Anatomy & Physiology: Histology (Epithelia)

tissue

collection of cells and cell products that performs a specific, limited function

4 basic tissue types

epithelial, connective, muscle, neural

epithelial tissue

covers exposed surfaces, lines internal passageways, forms glands (endocrine and exocrine)

connective tissue

fills internal spaces, supports other tissues (bone, cartilage), transports materials in the body (blood, lymph), stores energy (adipose)

matrix

a nonliving, noncellular part of all connective tissue

muscle tissue

specialized for contraction

types of muscle tisue

skeletal muscle (voluntary), cardiac muscle (heart exclusive), smooth muscle (in the walls of hollow organs)

neural tissue

carries electrical signals from one body part to another

epithelial tissue roles

epithelia and glands

epithelia

layers of cells that cover internal or external surfaces (ex: epidermis)

characteristics of epithelia

cellularity, polarity, attachment, avascularity, regeneration

cellularity

cells are bound closely together by cell junctions. may other tissue types have cells more widely spaced and separated by extracellular molecules

polarity

epithelial cells have a definite top and bottom. they have an apical surface (top) and a basolateral surface (bottom): attaches to either another cell or to the basement membrane. the sides contact neighboring cells

attachment

epithelia are anchored to a layer of extracellular material called the basement membrane (basal lamina)

avascularity

epithelia has no blood vessels unlike most tissues

regeneration

the rate of cell division and regeneration is much higher than in other tissues

functions of epithelial tissue

Physical protection (against abrasions, microbes, chemical, dehydration), controls permeability (epithelia lining lungs, intestines, and kidneys), produces specialized secretions

specializations of epithelial cells

moving or pushing materials over the epithelial surface, absorption, and secretion

moving or pushing materials over the epithelial surface

function of ciliated epithelia (ex: lining respiratory pathways to move mucus)

ciliated cell

has ~250 cilia that coordinate beating

absorption by epithelial cells

absorbing nutrients from the intestine

secretion by epithelial cells

endocrine cells secrete hormones, exocrine cells secrete sweat, tears, saliva, etc

microvilli

the bumps of cell membranes that aids secretion and absorption

- increase surface area by 20x compared to a normal cell

what is the small intestine lined by

villi and microvilli to help with absorbing nutrients

intercellular connections

connect neighboring epithelial cells, give strength to epithelial tissues, and help epithelial cells communicate with each other

what do intercellular connections involve

extensive areas of adjacent cell membranes or specialized cell junctions

cell adhesion molecules

transmembrane proteins resembling a zipper that bind to each other and to extracellular materials

- can exist on their own or as part of specialized cell junctions

intercellular cement

a sticky substance that reinforces the bond between cells / the interlocking CAMs

lumen

the space inside of a tube

cell junctions

form bonds between cells or between a cell and extracellular material

what are the 3 kinds of cell junctions

tight junctions, gap junctions, and desmosomes

tight junctions

between the apical portions of 2 neighboring plasma membranes

tight junction function

binds cells together in a way that prevents water and other substances from leaking in between the cells

what are tight junctions useful for

useful for when the epithelium lines a tube (ex: intestinal tract). they ensure that enzymes, acids, wastes, etc stay inside the lumen. if digestive juices leaked, an ulcer in intestinal wall would occur.

gap junctions

allow rapid communication between the two connected cells (communication helps coordinate functions eg the beating of cilia). the 2 neighboring cells are held together by channel proteins.

channel proteins

"little passageway": allow ions and small molecules to pass from cell to cell

gap junctions in heart muscle tissue

allow heart muscle cells to coordinate contractions so the heart can generate the fuel to pump

gap junctions in smooth muscle tissue

connect smooth muscle cells wherever smooth muscle tissue is found to coordinate the contracting and relaxing of smooth muscle cells

desmosomes

very strong, can endure stretching and twisting; present in the superficial layers of the skin

how do we know desmosomes are present in the superficial skin layers?

burned or damaged skin peels off in strips or sheets (still holding onto each other)

what are the two kinds of desmosomes?

spot desmosomes and hemidesmosomes

spot desmosomes (button desmosomes)

the usual kind; between 2 cells

hemidesmosomes

attach one cell to the basement membrane

basement membrane (basal lamina)

made of layers of proteins; gives strength to epithelia; helps form a barrier between an epithelium and the connective tissue below

what are the 3 shapes of epithelia

squamous, cuboidal, columnar

squamous

thin and flat cells; root meaning "scale" like fish scales

cuboidal

square shaped cells

columnar

tall, slender rectangles

what are the 2 layers of epithelia

simple and stratified

simple epithelia

single layer of cells; good for areas where substances need to be absorbed through the epithelium (ex: lung + intestinal lining)

stratified epithelia

multiple layers of cells; good for areas that need protection (ex: esophageal lining, epidermis)

what do new epithelial cells grow from?

germinative cells, a type of stem cell that is found in epithelial tissue. these new cells are located near the basement membrane

glands

collections of epithelial cells that produce secretions

exocrine glands

release secretions into ducts that empty onto an epithelial surface. ducts can empty onto the skin surface (via tears, sweat, oil, milk) or an epithelial lining (mucous glands, digestive enzymes) inside the body

endocrine glands (ductless glands)

secretions are chemical messengers called hormones which get released into body fluids (blood, interstitial fluid)/end up circulating the bloodstream.

merocrine secretion

the most common mode of secretion; the product is packaged into secretory vesicles then released by exocytosis

secretory vesicles

bubble-like organelles that store and transport substances in a cell

what products are released by merocrine secretions

sweat, mucus, saliva, etc

apocrine secretions

related to the apical part of the cell; the product is released when the apical portion of the cell pinches off and breaks down. the cell can regenerate a new apical region and start over

how can you recognize what products are released via apocrine secretion?

cytoplasm is lost with the secretory product, which is a key determiner

what product is released via apocrine secretion

breast milk from the cells of mammary glands

holocrine secretions

destroys the whole gland cell. the entire gland cell gets packaged with the secretory product then bursts, which releases the product and kills the cell.

how do gland cells get replaced after holocrine secretion?

they get replaced by the division of stem cells called germinative cells

what cells use holocrine secretion

sebaceous gland cells

what are the three types of secretions?

serous secretions, mucous secretions, and mixed exocrine secretions

serous secretions

watery solutions that contain proteins, especially enzymes

example of serous secretion

gastric chief cells secrete stomach acid, a serous secretion

mucous secretions

the gland cells secrete molecules called mucins, which mix with water to become mucus

example of mucous secretions

various digestive glands produce mucous secretions to protect the digestive tract from the harsh digestive juices

goblet cells

the only unicellular exocrine glands, which secret mucins

mixed exocrine secretions

exocrine glands may have more than one type of gland cell and produce both serous and mucous products

example of mixed exocrine secretion

salivary glands make saliva, which is a mix of serous and mucous secretions