Download epithelial tissue

Epithelial tissue
Histology = The study of tissues
Tissue = A collection of cells that perform related
functions, and are similar in structure
Tissues are aggregates or groups of cells organized to perform
one or more specific functions.
The Four Primary Tissue Types:
1. Epithelial
2. Connective
3. Muscular
4. Nervous
Epithelium (epithelial tissue) covers body surfaces, lines body
cavities, and forms glands.
Connective tissue underlies or supports the other three basic
tissues, both structurally and functionally.
Muscle tissue is made up of contractile cells and is responsible for
movement.
Nerve tissue receives, transmits, and integrates information from
outside and inside the body to control the activities of the body.
Epithelium covers body surfaces, lines body cavities, and constitutes glands
The cells that make up epithelium have three principal characteristics:
1.
2.
3.
They are closely apposed and adhere to one another by means of specific cell-tocell adhesion molecules that form specialized cell junctions
They exhibit functional and morphologic polarity.
Their basal surface is attached to an underlying basement membrane, a
noncellular, protein–polysaccharide-rich layer.
Classification of epithelium
simple when it is one cell layer thick
stratified when it has two or more cell layers
The individual cells that compose an epithelium are
described as:
squamous when the width of
the cell is greater than its
height;
cuboidal when the width,
depth, and height are
approximately the same;
columnar when the height of
the cell appreciably exceeds
the width
Two special categories of epithelium are pseudostratified and transitional.
Pseudostratified epithelium appears stratified, although some of the cells do not
reach the free surface; all rest on the basement membrane.
•Transitional epithelium (urothelium) is a term applied to the epithelium lining the
lower urinary tract, extending from the minor calyces of the kidney down to the
proximal part of the urethra.
Transitional Epithelium
Transitional Epithelium with rounded binucleated
surface cells
Specific names are given to epithelium in certain locations:
Endothelium is the epithelial lining of
the blood and lymphatic vessels.
Mesothelium is the epithelium that lines
the walls and covers the contents of the
closed cavities of the body (i.e.,the
abdominal, pericardial, and pleural
cavities).
Diverse epithelial functions can be found in different organs of the
body.
A given epithelium may serve one or more functions, depending on the activity
of the cell types that are present:
• secretion, as in the columnar epithelium of the stomach and the gastric glands;
• absorption, as in the columnar epithelium of the intestines and proximal
convoluted tubules in the kidney;
• transportation, as in the transport of materials or cells along the surface of an
epithelium by motile cilia or in the transport of materials across an epithelium to
and
from the connective tissue;
• protection, as in the stratified squamous epithelium of the skin (epidermis)
and the transitional epithelium of the urinary bladder;
• receptor function to receive and transduce external stimuli, as in the taste
buds of the tongue, olfactory epithelium of the nasal mucosa, and the retina of
the eye.
Epithelial cells exhibit distinct polarity.
They have an apical domain,
a lateral domain,
and a basal domain.
The apical domain and its modifications
• microvilli, cytoplasmic processes containing a core of actin
filaments;
• stereocilia
(stereovilli), microvilli of unusual length;
• cilia, cytoplasmic processes containing bundle of microtubules.
Summary of the maintenance of cell shape by
junctions and cytoskeletal elements
Microvilli are fingerlike cytoplasmic projections on the apical
surface of most epithelial cells.
The internal structure of microvilli contains a core of actin
filaments that are cross-linked by several actin bundling proteins.
Stereocilia are unusually long, immotile microvilli.
Cilia are capable of moving fluid and particles along epithelial surfaces
Cilia contain an axoneme, which
represents an organized core of
microtubules arranged in a 9 + 2
pattern.
Cilia movement originates from the
sliding of microtubule doublets, which
is generated by the ATPase activity of
the dynein arms.
The lateral domain and its modifications
Occluding junctions are
impermeable and allow epithelial
cells to function as a barrier. Also
called tight Junctions.
Anchoring junctions
Communicating junctions
Occluding Junctions
The zonula occludens (pl., zonulae occludentes) represents the most apical
component in the junctional complex between epithelial cells.
occluding junctions form the primary intercellular diffusion barrier between
adjacent cells
The zonula occludens is created by localized sealing of the plasma membrane of
adjacent cells.
