Olfaction

The respiratory system consists of the lungs and the passages that carry air to and from the lungs. The system is composed of three functional parts: conducting, respiratory, and ventilating portions. The conducting portion consists of the nasal cavities and associated sinuses, nasopharynx, oropharynx (which conducts both air and food), larynx, trachea, bronchi, bronchioles, and terminal bronchioles. The olfactory mucosa is associated with the conducting portion, being found in the superior part of each nasal cavity. The respiratory portion is specialized for the rapid exchange of gases between blood and air. It includes the respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli. Components of the ventilating portion include the thoracic cage, intercostal muscles along with certain other muscles, muscular diaphragm, and elastic tissue of the lungs. These components assist the conducting and respiratory portions of the respiratory system in performing their functions.

Respiratory and olfactory mucosae of nasal cavity

The olfactory mucosa is located in the uppermost part of each nasal cavity (mainly on the roof and superior nasal concha). Grossly, it has a brownish-yellow color in contrast to the red-dish color of the surrounding respiratory mucosa. Microscopically, the olfactory mucosa consists of an epithelium, which rests on a thin basement membrane, and a lamina propria.

Study slide C91, olfactory epithelium, (H&E). This is a section of the nasal septum and conchae lined with olfactory epithelium. Since it is difficult to obtain human olfactory epithelium, this sample is from an animal, which has an enormous olfactory area. Using Figure 13 below, compare the thin respiratory (nasal) epithelium with the features of the thick olfactory mucosa. Note that wherever the epithelium is olfactory in type, it is underlain by a thick lamina propria with numerous glands and bundles of axons, which will give rise to the olfactory nerve.

Figure 63: Left photo is a diagram of hard palate, nasal septum and cribiform plate as seen on low magnification on your slide. Right photo is a schematic diagram of the 3 different types of cells found in the olfactory receptors. Taken from Wheater’s Functional Histology, a text and colour atlas, p. 381, Figure, 21.2a.

Three cell types are present: olfactory cells (bipolar neurons), sustentacular cells, and basal cells (see Figure 63 above). In H&E preparations, it is not possible to distinguish clearly between the cell types. The location of the nuclei, however, gives some clue to their identification. Compare the rather large cytoplasmic zone, which is devoid of nuclei and lies just beneath the free surface, with the more deeply located nuclear zone. Nuclei of sustentacular cells lie in the most superficial part of the nuclear zone. They constitute 1-3 rows of nuclei, which are often elongated ovoid in shape. Nuclei of basal cells form a single row in the deepest part of the epithelium. They are located just above the lamina propria. Nuclei of olfactory cells form a broad zone, which occupies the area between the nuclei of the other two cell types. They are often round and contain a prominent nucleolus.

Olfactory cells are bipolar sensory ganglion cells, which serve as receptors for olfactory stimuli. The apical extension of the cell is a dendrite and the basal process is an axon. Olfactory cells constitute the only example in man of peripherally placed sensory ganglion cells. Because of their position, they are vulnerable to destruction from trauma or infection of the nasal mucosa. They are lost at the rate of about 1% per year and do not regenerate.

Study the lamina propria beneath the olfactory epithelium, noting the olfactory glands of Bowman, fila olfactoria, and the bone to which the lamina propria is attached. Since the lamina propria is supported by bone, the olfactory mucosa is called a mucoperiosteum. The glands of Bowman are simple branched tubular glands whose secretory portions are lined by cuboidal to pyramidal cells, which secrete continuously. Ducts lined by a single layer of somewhat flattened cells convey the secretion to the epithelial surface. Since the glands are continuously active, the surface is kept moist and a fresh supply of solvent is always available for dissolving odoriferous substances. Fila olfactoria are small nerves made up of axons (basal processes) of the bipolar olfactory neurons. The nerves pass through the cribiform plate of the ethmoid to enter the olfactory bulb of the brain.

Examine the respiratory mucosa on slide C91 and compare it to olfactory mucosa on the same slide, noting the following differences:

1. Thickness of epithelium. Olfactory epithelium is thicker than respiratory epithelium although both types are pseudo-stratified columnar.

2. Goblet cells. They are present in respiratory epithelium but absent in olfactory epithelium.

3. Cilia. Respiratory epithelium has numerous true cilia while olfactory epithelium has a few cilia associated with the olfactory cells, but these, as shown by electron microscopy, are atypical.

4. Basement membrane. Adult respiratory epithelium rests on a thick basement membrane. It is thin beneath olfactory epithelium. This difference may not be apparent in your section since the section was taken from a young monkey.

5. Olfactory glands of Bowman. These glands are present in olfactory mucosa; absent in respiratory mucosa. Although glands do occur in respiratory mucosa, they are fewer in number and they are not the glands of Bowman.

