DONALD R. ADAMS

The primary function of the respiratory system is to provide for the exchange of respiratory gases (oxygen and carbon dioxide) between the organism and the environment. The conducting airways provide a series of air passages for moving air to and from the gas exchange area in the lungs. The conducting airways also serve a protective function by conditioning incoming (inspired) air. This conditioning includes heating the air to body temperature, saturating it to 100% relative humidity, and filtering out noxious gases and particulates. The conducting airways also conserve body heat and water by extracting them from the air during expiration. The mucociliary blanket, which covers the mucosal surface of conducting airways, serves to trap inhaled particles and conveys them and cellular debris out of the system. Other structures, such as the nasolacrimal duct, vomeronasal organ, paranasal recesses and sinuses, auditory tube, and equine guttural pouch, connect to the conducting airways.

The distal, smallest conducting airways connect to the gas exchange area, which includes the respiratory bronchioles, alveolar ducts, and alveolar sacs. Gas exchange occurs in alveoli, where only a thin blood-air barrier is present between pulmonary capillary blood and respired air. An extensive pulmonary capillary bed receives the entire output of the right ventricle of the heart.

NASAL CAVITY, VOMERONASAL ORGAN, AND PARANASAL SINUSES

Rostrally, the cutaneous region (nasal vestibule) is lined by a relatively thick keratinized stratified squamous epithelium (Fig. 9-1). At midvestibule, the epithelium is thinner and nonkeratinized. Superficial cells have microridges on their free surface. The caudal portion of the cutaneous region and the rostral third of the nasal cavity proper are a transitional zone lined by an epithelium that varies from stratified cuboidal to nonciliated pseudostratified columnar. Surface epithelial cells in the transitional zone contain multilobated nuclei, have microvilli on their free surface, and are frequently spherical (Fig. 9-2).

FIGURE 9-1 Stratified squamous epithelium in the cutaneous region of the canine nasal cavity. Airway lumen (A); dermal papilla (B). 1 µm. Azure II (×385). (With permission from Adams DR, Hotchkiss DK. The canine nasal mucosa. Zentralbl Veterinarmed C Anat Histol Embryol 1983;12:111.)

The propria-submucosa of the cutaneous region interdigitates via papillae with the epithelium. The papillae contain small vessels, nerves, and numerous migratory cells, including mast cells, plasma cells, lymphocytes, macrophages, and granulocytes. Lymphocytes and other migratory cells are also frequently observed in the basal portion of the epithelium. Bundles of collagen fibers, larger blood vessels and nerves, and serous glands are located deep in the propria-submucosa.

In horses, a nasal diverticulum lined with skin opens into the cutaneous region of the nasal vestibule; this region is lined by an integument containing vibrissae, sebaceous glands, and sweat glands. The papillary layer of the vestibular propria-submucosa in dogs has particularly numerous papillae and capillary loops.

Respiratory Region

Epithelium lining the caudal two thirds of the nasal cavity proper, with the exception of the olfactory region, is classified as respiratory epithelium (i.e., ciliated pseudostratified columnar); that lining the middle nasal meatus is thinner and contains fewer ciliated and goblet cells. The ciliated pseudostratified epithelium of the nasal cavity contains several cell types, including ciliated, secretory, brush, and basal cells (Figs. 9-3, 9-4, and 9-5).

FIGURE 9-2 Stratified cuboidal epithelium in the transitional zone of the canine nasal cavity. Airway lumen (A); connective-tissue papilla (B). 1 µm. Azure II (×385). (With permission from Adams DR, Hotchkiss DK. The canine nasal mucosa. Zentralbl Veterinarmed C Anat Histol Embryol 1983;12:113.)

Individual ciliated cells are columnar and have 200 to 300 motile cilia and numerous microvilli projecting into the nasal lumen. The supranuclear portion of the cell contains basal bodies, a Golgi complex, and numerous mitochondria; small strands of rough endoplasmic reticulum (rER) are scattered throughout the cell. Defects in the fine structure of cilia may result in ineffective ciliary beat or immotility. Immotile cilia syndrome is a condition associated with congenital ciliary abnormality, resulting in respiratory tract infections.

Secretory cells of the respiratory epithelium extend from the basal lamina to the epithelial surface. Their luminal surface bears microvilli. The morphologic and histochemical appearance of these cells is both species and regionally variable. Their description as mucous or serous is based on their glycoprotein content. Granules of mucous epithelial cells are relatively electron-lucent and contain sialated or sulfated acid glycoproteins. The supranuclear portion of mucous epithelial cells varies in appearance with the secretory phase from tall and slender with few granules to wide and globular with numerous mucous granules. Globular mucous cells, known as goblet cells, have nuclei pressed to the base of the cell by the supranuclear mass of large mucous granules. Organelles usually present in the perinuclear region of the goblet cell include a Golgi complex, rER, and mitochondria. Goblet cells of most species secrete primarily sulfated glycoprotein as a major component of mucus.

