Epithelial Tissue Flashcards
Describe the Classification of Epithelial Tissues
1) SURFACE EPITHELIA
Cover a surface & vary in shape in relation to:
- environment they face
- function they perform (e.g. impermeable protective barrier, production of materials, permeability)
2) GLANDULAR EPITHELIA
Produce and secrete substances
- Endocrine: release products into bloodstream
- Exocrine: release products outside body or inside cavities that communicate with outside of the body
3) SENSORY EPITHELIA
Responsible for the 5 senses
- Many epithelial and nervous tissue have the same embryonic origin, so sensory epithelia shares some characteristics
How can we identify an epithelium under an optical microscope?
1) Tightly packed cells
2) Lack of vessels - take a needle parallel to skin there is no blood > gets nutrients by diffusion
3) Presence of an Amorphous Layer (basal laminar) - separates epithelium from underlying connective tissue
How do we classify surface epithelia?
1) NUMBER OF CELL LAYERS
- Simple: 1 layer
- Stratified: >1 layers
- Pseudostratified: 1 layer, different sized cells
- Transitional: apparent different number of layers according to status of organ (urothelium of bladder)
2) CELL SHAPE - of most superficial layer
Squamous, cuboidal or columnar
3) SURFACE SPECIALISATIONS
- Cilia, Microvilli, Pseudocilia, Keratinisation
What are the different specialisations of the free surface of an epithelium?
Cilia
- core of microtubule doublets in 9+2 arrangement
- lung
Microvilli
- core of actin-containing microfilaments
- intestine
Stereocilium
- branched
- core of actin-containing microfilaments
- epididymus of testicle, ear
Describe Simple Squamous Epithelium
Located: in blood & lymphatic vessels (endothelium), internal cavities (peritoneum), alveoli, and pericardium & pleura (mesothelium)
Mesothelium:
- Located in pericardium and pleura, surrounds male testis as tunica vaginalis
- Expresses vimentin, desmin, cytokeratin
- Secretes lamellar bodies rich in phosphatidylcholine
- Reabsorbs fluids through pinocytosis from pleural, peritoneal and pericardial cavities
Endothelium
- Located in inner surface of blood & lymphatic vessels, and heart
- Expresses vimentin and desmin (NOT cytokeratin)
- Produces vasoactive substances e.g. endothelin 1 (vasoconstrictor), prostacyclin and NO (vasodilator)
Respiratory Epithelium
- Type 1 pneumocytes: simple squamous
- Type 2 pneumocytes: larger cuboidal cells, can proliferate and differentiate to Type 1, situated at septa they secrete surfactant
What are the three types of Capillaries?
1) Continuous
- completely covered by endothelial cells which are held together by occluding junctions
- continuous basal lamina
- e.g. in nervous tissue, fat, muscle
2) Fenestrated
- have some holes between cells through which substances can pass
- continuous basal lamina
- e.g. kidney glomeruli
3) Discontinuous (sinusoids)
- Both discontinuous epithelial cells and basal lamina
- Allows passage of a lot of substances
- e.g. spleen, liver, bone marrow
Describe Simple Cuboidal Epithelium. Do we have any stratified cuboidal epithelium?
Located: glandular ducts, kidney tubules
Function: absorption, secretion into ducts or tubes
NB: vessels and always squamous while ducts are always cuboidal
Stratified cuboidal epithelium? Yes, rarely.
- conjunctiva, pharynx, anus, and male urethra
Describe Simple Columnar Epithelium. Do we have stratified columnar epithelium?
Located: stomach, intestine, uterine tube
Function: absorption and secretion
Apical specialisations:
- Microvilli: brush border of intestine
- Cilia: uterine tube
Polarised > nuclei are towards basal membrane
NB: Goblet cells are present in GI tract in simple columnar epithelium (with microvilli), and in respiratory tract within pseudostratified columnar epithelium (with cilia)
Stratified columnar? Doesn’t exist
Describe Pseudostratified Columnar Epithelium. How can we recognise pseudostratified columnar from stratified?
Located: tracheal-bronchial tree and epididymus (only these 2)
Functions: protection, transport of materials on surface
Surface specialisations:
- Cilia: trachea and bronchioles
- Stereocilia: epididymus
Recognition? In no cases do stratified epithelium have surface specialisations
Describe Transitional Epithelium
Urothelium: present in bladder
- depth changes according to different volume of organ
- when volume increases the number of apparent layers decreases
- when bladder empty, cells become almost round
- stratified > 1-3 when full bladder, 5-7 when empty
Compare the different types of Surface Epithelium
Describe Pluri-stratified Squamous Epithelium (non-keratinised)
- Location: oesophagus
- Distribution of nuclei is different between apical and basal portion of the tissue
- Cells have different shapes > to define epithelium type use the superficial layers
- If top layer of cells has nuclei it is non-keratinised
Describe keratinised pluri-stratified squamous epithelium
- Skin is the only example in humans
- Superficial layer cells lack nuclei
- Animals may have keratinised tissue in digestive tract as they eat very hard food
- Keratinised is more mechanically resistant
- Makes tissue impermeable > prevents loss of water
