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Questions and Answers
Which of the following correctly describes the primary function of microvilli?
Which of the following correctly describes the primary function of microvilli?
What distinguishes cilia from microvilli in terms of their functionality?
What distinguishes cilia from microvilli in terms of their functionality?
Which type of junction primarily functions to maintain strong connections between epithelial cells?
Which type of junction primarily functions to maintain strong connections between epithelial cells?
What is a characteristic feature of stereocilia compared to microvilli?
What is a characteristic feature of stereocilia compared to microvilli?
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Which statement about epithelial polarity is accurate?
Which statement about epithelial polarity is accurate?
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What component is part of the terminal web supporting microvilli?
What component is part of the terminal web supporting microvilli?
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What role does glycocalyx play in relation to microvilli?
What role does glycocalyx play in relation to microvilli?
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Which of the following best describes hemidesmosomes?
Which of the following best describes hemidesmosomes?
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What is the primary structural difference between cilia and flagella in terms of microtubule arrangement?
What is the primary structural difference between cilia and flagella in terms of microtubule arrangement?
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Which syndrome is associated with mutations in the proteins of cilia leading to immobility?
Which syndrome is associated with mutations in the proteins of cilia leading to immobility?
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What is the primary role of cadherins in cellular interactions?
What is the primary role of cadherins in cellular interactions?
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What is the primary function of tight junctions in epithelial tissues?
What is the primary function of tight junctions in epithelial tissues?
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What structural components are associated with desmosomes?
What structural components are associated with desmosomes?
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What happens to microvilli in the enterocytes during celiac disease?
What happens to microvilli in the enterocytes during celiac disease?
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Which junction type allows the rapid spread of chemical and electrical signals between adjacent cells?
Which junction type allows the rapid spread of chemical and electrical signals between adjacent cells?
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Which junction connects adjacent cells in a belt-like manner below tight junctions?
Which junction connects adjacent cells in a belt-like manner below tight junctions?
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In pemphigus vulgaris, which cellular structure is primarily targeted by antibodies?
In pemphigus vulgaris, which cellular structure is primarily targeted by antibodies?
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What is the cause of malabsorption in celiac disease?
What is the cause of malabsorption in celiac disease?
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What is a notable consequence of losing cadherins in cells?
What is a notable consequence of losing cadherins in cells?
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What type of junction is characterized by the fusion of outer cell membrane proteins?
What type of junction is characterized by the fusion of outer cell membrane proteins?
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What is the width of the intercellular space typically found in desmosomes?
What is the width of the intercellular space typically found in desmosomes?
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What is a common consequence of the dynein arm mutations found in cilia?
What is a common consequence of the dynein arm mutations found in cilia?
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What role do connexons play in gap junctions?
What role do connexons play in gap junctions?
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Which of the following conditions can disrupt tight junctions, leading to gastrointestinal issues?
Which of the following conditions can disrupt tight junctions, leading to gastrointestinal issues?
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What is the primary function of the basement membrane?
What is the primary function of the basement membrane?
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Which proteins form intercellular junctions that allow ions and small signaling molecules to pass between adjacent cells?
Which proteins form intercellular junctions that allow ions and small signaling molecules to pass between adjacent cells?
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Which structure is responsible for the attachment of the basal lamina to the underlying connective tissue?
Which structure is responsible for the attachment of the basal lamina to the underlying connective tissue?
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Which characteristic is associated with the structure of hemidesmosomes?
Which characteristic is associated with the structure of hemidesmosomes?
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What is the source of the reticular lamina in the basement membrane?
What is the source of the reticular lamina in the basement membrane?
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What role do basal and lateral infoldings in epithelial cells serve?
What role do basal and lateral infoldings in epithelial cells serve?
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Which of the following connexins is NOT involved in the formation of gap junctions?
Which of the following connexins is NOT involved in the formation of gap junctions?
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What type of collagen is specifically associated with anchoring fibrils in the basement membrane?
What type of collagen is specifically associated with anchoring fibrils in the basement membrane?
