2.3 - Epithelial Cells and Tissues

Question 1

Tissues and extracellular matrix

Answer 1

• tissues: a group of cells whose type, organisation and architecture are integral to its function. Tissues are made up of cells, extracellular matrix and fluid
• extracellular matrix (ECM) - material deposited by cells which forms the ‘insoluble’ part of the extracellular environment. Generally composed of fibrillar (/reticular) proteins (e.g. collagens, elastins) embedded in a hydrated gel (proteoglycans). May be poorly organised (e.g. loose connective tissue) or highly organised (e.g. tendon, bone, basal lamina)

Question 2

Five main cell types

Answer 2

• connective tissue cells: fibroblasts (many tissues), chondrocytes (cartilage), osteocytes (bones)
• contractile tissues: skeletal muscle, cardiac muscle, smooth muscle
• haematopoietic cells: blood cells, tissue-resident immune cells, and the cells of bone marrow from which they are derived
• neural cells: cells of the nervous system having two main types; neurones (carry electrical signals) and glial cells (support cells)
• epithelial cells: cells forming continuous layers, these layers line surfaces and separate tissue compartments etc

Question 3

Types of tumours from cell types

Answer 3

• epithelial cancers are carcinomas
• mesenchymal (connective tissue and muscle) cancers are sarcomas
• haematopoietic cancers are leukaemias (from bone marrow cells) or lymphomas (from lymphocytes)
• neural cell cancers are neuroblastomas (from neurones) or gliomas (from glial cells)

Question 4

Epithelial organisation

Answer 4

• epithelial cells make organised, stable cell-cell junctions to form continuous, cohesive layers
• epithelial layers line internal and external body surfaces and have a variety of functions e.g. transport, absorption, secretion, protection
• cell-cell junctions are key to the formation and maintenance of epithelial layers e.g. in separating tissue compartments, or lining the surface of a tissue
• epithelial characterisation can be by shape and by layering

Question 5

Epithelial characterisation by shape

Answer 5

• squamous - flattened, plate shapes
• columnar - arranged in columns
• cuboidal - cube-like

Question 6

Epithelial characterisation by layering

Answer 6

• single-layer - simple epithelium e.g. alveolus
• multi-layer - stratified epithelium

Question 7

Simple squamous epithelium

Answer 7

• found in the lung alveolar epithelium, mesothelium (lining major body cavities), endothelium (lining blood vessels and other blood spaces)
• form a thin epithelium that allows exchange to occur e.g. gas exchange at the alveoli

Question 8

Simple cuboidal epithelium

Answer 8

• typical of the linings found in ducts e.g. those lining kidney collecting ducts

Question 9

Simple columnar epithelium

Answer 9

• typical of surfaces involved in absorption and secretion of molecules
• e.g. enterocytes lining the gut, involved in the take up of the breakdown products of digestion

Question 10

Stratified squamous epithelium

Answer 10

Two main types:

• keratinising - epithelial cells which produce keratin and in doing so die, becoming thicker, stronger, protective structures e.g. epidermis. Such cells lose their cellular organelles and nuclei, not visible under light microscopy. They can form thick layers that protect underlying tissues for various physical and chemical insults
• non-keratinising - epithelial cells which do not undergo keratinisation, retaining their nuclei and organelles (e.g. epithelium lining mouth etc)
• in the various layers, the cell shapes vary - the squamous classification relates to the surface cells

Question 11

Pseudo-stratified epithelium

Answer 11

• this epithelium appears to be multi-layered but is not
• on close examination, the surface cells have contact with the basal lamina
• e.g. airway epithelium, various ducts in urinary and reproductive tracts

Question 12

Epithelial cell polarity

Answer 12

• most epithelial functions require directionality which comes from plasma membrane polarity
• the membrane of the epithelial cell is organised into discrete domains by the formation of junctions
• this membrane polarity is key to generating a distinct polarity, with an apical domain at the lumenal (open) surface and a basolateral domain (in contact with ECM)

