4 types of tissues:
epithelial – covers exposed surfaces, lines passages, forms glands
connective – fills spaces, provides structural support, transports materials, stores
muscle – contraction – muscles, organs, heart
nervous – carries information in the form of electrical impulses
cover surfaces, form glands – which produce fluid secretions.
Epithelia cover every exposed surface of the body – respiratory, digestive, reproductive
Also internally, surfaces not exposed to outside world, so don’t have to be as durable.
Cellularity – bound together by junctions – tight sheets
Polarity – cells have distinct “sides” – one facing out and one going in to adjacent tissue.
This polarity extends to location of cell organelles, etc.
Attachment – bound to basal lamina or basement membrane
Avascularity – don’t have blood vessels of their own, thus must rely on diffusion for
nutrients and O2 – some epithelia are dead however and don’t require any nutrients
Regeneration – continuously replaced – more quickly than most cells (think of burning
the roof of your mouth)
physical protection from abrasions, dehydration, other damage.
control permeability – some are very permeable, some impermeable, some
This can be regulated and changed.
provide sensation – many sensory nerve endings since many are exposed to
outside, this is a good way to sense environment
secretion – most epithelia have some glands – some are almost all glands –
Epithelial repair and maintenance:
it’s not easy being epithelial!
Have to replace and
renew constantly because they are always getting messed up – abrasion, etc.
stem cells, near basal lamina (basement membrane), continually sending up new cells.
Classification of epithelium – can be done with 2 categories:
Shape – squamous, cuboidal, or columnar
Number of layers – simple or stratified
Simple – one cell layer – thin, also fragile.
Needed where excretion/absorption take
But can only have on inside lining of cavities, not in places open to outside world.
Stratified – multiple cell layers – takes abrasion better.