Epithelia Flashcards
4 basic types of tissue
muscular
nervous
connective
epithelia
where is epithelia found?
at boundaries of body - cover surfaces, line cavities, form glands
separate controlled internal environment from uncontrolled external
how does epithelia develop?
from all germ layers:
endoderm = GI lining
mesoderm = lining of CV system
ectoderm = epidermis
epithelia functions
protection - skin
diffusion - lungs
absorption - small intestine
secretion - glands
common properties (4)
1 - polarity
2 - basement membrane
3 - cell adhesion and communication
4 - cell replacement
polarity in epithelium
entirely cellular, avascular, without extracellular fibres
apical surface to external environment, specialisation on basolateral or apical related to function
epithelia basement membrane
vital component, separates from underlying connective tissues (collagen IV)
consists of:
basal lamina - secreted from epithelial cells
reticular lamina - secreted by fibroblasts - reticular fibres anchoring basal lmaina to underlying connective tissue
cell adhesion and communication in epithelia
lateral communication throu gap junctiond of water, ions and small molecules
cell matric attacements bond epith tissue ti connective tissue
strong adhesion between cells
stress bearing cytoskeletons linked from cell to cell by adhesive junctions
adhering junctions form belt around cell linked to bundles of actin filaments
myosin filaments can pull on actin to contract cell
tight junctions
seals gap between epithelial cells
forms network of sealing strands, more than 50 types if proteins, myosin can loosen junction
transmembrane Claudius in stomach link to small intestine, claudins change, each part of GI tract has diff function so diff tightness of junctions needed
cell replacement in epithelia
more hostile external env = greater cell death
tissue homeostasis through continual cell replacement from stem cellssubstantial variation among diff epithelia:
epithlia of intestine completely self renews in 5 days
inter follicular epidermis takes 4 weeks
lung wpithelia takes up to 6 months
regulation of epithelial cell fate
epithelial - meschymal interactions
2 way process
assembly of ep cells is reversible,
epithelium = surface tissue
mesenchymal = underlying tissue
transition allows polarised epith cells that interact with basememnt membrane to undergo multiple changes and assume mesenchymal cell phenotype
after degredation of basement membrane, cells have:
- enhanced migratory potential
- invasiveness
- increased resistance to apoptosis and inc production of ECM components
naturally happens in embryogenesis and tissue repair
most cancers originate in epithelia and only become malignant when it escapes epithelia and infects other cells
classification of epithelia
2 types
1 - simple = single layer of cells e.g. lung
2 - stratified = many layers e.g. skin
(some dont easily fit into either, pseudostartified = upper respiratory tract, transitional = urothelium)
types of epithelial cells (7)
simple squamous simple cuboidal simple columnar pseudostratified columnar stratified squamous stratified cuboidal stratified columnar
simple squamous
thin scales facilitates rapid passage of molecules lining of capillaries, lung air sacs etc nuclei as flat as cell e.g. alveoli and serosa
simple cuboidal
secretion and absorption of molecules requiring active transport
nucleus in centre of cell
e.g. lining of kidney tubules and ducts of glands
simple columar
forms most of digestive tract with and without cilia/microvilli absorption and secretion of molecules requiring active transport nucleus at bottom, basal end e.g. most of GI tract, Fallopian tubes
pseudostratified columnar
irregular nature of nuclei - stratified but all cells in touch with basal membrane, only some reach apical.
cilia/no cilia, ciliated cells can be interspersed with goblet cells
e.g. respiratory tract, male reprodcutive tract
stratified sqaumous
most common in body
thin and flattened apical cells with cuboidal cells on basal membrane
top layer often dead, with keratin
in high ares of abrasion
e.g. upper skin layers - keratinised, oesophagus - not keratinised
stratified cuboidal
rare
found in glands
e.g. ducts, tubes of glands
stratified columnar
rare
allows tissues to stretch and contract
e.g. conjunctiva, pharynx, anus, male urethra and embryos
transitional epithelia
round when relaxed e.g. when bladder is empty
allows change in distention
glandular epithelia
- glands definition
- secretion pathways for exo and endocrine
gland = structure made up of one or more type of cells that have been modified to secrete chemicals
secretion - ducts - exocrine - tissues that secrete through ducts
secretion - ductless - endocrine - hormones in interstitial fluid into blood stream, straight into tissues
goblet cells
embedded in other tissues
individual cells that may be surrounded by ciliated columnar etc
glands that secrete mucus
mucus glands
glands that secrete proteins
serous glands
types of goblet cells
unicellular - seen around edge as bulbous
multicellular = acinar/acinus
