HISTOLOGY FULL-TEXT

William A Beresford MA, D Phil ©
Professor of Anatomy
Anatomy Department, West Virginia University, Morgantown, USA

Chapter 24 ALIMENTARY SYSTEM

Long, muscular, tubular structure for ingesting food and water, separating them from the intake of air, breaking the food down mechanically and chemically (digestion) for absorption, while propelling it anally. Ancillary glands, liver and pancreas (Chapter 25), are included, since they produce materials used for digestion or to be excreted via the tube, and they participate metabolically and in the control systems. Powerpoint

A ORAL STRUCTURES

l Salivary glands
l Generally compound tubulo-alveolar, with intralobular intercalated ducts and secretory ducts (with basal striations), leading to interlobular excretory ducts.
2 Parenchyma is divided by CT septa into lobes and lobules.
3 Saliva is water, salts, and organic materials (mainly mucin and salivary amylase/ptyalin and maltase), with suspended lymphocytes (salivary corpuscles), epithelial cells, and bacteria.
4 Mucin is formed by mucous cells (pale in HE staining).
5 Enzymes are formed by serous cells (basophil, with zymogen granules).
6 Parotid gland is serous; submandibular/submaxillary has serous alveoli, and mixed tubules with serous demilunes/crescents; and the sublingual gland has mucous and mixed branched tubules, but lacks intercalated and secretory ducts.( The tubules are long enough to reach the excretory ducts.)
7 Smaller mucous and mixed glands are in lingual, labial, buccal, pharyngeal and palatine sites.
8 Chapter 11 gave more details of salivary glandular structure.

2 Lip
l Core of fibro-elastic CT and skeletal muscle.
2 Outside is thin skin with hairs and glands.
3 Transition zone is the red margin/vermilion border, where the skin's cornified layer thins out; a rich capillary plexus is responsible for the colour. Glands are absent.
4 Inside is a thick stratified squamous epithelium, with mucous glands in its lamina propria.
5 The cheek is similar, but has more adipose tissue, and no red margin.

3 Gingiva/gum and raphe of hard palate
Stratified squamous epithelium (partly keratinized) on a dense CT lamina propria, with deeply penetrating papillae, and fastened tightly to tooth or bone.

4 Soft palate
l Fibrous and skeletal muscle core, with mucous glands;
2 pseudostratified, columnar, ciliated epithelium covers the pharyngeal side, and stratified squamous the oral surface.
3 Functions in deglutition (swallowing), speech, blowing, coughing, and sneezing.

5 Tongue
l Core is interlaced skeletal muscle bundles oriented in three directions, with attendant nerves and blood vessels.
2 Covered by stratified squamous epithelium, modified over the anterior dorsum by being thrown up with the dense lamina propria into projections called
3 papillae of various kinds, with special distributions:

4 Lingual glands - (a) posterior mucous; (b) posterior serous of von Ebner, opening into the trenches; (c) anterior mixed sero-mucous.
5 Lingual tonsils are stratified squamous epithelium-covered aggregations of lymphoid nodules, with shallow crypts flushed out by mucous secretions of the posterior lingual glands.

6 Palatine/faucal tonsils
l Covering is stratified squamous epithelium.
2 Deep, branching, epithelium-lined pits or crypts run down from the surface into the tonsils, but the epithelium is infiltrated by
3 lymphocytes produced in germinal centres of lymphoid nodules (often confluent) in the lamina propria, and by macrophages.
4 Immunoglobulins and lysozyme are present.
5 Glands and skeletal muscle lie nearby, outside the underlying CT capsule.
6 The palatine tonsils have substantial depth; the lingual are a narrow region interposed between the epithelium and the muscular core of the tongue.

7 Tooth
l Anatomical features: crown, cervix/neck, root, apical foramen, pulp cavity, bony alveolus/socket, attaching periodontal ligament and the gingiva.

