What are mucous membranes?
Mucous membranes: line certain internal tubes which open to the exterior and bear mucus-secreting cells to varying degrees e.g. the alimentary tract, the respiratory tract, the urinary tract
What do mucous membranes consist of?
- Epithelium: lining the lumen of the tube
- Lamina propria: an adjacent layer of connective tissue
- Muscularis mucosae: a third layer consisting of a smooth muscle cells
What are serous membranes?
Serous membranes are thin, two-part membranes which line certain closed body cavities (spaces which do not open to the exterior)
What do serous membranes do?
Serous membranes secrete a lubricating fluid which promotes relatively friction-free movement of the structures they surround
Where can serous membranes be found?
- Line the peritoneum (envelopes many abdominal organs)
- Line the pleural sacs (envelopes the lungs)
- Line the pericardial sac (envelopes the heart)
What do serous membranes consist of?
- Visceral serosa – a simple squamous epithelium which secretes a watery lubricating fluid
- Parietal serosa – a thin layer of connective tissue which attaches the epithelium to adjacent tissues
What are epithelia?
Epithelia are sheets of contiguous cells, of varied embryonic origin, that cover the external surface of the body and line internal surfaces
Identify 3 types of surfaces which have epithelial linings
- Exterior surface: skin
- Open interior surfaces: gastrointestinal, respiratory & genitourinary tracts
- Closed interior surfaces: pleura, pericardium, peritoneum, lymphatic & blood vessels
What is a basement membrane?
Basement membrane is the thin, flexible, acellular layer which lies between epithelial cells and the subtending connective tissue
What are the three principal shapes associated with epithelial cells?
- Squamous epithelium
- Cuboidal epithelium
- Columnar epithelium
What are the three ways of describing the layering of epithelium?
What are the characteristics of simple epithelia?
- Single layer of cells
- Found where absorption, secretion and filtration occur
Describe the structure and function of simple squamous epithelium
- Structure: epithelium cells are flat in shape and arranged in a single layer
- Function: thinness facilitiates processes of lubrication, gas exchange, active transport
Identify 3 locations where simple squamous epithelium can be found
- Lining of blood and lymph vessels (endothelium)
- The lining of alveoli in the lungs
- Lining of closed body cavities (mesothelium)
- Bowman's capsule
Describe the structure and function of simple cuboidal epithelium
- Structure: epithelium consists of a single layer cells that are as tall as they are wide
- Functions: secretion and absorption
Identify 3 locations where simple cuboidal epithelium can be found
- Small ducts of many exocrine glands (pancreas and salivary glands)
- Kidney tubules
- Germinal epithelium of the ovary
Describe the structure and function of simple columnar epithelium
- Structure: a single row of tall, closely packed cells, aligned in a row.
- Functions: absorption, secretion, lubrication (small intestine and colon), transport (oviduct)
Identify 3 locations where simple columnar epithelium can be found
- Areas with high secretory function (stomach lining and gastric glands)
- Absorptive areas (small intestine and colon)
Describe the structure of pseudostratified epithelium
- Simple columnar epithelial cells with nuclei appearing at different heights, giving the misleading 'stratified' impression
- Can also possess cilia on their apical membrane
Identify 3 locations where ciliated pseudostratified epithelium can be found
- Lining of the nasal cavity
- Trachea and bronchi
- Epididymis and ductus deferens
What are the functions of pseudostratified ciliated epithelium?
- Mucus secretion, particle trapping and removal
What are the characteristics of stratified epithelia?
- The basal cells divide and push towards the apex, and in the process flatten the apical cells
- Found where body linings withstand mechanical/chemical insults (more durable)
Identify 4 locations where stratified squamous non-keratinised epithelium can be found
- The oral cavity
- The oesophagus
- The vagina
- The anal canal
What are the functions of stratified squamous non-keratinised epithelium?
- Protection against abrasion
- Reducing water loss but maintaining moisture
What are the characteristics of keratinised epithelia?
- Apical layers of cells are dead and lose their nucleus & cytoplasm
- Cells contain keratin (tough & resistant) making the epithelium waterproof
Where can keratinised epithelia be found?
What are the functions of keratinised epithelium?
- The prevention of water loss
- The prevention of ingress of microbes
- Protection against abrasion and physical trauma
- Shielding against UV light damage
What are the characteristics of transitional epithelia?
- Appear to be stratified cuboidal when the tissue is not stretched
- Appears to be stratified squamous when the organ is distended
Where is transitional epithelium found?
Found in tissues that undergo distension e.g. renal calyces, bladder, ureters, and urethra
What is another name for transitional epithelium?
What are the functions of transitional epithelia?
- Protection of underlying tissue from toxic chemicals
Stratified squamous keratinised epithelium is made up of keratinocytes and their products.
What are its four layers?
- The horny layer (stratum corneum)
- The granular layer (stratum granulosum)
- The prickle cell layer (stratum spinosum)
- The basal layer (stratum basale)
Describe keratinocyte mitosis
- Occurs mainly in the basal layer
- Daughter keratinocytes then move upwards to form the prickle cell layer
- Here, terminal differentiation begins and keratinocytes lose their ability to divide
What do keratinocytes produce?
Keratinocytes synthesise keratins (heterodimeric fibrous proteins) which contribute to the strength of the epidermis
What abrupt changes occur in the granular layer?
- Keratinocytes lose their plasma membrane (phospholipid bilayer)
- Keratinocytes begin differentiating into corneocytes, the main cells of the stratum corneum
Describe the composition of the stratum corneum
The stratum corneum (horny layer) is made up of layers of flattened corneocytes
Describe the structure, function and location of melanocytes
- Structure: dendritic cells of neural crest origin
- Function: produce melanin
- Location: occur at intervals along the basal layer of the epidermis
Melanin is the pigment that gives skin its colour.
How does melanin production vary across races?
Melanocytes produce more melanin in darker skin but are not increased in number
Describe the structure, function and location of Langerhans cells
- Structure: dendritic cells of bone marrow origin
- Function: antigen presentation and mediating immune reactions
- Location: scattered throughout the prickle cell layer
What is a gland?
A gland is an epithelial cell or collection of cells specialised for secretion
How do we classify glands by destination of secretion?
- Exocrine glands: have ducts
- Endocrine glands: ductless, secrete into the bloodstream
How do we classify glands according to their structure?
How do we classify glands according to the method of secretion?
- Merocrine: exocytosis e.g most glands
- Apocrine: secreted droplets covered by plasma lemma e.g. fat droplets in milk
- Holocrine: whole cell breaks down e.g. sebaceous gland of skin
What is cystic fibrosis?
Cystic fibrosis is a condition resulting from the absence of the CFTR molecule in the apical membrane of the epithelial cells lining the duct of the sweat gland, resulting in poor reabsorption of chloride ions
What is psoriasis?
Psoriasis is a long-lasting autoimmune disease which is characterised by patches of abnormal skin (red, itchy, and scaly)