6. Fila olfactoria. These special nerves are present in olfactory mucosa but absent in respiratory mucosa.

7. Pigment. Pigment is present in olfactory mucosa (sustentacular cells and glands of Bowman); absent in respiratory mucosa.

Study slide B6, nasal epithelium, (H&E). This slide is composed of respiratory type epithelium, rich in goblet cells, overlying a rich venous plexus, which covers the nasal conchae. An occasional gland is found in the connective tissue.

Keep in mind that the paranasal air sinuses (frontal, ethmoid, sphenoid, and maxillary) communicate with the nasal cavities and that the respiratory mucosa of the nasal cavities continues into the paranasal air sinuses. The ciliated pseudo-stratified columnar epithelium of the sinuses, however, is thinner than that in the nasal cavities and it contains fewer goblet cells. The lamina propria is also thinner and contains fewer glands.

Study slide BB-6, soft palate, monkey, (H&E). Examine the respiratory surface of the soft palate and compare it to the oral surface. What differences do you see between the two?

Taste

Tongue

The tongue is a mobile muscular organ surrounded by stratified squamous epithelium and filled with skeletal muscle, glands, and connective tissue. See Figure 71 below. It has several functions, including taste, chewing, swallowing, and speaking.

Figure 71: Micrograph of the tongue. C = circumvallate papilla, F = filiform papilla, LP = lamina propria, M = muscle, MG = mucous glands, SG = serous glands. Taken from Wheater’s Functional Histology, a text and colour atlas, p. 244, Figure 13.10a.

Grossly, the tongue can be divided into root and body. The root (pharyngeal portion) is the immovable, posterior one-third of the tongue, which is derived from endoderm. Muscles connecting the tongue with the hyoid bone pass through the root. The body is the movable, anterior two-thirds of the tongue, which is derived from ectoderm.

Several landmarks can be seen on the dorsal surface of the tongue.

1. The sulcus terminalis is an ill-defined v-shaped groove at the junction of the anterior two-thirds and posterior one-third of the tongue. The apex of the groove is directed posteriorly.

2. The foramen cecum is a slight pit at the apex of the sulcus terminalis. It is the site where the thyroid gland arises in the embryo and where the thyroglossal duct attaches as the thyroid moves to a cervical position.

3. The median sulcus is a depression, which runs forward from the foramen cecum to divide the body of the tongue into two symmetrical halves.

4. The circumvallate papillae, usually 8-12 in number, lie anterior and correspond in

arrangement to the V of the sulcus terminalis. Each is 1-2 mm in diameter and is surrounded by a moat-like circular sulcus.

5. The lingual tonsils are represented by small bulges on the root of the tongue. The bulges are produced by lymph nodules, which lie beneath the stratified squamous epithelium. Ducts from underlying glands open into the crypts or grooves surrounding the nodules and the glandular secretions help to keep the crypt washed free of debris. Perhaps this arrangement prevents the lingual tonsils from becoming infected as frequently as the other tonsils where there are no underlying glands to keep the crypts clean.

Examine slide B21, tongue, monkey, (H&E). This is a full cross section of the tongue with both dorsal and ventral surfaces. Identify the skeletal muscle, which constitutes the bulk of the tongue. The mucous membrane includes the stratified squamous epithelium and the lamina propria. Identify the smooth ventral (lower) surface lined by non-cornified stratified squamous epithelium and compare it to the dorsal (upper) surface, which is rough due to the presence of cornified filiform papillae. Fungiform papillae can also be identified in this section. Compare Figure 72 to your slide. They are far fewer than filiform papillae and are scattered among them. A few barrel-shaped collections of pale columnar cells, called taste buds, are present in the walls of fungiform papillae and should be identified.

Figure 72: Micrograph of the tongue showing both fungiform (Fg) and filiform (FL) papillae. Taken from Wheater’s Functional Histology, a text and colour atlas, p. 244, Figure 13.11.

Study slide B22, tongue, taste buds, rabbit, (Fe Hematoxylin). Rabbits have numerous foliate papillae as seen in abundance on this slide. Taste buds, tastes pores and sustentacular cells are well demonstrated. Many glands, both serous and mucous, muscle and nerves are present.

Study slide B23, tongue, monkey, (H&E). Circumvallate papillae and filiform papilla are present. Identify taste buds, taste (neuroepithelial) cells with dark ovoid nuclei, sustentacular cells with pale nuclei, a taste pore, and the serous glands of von Ebner. Compare your slide with Figure 73 below.

Figure 73: Micrographs of a circumvallate papilla, CV, (left photo) and the surrounding deep cleft, C, (right photo). VE = von Ebner’s glands, B = taste buds. Taken from Wheater’s Functional Histology, a text and colour atlas, p. 245, Figure 13.12a & b.