Tissues - classsification, general
structure and function
Epithelial tissue
Institute of Histology and Embryology
Doc. MUDr. Zuzana Jirsová, CSc.
Histology and Embryology – B81131
Lecture - 16. 10. 2013
TISSUES
Aggregates of cells organized to perform one
or more specific functions
Common origin and morphological characteristic
Four basic types of tissues:
EPITHELIAL TISSUE
CONNECTIVE TISSUE
MUSCLE TISSUE
NERVE TISSUE
EPITHELIAL TISSUE
Origin: tissue developing from the ectoderm,
mesoderm and endoderm (all three germ layers )
Structure: tissue composed of closely
aggregated cells
Function: protection, secretion, absorption,
transport, respiration, reception
CONNECTIVE TISSUE
Origin: tissue develops from mesenchyme
Structure: CELLS
EXTRACELLULAR MATRIX
protein fibers and ground substance
Types: connective tissue proper
(loose and dense)
CT with special properties
(mucous, reticular, elastic and adipose)
supporting CT (cartilage, bone, dentin)
Functional specialization: supporting, structural,
storage, defense, nutrition
MUSCLE TISSUE
Origin: mesoderm (skeletal and cardiac muscles)
mesenchyme (smooth muscles)
Functional property: contractibility. Actin and
myosin are organized into the myofilaments,
actin–myosin interaction is responsible
for contraction.
Two types: striated muscles - skeletal and cardiac
muscles
smooth muscles
Structural unit: cell (cardiac and smooth muscles)
muscle fiber (skeletal muscle)
NERVE TISSUE
Origin: tissue develops from neuroectoderm
Structure: NT consists of two principal types of
cells: neurons (nerve cells)
supporting glial cells (neuroglia)
Function
Neurons: functional units of nervous system,
gather and process information and generate
signal, conduct electrical impulses
Glial cells: protection, electric insulation,
metabolic exchange, defense
EPITHELIAL TISSUE
Cell adhesion and intercellular
junctions
Cell surface modifications
Cell polarity
Classification of the epithelial tissue
EPITHELIAL TISSUE
Cells are closely apposed and adhere to one another by
means of specific cell adhesion molecules (CAMs)
Specialized cell junctions:
OCCLUDING JUNCTIONS - link forming an impermeable
barrier: zonula occludens, tight junction
ANCHORING JUNCTIONS - provide mechanical stability
by linking of the cytoskeleton of one cell to the cytoskeleton of adjacent cell or cell-to-extracellular matrix (*)
zonula adherens (focal contacts*) - actin filaments
macula adherens/desmosome (hemidesmosome*)
- intermediate filaments
COMMUNICATING JUNCTIONS: gap junctions – allow
selective diffusion of molecules between adjacent cells
CELL POLARITY
Free surface/ apical domain
Special structural surface modifications:
Microvilli – cytoplasmic processes (core consisting
of actin filaments)
closed packed microvilli – brush border (absorptive
epithelium)
Stereocilia – long immotile microvilli: hair cells of the
inner ear, epithelium of the male genital ducts
Cilia – motile cytoplasmic processes (consisting of
microtubule axoneme, basal bodies); epithelium of
respiratory ways, and uterine tube
CELL POLARITY
Lateral domain
In close contact with the opposed lateral domains of
neighboring cells. Cell adhesion molecules: CADHERINS
Cell-to-cell junctions: (a) occluding, (b) adhesive or
anchoring, (c) and communicating
Folding - invagination and evagination of lateral cell
membrane, interdigitation
Basal domain
Attached to the basement membrane, cell adhesion
molecules: INTEGRINS
Cell-to-extracellular matrix junctions: hemidesmosomes
and focal contacts
Infoldings of basal cell membrane: ion – transporting
epithelium (striated ducts of salivary glands, kidney tubules)
BASEMENT MEMBRANE (term used in light microscopy)
Basement membrane mediates attachment of epithelial
cells to underlying connective tissue
Basal lamina (in electron
microscope)
Lamina rara is an electron-lucent area (40 nm thick)
between the lamina densa and epithelium; contains
of extracellular portions of intergrins (CAM), mainly their
fibronectin and laminin receptors
Lamina densa is an electron-dense layer of 40 – 60 nm
thick between the epithelium and the adjacent connective
tissue composed of laminins, type IV collagen,
proteoglycans (heparan sulfate and chondroitin sulfate
proteoglycans), entactin
BASEMENT MEMBRANE
Attachment of the basal lamina to the underlying
connective tissues mediated by anchoring fibrils
(type VII collagen) and fibrillin microfibrils
A layer of reticular fibers underlies the basal lamina reticular lamina (is not produced by the epithelium)
belongs to the connective tissue
Basement membrane is PAS positive and stains with
acidic dyes
Zonula occludens
CELL JUNCTIONS
Zonula adherens
Free surface
Apical domain
Junctional
complex
Terminal
bars in LM
Scheme: Ross, Pawlina, Histology, 2003
Lateral
domain
Focal contacts
Hemidesmosomes
Desmosomes
cell membranes
b
Scheme, electron micrographs: Ross, Pawlina, Histology, 2003
ZONULA OCCLUDENS
Tight junction, that separates luminal space from the intercellular compartment. Focal junctions between cells by transmembrane proteins (claudin,
occludin) form sealing strands which are arranged as intertwining lines.
Electron micrograph of ZO (a) - close apposition of adjacent areas of cell membrane
Freeze fracture technique (b) shows ZO as anastomosing network of ridges – arrows
ZONULA ADHERENS
(Scheme, electron micrograph: Ross, Pawlina, Histology, 2003)
ZA is belt-like junction encircling the cell localized just bellow ZO. Actin filaments
of terminal web of adjacent cells are attached to E cadherin-catenin complex by
alfa-actinin and vinculin. Intercellular space measures 10-20 nm.