FIGURE 9-3 Ciliated pseudostratified columnar epithelium with goblet cells lining the respiratory region of the nasal cavity. Goblet cells (A); basal cells (B); ciliated cells (C). 1 µm. Azure II (×590).

FIGURE 9-4 Scanning electron micrograph of respiratory epithelium (ciliated pseudostratified columnar epithelium). Ciliated cells with cilia and microvilli (A); secretory cells with apical microvilli (B) (×3500).

Granules of serous epithelial cells have electron-dense cores, contain neutral glycoproteins, and are smaller than those of mucous cells.

Brush cells have long, thick microvilli and a cytoplasm containing mitochondria and many filaments. These cells may be sensory receptors associated with endings of the trigeminal nerve.

Basal cells are small polyhedral cells located along the basal lamina. The cytoplasm of basal cells contains numerous bundles of tonofilaments and free ribosomes. Basal cells are characterized by anchoring attachments (desmosomes) to other cell types and to the basal lamina (hemidesmosomes). While basal cells appear to have some role in replacing other cell types, the rate of cell proliferation is very low and most of the replacement cells are derived from the other cell types in the epithelium.

Another unnamed cell in the nasal mucosa has surface microvilli and contains much smooth endoplasmic reticulum (sER) and little secretory material; this cell type is believed to function in the metabolism of xenobiotic compounds (see below).

The respiratory mucosa (respiratory epithelium plus underlying propria-submucosa) of the nasal cavity is more vascular than the mucosae of the cutaneous, transitional, or olfactory regions. The highly vascular propria-submucosa, in which arteries and large, thin-walled veins are oriented rostrocaudally, is called the cavernous stratum (Fig. 9-6). The veins anastomose profusely and are referred to as capacitance vessels because they determine the degree of mucosal congestion and, inversely, nasal patency. Constriction of nasal blood vessels is effected by α-adrenergic stimulation via the sympathetic nervous system. Periods of vascular engorgement varying from 30 minutes to 4 hours followed by periods of decongestion normally occur in the cavernous stratum of mammals; during this nasal cycle, the vascular activity in one side of the nose alternates with that in the other side.

Serous or mixed nasal glands are present between the numerous veins of this stratum (Fig. 9-7). Acini of nasal glands also secrete secretory immunoglobulin A, lysozyme, and odorantbinding protein.

Nerves in the nasal mucosa include sensory fibers arising from the terminal, olfactory, vomeronasal, and maxillary division of the trigeminal nerves and efferent fibers of the autonomic nervous system. Nerves are distributed throughout all compartments of the nasal mucosa, including within epithelium.

Lymphatic nodules are commonly present in the caudal part of the nasal cavity, adjacent to the choana, the opening between the nasal cavity and nasopharynx.

Metabolically active exogenous compounds (xenobiotics) that reach nasal tissues via air or blood pathways may remain firmly bound to tissue elements unless degraded. Cytochrome P-450-dependent monooxygenase enzymes in surface epithelium and in acinar cells of the lateral nasal gland (see below) actively metabolize endogenous compounds (e.g., progesterone and testosterone) and exogenous compounds. These enzymes convert lipid-soluble exogenous compounds, some of which are highly toxic (e.g., formaldehyde and acetaldehyde), to water-soluble metabolites.

FIGURE 9-5 Drawing of the fine structural characteristics of respiratory epithelial cells.

Olfactory Region

The olfactory region comprises the dorsocaudal portion of the nasal cavity, including some of the surfaces of the ethmoid conchae, dorsal nasal meatus, and nasal septum. Olfactory mucosa may be discerned from adjacent respiratory mucosa because it has a thicker epithelium, numerous tubular glands, and many bundles of nonmyelinated nerve fibers in the lamina propria.

The olfactory mucosa is lined by a ciliated pseudostratified columnar epithelium, the olfactory epithelium, consisting of three primary cell types: neurosensory, sustentacular, and basal (Fig. 9-8).

Neurosensory olfactory cells are bipolar neurons with perikarya in a wide basal zone of the epithelium, dendrites extending to the lumen, and axons reaching the olfactory bulb of the brain. A club-shaped apex, the dendritic bulb, protrudes from each dendrite into the lumen (Fig. 9-9), from which 10 to 30 cilia emanate. Each cilium is 50- to 80-µm long and consists of a wide, short basal portion and a long, thin, tapering distal portion. The number of microtubules decreases from the typical nine peripheral doublets (fused pairs of microtubules) plus two single central microtubules in the basal portion to singlets of one to four micro-tubules distally. The perikaryon has typical neuronal structural characteristics. Individual axons converge as they pass into the lamina propria, thereby forming bundles of nonmyelinated nerve fibers. Neurosensory cells are continuously replaced during the life of the animal by cells derived from basal cells.

Sustentacular cells are columnar cells with a narrow base and a wide apical portion. Their oval nuclei form the most superficial nuclear layer in the epithelium. Microvilli, often branched, cover the luminal surface of sustentacular cells. Juxtaluminal junctional complexes occur between sustentacular cells and the adjacent dendrites of neurosensory cells. Pigment granules are present in the infranuclear cytoplasm. Sustentacular cells are also replaced by basal cells.

