Days 1 +2: Tissues, Organs, & Epithelia (Membranes + Glands) Flashcards
lumen
interior cavity of an organ
luminal / adluminal: free side of epithelium
abluminal: attached side of epithelium
components of mucous membrane (mucosa)
- epithelium (and its underlying basal lamina)
- lamina propria (loose connective tissue)
- muscularis mucosae (thin layer of smooth muscle
Deep to the mucosa
- lumen
- epithelium
- lamina propria
- muscularis mucosae
- submucosa : supportive (dense irregular) connective tissue
- muscularis externa (if an organ has no muscularis mucosae, it is just called the muscularis)
- With respect to the long axis of the gut:
- innermost layer: circular layer
- outermost layer: longitudinal layer
- Transverse secton of the gut:
- innermost layer: smooth muscle circular layer sectioned longitudinal
- outermost layer: longitudinal layer will be cut transverse
nervous tissue in the wall of an organ
visible as neuronal cell bodies
(of ANS that direct action of smooth muscles)
- cell bodies are clustered in parasympathetic ganglia
- located in two places:
- submucosa
- between the two layers of the muscularis externa
- located in two places:
mesothelium
membrane that covers and protects most of the internal organs
different names:
- peritoneum: abdominal cavity
- pleura: lungs + chest cavity
- pericardium: heart
serosa / serous membrane
mesothelium + subjacent connective tissue
permits low-friction movement of the surfaces against each other
outermost coat of an organ that lies in a body cavity
have a parietal + visceral layer
adventitia
- organs embedded in the body wall don’t have a serosa*
- outermost layer of organs that connects the organ to surrounding organs or body wall*
- adventitia: the connective tissue layer located beyond the muscularis externa that connects directly with the tissues of the body wall
epithelium
- located on free surfaces within and on the body
- always attached to and supported by underlying connective tissue
- renewable in some tissues (gut, skin, conditionally liver) and never in core of eye lens
- most common source of cancers in adults
- avascular: connective tissue carries bld vessels + nerves necessary for metabolic maintenance + proper functioning
- tightly apposed cells: no extracellular matrix between epithelial cells
two broad types:
- membranous
- number of cell layers: simple, stratified, pseudostratified
- cell shape: squamous, cuboidal, columnar
- glandular
simple, stratified, pseudostratified epithelium
classification of epithelium based on cell layers
simple epithelium: single layer of cells
stratified epithelium: two or more layers of cells
pseudostratified epithelium: all cells contact the basement membrane, but not all cells extend to free surface; the nuclei are often located at different levels
squamous, cuboidal, and columnar epithelium
- Cell shape epithelium classification*
squamous: flat nucleus
cuboidal: round nucleus
columnar: tall oval nucleus
basement membrane
thin, specialized region of extracellular matrix produced by epithelial cells so it can bind to underlying connective tissue
- cells attach by means of transmembrane linker proteins (link cell’s cytoskeleton to BM’s laminin)
- in stratified epithelium, only the deepest layer of basal cells contacts the basal membrane
- typically not visible under H&E stains, but is magenta with PAS stains
*known as the basement membrane under a light microscope, basal lamina under electron microscope
polarization of epithelium
polarization: having two different sides, in terms of function, location, morphology, etc.
Regarding individual epithelial cells:
apical surface: adjacent / oriented to outside air or organ lumen
basal surface: oriented toward/attached to underlying connective tissue
lateral surface: sides of the cell
Regarding epithelial surface as a whole:
free/luminal/adluminal surface: facing lumen of organ or vessel
abluminal (attached) surface: facing underlying connective tissue
pseudostratified epithelium
single layer made up of tall and short cells, all of which are in contact with the basement membrane
- only the tall cells reach the free surface of the epithelium
- distribution of the nucleus gives epithelium a striated appearance
stratified squamous epithelium
(nonkeratinized + keratinized)
basal layer cells may be cuboidal or columnar, but surface cells are squamous
if the cells are living: nonkeratininized
non-living cells: keratinized (nucleus and cytoplasm have been replaced by the intracellular intermediate filament keratin)
stratified epithelia
- have two or more layers of cells
- appearance of cells in deeper layers differ from surface
- classified based on cell shape in most superficial layer
- progenitor cells rest on the basement membrane, give rise to other cells
- cells held together by intercellular adhering junctions
transitional epithelium
- also known as urothelium (only found in the urinary system)
- consists of a multilayer of cells that remain functionally and morphologically intact and stretch as the organs that it lines expand
- transitions between stratified squamous and stratified cuboidal
metaplasia
epithelia produce progeny that are specialized, but not what is normally found in that location
dysplasia
abnormal differentiation results in altered size, shape, and organization of mature cells; may produce neoplasia
neoplasia
(ie, cancer)
occurs if cell proliferation is not controlled and results in an abnormal mass of proliferating cells (tumor)
benign (noncancerous, localized)
vs.
