Epithelial Tissues And Exocrine Glands Flashcards
What does a mucous membrane line?
MUCOUS MEMBRANES line certain internal tubes which open to the exterior.
i.e. the alimentary tract
the respiratory tract
the urinary tract
(these mucosae bear mucus-secreting cells to varying degrees)
What 3 layers dos a mucous membrane consist of?
A MUCOUS MEMBRANE consists of:
- an epithelium (type depends on site) lining the lumen of a tube.
- an adjacent layer of connective tissue often referred to as the
lamina propria (literally ‘one’s own layer’).
- in the alimentary tract, a third layer consisting of smooth muscle
cells, referred to as the muscularis mucosae.
What is a serous membrane and what do they line?
SEROUS MEMBRANES are thin, two-part membranes which line certain closed body cavities (spaces which do not open to the exterior). They envelop the viscera.
- They line the peritoneum……….which envelops many abdominal organs.
- the pleural sacs……..which envelop the lungs.
- the pericardial sac…..which envelops the heart.
What do serous membranes secrete?
These serous membranes (or serosae) secrete a lubricating fluid which promotes relatively friction-free movement of the structures they surround.
What does a serous membrane consist of?
A SEROUS MEMBRANE consists of:
- a simple squamous epithelium (mesothelium) which secretes a
watery lubricating fluid.
- a thin layer of connective tissue which attaches the epithelium
to adjacent tissues (also carries blood vessels and nerves).
When do many organs become surrounded by a serous membrane?
During embryonic development the heart, lungs, and gut develop next to a bag-like cavity into which they invaginate. So they each become surrounded by serous membrane.
(NB - they do not lie within the serous cavity itself, but are surrounded by it).
See analogy………
What is the outer “wall” or a serous membrane called?
Parietal serosa
What is the inner wall of a serous membrane called?
Visceral serosa
What lies in the serous cavity and what is its function?
Lubricating fluid; allows movement
What are epithelia?
Sheets of contiguous cells, of varied embryonic origin, that cover the external surface of the body and line internal surfaces.
Name some surfaces with epithelial linings
Exterior surface:
skin
Interior spaces opening to exterior:
gastrointestinal tract
respiratory tract
genitourinary tract
Interior spaces which do not open to the exterior: pericardial sac pleural sacs peritoneum blood vessels lymphatic vessels
Name some terms to classify the nature of the epithelial layer
Simple
Stratified/compound
Pseudostratified
Name some terms to classify the shape of epithelial cells
Squamous
Cuboidal
Columnar
Transitional
What is simple epithelium?
One cell layer thick
What is stratified/compound epithelium?
More than one cell layer thick
What is the basement membrane?
Basement membrane is the name given to the thin, flexible, acellular layer which lies between epithelial cells and the subtending connective tissue.
What is the function of the basement membrane
The basement membrane serves as a strong flexible layer to which epithelial cells adhere. It also serves as a cellular and molecular filter.
The degree to which malignant cells penetrate basement membrane is highly relevant to prognosis (see Mechanisms of Disease Unit).
NB: the terms basement membrane and basal lamina are used
inconsistently in the literature.
Define simple squamous epithelium
A single layer of thin squamous cells
Give examples of vessels which are lined by simple squamous epithelium
Blood vessels, lymphatic vessels
What is the name given to the simple squamous epithelium lining all vascular elements?
ENDOTHELIUM
What is mesothelium?
The name given to simple stratified epithelium lining the pleural sac, the pericardial sac and the peritoneum
What is the function of endothelium?
Endothelium can exchange gasses and nutrients, allow certain blood cells to enter the tissues, and regulate platelet coagulation.
Name 4 functions of simple squamous epithelium
1) Lubrication (pericardium, pleura, peritoneum/viscera)
Secretion of fluid can be excessive. e.g.:
- hydrocoele around testis – accumulation of clear serous fluid between visceral and parietal layers of tunica vaginalis
- excessive fluid in pleural space around lungs, requiring drainage (see Respiratory system lecture)
2) Gas exchange (Type 1 pneumocytes of pulmonary alveoli)
3) Barrier (Bowman’s capsule)
4) Active transport by pinocytosis (mesothelium & endothelium)
What type of epithelium lines a pancreatic duct?
Simple cuboidal
What type of epithelium line thyroid gland follicles?
Simple cuboidal
What do epithelial cells in the thyroid gland synthesise?
The epithelial cells synthesise thyroxine, store it in colloid, and transport it to the bloodstream.
What type of epithelium lines the collecting ducts of the kidneys?
Simple cuboidal
What are 4 functions of simple cuboidal epithelium?
Absorption & Conduit (exocrine glands)
Absorption & secretion (kidney tubules)
Barrier/covering (ovary)
Hormone synthesis, storage and mobilisation (thyroid)
Describe simple columnar epithelium
Simple columnar: a single layer of cells, whose heights are significantly greater than their widths.
Give an example of where simple columnar epithelium can be found?
Stomach lining and gastric glands
Small intestine and colon e.g. Crypts of liberkuhn
What are microvilli
Projections on the surface of cells which increase the surface area available for absorption
Name some functions of simple columnar epithelium
Absorption (small intestine and colon, gallbladder)
intestine and colon)
Secretion (stomach lining & gastric glands, small Lubrication (small intestine and colon)
Describe pseudostratified epithelia
Those in which all cells make contact with the basement membrane, but not all cells reach the epithelial surface. This results in nuclei lying at different levels, giving a false impression of multiple layers of cells.
