Flashcards in Lecture 30: Upper GI Histology I Deck (17)
What are generalizations of the GI tract?
* Epithelium of the GI tract:
- Stratified squamous at either end for protection and resisting friction
- Simple columnar epithelium most of length
- With glands along most of length of tract
* Musculature of the GI tract:
- Skeletal muscle at either end, including upper esophagus
- Smooth muscle throughout most of length
* The GI tract possesses an intrinsic rhythmicity:
- Mostly due to presence of an enteric nervous system
-- Exists independently of external influences including ANS
- Also receives input from sympathetic and parasympathetic nervous system.
* Peptide neurotransmitters:
- Vasoactive intestinal peptide
* Unicellular endocrine cells:
- Secrete neuropeptides
- Derived from endoderm unlike intrinsic nerve fibers, which are derived from neural crest
* Lymphoid tissue:
- MALT (mucosal associated lymphoid tissue)
-- Referred to as GALT (gut associated lymphoid tissue) in the gut tube
* Protective mechanisms:
- Production of large amount of mucous:
-- Assists in lubrication and friction reduction
- Rapid turnover of epithelial cells in harsh environments such as stomach and small intestine
Describe the lip
* The oral cavity is lined with a mucous membrane lying on a vascular FECT layer; this mucosa consists of:
- Stratified squamous epithelium
- Basement membrane
- Lamina propria
* The lip is composed of a core of skeletal muscle (obicularis oris) cells embedded in FECT and covered externally by skin.
* An internal poorly keratinized, modified, stratified squamous epithelium lies on a lamina propria with high papillae.
* Within the lamina propria are mixed and mucous glands.
* Dermis has a rich plexus of capillaries which are prominent at the free margin of the lip.
Describe the cheek
* Structure is similar to the lip.
* One exception is the abundant elastic fibers present in the submucosa:
- These are continuous with the fibers surrounding the striated muscle and the lamina propria.
- This arrangement gives a maximum of distensibility to the oral mucous membrane.
Describe the structure of teeth
* Oral cavity contains teeth and tongue (including taste buds).
* Three embryonic tissues interact in the development of teeth:
- Ectoderm forms the enamel organ:
-- Consists of cells committed to formation of enamel
-- Formed from ectoderm under influence of mesoderm and neural crest
* Neural crest and mesoderm give rise to dental papilla.
- Forms dentin, cementum, and pulp as well as surrounding connective tissue and periodontal pulp
Describe the cells found in tooth development
- Form enamel which covers only the tooth crown
- Enamel is the hardest substance in the body.
- Enamel is 96% inorganic salts, about 90% of which is calcium phosphate in the form of apatite crystals and 4% organic matter and water.
- Enamel is laid down in prisms.
- Each prism is formed by one ameloblast.
- Secreting apical domains = Tomes’ processes.
- Increment lines of Retzius are periods of rhythmic growth.
* Specific organic components, rather than collagen, are unique to enamel and are removed after calcification:
- Amelogenins and Enamelins
- See Slide 17
Describe the pulp and cementum in the development of teeth
- Originates from the dental papilla containing condensed mesenchyme
- Consists of fibroblasts, macrophages, peripheral odontoblasts, reticular fibers, nerve fibers, and blood vessels that pass via the apical foramen
- Cells of the dental sac differentiate into cementoblasts which deposit cementum on the dentin of the root from neck to apex. Cementum has coarse collagen fibers (Sharpey's) in a bone-like calcified matrix.
Describe the periodontal membrane in the development of teeth
* CT formed from dental sac with fibroblasts, osteoblasts, cementoblasts, collagen fibers, blood vessels, and nerve fibers.
* Highly metabolically active tissue
* Binds cementum to bony socket.
* Sharpey's fibers extend from cementum to alveolar wall via the membrane.
* Allows limited movement
* Absorbs pressures of mastication and prevents this pressure from damaging alveolar bone.