malignant (cancerous, metastatic)
carcinoma
malignant tumor of epithelial tissue
adenocarcinoma
glandular growth pattern of cells
cytoskeleton
- provides structural integrity, organizes cell spatially, and carries out coordinated intracellular movements
- composed of 3 types of protein filaments:
- actin
- microtubules
- intermediate filaments
actin filaments
(microfilaments)
- flexible, double-stranded helical polymer of one of several types of actin
- globular g-actin monomers polymerize into filamentous f-actin (this change is necessary for cell movements/phagocytosis)
- fibril diameter 5-7 nm
- in non-muscle cells, actin microfilaments are highly concentrated beneath the plasma membrane
- small bundles form the cores of microvilli
- actin binding proteins: myosin, alpha-actinin, spectrin, fimbrin, filamin, gelsolin, talin
microtubules
- long, rigid, hollow polymeric cylinders; outer diameter 25 nm
-
polarized: rapidly growing + end, depolarizing - end (embedded in the centrosome)
- subunits: tubulin heterodimers
- one alpha tubulin, one beta tubulin
- subunits: tubulin heterodimers
- function to maintain cell shape, provide rigidity, regulate intracellular movement of organelles and vesicles, and comprise core of cilia + flagella
-
microtubule associated proteins: prevent depolarization, assist in intracellular movement of organelles and vesicles
- motor proteins: ATPases that bind + transport organelles/large molecules
- kinesins: towards + end
-
dyneins: towards - end
- cytoplasmic: essential for cytosolic trafficking of vesicles
- axonemal: involved in sliding microtubules necessary for the beating of cilia + flagella
- motor proteins: ATPases that bind + transport organelles/large molecules
intermediate filaments
- rope-like polymer fibers (10-12nm in diameter)
- maintain cell shape + structural support
- high tensile strength, protect cell from stresses + strains
- occur throughout cytoplasm + sometimes attached to the cytosolic side of the plasmalemma
- typically associate with specific cells
- lamins: cell nuclei
- keratins: epithelia + related structures
- neurofilaments: neurons
- vimentins: fibroblasts
- glial fibrillary acidic proteins (GFAPs): glia
- desmin: skeletal, cardiac, smooth muscle
apical cell surface specializations
microvilli
stereocilia
cilia
cilia
- uniform, individually distinct, hair-like, 5 - 15 um length
- composed of a highly-organized core of microtubules (axoneme) that inserts into the basal body (locted in the apical cytoplasm of the ciliated cell)
- axoneme: complex of microtubules (9 +2) and MAPs
- basal body: short cylinder of 9 microtubule triplets
- moves material along the lumen parallel to the epithelia
microvilli
- core composed of actin filament bundles (25-30), inserts into the terminal web in apical cytoplasm
- increases surface area of the cell
- may be short, irregular bleb-like, or tall, closely-packed uniform projections
- the number of shape of the cell’s microvilli reflect the cell’s absorptive capacity
stereocilia
- longest microvilli, uneven length, branching, often get clumped together
- core of actin filaments
- non-motile, limited distribution
lateral cell specializations
(aka intercellular junctions)
junctions not restricted to epithelia
-
occluding:
- zonula occludens (tight junctions)
-
adhering: transmembrane junctional proteins attach cytoskeletal filaments of one cell to cytoskeletal filaments of adjacent ones
- zonula adherens (intermediate junction)
- macula adherens (desmosome)
- hemidesmosome
-
communicating:
- gap junction/nexus
zonula occludens
(tight junction)
- prevents passage of material between adjacent cells
- keeps apical membrane domain separate from basolateral membrane domain
- rows of tight junction proteins fuse adjacent plasma membranes together (forms a belt around cell)
- transmembrane proteins (claudins + occludins) bond at fusion sites
zonula adherens
intermediate junction (adhering junction)
- uniform intercellular gap of 15-20 nm
- encircling attachment site; encircles cell near lumen
- links cadherin complex and actin to that of adjacent cell
macula adherens
(desmosome)
- dense attachment plaque of ~12 proteins (including desmoplakins and plakoglobins) on cytoplasmic surfaces of opposing cell membranes
- hairpin loops of tonofilaments (cytokeratin intermediate filaments) insert into + link the plaques, providing linked cells with resistance to shearing
gap junction
(nexus)
- disk-shaped communicating junction
- very narrow (2nm) intercellular space
- bridged by transmembrane connexons
- each composed of 6 connexins
- bridged by transmembrane connexons
- sites of electronic coupling (reduced resistance to ion flow)
- regulated (open/close with signal)