2 Tooth components

3 Histological details of tooth
(a) Decalcification for sectioning destroys mature enamel. It can be studied in the ground section.
(b) Enamel prisms have a spiral curvature to better withstand masticatory forces.
(c) Bands/striae of Retzius are growth/incremental lines across the enamel; Owen's contour lines are analogous features in dentine.
(d) Interglobular areas are poorly mineralized regions in the dentine.
(e) Dentinal tubules branch, and may penetrate a little way into the enamel as enamel spindles.
(f) Von Korff's 'fibres' seen in the pulp by young odontoblasts are either collagen awaiting incorporation into the matrix of the dentine, or are an artefact of silver impregnation.
(g) Secondary dentine (sometimes reparative) may be formed later to increase the thickness of the dentine.
(h) Epithelial attachment is a cuff-like extension of the gingival epithelium, attached to the neck of the tooth by glycoprotein.
(i) Acellular cementum lacks cementocytes.

4 Tooth development
(a) Two stages with (i) 20 deciduous/milk teeth, (ii) followed by 20 successional teeth and l2 permanent or accessional molars, totalling 32. Powerpoint
(b) Involves complex inductive processes:

8 Functions of oral structures
l Obtaining, approving, masticating, and swallowing food and water.
2 General exploration of the environment.
3 Vocalization and communication. (Many systemic diseases have oral signs).
4 Preening, mating, fighting, etc, where chewing, licking, grasping or biting is needed.
5 Oral glands contribute to digestion, and the lymphoid tissues to protection.
6 Breathing in exercising, and when the nose is blocked; coughing, and blowing.

B GASTROINTESTINAL TRACT

l General plan
l Mucosa (innermost) 2 GI submucosa 3 GI muscularis externa 4 GI serosa or adventitia/fibrosa (outermost) 2 Oesophagus
l Mucosa has stratified squamous epithelium ending sharply, but along a jagged line, at the gastric junction, creating a white-red distinction between proximal and distal sides of the Z-line in endoscopy. Here, abnormalities of the oesophageal epithelium and the position of the epithelial junction are quite common - Barrett's oesophagus, where the stratified squamous epithelium is replaced metaplastically by simple columnar epithelium with some or all of the small-intestinal cell types.
2 Muscularis mucosae - longitudinal smooth muscle.
3 Cardiac glands - make neutral mucus and are branched tubular, in the mucosa near the gastric cardia, and in mucosa of the upper oesophagus; inconsistently present.
4 Oesophageal glands - acidic mucous, compound, tubulo-alveolar, and lying in the submucosa, less numerous in the middle segment of the oesophagus.
5 Circular and longitudinal external muscle coats of skeletal muscle in the upper fifth or so give way progressively to only smooth muscle in the lower half.
6 Outermost coat is CT adventitia, except on a small piece below the diaphragm.
7 Function - rapid passage of food to (and from) the stomach.

3 Stomach
l General structure

2 Stomach mucosa 3 Gastric secretions and cell types responsible
(a) Surface mucous cells - mucus, to prevent auto-digestion of the mucosa, and bicarbonate ions held in the mucus.
(b) Chief/zymogenic cells - enzymes, e.g., pepsin, rennin, gastric lipase.
(c) Oxyntic/parietal cells - Cl-/HCO3- is exchanged basolaterally to balance the apical Na+/H+ proton pump used to form the hydrochloric acid of the digestive juice.
(The stimulated active parietal cell has greatly extended canaliculi.)
(d) Mucous neck cells - mucus and enzymes, e.g., dipeptidases.
(e) Endocrine cells - hormones and amines; e.g., a hormone - gastrin - produced by the pyloric antral G cells controls the release and formation of acid from parietal cells, and of digestive enzymes from chief cells.
(f) Parietal cells - intrinsic factor - to assist in the absorption of vitamin B12: this role is upset when the parietal cells' proton pump is an autoimmune target in pernicious anaemia, leading to the cells' destruction.

4 Gastric protective mechanisms

4 Small intestine
l General structure 2 Cytology of small-intestinal mucosa 3 Functions of small-intestinal mucosa

4 Changes within small intestine during descent:
(a) Goblet cells increase in number.
(b) Villi become more finger-like.
(c) Lymphoid tissue increases.
(d) Plicae circulares diminish.

5 Protective mechanisms of the gut:

5 Large intestine
l General features

2 Regional details of large intestine
William A Beresford, Anatomy Department, School of Medicine, West Virginia University, Morgantown, WV 26506-9128, USA - - e-mail: -- wberesfo@wvu.edu -- wberesfo@hotmail.com -- beresfo@wvnvm.wvnet.edu -- fax: 304-293-8159