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List the three qualifying characteristics of epithelial tissue.
- 1. Closely adhering cells with little ECM between cells.
- 2. May arise from all 3 germ layers (ectoderm, mesoderm, or endoderm) - it is the only tissue that can do this.
- 3. Avascular - no blood vessels - it exchanges nutrients and wastes with connective tissue via diffusion through the basal lamina.
What are the two types of epithelium?
- 1. Surface Epithelium - covers external and internal body surfaces ie. skin, digestive system, respiratory system, body cavities, etc
- 2. Glands - form the functional units of glands
What are the three functions of the surface epithelium?
- 1. Protection of the underlying tissues and organs
- 2. Transportation of materials between tissues and organs- ie. blood vessels
- 3. Absorption of substances from a free surface or lumen - ie GI tract, kidney tubules
The epithelial lining of the pleural (lungs), pericardial (heart), and peritoneal (GI tract) cavities.
The epithelial lining of the lumen of blood vessels and lymphatics.
What is the classification of surface epithelium based on?
- 1. Number of cell layers between the free surface and the basal lamina - simple (one) or stratified (several)
- 2. Shape of the cells at their free surface - squamous (flat), cuboidal, columnar
Fill in the blanks.
- 1. Simple squamous epithelium: single layer of flat cells ie. lining of blood vessels and body cavities- good for transport
- 2. Simple cuboidal epithelium: single layer of cuboidal cells ie. kidney tubules - good for absorption and processing
- 3. Simple columnar epithelium: single layer of columnar cells ie. lining of digestive tract - good for absorption and processing
- 4. Stratified squamous epithelium: several layers of cells with the layer adjacent to the free surface being flat cells.
- 5. Stratified cuboidal epithelium: 2 or more layers of cells with the top layer being cuboidal, ie. ducts of sweat glands
- 6. Pseudostratified epithelium: single layer of cells appear stratified as cells are different heights; all rest of basal lamina, but not all reach surface. ie. trachea
- 7. Transitional epithelium: stratified cells in which the surface layer changes shape. When the tissue is relaxed, the cells are rounded. When the tissue is distended, the cells are squamous. ie. most of urinary tract and bladder
Emphasize the differenced between the two kinds of stratified, squamous epithelium.
Stratified, squamous epithelium can either be non-keratinized or keratinized. In keratinized epithelium, the superficial layer of cells are dead; the nuclei and cytoplasm are replaced by keratin (insolube protein).
What kinds of polarity do epithelial cells possess?
Surface polarity and intracellular polarity
What are the three surfaces of an epithelial cell and how are they polarized?
- 1. Apical surface: free surface facing lumen, may have microvilli and cilia
- 2. Lateral surface: has occluding (tight) junctions at apical part of surface to separate apical and basolateral suraces and other junctions to maintain contact with adjacent cells
- 3. Basal surface: specializations to interact with the underlying basal lamina
Give a brief description of microvilli.
- cytoplasmic projections of apical surface into the lumen
- increase surface area
- contain mostly actin filaments
Give a brief description of cilia.
- Motile hair-like projections that move substances over the surface
- allow the movement of materials from one area to another
- composed of microtubules
How does an epithelial cell exhibit intracellular polarity?
The nucleus, RER, and mitochondria and located in the basal end of the cell while the Golgi apparatus and secretory vesicles and located in the apical end of the cell.
Simple columnar and simple cuboidal epithelial sheets are both used for the absorption and processing of materials. Explain the advantage of columnar cells over cuboidal.
Columnar cells have more room to accomodate the larger number of organells necessary to absorb and process more materials at once.