Esophageal tonsil in the studied bird species is located in the tunica mucosa of caudal part of esophagus in the region of its transition into the proventriculus. Such birds as goose and brant goose have a transition zone between them. In the majority of studied species of birds (chicken, guinea-fowl, quail, turkey, duck, goose, pheasant, hazel grouse, peacock, brant goose, magpie, gray crow and white stork), the tonsil is macroscopically detected. It looks like a folded, hilly strip light pink colored with a little bit of yellow, which is covering its perimeter.
The length and width of esophageal tonsil of birds may vary. The largest length is registered in the brant goose (47.5±0.44 mm) and white stork (46.23±0.37 mm), and the smallest – in quail and magpie (as relevant 9.26±0,09 and 8.19±0.13 mm).
The maximum values of the width of the tonsil are peculiar to the duck (9.25±0.08 mm), and the minimum are to the magpie and gray crow (respectively 1.16±0.06 and 1.04±0.06 mm). In willow ptamigan, jay, common moorhen, coot and pigeon, esophageal tonsil is not macroscopically visible.
The microscopic structure of the wall of the esophagus in the area of tonsil is the same as in its other parts. So, it is formed by the tunica mucosa, tunica muscularis and tunica serosa. The tunica mucosa forms folds, which are a continuation of the folds of the esophagus. White stork has the largest number of that folds (11–13), and the Galliformes – the smallest number (5–7).
The tunica mucosa in the tonsil region is represented by a stratified squamous epithelium, which in some species of birds (white stork, turkey, hazel grouse, willow ptarmigan, pheasant and peacock) can be keratinized, lamina propria, weakly expressed lamina muscularis and submucosa. Lamina propria and submucosa are formed by a loose connective tissue, and the lamina muscularis by the smooth muscle. The tunica muscularis is formed by smooth muscle tissue, in goose, magpies, jay and gray crow it has a two-layer structure, and in other species of birds – threelayer.
The tunica serosa is formed by a loose connective tissue that is covered with a
mesothelium.
The lymphoid tissue of birds tonsils locate in the lamina propria and submucosa of tunica mucosa. In some species of birds it is compact, in others – diffused. Therefore, it was proposed to classify the esophagus tonsils on compact and diffuse ones. The compact includes the tonsils of chicken, guinea-fowl, quail, turkey, hazel grouse, peacock, pheasant, duck, goose, brant goose, white stork, magpie and gray crow, and the diffuse – willow ptamigan, common moorhen, coot, jay, pigeon. Lymphoid tissue occupies an unequal area in the tunica mucosa of the esophagus tonsil area. It is the largest among in the birds with a compact location (from 68.64±0.70 % in duck to 25.16±0.29 % in gray crow) and much smaller with diffuse location (from 2.31±0.05 % in willow ptamigan to 12.79±0.18 % in the jay). It replaces the part of the adenomeres of the esophageal glands and partially their ducts. As a result, adenomeres become crypth-like formations. The replacement process begins with the infiltration of glandulocytes by lymphoid cells. The latter also infiltrate the surface epithelium of the tunica mucosa.
Esophageal tonsil lymphoid tissue of the most researched species of birds is represented by all levels of its structural organization: a diffuse form, prenodules, primary and secondary nodules. Its area in tonsil may vary. The largest area is characterized for diffuse form (from 86.12±0.15 to brant goose to 40.02±2.12 % of hazel grouse), and the smallest – to prenodules (from 13.85±1.39 % (in hazel grouse) to 0.81±0.05 % (in the brant goose).
The general plan of topography, macroscopic and microscopic structure of the esophageal tonsil of domestic duck in the early stages of its postnatal period of ontogenesis is the same as in the mature duck. It is detected microscopically at 5-day age. Its length and width reach the maximum values (respectively 37.8±0.2 and 8.3±0.1 mm) in 120-day-old of birds, and in the elderly almost does not change.
The structural levels of its lymphoid tissue are formed gradually. The diffuse lymphoid tissue is found in one-day-old birds, the prenodules – in 5-days-old, the 27 primary nodules – in 10-days-old, and the secondary ones – in 15-days. The area of lymphoid tissue reaches the maximum values in the duck of the 25-day age and significantly decreases in the poultry older than 180 days.
The obtained data on the morphofunctional features of the esophageal tonsil and the immune formations of the stomach of birds are recommended for researchers who are investigating the immune organs and in educational work.
The results of research on the development of the esophageal tonsil in the early stages of the postnatal period of ontogenesis of domestic duck are recommended for veterinarians to determine the optimal time for vaccination of this bird against
infectious diseases.
