Epithelial tissue and cell junctions Flashcards
Functions of the Basement Membrane/Basal Lamina
- Attachment of the epithelial cells to connective
tissue - Isolates epithelial cells from muscle cells
(compartmentalization) - Filtration/movement of molecules regulated by
ionic charge - Important in the kidneys
- Maintains polarity of epithelia
- Aids in tissue scaffolding and scaffold
regeneration
Composition of the Basement Membrane
- Proteoglycans
- Glycoproteins
- Collagen
- A layer of underlying reticular fibres
Characteristics of Epithelial Tissues
- Cells are closely associated, leaving little extracellular space
- Cells are strongly attached by intracellular junctions
- Epithelial tissues line body surfaces and cavities
- Epithelial tissues can be specialized
o Absorption
o Secretion
o Sensation
o Contractility - Cells are polarized
- All epithelial tissues rest on a basement membrane
Epithelial Tissue Composition
- Zonula occludens
- Zonula occludens
o Occludens means to “keep out”
o These are tight junctions
o Membrane proteins such as occludin and claudin present here
o Membrane proteins make a tight seal to form barriers in epithelial tissue
o Cytoplasmic proteins such as ZO-1 and actin present within cell link membrane
proteins
Zonula adherens
- Zonula adherens
o Form band around cell
o Cytoplasmic proteins alpha-actinin and vinculin present here
o Contain a terminal web of actin bundled proteins
o Intercellular space of 15nm
o Cadherins, transmembrane proteins with cytoplasmic
domains, are present here
Macula adherens (desmosome)
Intercellular space of 25nm
- Contains transmembrane proteins desmocollin and
desmoglein
- Has intermediate filaments called tonofilaments
- Filaments form a hairpin loop which provides strength
-These filaments are non-polar due to their self-assembly process
- Plaque proteins are found here, and in hemidesmosomes
- Density of proteins such as desmoplakins I
and II, plakoglobin, and desmocalmin
o This is the tightest type of junction
o Make skin tough
o Wide intercellular space due to transmembrane
proteins
- Hemidesmosomes
Hemidesmosomes
- Plaque contains BP 200 a protein related to
bullous phemphigoid
- If hemidesmosomes are faulty, the
stratum germinativum can flake off of
the basement membrane, forming blisters
- Has tonofilaments
- Transmembrane proteins alpha-6
integrin, beta-4 integrin, and BP 180 present
- Collagen VII is found here
Gap junction
-2nm of intercellular space
- Contain an ion channel which is 1.5nm in diameter
- Coupled connexons are 6 individual connexins which couple with 6
others
summary of epithelial tissue
- Highly polarized
- Can be specialized for
structure, migration, and secretion, which start on a
basement membrane - All epithelial tissues rest on a basement
membrane - Very little intracellular space
- Junctional complexes found here
o Zonula occludens
o Zonula adherens
o Desmosome
o Hemidesmosome
o Gap junction
Epithelial Tissue of the Skin
- Cells of the stratum spinosum are
connected by desmosomes - The stratum granulosum contains
keratohyalin granules and lamellar bodies
which are waxed in lipids - When cell lysis occurs and the cells die, forming the stratum corneum, keratin forms sheets which make the skin tough and lamellar bodies form lipid droplets which allow for waterproofing
Stem Cells of the Stratum Germinativum
- Stem cells can differentiate or self-renew
- Steady state refers to when the number of newly originated cells by the stem cells are
equal to the number of dead cells on the surface of the epithelium
o Skin is in a steady state - Turnover time refers to the time required to change the whole population of epithelial
cells - In skin, this is 30 days
- Radioautography is the means of studying the turnover of epithelial cells
o In an experiment, mice were fed radioactive thymine, which appeared in skin cells
over time, proving the existence of stem cells and skin turnover
During adulthood, cells/tissues behave in 3 ways
- Static (neurons, skeletal muscle)
- Expanding (liver)
- Renewing (epidermis and most epithelia)
The cell cycle
o G1
- Pre-synthesis of DNA
-25 hours
- Diploid cells
o S
- Synthesis of DNA
- 8 hours
o G2
- Post-DNA duplication
- 3 hours
-Tetraploid cells
o Mitosis
- 2.5 hours
- Diploid cell state
restored
* Human chromosomes
o 23 pairs of homologous
chromosomes, yielding 46
individual chromosomes
explain all the steps of Mitosis briefly
(PMAT)
- Preprophase
o Condensation of chromosomes within nucleus
* 2. Prophase
o Chromosomes and mitotic spindle become visible
* 3. Metaphase
o Chromosomes line up in the equatorial plane
o Nuclear envelope and nucleolus are lost
* 4. Early anaphase
o Longitudinal splitting of chromosomes
* 5. Late anaphase
o Chromosomes migrate to poles
* 6. Telophase
o Nuclear envelope and nucleolus reform
o End of cell division