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Study Notes
Epithelial Cell Specializations
- Epithelial cells have different polarities
- Apical domain - faces the exterior surface
- Lateral domain - interacts with adjacent cells
- Basal domain - anchors the cell to underlying connective tissue
Apical Specializations
-
Microvilli: Finger-like extensions of cytoplasm covered by plasma membrane
- Each microvillus contains an actin core that is cross-linked to other microfilaments
- The basal ends of these microfilaments intermingle with the terminal web
- Terminal web: a dense layer of horizontal filaments in apical cytoplasm beneath the microvillus that provides structural support for the cell apex
- Glycocalyx: Filamentous coat (glycoproteins) covering the microvillus, PAS positive, known as the cell coat
- With light microscopy, the microvillus and glycocalyx appear as a brush border (striated)
- Sites: Absorptive cells of the small intestine, cells of the proximal renal tubules
- Function: Increase surface area of the cell
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Stereocilia: Long, branched, non-motile, irregular microvilli
- Found in the epididymis and inner ear
- Function: Increase the surface area; facilitate the movement of molecules into and out of the cell
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Cilia: Elongated, motile structures on the surface of epithelial cells
- Sites: Ciliated cells of the trachea where each cell contains about 250 cilia
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EM of cilia:
- Basal body: 27 microtubules (9+0)
- Axoneme (shaft): 20 microtubules (9+2)
- Axoneme is formed of outer nine doublets of microtubules surrounding two central singlets
- The two singlets are surrounded by a central sheath
-
Flagella: Present in the human body only in spermatozoa
- Similar in structure to cilia but much longer
Medical Applications (Apical Specializations)
-
Kartagener's Syndrome or Immotile Cilia Syndrome (Primary Ciliary Dyskinesia): Rare autosomal recessive disorder caused by a defect in the microtubules (dynein arm mutations)
- Characterized by absent or dysmotile cilia
- Symptoms: Immotile spermatozoa, male infertility, and chronic respiratory infections because of lack of ciliary cleansing action in the respiratory tract
- Abnormal cilia are also present in polycystic kidney disease
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Celiac Disease (Gluten-Sensitive Enteropathy or Sprue): Disorder of the small intestine where one of the first pathologic changes is the loss of microvilli (brush border) of the absorptive cells (enterocytes)
- Caused by an immune reaction to the wheat protein gluten during digestion which produces diffuse enteritis
- Leads to malabsorption and eventually to pathologic changes in the intestinal wall
- The malabsorption problems and structural changes are reversible when gluten is removed from the diet
Lateral Specializations (Intercellular Junctions)
-
Tight Junction (Zonula Occludens):
- Site: Apical parts of the cells
- Structure: Formed by the fusion (sealing) of outer cell membrane proteins (claudins and occludins) of two adjacent cells
- Function:
- Prevents fluid from escaping between cells (e.g. in the digestive)
- Forms the impermeable barrier of the blood-brain barrier
- Provides strength and stability to the tissue
-
Adherens Junction (Zonula Adherens):
- Description: Encircles the apical parts of two adjacent cells below the tight junction (belt-like)
- Structure:
- Intercellular space between adjacent cell membrane is 20nm.
- Cadherins adhere the two cells together.
- The cytoplasmic part of cadherins is attached to actin filaments inside the cells.
-
Desmosomes (Macula Adherens):
- Description: Spot-like specialization of the cell membrane that is the strongest type of cell junction and prevents cells from separation
- Structure:
- Two plaques are opposite each other on the cytoplasmic aspects of adjacent cell membranes, to which intermediate filaments (keratin) are inserted.
- Cadherins connect the desmosomal plaques of two cells.
- Intercellular space is 30nm.
-
Junctional Complex:
- Several types of junctions between adjacent epithelial cells to maintain their structural and functional integrity
Medical Applications (Lateral Specializations)
-
Pemphigus Vulgaris: Autoimmune disease where antibodies are produced against the desmosomal proteins of the epidermal cells
- Leads to widespread skin and mucous membrane blistering.
-
Gap Junction (Communicating Junction; Nexus):
- Site: Cardiac and smooth muscles
- There are many intercellular channels that connect two adjacent cells.
- Structure: Formed by the interaction of connexons of two neighboring cells.
- Connexon: Composed of six integral proteins called connexins that have a central pore.
- Function:
- Allows cells to communicate with each other.
- Facilitates rapid spread of chemical and electrical signals between cells.
- The contraction of cardiac muscles, muscle of the gut, and uterus depends on this junction.
Medical Applications (Gap Junction)
-
Clostridium Enterotoxin: Secretion of this bacterial enterotoxin causes food poisoning, binds to claudine molecules of intestinal cells, and disrupts tight junctions
- Leads to loss of interstitial fluid into the intestinal lumen (diarrhoea) and intestinal ulcers.
- Helicobacter pylori: Bacterial cause of gastric ulcers, binds to extracellular domains of tight-junction proteins in cells of the stomach, leading to the loss of the tight junction's integrity.
- Mutations in various connexin genes have been implicated in certain types of deafness and peripheral neuropathy.
Basal Specializations
-
Basement Membrane: Located at the interface between the epithelium and connective tissue
-
Basal Lamina:
- Synthesis: By epithelial cells
- Structure: Proteins (collagen type IV), glycoproteins (laminin), proteoglycans (heparan sulfate)
-
Reticular Lamina:
- Synthesis: By cells in the connective tissue
- Structure: Network of reticular fibers (collagen type III)
- Anchoring Fibrils: Type VII collagen that anchors the basal lamina to the underlying connective tissue
- Function:
- Anchors the epithelium to the connective tissue
- Acts as a semipermeable filter, playing a role in nutritional function
- Provides support.
-
Basal Lamina:
-
Hemidesmosome: Attaches cells to the extracellular matrix
- Structure: Integrins connect the intracellular cytoskeleton (keratin) with molecules of the basement membrane (laminin, fibronectin, and collagen)
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Basal & Lateral Infoldings:
- Structure: Basal and lateral membrane of cells is thrown into folds (deep invaginations)
- Numerous mitochondria are located near these folds to produce ATP necessary for ion transport.
- Site: Found in cells involved in fluid or ion transport, such as gut cells and kidney tubules.
- Function: Increase surface area of the plasma membrane.
Case Scenario
- The proteins that form intercellular (gap) junctions are connexins
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Description
Test your knowledge on the different specializations of epithelial cells, including their polarities and the structure and function of apical specializations like microvilli. This quiz covers essential concepts related to cell biology and histology, including the roles of microvilli and glycocalyx in absorptive cells.
