histology of epithelial cells Flashcards
epithelial cells:
they are lining cells. they line parts of the body that come into contact with an external environment.
endothelial cells are considered to be a specialised type of epithelial cell, specialised molecularly and physically to coordinate blood homeostasis.
functions of epithelial cells:
barrier/protection- they provide a barrier from our internal tissues and external environment, preventing unregulated movement of microbes and macromolecules into our bodies.
absorption- epithelial cells transport the substances we do need from the external environment into our bodies tissues.
functions of epithelial cells: 2
lubrication- epithelial cells secrete mucus and/or other fluids which minimise the sheer stress our internal tissues are subject to, either by external factors such as food moving down the oesophagus or internal organs sliding over one another.
movement- some epithelial cells contain specialised structures called cilia which act like a conveyor belt in the trachea which moves foreign particles and microbes away from the lungs.
basement membrane:
epithelial cells are kept in place by adhesion to an underlaying layer of proteins and glycoproteins. the underlay is called the basement membrane and is an integral part of all epithelial structures.
epithelial cells are polar. their sides, tops and bottoms all have distinct roles, and therefore display expression of specific proteins which allow them to perform their function.
top side is called the apical surface- by the lumen. basolateral surface is in contact with the basement layer. lateral is on the sides touching other cells.
molecules passing through an epithelial cell:
passing directly through the cell- transcellular. passing between the lateral sides of the cells-paracellular
epithelial cell junctions
epithelial cells need to tightly regulate the passage of substances from the lumen of a vessel to the tissue below. proteins that stick epithelial cells together in their lateral membranes are called tight junctions. they control passage of substances between cells. they are a complex mixture of proteins.
shape:
simple= 1 cell thick.
stratified= layered.
cuboidal= square.
columnar= tall.
squamous= scale like/flattened
pseudostratified= false layered.
transitional= changeable/dynamic (bladder).
simple squamous:
simple squamous is the thinnest of all epithelia. they are flat and spread out, meaning they have a large surface area in contact with the exterior environment and the basement membrane. they are present in areas where easy transmembrane movement is advantageous (alveoli). endothelium is a simple squamous epithelium which supports the efficient movement of substances into and out of the blood. don’t provide much protection from mechanical abrasion.
simple cuboidal:
intermediate between squamous and columnar epithelium. simple epithelia have an increased cytoplasmic volume to their squamous counterparts which means they are capable of fulfilling some complex roles such as secretion and can withstand more trauma. they are found in glands which specialise in secretion including salivary glands. they are also in tissues that specialise in diffusion including the tubules of the kidney.
simple columnar:
cells are taller than they are wide. cell nucleus is polarised towards the basement membrane. very capable of dealing with wear and tear eg stomach and intestines.
capable of secreting large amounts of substances, so they are found in areas where lubrication is required- intestine. they secrete mucus in the stomach to protect them from harsh acidic environment.
stratified squamous:
multi layered epithelium where the outer layer is flat and scale like. cells in the basal layers may be cuboidal. only initial layer of cells is in contact with the basement membrane. found in surfaces that are subject to high levels of mechanical abrasion. cells on the top layers are continually replaced by those underneath, which become increasingly more specialised as they migrate up the strata from the base layer. cells at the base are very mitotically active.
2 varieties of stratified squamous: keratinising
keratinising (dry)- they produce a tough protein called keratin, theyre specialised cells are given the name keratinocytes. the top layer of keratinising epithelial cells are dead. this produces a dry surface, which protects underlying delicate tissues from external forces- skin.
2 varieties of stratified squamous: non keratinising
non keratinising (wet)- occur in moist environments when abrasion is likely. the top layer of keratinising epithelial cells are alive. this produces moist surfaces, which protects underlying delicate tissues from external forces- oesophagus, inside of mouth.
pseudostratified columnar:
the presence of misaligned nuclei. all the cells in the layer are in touch with the basement membrane, meaning no stratification, cells are at different heights, some of which do not reach the lumen of the organ they are lining. common in the upper respiratory system. mucus helps trap microbes and foreign particles in the respiratory system.
transitional epithelium:
this epithelium changes shape to allow an organ or tube to increase in size or shrink. cells may appear rounded/cuboidal when the organ is relaxed and flattened/squamous when it is stretched. common in the urinary tract (allows bladder to increase/decrease as it fills.)
ciliated epithelial cells:
some epithelial cells contain hair like projections on their surface called cilia. each ciliated cell may have over 200 cilia. ciliation is most common on columnar epithelial cells. common in the respiratory tract where they move mucus laden dust or microbes away from lungs. and in the fallopian tubes where they move ova.
cilia move to help substances migrate across the epithelial layer.
epithelial cells with microvilli:
some epithelial cells contain finger like protrusions on their surface. these projections are called microvilli. microvilli dont move and their main role is absorption. microvilli are shorter and wider than cilia. they are so tightly packed they have a fuzzy appearance. in histology this is referred to as a brush boarder, the epithelium in the intestine has microvilli to increase surface area for absorption of nutrients from the gut lumen.
goblet cells:
modified columnar epithelial cells. synthesise and secrete mucous. respiratory and gastrointestinal tracts.
