Veterinary Histology UFF
Department of Morphology - Biomedic Institute
LaBEc - Laboratory of Cellular and Extracellular Biomorphology 		 
Veterinary Histology Atlas
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Introduction
Respiratory System of Birds

The respiratory system of birds is formed by nostrils, a nasal cavity, a pharynx, a trachea, a syrinx, bronchi, air capillaries and air-sacs.

• The skin enters the nostrils up until the first part of the nasal cavity (the vestibule), that is lined by modified keratinized stratified squamous epithelium. It is characterized by epithelial cells organized into columns, giving it a ceruminous aspect on the surface.
• The respiratory region of the nasal cavity is lined by a ciliated pseudostratified columnar epithelium. There are mucous glands in the respiratory epithelium.
• The olfactory epithelium is pseudostratified and columnar. It is located at the upper portion of the respiratory regions. Its structure is similar to that of mammals, it is formed by basal, sensorial and supporting cells. They are found in Bowman’s glands.

Pharynx

• The pharynx is lined by a stratified squamous epithelium.
• A dense lamina propria and a less dense submucosa are positioned under the epithelium.
• There are salivary (mucous) glands inside the lamina propria or the submucosa.
• There are bundles of skeletal muscle beneath the floor of the pharynx.

Trachea

• At the anterior extremity of the trachea there is a cranial larynx that is reinforced by a cartilage ring.
• A caudal larynx (syrinx) is located at the posterior extremity of the trachea.

 

• It is lined by a ciliated pseudostratified columnar epithelium that contains numerous simple mucous alveolar glands.
• At the posterior portion of the trachea, the glands are substituted by goblet cells.

 


• The trachea is supported by complete and overlapped cartilage rings.

 

• They are found in the lamina propria and the submucosa. Each one of them consists of a dense connective tissue.
• The submucosa is rich in elastic fibers.

Syrinx

• The syrinx or voice-box is located in the thoracic cavity , at the point of bifurcation of the trachea into two bronchi.
• The inner and outer tympanic membrane, located in the tracheal bifurcation region, characterize the syrinx wall.
• The intersyringeal cartilages and a bony wedge (the pessulus) offers support to the syringeal region.

Bronchi

Each extrapulmonary primary bronchus enters the lung as a primary intrapulmonary bronchus (mesobronchus).
The secondary bronchi originate from the primary ones and branch into numerous parabronchi (tertiary bronchi) inside the lungs. The latter anastomose with each other.
The small air capillaries form large networks that interconnect the tertiary bronchi.

Primary bronchi
• The primary bronchi are lined by ciliated pseudostratified columnar epithelium with mucous glands and goblet cells.
• The primary extrapulmonary bronchi have C-shaped cartilages
• While the walls of the primary intrapulmonary bronchi have cartilage plates that become scarce distally
• There are bundles of smooth muscle, mostly circular, below the lamina propria.
• Numerous elastic fibers are found throughout the bronchial connective tissue.

Secondary bronchi
• The secondary bronchi are lined by ciliated columnar epithelium with mucous cells.
• There is a lamina propria and a well developed muscle layer.

Tertiary bronchus
• The parabronchi are lined by a cuboidal epithelium.
• A thin layer of connective tissue is positioned under the epithelium.
• Bundles of smooth muscle cells are located under the connective tissue.
• The inner wall of each tertiary bronchus is riddled with numerous openings.


Air-sacs

The air sacs are thin-walled, paired or not structures that occur in the cervical, clavicular, thoracic and abdominal regions of the body.
They connect themselves to the lungs through the bronchi.
Many hollow bird bones contain extensions of the air-sacs. Amongst other bones there is the sternum, humeri, pelvis and most of the thoracic and cervical vertebrae.

• The air sacs are lined by squamous columnar, ciliated cuboidal and ciliated columnar cells.
• The epithelium is supported by a thin layer of connective tissue that consists of collagen and elastic fibers.

The sacs are poorly vascularized and do not participate in the gas exchange.

 
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