Overview
This lecture works down the first part of the gut tube histologically. It starts by revising epithelial classification and establishing that the gut tube is lined by only two epithelia, then applies that to the lip and oral cavity and to the tongue with its four papilla types. It then treats the salivary glands as a worked example of exocrine gland architecture, splitting them into a secretory part (serous and mucous cells, myoepithelial cells) and a conducting part (the duct sequence, with the striated duct as the functionally active segment). Finally it sets out the general four-layer plan of the gut wall and applies that plan to the oesophagus, including its regional skeletal-to-smooth muscle transition.
Epithelium and the lining of the gut tube
Epithelial classification as presented:
- Simple: squamous, cuboidal, columnar (starred and bolded on the slide).
- Stratified: squamous (keratinised, non-keratinised, the latter starred and bolded), cuboidal, columnar.
The lining epithelium of the gut tube is either SSNKEpi (stratified squamous non-keratinised epithelium) or simple columnar. This is why those two are the highlighted entries in the classification.
The accompanying line drawing of a fetus traces the gut tube from mouth to anus: the length of the tube is coloured yellow and the two ends (the oral region and the anal region) are coloured pink/red, orienting the SSNKEpi versus simple columnar distinction to position along the tube.
Lip and oral cavity
- Lined by stratified squamous epithelium.
- The lip is the site of transition to mucous membrane; in section it shows an oral surface and a cutaneous surface.
- Not all oral mucosa is SSNKEpi. Masticatory mucosa is parakeratinised.
- Layers of the lip and oral cavity wall:
- Mucosa (mostly SSNKEpi)
- Lamina propria
- Submucosa, containing salivary glands
- Skeletal muscle (in the lip and soft palate)
On the lip micrograph the salivary gland acini are seen as darker-staining glandular tissue lying deep to the epithelium, in the submucosa.
Tongue and lingual papillae
Tongue mucosa:
- General: SSNKEpi.
- Special: taste buds.
Functions of the papillae:
- Generate friction for the food during chewing.
- House taste buds.
Papillae are not taste buds, and not all papillae have taste buds.
The four papilla types:
| Papilla | Features | Taste buds |
|---|---|---|
| Filiform | Conical shape, keratinised | No |
| Fungiform | Mushroom shaped | Yes |
| Foliate | Poorly developed in adults | Yes |
| Vallate / circumvallate | Largest lingual papillae; sunk into the mucosa with a surrounding trench, with serous glands opening at the base that facilitate taste sensation | Yes |
A taste bud sits within the epithelium as a barrel or onion-shaped cluster of elongated pale cells, with nerve fibres entering at its base [the enlarged taste-bud diagram carries no printed labels, so the individual cell types are not named].
The circumvallate micrograph runs off the right edge of the slide and is partly cut off.
Glands: general organisation
- All the glands of the GI tract have an epithelial origin.
- Every gland has a secretory part and a conducting part. Rounded secretory acini (serous and mucous types) drain into a branching duct system, with secretion flowing away from the acini towards the larger ducts. The architecture is likened to a bunch of grapes.
- Glands are covered by connective tissue, which forms a capsule that gives off septa/trabeculae passing inwards between the acini.
- Functions of the connective tissue:
- Provide structural integrity.
- Conduct neurovascular bundles and ducts (a blood vessel and a nerve are shown running within the septa).
[The small coloured profiles drawn within the connective tissue, and the coloured markings on the accompanying gut-tube line drawing, are unlabelled on the slides.]
Secretory part: the acinus
Cells of the secretory part of the salivary glands:
- Mucus secreting cells
- Serous secreting cells
- Myoepithelial cells (basket cells), which force secretions from the acinar lumen into the ducts.
Gland-by-gland secretory type
| Gland | Secretion |
|---|---|
| Parotid | Serous |
| Sublingual | Mucus |
| Submandibular | Both mucous and serous |
On H&E: serous acini are densely eosinophilic with round basal nuclei; mucous acini are pale, almost unstained and frothy with nuclei pushed to the base. In the submandibular gland a serous demilune appears as a crescent of darker-staining serous cells capping a mucous acinus, which is how a mixed gland is recognised.
Serous cells
- Pyramidal shape.
- Round (spherical) basal nucleus.
- Zymogen granules at the apex.
- Secrete watery/serous saliva.
Ultrastructure: apical microvillus, heterogeneous electron-dense secretory granules, intercellular secretory canaliculus, tight junction, spherical nucleus. Stacked parallel arrays of rough ER fill the basal cytoplasm.
Mucus cells
- Cylindrical shape.
- Flat basal nucleus.
- Pale mucus droplets at the apex.
- Secrete thick viscous saliva.
Ultrastructure: tight junction, homogeneous electron-translucent secretory vesicles filling the apex, flattened basal nucleus. Rough ER lies basally around the nucleus.
The key discriminator: serous = dense heterogeneous granules with a round basal nucleus; mucous = pale homogeneous vesicles with a flattened basal nucleus.