DESMOSOM, MACULA ADHAERENS
Disc-like intercellular attachment. Intermediate filaments (CF) are anchored to the
attachment plaque (P) located on the cytoplasmatic side of cell membrane (CM).
CM
Cytokeratin
filaments
P
Plaque: desmoplakins
desmoglobin Transmembrane proteins:
desmogleins
X = intercellular space (20-30 nm) desmocollins
Scheme, elektron micrograph: Histology, Ross, Pawlina, 2006
GAP JUNCTION, NEXUS – permit the direct passage of the ions, signaling molecules
nexus
Gap junction proteins (connexins) form hexamers CONNEXONS
with hydrophilic pore (2 nm in diameter). Connexons in adjacent cell
membranes are aligned to form a cylindrical transmembrane
channel. Conformational changes in connexins lead to opening
or closing of channels. Epithelium, smooth and cardiac muscles.
CM
CM
1: Electron micrograph
(TEM): close apposition
of the cell membranes
in the gap junction (IS
is only 2 nm)
CM – cell membrane
IS – intercellular
space
Ross, Pawlina, Histology,
2006
CM
c
1
Cx
2: Freeze fracture
technique: dense
particles correspond
to connexons
Junquiera´s Basic Histology,
Mescher, 2010
A
3: Scheme of a gap
junction
Ross, Pawlina, Histology, 2006
A – closed, B – open
B
2
3
IS
Diameter of connecting
channel is regulated
by reversible changes
in the conformation
of the individual connexins.
Organization of actin filament in microvillus
Brush (striated) border of the absorptive epithelium
Microvillus is
about 1µm
heigh and
0.08 µm wide
villin
espin
fimbrin
Attachment
of AF to the
CM
AF
fascin
Freeze fracture technique
myosin I
Bundles of
AF constituting the
core of
microvillus
spectrin
AF rootlets
terminal
web (actin
cortex)
myosin II
Spectrin binds
AF to the actin
cortex and to CM
Cytokeratin
intermediate
filaments
cytokeratin
IMF
Scheme: Histology, Ross, Pawlina, 2011
EM, Freeze fracture technique showing the arrangement of AF
in the microvilli. Ross, Pawlina, Histology, 2006
STEREOCILIA - long immotile microvilli
Epididymis, pseudostratified columnar
epithelium - sterocilia (arrows)
ezrin
fimbrin
actin filaments
espin
cytoplasmic
bridges –
α-actinin
actin
filaments
α-actinin
Scheme: Histology, Ross, Pawlina, 2011
Photomicrograph: W. Kühnel, Atlas, 2003
CILIA (lenght: 5-10 µm, diameter: 0,2 µm)
Cilia are elongated, highly motile structures on the surface of some epithelial cells, inserted
into basal bodies (BB). BB have a structure similar to centrioles and MTOC function. Cilium
is composed of microtubules which form axoneme: 9 peripheral dublets (A microtubulus
consists of 13 protofibrils, B – 10 protofibrils) and one central pair of microtubules.
The motion of the cilia occurs due to activity of dynein (protein with ATP-ase activity), with
ATP as the energy source.
Occurrence: epithelium lining the respiratory tract and uterine tube
B
A
Scheme, electron micrographs: Stevens, Lowe, Histology, 1993
A cross (b) and longitudival (c) section of cilia in EM
CM = cytoplasmic membrane, BB = basal bodies, Cy = cytoplasm
Basal surface modification: ion-transporting epithelium (proximal and distal tubules
of the kidney, striated ducts of the salivary glands). Infoldings at the basal cell
membrane enlarge basal surface, mitochondria provide energy for active transport
of ions.
microvilli
tight junction
G = Golgi complex
tight junction
longitudinally
oriented
elongated
mitochondria
infoldings of the
basal cell
membrane
Na+K+ ATP-ase
basement
membrane
Basal labyrinth
Sodium pump
Scheme: Krstic, Illustrated Encycl. of Human Histology, 1984
Classification of epithelium
according arrangement of cells:
a) PLANE: cells form tightly cohesive sheets
traditional nomenclature is based on the:
shape of cells:
squamous, cuboidal, columnar
number of layers:
simple, stratified, pseudostratified
b) TRABECULAR: cells are arranged in cords or plates
(liver, endocrine glands)
c) RETICULAR: cells form three-dimensional network
(stroma of thymus, epithelium of crypts in tonsils,
stellate reticulum of enamel organ)
Classification of epithelium
according functional specialization
Covering epithelium
Simple squamous, cuboidal, columnar,
pseudostratified columnar
Stratified squamous, columnar
Transitional epithelium (urothelium)
Secretory epithelium – exocrine and endocrine;
polarization of cells
mechanism of secretion: merocrine, apocrine,
holocrine
Absorptive epithelium
Respiratory epithelium: exchange of respiratory
gases, in the lung alveoli
Sensory epithelium: cells react on the external
stimuli by change of membrane potential
Primary and secondary sensory cells
Myoepithelium: cells with ability to contract
Germinal epithelium - production of cells
(seminiferous epithelium of testis produce
spermatozoa)
Ion-transporting epithelium (modification of the basal
cell surface)
REQUESTED KNOWLEDGE
Epithelial tissue, overview of the structure
and function, classification
Secretory epithelium (endocrine, exocrine –
types of secretion)
Sensory epithelium - structure and function.
Myoepithelium