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Epithelial Tissue, Epithelia: Closely applied sheet of cells with minimal…
Epithelial Tissue
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Types of Epithelia
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Covering Epithelia
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- Cells organized in layers
- Cover the external surface / line the cavities of the body
- Classified according to number of cell layers & morphologic features(shape) of the cells in the surface layer
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Epithelia: Closely applied sheet of cells with minimal intercellular substance
- Covers all free surface of the body
- Large internal body cavities
- Internal surface of blood & lymph vessels
Derived from 3 germ layers
- Ectoderm
- Mesoderm
- Endoderm
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- Single layer of thin flat cells
- Width > height
- Nuclei are flat/oval/egg-shaped
- Central nuclei appear as protrusion
- Easy movement across single thin cell (exchange)
- Sigle layer of cells
- Height & width almost equal
- Nuclei rounded and centrally located
- Cells appear cuboidal in sections perpendicular to the surface of the epithelium
- Viewed from the surface of epith, (look rather like small polygons)
- Associated with secretion
Commonly found in glands and ducts within body
- kidney tubules
- sweat gland
- thyroid gland
Commonly found in digestive tract
- Stomach
- Intestine
- Gall bladder
Uterus & uterine tube
- Single layer of cells
- Height> width
- Nuclei elongated and basally located
- Associated with absorption/secretion
- Often associated with goblet cells
- Have microvilli/ cilia to increase surface area
- More than 1 layer of cells
- Classified according to the cell shape of its superficial layer : squamous, cuboidal, columar, transitional
- Pseudostratified epithelium :
- Many layers of thin "flat" cells vary in thickness
- Function : Protection, barrier
- Larger ducts of glands
- Testicular tubules
- Graafian follicles
- Deeper layers : Columar/ polyhedral
- Superficial layer: Cuboidal
- Rare
- Penile urethra
- Conjuctiva of the eye
- Ducts of glands
- Deeper layers: Polygonal + Cuboidal
- Surface cells: Columnar
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- 3-5 layers
- Contracted & distended morphological transitions of TE to accomodate changes in volume (e.g: urinary bladder)
- Urinary bladder
- Ureter
- Renal pelvis
- Major & minor calyses
- Trachea
- Larger branches of lungs
- Parts of reproductive system
- Appear as many layers(actually only 1 layer)
- All cells lies on same basement membrane, but not all cells reach free surface
- Nuclei at different levels in cell (giving false impression of stratification)
- Have cilia (important for movement)
- A sheet-like extracellular structure
- Separate basal surfaces from subjacent connective tissue
- Visible only with electron microscope
- Attached to underlying connective tissue by anchoring fibrils
- 200-100 nm thick
- Lamina densa (delicate network of fine fibrils)
- Laminae lucidae/rarae (electron lucent layer)
- type IV collagen
- glycoproteins (laminin&entactin)
- proteoglycan
- Around muscle, adipose &Schwann cells basal laminae : provide barrier that limits /regulates exchanges of material
- Between adjacent epithelial layers. e.g. in lung alveoli
- Structural attachment: Intermediary structure in the attachment of cells to adjacent connective tissue
- Compartmentalization: Separates/isolates the CT from epithelia, nerve& muscle tissue
- Provide a better barrier that regulates exchanges of macromolecules
- Polarity induction: Epithelial cells exhibit functionally different membrane properties as a result of surface exposure,
specific properties attributable to basal membrane surface are induced by the presence of basal lamina
- Tissue scaffolding: -Serves as a guide/ scaffold during regeneration & helps to maintain the original tissue architecture