Lecture 1

what are tissues

specialised cells embedded in an extracellular matrix

4 key types of tissues

epithelial tissue, connective (support) tissue, muscle tissue, nervous tissue

importance of ECM

to maintain differentiated state of cells - e.g. mouse mammary gland epithelium cultures without ECM are flat and do not produce milk

to maintain normal overall development - e.g. inactivating genes for ECM proteins results in defective skeletal development

epithelial tissue

lines organs and cavities in the body and covers external body (epidermis)

• protective barrier

• absorptove and secretory adaptations

has polarity, has a basement membrane as a scaffold and anchor

avascular

regenerative

classification of epithelium

based on morphological criteria

based on surface specialisations

based on surface or secretory

classifying epithelium based on morphological criteria

cell shape (squamous, cuboidal, columnar)

number of cell layers (simple, stratified, pseudostratified, transitional)

classifying epithelium based on surface specialisations

microvilli - very small

cilia - much longer

classification of epithelium based on whether surface of glandular

surface = covering, glandular = secretory

glandular epithelium can be single cells or grouped into glands to allow focused production of a secreted product

types of cellular junctions

occluding (tight junctions)

anchoring (zonula adherens, desmosomes, hemidesmosomes)

communicating (gap junctions)

tight junctions (zonula occludens)

at the apical region of the cell

controls paracellular pathway

transmembrane proteins - occludin and claudins

importance of claudins

different claudins = different permeabilities in different tissues

in kidney only specific regions permeable to Mg2+ ions due to claudin-16, a mutation in claudin-16 gene means impermeable to ion so low Mg2+ reabsorption leading to renal failure

also found in 2002 that claudin-1 makes the skin waterproof

zonula adherens

links cytoskeleton of adjacent cells

through actin microfilaments (band of actin filaments run across apical region of epithelial cells)

main transmembrane proteins are E-cadherins

cadherins importance

loss of E-cadherins linked to metastisis of cancer cells

gene can be screened for prevention of cancer

desmosomes

link cytoskeletons of adjacent cells

connect to cytokeratin (intermediate filaments)

main transmembrane protein is cadherins such as desmoglein

pemphigus vulgaris

autoimmune disease - antibodies attack desmoglein 3 which keeps cells bound together

cells fall apart, skin sloughs off

hemidesmosomes

modified desmosomes at basal surface of cell anchor it to basement membrane

bind to cytokeratin - intermediate filaments inside the cell and integrins outside cell

main transmembrane protein is integrins

gap junctions

communicating junctions

main transmembrane proteins are connexins which form channels called connexons that can open/close

allow passage of ions and small molecules

polarisation of epithelial tissue

linked as sheets through lateral junctions

linked to basement membrane through lateral junctions

different specialisations linked to polarity of cell - provide hints as to function

epithelial apical characteristics - microvilli

fingerlike projection of the plasma membrane

very short

provides increased surface area e.g. for absorption in intestine and kidney

core of parallel actin bundles

show as brush border in light microscopy and individual microvilli with shape maintained by actin filaments under electron microscopy

epithelial apical characteristics - cilia

non-motile = sensory information

motile = beat in rhythm to move substances across surface of epithelium

length of up to half the cell length

has cytoskeleton composed of a core of microtubules

basal characteristics - basement membrane

binds epithelial cells to connective tissue

formed and maintained by both epithelia and connective tissue

sheet like arrangement of ECM proteins

forms barrier but also lets nutrients pass to epithelia

controls growth and differentiation as well as stops epithelial cells from metastisising in cancer

basement membrane microscopy

light microscope: not easily distinguished with H and E staining - immunocytochemistry can be used to stain specific components

EM: three visible layers; lamina lucida, lamina densa (also basal lamina), lamina fiibroreticularis

basal folding

increases surface area on basal surface of cell