Several proteins are involved in the formation of zonula occludens strands
Junctional adhesion molecule (JAM)
Occludin
zonula occludens proteins
ZO-1, ZO-2, and ZO-3
Claudins
Anchoring junctions provide lateral adhesions between
epithelial cells
zonula adherens ( pl., zonulae
adherentes), which interacts
with the network of actin
filaments inside the cell; and
macula adherens (pl., maculae
adherentes) or desmosome,
which interacts with
intermediate filaments
The zonula adherens
The zonula adherens is composed of
the transmembrane cell adhesion
molecule E-cadherin.
On the cytoplasmic side, the tail of
E-cadherin is bound to catenin .
The resulting E-cadherin–catenin
complex binds to vinculin and actinin and is required for the
interaction of cadherins with the
actin filaments of the cytoskeleton.
The macula adherens (desmosome) provides a localized
spotlike junction between epithelial cells.
In the area of the macula
adherens, desmogleins and
desmocollins provide the
linkage between the plasma
membranes of adjacent cells.
Desmosome Structure
Communicating junctions, also called gap junctions or nexuses
Gap junctions are formed by 12
subunits of the connexin protein
family.
Conformational
changes in connexins
leading to opening or
closing gap junction
channels
Gap Junction
2 nM gap
between
membranes
Open pores for ions,
molecules, etc
Connexin
molecules
The basal domain
The basement membrane is a specialized structure located next to the
basal domain of epithelial cells and the underlying connective tissue
stroma.
The basal lamina contains molecules that come together to form a sheet
like structure.
composed of laminins, a type IV collagen molecule, fibronectin
Cell-to-extracellular matrix
junctions anchor the cell to the
extracellular matrix; they are
represented by focal adhesions
and hemidesmosomes.
Junctional Complexes
Epithelium (epithelial tissue) covers body
surfaces, lines body cavities, and forms glands.
Typically, glands are classified into two major groups according to how
their products are released:
• Exocrine glands secrete their products onto a surface directly or
through epithelial ducts or tubes that are connected to a surface. Ducts
may convey the secreted material in an unaltered form or may modify the
secretion by concentrating it or adding or reabsorbing constituent
substances.
• Endocrine glands lack a duct system. They secrete their products into
the connective tissue, from which they enter the bloodstream to reach
their target cells. The products of endocrine glands are called hormones.
Cells of exocrine glands exhibit different mechanisms of secretion
• Merocrine secretion. This secretory product is delivered in membrane-bounded vesicles to the
apical surface of the cell. Here vesicles fuse with the plasma membrane and extrude their
contents by exocytosis. This is the most common mechanism of secretion.
• Apocrine secretion. The secretory product is released in the apical portion of the cell,
surrounded by a thin layer of cytoplasm within an envelope of plasma membrane. This
mechanism of secretion is found in the lactating mammary gland, where it is responsible for
releasing large lipid droplets into the milk. It also occurs in the apocrine glands of skin, ciliary
(Moll’s) glands of the eyelid, and the ceruminous glands of the external auditory meatus.
• Holocrine secretion. The secretory product accumulates within the maturing cell, which
simultaneously undergoes programmed cell death. Both secretory products and cell debris are
discharged into the lumen of the gland. This mechanism is found in sebaceous glands of skin
and the tarsal (Meibomian) glands of the eyelid.
Exocrine glands are classified as either unicellular or multicellular
Unicellular glands are the simplest in
structure
A typical example is the goblet cell, a mucus-secreting cell positioned
among other columnar cells
Multicellular glands are composed of more than one cell. They exhibit
varying degrees of complexity. Their structural organization allows
subclassification according to the arrangement of the secretory cells
(parenchyma) and the presence or absence of branching of the duct
elements.
If the duct is unbranched, the gland is
called simple;
if the duct is branched, it is called
compound.
If the secretory portion is shaped like a
tube, the gland is tubular;
if it is shaped like a flask, the gland is
alveolar or acinar;
if the tube ends in a saclike dilation, the
gland is tubuloalveolar.
Most epithelial cells have a finite life span less than that of
the whole organism
The replacement cells are produced by mitotic activity of self-maintaining
adult stem cells
The stratified squamous
epithelium of skin is
replaced in most sites
during a period of
approximately 28 days.
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