Basal cells of the olfactory mucosa are similar in structure to those of the nonolfactory epithelium.

Olfactory glands, the cells of which contain pigment granules, are located in the propria-submucosa. The intraepithelial portion of their ducts is lined by squamous cells. The glands secrete a watery product, which may serve to enhance the solubility of airborne odorants and cleanse the cilia, facilitating access for new odorants.

FIGURE 9-6 Scanning electron micrograph of a section of the respiratory mucosa of the bovine nasal cavity. Epithelium (A); perichondrium (B); lumina of blood vessels (C) in the cavernous stratum (×40).

FIGURE 9-7 Nasal gland acini (A) occupy the connective tissue between the veins of the cavernous stratum (B) of the respiratory mucosa. 1 µm. Azure II (×425).

FIGURE 9-8 Mucosa of the canine olfactory region. Nuclei of neurosensory cells (A); nuclei of sustentacular cells (B); olfactory glands (C); olfactory nerves (D). 1 µm. Azure II (×410).

The olfactory mucosa also has very high levels of cytochrome P-450-monooxygenase activity and is the primary site for chemically induced nasal tumors.

Vomeronasal Organ

Located in the mucosa of the ventral portion of the nasal septum, the tubular, blind-ended bilateral vomeronasal organ consists of an internal epithelial duct (vomeronasal duct), a middle propria-submucosa, and an external cartilaginous support. Rostrally, the vomeronasal duct joins the incisive duct, which connects the nasal cavity with the oral cavity, except in horses, in which the ventral end is blind.

The vomeronasal duct is crescent-shaped in transverse section with a lateral convex and a medial concave mucosal wall. The epithelium transitions from a stratified cuboidal lining rostrally near the incisive duct to a ciliated pseudostratified columnar epithelium over much of the caudal portion of the vomeronasal duct.

FIGURE 9-9 Schematic drawing of the olfactory epithelium. Sustentacular cells (A); basal cell (B); axon of the receptor cell (C); dendritic bulb (D); thin distal portion of cilium (E); thick proximal portion of cilium (F); junctional complex between receptor and sustentacular cells (G).

The medial pseudostratified columnar epithelium has neurosensory, sustentacular, and basal cells (Fig. 9-10). The dendritic portions of vomeronasal neurosensory cells lack dendritic bulbs and, with the exception of those in dogs, have microvilli instead of cilia on their apical surfaces. Neurosensory cells are periodically replaced in the adult mammal. The lateral pseudostratified columnar epithelium has ciliated and nonciliated columnar, goblet, and basal cells.

Vomeronasal glands, located in the highly vascular propria-submucosa, secrete into the vomeronasal duct most commonly through the commissures between lateral and medial mucosal walls. Secretory granules of the acinar cells contain neutral glyco-proteins. The hyaline vomeronasal cartilage is J-shaped, enclosing all but the dorsolateral portion of the organ.

The vomeronasal organ functions in the chemoreception of liquidborne compounds of low volatility. Sensing of these compounds is believed to function in sexual behavior of both the female and the male, in maternal behavior, and in the interaction of the fetus with its amniotic environment. In several mammals, vomeronasal detection of the odor of a female results in an elevation of plasma testosterone in the male. The vomeronasal organ is associated with the lip-curl type of facial grimace (Flehmen response) used by some male mammals to sample substances in the urine of the female; odorant particles may reach the incisive duct with inhaled air, through contact with the tongue, or during passage through the mouth with food or water. These substances, dissolved in fluid in the incisive duct, are sucked into the vomeronasal duct by constriction of blood vessels within the propria-submucosa of the vomeronasal organ. Upon dilation of these vessels, the dissolved substances are expelled from the vomeronasal lumen.

FIGURE 9-10 Canine vomeronasal duct. The lateral epithelium (A) includes ciliated and nonciliated cells, whereas the medial epithelium (B) contains neurosensory and sustentacular epithelial cells (×158). (Courtesy of A. W. Stinson.)

Paranasal Sinuses

The mucosae of the paranasal sinuses are thinner than those of the respiratory region of the nasal cavity with which they are continuous. Glands and blood vessels in the propria-submucosa are scant. The epithelium is ciliated pseudostratified columnar, containing a few goblet cells. The ciliary beat carries mucus toward openings connecting the sinuses with the nasal cavity.

The lateral nasal gland (Fig. 9-11) is a relatively large compound gland that secretes neutral glycoproteins via a long duct into the nasal vestibule. The lateral nasal gland is present in the maxillary recess of carnivores, in the maxillary sinus of pigs, and at the nasomaxillary aperture in horses and small ruminants; it is absent in cattle. In addition, separate maxillary recess glands are present in carnivores.

NASOPHARYNX

FIGURE 9-11 Two striated ducts (A), an intercalated duct (B), and acinar cells (C) in the canine lateral nasal gland. 1 µm. Azure II (×425).

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