Components of the secretory unit
The labelled schematic of a salivary secretory unit names: myoepithelial cells (flattened cells applied to the outside of the acini), serous acinus, serous demilune, mucous tubule, intercalated ducts, striated ducts, terminal bars, and basal laminae.
Conducting part: the duct system
- Ducts are generally lined by simple cuboidal or columnar epithelium.
- Larger ducts have stratified cuboidal or columnar epithelium.
- Ducts are not lined by simple squamous epithelium.
Order of flow:
- Secretory cells (serous, mucous, or mixed)
- Small intralobular duct = intercalated duct, simple low cuboidal epithelium
- Striated duct
- Interlobular duct, stratified columnar epithelium
- Main duct
Striated ducts
- Nucleus located towards the apex (the accompanying schematic labels it a centrally located nucleus).
- Basal striated cytoplasm, produced by basal interdigitations of cytoplasmic processes, that is, deep infoldings of the basal plasma membrane.
- Columns of mitochondria packed between the basal infoldings, which is what gives the striated appearance.
- Functions:
- Modify the saliva and regulate its pH.
- Secrete lysozymes and IgA.
On H&E the striated ducts appear as large pale eosinophilic profiles around a central lumen, surrounded by darker acini, with radial basal striations running from the basement membrane towards the lumen.
General plan of the gut wall
From the lumen outwards:
- Mucosa
- Epithelium
- Lamina propria
- Muscularis mucosae
- Submucosa
- Submucosal glands
- Submucosal nerve plexus (Meissner’s)
- Blood vessels
- Muscularis externa
- Inner circular layer
- Myenteric (Auerbach’s) plexus, lying between the two muscle layers
- Outer longitudinal layer
- Adventitia / serosa
On the mesenteric side the wall is continuous with the mesentery, which carries a vein, an artery and a lymph vessel. The two nerve plexuses are drawn as networks: the submucosal plexus within the submucosa, and the myenteric plexus between the inner circular and outer longitudinal muscle.
Oesophagus
Mucosa
- Epithelium: SSNKEpi. Function: mechanical barrier.
- Lamina propria: mucus glands, lymphoid aggregates.
- Muscularis mucosae: barely visible.
Submucosa
- Mucus and serous glands, prominent in the upper and lower third of the oesophagus.
- Arteries, veins, nerve plexus, elastic fibres.
In low-power transverse section the oesophagus has a characteristic stellate/folded lumen, and the layer sequence reads outwards as epithelium, then lamina propria and muscularis mucosae, then submucosa with its seromucous glands, then inner circular and outer longitudinal muscularis externa.
Muscularis externa: skeletal to smooth
| Region | Muscle type |
|---|---|
| Upper 1/3 | Skeletal muscle |
| Middle 1/3 | Skeletal and smooth muscle |
| Lower 1/3 | Smooth muscle |
- Longitudinal layer: thin posteriorly, deficient near the pharynx; the myenteric plexus lies here (between the layers).
- Circular layer: forms the upper sphincter.
Distinguishing the two muscle types in the same field: smooth muscle shows uniform pink fibres with small elongated central nuclei and no cross-striations; skeletal muscle shows larger fibres with visible cross-striations and peripherally placed nuclei.
Outer layer
- Adventitia.
References given
Gray’s Anatomy 41st ed; Junqueira’s Basic Histology 14th ed; Kerr’s Functional Histology 2nd ed; departmental slides and image collection.
Self-test
- List the epithelial types in the classification given, grouped as simple and stratified.
- Name the two epithelia that line the gut tube, and give the full expansion of SSNKEpi.
- List the four layers of the lip and oral cavity wall, from the lumen outwards.
- Explain why “all oral mucosa is SSNKEpi” is incorrect, naming the exception given.
- State the two functions of the lingual papillae.
- List the four types of lingual papilla with one distinguishing feature of each, and state which lack taste buds.
- Describe what is seen around a circumvallate papilla and what the glands at its base contribute.
- Explain why the glands of the GI tract are described as having an epithelial origin, and name the two functional parts every gland has.
- Describe the arrangement of connective tissue around a gland and give its two stated functions.
- Name the three cell types of the secretory part of the salivary glands and state the function of the myoepithelial cells.
- Distinguish a serous cell from a mucous cell on shape, nuclear position, apical contents and the nature of the secretion.
- Distinguish serous from mucous secretory cells on ultrastructure, that is, on the appearance of their secretory granules or vesicles.
- State the dominant secretory type of the parotid, sublingual and submandibular glands.
- Define a serous demilune and explain what its presence tells you about the gland you are looking at.
- Describe the duct system of a salivary gland in order from the acinus to the main duct, giving the epithelium of the intercalated and interlobular ducts.
- Explain why simple squamous epithelium is not expected in a salivary duct.
- Explain how the basal striations of a striated duct cell arise, and state the two functions of the striated duct.
- List the layers and sublayers of the general gut wall from the lumen outwards, including the position of both nerve plexuses.
- Describe the mucosa and submucosa of the oesophagus, including where the submucosal glands are most prominent.