* Affected by diseases such as diabetes and scurvy
Describe the tongue
* The tongue is composed primarily of a core of skeletal muscle and glands, and is covered by a mucous membrane.
* The anterior two-thirds of the upper oral portion is separated from the posterior one-third by the sulcus terminalis.
* Lingual papillae appear on the oral portion as surface projections; they are formed of a central core of CT lamina propria covered by stratified squamous epithelium.
Describe the lingual papillae
* Filiform papillae:
- Most numerous of all the papillae with a conical appearance
- Evenly distributed over the entire oral upper portion
- Lack taste buds
* Fungiform papillae:
- Relatively few in number;
- Interspersed singly among the parallel rows of filiform papillae;
- Have a mushroom appearance;
- Taste buds are present only on the oral surface of the epithelium in contradistinction to the position of the taste buds on he circumvallate papillae whose taste buds are primarily located in the lateral walls
* Foliate papillae:
- Rudimentary in humans, but well developed in lower animals.
- The pharyngeal portion is free of papillae but contains the lingual tonsils.
* Circumvallate papillae:
- Located along the sulcus terminalis as projections surrounded by a moat (circular furrow).
- Taste buds are present on the lateral walls.
- Ducts of von Ebner's glands (serous) open into the moat.
Describe the taste buds
* Contain two types of cells derived from a single stem cell:
* Sustentacular cells:
- Spindle-shaped support cells and arranged like barrel-staves to surround the inner taste pore at the base.
* Taste cells:
- Each cell is long and slender with an elongated central nucleus and terminates as a short taste hair which projects into the external opening called the outer taste pore.
- Possess apical microvilli with taste receptors
- Basal part of cell releases neurotransmitters
- Taste cells are distributed between the supporting cells.
- See Slides 27-28
Describe Taste Sensation
* Except for bitter taste, taste sensations utilize various signal transduction pathways that result in depolarization of the taste cell.
- Salt and sour utilize ionic transport as a signal transduction mechanism.
- Sour involves hydrogen ion blockage of potassium ion channels to cause depolarization.
* Bitter taste involves a hypopolarized state similar to retinal cell signal transduction; sweet taste involves a hyperpolarized state.
- Involves gustucin (a homologue of retinal transducin)
- Involves activation of Gα subunit, decrease in cGMP, and closure of 29 sodium channels and hypopolarization of the taste cell.
Describe the first layer of the gut tube
* Mucosa (innermost):
- Stratified squamous transitioning to simple columnar
- May have secretory, absorptive, and/or protective functions
* Lamina propria:
- Loose areolar CT associated with epithelium
- Contains various glands and GALT (gut-associated lymphoid tissue)
* Muscularis mucosa:
- Made up of one to three layers of smooth muscle
Describe the second layer of the gut tube
- Dense, irregular connective tissue
- Contains a nerve plexus: Meissner’s plexus (submucosal plexus)
Describe the third layer of the gut tube
* Muscularis externa:
- Inner circular layer of smooth muscle
- Outer longitudinal layer of smooth muscle
- Myenteric (Auerbach’s) nerve plexus lies between the two layers of muscle tissue.
-- Regulates the size of the lumen (circular layer of smooth muscle)
-- Regulates rhythmic movement of the GI tract (longitudinal layer of smooth muscle)
Describe the fourth layer of the gut tube
* Outermost layer:
- Composed of dense irregular CT
- Consists of a mesothelial lining and a layer of submesothelial connective tissue
- Forms visceral peritoneum
- Covers intraperitoneal portions of abdominal organs
- Dense irregular CT with adipose tissue
- Covers retroperitoneal portions of digestive system.
- See Slide 35-37
Describe Gut Wall Innervation
- Sympathetic postganglionic fibers pass through gut wall to glands and smooth muscle.
- Preganglionic fibers synapse with postganglionic fibers in ganglia within the gut wall itself.
- Meissner’s plexus: Regulates local secretions, blood flow, and absorption
- Auerbach’s plexus: Coordinates muscular activity of gut wall