Epithelia

Question 1

Classification of epithelia

Answer 1

1. Epithelia are tissues that line free surfaces of the body
2. They are avascular (nourished by connective tissue)
3. Classified based on no. of cell layers
4. Simple epithelia: have only cell layer
   Stratified epithelia: have two or more layers
5. Stratified epithelia are designed for protection; basal layer is cuboidal; always sits on a basement membrane; undergoes mitosis to form layers; close to surface cells become more flattened
6. Stratified epithelia are classified according to shape of cells in luminal surface:
   → stratified squamous
   → stratified columnar /cuboidal
   → transitional
7. Stratified squamous: lines surfaces exposed to friction → keratinizing & non-keratinizing
   Keratinizing → have cells with cytoplasm filled with keratin
   Non-keratinizing → contains basal cells which become polygonal in shape and later squamous when it ———————————————————— reaches the surface
8. Stratified columnar/cuboidal: very uncommon, found mainly in large ducts in the glands, permitted up to 2-3 cell layers
9. Transitional: → also called urinary epithelium, found exclusively in urinary tract
   → cells in luminal surface are umbrella cells, mainly because this layer is urine impermeable
   → designed for max. stretching; provides protection against acidic urine

Question 2

Surface specializations and their function

Answer 2

1. Cilia - beats like a wave, propels a film of mucus on surface (fallopian tube)
2. Microvilli - increases SA of cell for absorption (SI lining)
3. Stereocilia - long microvilli (epididymis)

Question 3

Cell junctions and their function

Answer 3

1. Tight junction - prevents luminal content from leaking in between cells (cell membrane)
   - found just below the apical surface of the epithelial cells
   → intestinal epithelium
2. Adhering junction - makes strong transcellular network lining all epithelial cells together (cytoskeleton)
   → most tissues
3. Gap junction - allows passage of inorganic ions and + charged small molecules to pass (cytoplasm)
   - important in cell to cell recognition
   → cardiac muscles
4. Hemidesmosomes connect basal cells to basement membrane
5. Junctional complex can be seen in intestinal epithelium

Question 4

How is the SI adapted to perform its function?

Answer 4

1. Simple columnar, epithelium; adapted for absorption
2. Cells are tall to accommodate more organelles; nucleus shifted to base; microvilli on luminal surface to↑SA
3. Basal surface tightly bound to basement membrane for integrity
4. Single layer thick for effective absorption; forms villi for effective absorption

Question 5

How are salivary glands and secretory cells adapted to form their function?

Answer 5

1. Simple cuboidal; adapted for secretion
2. Large nucleus & lots of RER, Golgi apparatus & vesicles for producing secretions. Single layer
3. Basal surface tightly bound to basement membrane for integrity

Question 6

How are neurons adapted to perform their function?

Answer 6

1. Conducts nerve impulses according to stimulations
2. Multiple short dendrites to gather more info from sensory organs
3. Long axon to convey info to CNS/effectors
4. Large nucleus, RER, Golgi, vesicle for neurotransmitters
5. Multiple mitochondria to generate energy for nerve impulse conduction
6. To↑ conduction velocity, axons are myelinated
7. Have specialized cells: Schwan cells to produce myelin

Question 7

How are skeletal muscles adapted to perform their function?

Answer 7

1. Function is contractility + movement
2. Contains myofilaments which make myofibril which produce muscle fibre
3. Muscle fiber is covered by endomysium
4. Arranged in bundles (fascicles) covered by perimysium. Arranged longitudinally
5. Muscles contain Creatine phosphate & Glycogen as energy sources
6. Myoglobin for effective oxygen transport
7. Rich capillary density for nutrient + oxygen transport; lots of mitochondria for energy

Question 8

Functions of smooth endoplasmic reticulum

Answer 8

1. Synthesis of cholesterol and phospholipids
2. Intra-cellular transport
3. Metabolism of glycogen
4. Detoxification of drugs and alcohol (in liver)
5. Storage and release of calcium ions (in muscles)

Question 9

What is the Golgi apparatus involved in?

Answer 9

1. Packing and labeling of substances
2. Modification of proteins
3. Synthesis of glycoproteins
4. Recycles cell membrane and returns it to the common pool
5. Replaces damaged membrane

Question 10

Sites of epithelia

Answer 10

1. Simple squamous → lungs, kidneys, blood capillaries
2. Stratified squamous → esophagus, vagina, rectum, oral cavity, glans penis, skin
3. Simple cuboidal → tubules of kidney, ovary, exocrine, glands (small), thyroid
4. Stratified cuboidal → exocrine glands (large), salivary glands, sweat glands, mammary glands, pancreas
5. Simple columnar → fallopian tube, small bronchi, tract from stomach to anal canal
6. Stratified columnar → male reproductive tract, pharynx
7. Pseudostratified columnar → respiratory tract, trachea, vas deferens, epididymis
8. Transitional → urinary tract

Question 11

Functions of epithelia

Answer 11

1. Simple squamous → transport, filtration
2. Stratified squamous → protects tissue from abrasion
3. Simple cuboidal → secretion, absorption
4. Stratified cuboidal → protection
5. Simple columnar → secretion of enzymes, propels mucus (ciliated)
6. Pseudostratified columnar → propels mucus
7. Transitional → distention, withstands toxicity of urine