) within the connective tissue immediately surrounding the taste bud. Taste bud morphology in the emu was similar to that described in other birds. However, when sectioned tangentially they were indistinguishable from the surrounding epithelium with H&E staining. By using IHC labelling, concentrations of nerve fibres could be demonstrated beneath apparently nondescript epidermal structures, thus indicating the presence of a taste bud. The distribution of taste buds in the oropharynx could be linked to the particular feeding method of the emu. Based on information from GenBank, it would appear that the relatively few taste buds present in the emu oropharynx would mainly function in distinguishing bitter taste. As bitter-tasting compounds can cause a negative association with a particular food type, it would appear that the sense of taste in the emu would predominantly function for protection and not food selection. This study revealed various unique findings regarding the mechanical and sensory specialisations in the upper digestive tract of the ostrich and emu.  The ostrich and emu possess a combination of structures which functionally replace an epiglottis, namely the linguo-laryngeal apparatus.  Herbst corpuscles are widely distributed in the oropharynx of the ostrich and emu and their distribution is related to the particular feeding habits of these birds.  The capsule of Herbst corpuscles in the ostrich and emu is composed of contractile elements, a feature not reported in other birds.  The ostrich and emu possess a well-developed bill tip organ, which is an unusual feature amongst pecking birds.  Taste buds are present in the emu and no structures resembling taste buds were identified in the ostrich. / Thesis (PhD)--University of Pretoria, 2013. / gm2014 / Anatomy and Physiology / Unrestricted