Name some locations of pseudostratified epithelium
Lining of nasal cavity, trachea and bronchi
What are some functions of pseudostratified epithelium?
Secretion and conduit (respiratory tract, ductus deferens) Mucus secretion (respiratory tract) particle trapping and removal (respiratory tract)
Describe stratified squamous epithelia
Multiple layers of camels, the outermost of which are thin squamous cells
Where is stratified squamous non-keratinised epithelium found?
Oral cavity Oesophagus Vagina Part of anal canal Surface of cornea Inner surface of eyelid
Moist surfaces, prone to abrasion
What are functions of stratified squamous non-keratinised epithelium?
Protection against abrasion
Reduces water loss but remains moist
Why are stratified squamous non-keratinised cells of the vagina rich in glycogen
Glycogen is a substrate for lactobacilli which produce lactic acid and thereby maintain a suitably low pH
Describe stratified squamous keratinised epithelium
Stratified squamous keratinized epithelium contains multiple layers of cells, the outermost of which are squamous cells that have lost their nuclei and
cornified (i.e. become ‘squames’ of keratin).
What is the stratum corneum and what are its functions?
The outermost dead cells of the epithelium are collectively called the stratum corneum. This layer - greatly reduces water loss and ingress - prevents ingress of toxins - protects against abrasion - reduces microbial colonisation.
What is the primary site of stratified squamous keratinised epithelium?
The epidermis of the skin
Is epidermis vascular or avascular?
Avascular
Describe the dermis
Dense irregular connective tissue consisting of collagen and elastin bundles orientated in all directions.
What is the main cell type which makes up stratified squamous keratinised epithelium?
Keratinocytes
What are the 4 layers of the epidermis?
- horny layer (STRATUM CORNEUM)
- GRANULAR LAYER (stratum granulosum)
- prickle cell layer ( stratum spinosum)
- BASAL LAYER (stratum basalt)
(Common terms = capital)
Where does keratinocyte mitosis occur primarily, and what then happens to the keratinocytes?
Basal layer
Daughter keratinocytes then move upwards, differentiating and losing their ability to divide
What are keratins?
Keratinocytes synthesise keratins (fibrous proteins) which contribute to the strength of the epidermis. Keratins are also the main constituents of hair and nail, and animal horns.
What is the transit time of a keratinocyte from basal layer to stratum corneum?
28-40 days
However in diseased like psoriasis the transit time is reduced
What are melanocytes?
- occur at intervals (1:8 of basal layer of epidermis)
- difficult to see histologically without special stains
- produce melanin, the main pigment that gives the skin its colour
- produce more melanin but are not increased in number in black or tanned skin
Same ratio of melanocytes to keratinocytes in all skin colours, it’s just the amount of melanin. Paler skin - melanocytes destroyed faster
What are Langerhans cells?
- difficult to see histologically without special stains
- highly specialised capacity to present antigens to T lymphocytes
- mediate immune reactions e.g. Allergic contact dermatitis
Where are melanosomes found?
Mature melanosomes containing melanin are transferred to neighbouring keratinocytes by pigment donation, involving phagocytosis of the tips of the dendritic processes
Where is stratified squamous keratinised epithelium found?
Surface of the skin
Limited distribution in oral cavity
Name some functions of stratified squamous keratinised epithelium
- protection against abrasion and physical trauma
- prevention of water loss
- prevention of microbial ingress
- shielding against UV light damage
Describe transitional epithelium
Surface ce;;s vary in shape from columnar/cuboidal to flattened
Where is transitional epithelium found?
Urinary tract; renal calyces, ureters, ladder, urethra
Name some functions of transitional epithelium
Protection of underwing tissue from toxic chemicals
What is a gland
An epithelial cell or collection of cells specialised fo secretion
The secretion can be into the bloodstream or onto an epithelial surface (i.e. Of the gut, respiratory or reproductive tracts)
What is an adenocarcinoma?
A malignant growth derived from glandular cells of epithelial origin
Define exocrine glands
Glands with ducts which secrete onto an epithelial surface (e.g. Salivary glands and the breast)
Define endocrine glands
Ductless glands which secrete directly into the bloodstream? (E.g. Thyroid and adrenal glands)
Can glands be both endocrine and exocrine?
Yes, e.g. The pancreas
Are glands unicellular or multicellular?
Can be either
What are goblet cells?
Unicellular exocrine glands which reduces and secretes mucus into the respiratory and intestinal tracts
What is the name given to the endocrine tissue of the pancreas?
Islets of Langerhans
What is the name given to the exocrine tissue of the pancreas?
Acini
Describe the problems associated with mucus in cystic fibrosis
Insufficiently hydrated mucus is formed in the respiratory tract and gut with resultant pathologies
What is an acinus?
The secretory unit of many exocrine glands
What are myoepithelial cells?
Cells around each acinus which constrict simultaneously to assist the secretion of:
- milk from the lumens of the acini in the breast
- unmodified sweat of eccrine sweat glands towards the duct