- permits intercellular passage of small molecules ( < 1kD)
- essential in intercellular communication + coordination
junctional complex
three intercellular junctions that arise in intestinal epithelial cells:
- zonula occludens (tight junction)
- zonula adherens (intermediate junction)
- macula adherens (desmosome)
the junctional complex corresponds to the terminal bar of light microscopy
terminal web: dense mesh of apical cytoskeleton associated with the intercellular junctions
hemidesmosome
- half of a desmosome on the basal surface of some cells
- links cytoskeletal intermediate filaments (tonofilaments) to transmembrane integrins
- which links epithelium to basement membrane
basement membrane (LM) & basal lamina (EM)
- occupies th extracellular interface between an epithelium and its underlying connective tissue
- LM: visible with PAS or silver staining
- bases of epithelial cells (type IV collagen) are attached by binding sites (transmembrane integrins) to extracellular glycoproteins (ie, laminin)
-
two components:
- lamina lucida: laminins, glycoproteins, proteoglycans
- lamina densa: type IV collagen underlain by reticular fibers anchored by type VII collagen
-
functions:
- attachment
- filtration barrier (physical and polyanionic)
- compartmentalization/separation of tissues
- tissue scaffolding/structure
- regulation & signaling (morphogenesis, development, wound healing)
- referred to as external lamina surrounding muscle, adipose, Schwann cells
basal infoldings (enfoldings)
- deep invaginations of the cell membrane on the basal surface of epithelial cells; typically those forming the walls of ducts
- enfolding membranes contain ion (Na-K ATPase) pumps to transport ions from the lumen to the underlying connective tissue (so it can return to the body)
- enfoldings increase the surface area of the basal membrane, and thus the number of ion pumps
classification schemes of epithelial glands
- cell number (unicellular/multicellular)
- mechanism of secretion (merocine/apocrine/holocrine/active transport)
- control of secretion (regulated / constitutive)
- direction of secretion (endocrine / exocrine)
- product of secretion (mucous / serous / lipid / other)
cell number:
unicellular vs. multicellular glands
- unicellular: single cell
-
multicellular: many cells
-
simple: lack ducts / have unbranched ducts
- tubular (U-shaped)
- coiled tubular
- branched tubular
- branched acinar
- compound: branched duct system
-
simple: lack ducts / have unbranched ducts
- ducts are arranged in shape of a tube or acinus (grape-like)
- both exocrine + endocrine glands can be uni/multicellular
merocrine (eccrine) glands
- secretions are released from membrane-bound secretory granules that fuse with the apical cell membrane, open, and release product onto the surface by exocytosis
- often proteinaceous secretions
- Most glands secrete in this manner
apocrine secretion
- released lipid product is surrounded by thin layer of cytoplasm surrounded by plasmalemma
- mammary glands release the lipid component of milk this way
active transport method of secretion
- transports acids, salts, etc
- parietal (acid-producing) cells of stomach actively transport H+ and Cl- across the cell membrane into the lumen of the gastric gland
- osteoclast (resorbs bone): secretes H+
holocrine secretion
- product accumulates within maturing cell, secretory cell bursts, secreting entire content along with cell debris
- sebacous glands secrete in this manner
mucus vs. serous secretion
Mucus:
- mucus cells and glands synthesize and secrete mucinogen
- becomes mucin upon hydration
- mucin: viscous secretion with high concentration of glycoproteins and low protein content
- becomes mucin upon hydration
Serous:
- secrete less viscous fluid, mostly proteins (enzymes/proenzymes) with low [glycoproteins]
exocrine vs. endocrine glands
-
exocrine: toward lumen; release product onto epithelium surface either directly or through a duct
- secrete apically
-
endocrine: toward the connective tissue (or the general circulation)
- ductless, but highly vascularized
- secrete basally
regulated vs. constitutive secretion
-
regulated secretion:
- cells secrete in response to external signals
- soluble product is stored in high concentration in intracellular secretory vesicles (storage granules) until stimulated by extracellular signal
-
constitutive secretion:
- membranes of transport vesicles move to + fuse with cell’s plasma membrane and contents of vesicle is immediately released into extracellular space
external lamina
similar to the basal lamina, surrounds Schwann cells, muscle cells and fibers, and adipocytes
simple tubular gland
simple acinar (alveolar) gland
also a holocrine gland
simple coiled tubular
(eccrine glands)
