Respiratory system

A histological section of an avian trachea stained with Hematoxylin and Eosin (H&E).
Respiratory Epithelium: The luminal surface (top-left) is lined by a pseudostratified ciliated columnar epithelium. Interspersed goblet cells and intraepithelial mucous glands are visible.
Lamina Propria and Submucosa: Beneath the epithelium lies a thin, loose connective tissue layer containing small blood vessels.
Hyaline Cartilage Rings: The prominent, thick, basophilic bands running diagonally across the section are hyaline cartilage. In avian anatomy, the tracheal rings are complete (overlapping circles) rather than C-shaped as seen in mammals. The distinct lacunae housing chondrocytes embedded within the extracellular matrix are clearly visible.

Avian lung.
Parabronchi: The large, open lumen structures visible in the section are parabronchi (specifically paleopulmonic parabronchi).
Atria: Branching radially outward from the lumen of each parabronchus are small, pocket-like chambers called atria. They form the initial subdivisions of the parabronchial wall.
Infundibula and Air Capillaries: Extending from the atria are fine, microscopic tubules called infundibula, which lead into an intricate, sponge-like network of air capillaries.
Interlobular Septa: Thin connective tissue bands are visible, partially delineating the hexagonal or irregular boundaries of individual parabronchial units (lobules).