- Describe the muscle composition of the oesophageal muscularis externa by thirds, and state which layer forms the upper sphincter.
- A histology slide shows a muscle field in which some bundles have cross-striations and peripheral nuclei while adjacent bundles have uniform fibres with small central nuclei and no striations. Which organ and which region of it are you most likely looking at, and why?
- A transverse section of a hollow organ shows a stellate lumen lined by stratified squamous non-keratinised epithelium, seromucous glands in the submucosa, and a barely visible muscularis mucosae. Identify the organ and justify from three of those features.
- Integrative: trace saliva from its site of production to the oral cavity, naming each cell type and duct segment it passes and stating what happens to the saliva along the way.
Answers
Reveal answers
- Simple: squamous, cuboidal, columnar. Stratified: squamous (keratinised and non-keratinised), cuboidal, columnar.
- Stratified squamous non-keratinised epithelium (SSNKEpi) and simple columnar epithelium.
- Mucosa (mostly SSNKEpi), lamina propria, submucosa (containing salivary glands), skeletal muscle (in the lip and soft palate).
- Not all oral mucosa is SSNKEpi: masticatory mucosa is parakeratinised.
- Generate friction for the food during chewing, and house taste buds.
- Filiform, conical and keratinised, lacking taste buds; fungiform, mushroom shaped, with taste buds; foliate, poorly developed in adults, with taste buds; vallate/circumvallate, the largest lingual papillae, with taste buds. Only the filiform lack taste buds.
- The circumvallate papilla is sunk into the mucosa with a surrounding trench, and serous glands open at its base; these facilitate taste sensation.
- All GI glands derive from epithelium. Every gland has a secretory part (the acini) and a conducting part (the ducts).
- Connective tissue covers the gland as a capsule, which gives off septa or trabeculae passing inwards between the acini. It provides structural integrity and conducts neurovascular bundles and ducts.
- Mucus secreting cells, serous secreting cells, and myoepithelial (basket) cells; the myoepithelial cells force secretions from the acinar lumen into the ducts.
- Serous cell: pyramidal, round basal nucleus, zymogen granules at the apex, secretes watery/serous saliva. Mucous cell: cylindrical, flat basal nucleus, pale mucus droplets at the apex, secretes thick viscous saliva.
- Serous cells contain heterogeneous electron-dense secretory granules with a spherical nucleus, plus apical microvilli and an intercellular secretory canaliculus. Mucous cells contain homogeneous electron-translucent secretory vesicles with a flattened basal nucleus. Both show tight junctions apically.
- Parotid, serous. Sublingual, mucus. Submandibular, both mucous and serous.
- A serous demilune is a crescent of serous cells capping a mucous acinus. Its presence indicates a mixed gland containing both mucous and serous elements, for example the submandibular gland.
- Secretory cells (serous, mucous or mixed), then the small intralobular or intercalated duct lined by simple low cuboidal epithelium, then the striated duct, then the interlobular duct lined by stratified columnar epithelium, then the main duct.
- Ducts are lined by simple cuboidal or columnar epithelium, and stratified cuboidal or columnar in the larger ducts; simple squamous is explicitly excluded.
- The basal third of the cell has deep infoldings of the basal plasma membrane (basal interdigitations of cytoplasmic processes) with columns of elongated mitochondria packed between them, which produces the striated appearance. Functions: modify the saliva and regulate its pH, and secrete lysozymes and IgA.
- Mucosa (epithelium, lamina propria, muscularis mucosae); submucosa (submucosal glands, submucosal or Meissner’s nerve plexus, blood vessels); muscularis externa (inner circular, then outer longitudinal, with the myenteric or Auerbach’s plexus between the two layers); adventitia or serosa.
- Mucosa: SSNKEpi acting as a mechanical barrier, a lamina propria containing mucus glands and lymphoid aggregates, and a barely visible muscularis mucosae. Submucosa: mucus and serous glands, most prominent in the upper and lower third, plus arteries, veins, nerve plexus and elastic fibres.
- Upper third skeletal, middle third skeletal and smooth, lower third smooth. The circular layer forms the upper sphincter.
- The oesophagus, in its middle third, because that is where skeletal and smooth muscle coexist in the muscularis externa; skeletal fibres are larger with cross-striations and peripheral nuclei, smooth fibres are uniform with small elongated central nuclei and no striations.
- The oesophagus. The stellate or folded lumen, the SSNKEpi acting as a mechanical barrier, and the seromucous glands in the submucosa (prominent in the upper and lower thirds) are all oesophageal features, as is the barely visible muscularis mucosae.
- Saliva is produced by serous cells (watery secretion, zymogen granules) and mucous cells (thick viscous secretion) in the acini, which may be purely serous (parotid), purely mucous (sublingual) or mixed with serous demilunes (submandibular). Myoepithelial cells force the secretion out of the acinar lumen. It then passes into the intercalated duct (simple low cuboidal), then the striated duct, where it is modified and its pH regulated and where lysozymes and IgA are added, then the interlobular duct (stratified columnar), then the main duct into the oral cavity.