Question 13

Methods used by solutes to cross membranes

Answer 13

• gases and hydrophobic molecules diffuse across the PLB
• most molecules require particular transport proteins, which uses ATP hydrolysis in the case of active transport against an electrochemical gradient, or is passive
• when transporter epithelia is unpolarised, there is zero net flow of molecules as the movement is the same in both directions
• when transporter epithelia is polarised (uneven active transporters on each side of the membrane), there is movement (passive ion and fluid flow)
• secretory epithelia - polarisation allows directional secretion of molecules

Question 14

Transporting epithelium

Answer 14

• tissues with high concentrations of ion transporters in the plasma membranes
• mitochondria often closely associated providing energy for active transport across membranes
• membrane infoldings increase the amount of basal membrane that can pump ions and water
• mitochondria are concentrated in the basal aspect of the cell, close to the basal infoldings which contain the active transporters
• mitochondria provide ATP required for active transport
• because active transport is mainly confined to the basal membranes, ion and water transport will have directionality

Question 15

Absorptive epithelium

Answer 15

• carriers transporting nutrients are found at brush-border membranes e.g. absorptive intestinal cells (enterocytes)
• the small intestine surface area is increased by being long, and by the interior surface of the wall being folded into numerous villi, which contain intestinal epithelial cells
• standard stain: darker epithelial cells are enterocytes (absorptive) and lighter cells are goblet cells
• special mucus stain: goblet cells are stained dark purple
• brush border is rich in active transporters and channels for the uptake of nutrients from the gut lumen
• as the concentration of nutrients increases in the cytoplasm, it diffuses down its concentration gradient into the basal interstitial space to be collected in the capillaries and distributed in the circulation

Question 16

Secretory epithelium

Answer 16

• often arranged in tubules and glands of varying complexity
• alternatively in many epithelial tissues, individual, dispersed secretory cells can be present
• two main types of secretion: exocrine (into a duct/lumen) and endocrine (into the bloodstream) - the two types of cells have distinct arrangements of the organelles
• exocrine secretory cells - organelles are arranged for secretion from the apical plasma membrane (nucleus bottom, then Golgi, then secretory granules)
• endocrine secretory cells - organelles arranged for secretion from the basal membrane (secretory granules bottom, then Golgi, then nucleus) - the secretory vesicles are positioned so their contents have close access to the blood circulation when released

Question 17

Epithelial cell proliferation

Answer 17

• many epithelia are constantly ‘turning over’ i.e. cells that are lost by cell death or mechanical removal (abrasion) are replaced by proliferation (multiplication and reproduction) of stem cells within the epithelium
• a balance between cell proliferation and cell death is key to maintaining the architecture of tissues
• cells in intestinal crypts replace cells lost from the tips of intestinal villi
• inhibition of the proliferation of intestinal crypt cells e.g. in cancer chemotherapy, results in loss of villi and flattening of the intestinal mucosa - this is responsible for many of the gastro-intestinal disturbances that are side effects of chemotherapy
• increased cell proliferation leads to tumour formation - rate of cell loss is not sufficient to maintain normal tissue volume and too much tissue is formed - benign tumour called an adenoma (polyp) - high risk of mutating and becoming cancerous
• proliferation of epidermis - cells of the basal layer of stratified squamous epithelia divide to replace cells lost from the surface. Surface squame are replaced by new cells formed in the basal layer which migrate to the surface, and while migrating they differentiate and flatten out and keratinise

Question 18

Hyperproliferation of epithelial cells

Answer 18

• results in increased cell numbers and a thickening of cell layers e.g. in response to repeated or constant pressure
• if the increase in cell production is greater than the cell loss from the surface, cells will accumulate creating an increased thick hard layer (localised proliferation due to mechanical pressure)
• infectious agents e.g. papilloma virus can also induce hyperproliferation - by hijacking the cellular machinery of stratified squamous epithelia and inducing increased cell proliferation, which results in surface growth