Download Tissues. Epithelial tissue. Glands.

Volgograd state medical university
Department of histology, embryology, cytology
Tissues. Epithelial tissue. Glands.
For the 1st year General Medicine Students
Volgograd, 2017
THE OBJECTIVES:
1.To define the term “tissue” and to characterize tissue
types.
2.To develop classification of the epithelial tissue.
3. To provide morphological and functional
characteristics of the epithelial tissue and its subtypes.
4. To understand the difference between the exocrine and
endocrine glands and to characterize exocrine glands
(classifications, structure, types).
DEFINITION AND CLASSIFICATION OF TISSUES.
A tissue is a complex assemblage of cells and cell products,
having a common function. The approximately 200 distinctly types of
cells composing the human body are arranged and cooperatively
organized into four tissues:
1. Epithelium.
2. Connective tissue.
3. Muscle tissue.
4. Nervous tissue.
Body tissues are grouped according to their cells and cell
products into organs. These tissues exist and function in close
association with one another.
Epithelial tissue is present in the two major forms: as sheets of
contiguous cells (epithelia) that cover body on its external surface and
as glands which originate from invaginated epithelial cells.
Epithelial Tissue. Definition.
The epithelial tissue consists of sheets of cells that
cover the external surfaces of the body, line the internal
cavities, form various organs and glands and line their
ducts.
The are 4 major types of epithelial tissue:
1.Lining epithelium (it lines or covers all body surfaces
and cavities except joint cavities).
2.Glandular epithelium (it comprises glands).
3.Germinative epithelium (it is present in the sex glands).
4.Sensory epithelium (it is present in the sensory organs).
ORIGIN OF THE EPITHELIAL TISSUE
Epithelia are derived from all three embryonic germ layers,
although most of the epithelia are derived from ectoderm and
endoderm.
ectoderm
endoderm
mesoderm
epithelia: oral and nasal
mucosae, cornea, the
epidermis of skin, nails,
hairs, anal canal,
the liver, pancreas, and
lining of the respiratory
and (larynx, trachea,
lungs) digestive tract
(including pharynx),
eustachian tube, urinary
bladder, urethra,
covering of posterior
1/3 of the tongue.
the uriniferous tubules
of the kidney, the lining
of
the
male
and
reproductive
system,
the endothelial lining of
the circulatory system,
and the mesothelium of
the
body
cavities,
ureter, joint cavities.
glands: the glands of
the skin (sweat,
sebaceous), mammary
glands. and salivary
glands,
adenohypophysis
thymus, thyroid,
parathyroids,
esophageal, duodenal
and intestinal glands.
kidneys, gonads,
adrenal cortex
STRUCTURAL AND FUNCTIONAL CHARACTERISTICS OF THE EPITHELIA
Epithelium is a structurally and functionally important component of the
complex organs.
The histology of the epithelium differs from organ to organ depending on
its location and function. Irrespective of location all epithelial tissues
possess certain features:
1. Intercellular substance. It is almost absent
2. Derivation: ectoderm, mesoderm and endoderm can give rise to
epithelium. Ex.: intermediate mesoderm – epithelium of the genitourinary
system.
3. Basement membrane. – all epithelia rest on it.
4. Nutrition. – It is provided through the basement membrane by diffusion
as epithelia lack blood vessels.
5. Diversity: epithelium ranges from one to many layers - for protection,
secretion, absorption, etc.
6. Intracellular polarity: the apical and basal poles of the cells have different
structure.
7. Metaplasia: facing a chronic change of environmental conditions,
epithelia undergo metaplasia, i.e. they under transition from one type to
other (simple to stratified).
8. Regeneration. Regenerative capacity of epithelial cells is very high. Cells
readily divide by mitosis.
Simple Squamous Epithelium – Mesothelium, Silver impregnation
cell boundaries
cytoplasm
nuclei
1. Intercellular substance. The epithelial tissue have little intercellular
substance. The cells are densely packed or closely apposed to
each other connected by the specialized intercellular junctions.
Intercellular space may be only 25 nm.
On the slide cell boundaries are revealed by silver impregnation.
Intestinal Villus lined with Simple Columnar Epithelium with Striated Border
3. Basement membrane.
Epithelium rests on the
basement membrane which
binds it to the underlying
tissue hence polarity of the
epithelial cells: basal surface
is
connected
to
the
basement membrane while villus
apical surface is free. The
basement
membrane
provides scaffolding and is
composed of a selective
mortar which connects the
epithelium to the underlying
connective tissue.
The basement membrane
includes
basal
lamina
composed of the type IV
collagen and extracellular
matrix
molecules
(fibronectin and laminin).
Basal lamina components
are secreted by epithelium.
simple columnar
epithelium
lacteal
blood capillary
network
intestinal gland
goblet cell
arteriole
venule
lymph vessel
BM
C
L
D
FR
a
b
a) Basement membrane may not be seen under LM as it is only 0.05
mcm thick, however a high content of glycoprotein renders it stainable with
PAS, when ir appears as a faint magenta stained line (BM).
b) By EM basement membrane resolves into several layers (laminae).
The lamina densa (D) is a dark staining band 30-100 nm thick. Between this and
the attached cell (C) is a lucents zone, the lamina lucida (L), which is usually 60
nm wide. On the other side of the lamina densa is a rarefield layer of variable
thickness, the fibroreticular lamina (also included in the basement membrane)
which merges with fibrous proteins of the extracellular matrix. Though usually
the terms basal lamina and basement membrane are interchangable, the term
basal lamina should refer to the lamina densa only.
BASAL LAMINA = LAMINA DENSA + LAMINA RARA
BASEMENT MEMBRANE = BASAL LAMINA + FIBRORETICULAR LAYER
Stratified Squamous Non-keratinized Epithelium of of the Esophagus
squampous
cells
polyhedral
cells
epithelium
basal cells
basement
membrane
connective
tissue
capillary
capillary
arteiole
connective tissue
collagen fibers fibroblasts
4. Nutrition. There are no direct contact between blood vessels and
epithelium. All epithelia lack blood vessels. All nutrients and oxygen pass
through basement membrane from the capillaries of the adjacent connective
tissue because blood vessels do not penetrate the epithelium. Metabolic
waste products and carbon dioxide also diffuse from the epitheliocytes to
the capillaries in the connective tissue. To shorten the diffusion distance
basal surface of the epithelia is often folded.
SMALL INTESTINE, H & E
mitosis
mitosis
intestinal
enteroendoglands
crine cells
enteroendo
Paneth
crine cells
ells
Paneth cells
muscularis
muscularis
mucosae
mucosae
5.Intracellular polarity: the nucleus and the organelles are often found in
characteristic regions of epithelium due to intracellular polarity (nuclei are
located in the basal part of the cell, the secretory granules – in the apical pole).
7. Regeneration. Because most epithelia cover and line surfaces, the cells are
subject to mechanical abrasion, therefore it is necessary for the epithelial cells
to be capable of replacement. Regenerative capacity of epithelial cells is very
high (mitoses are frequent). Ex.: the turnover rate for intestinal epithelial cells in
the colon is about 6 days. Specialized cells that can not divide are provided
with stem cells that have the capacity to produce their different kinds of
specialized cells. Epithelial cells are capable of division to maintain the integrity
of the cellular sheet. The entire epithelial lining the interior of the small intestine
is replaced over a period of less than 6 days, of the epidermis – around 28 days.
goblet cells
FUNCTIONS OF EPITHELIA
function
example
protection from
stratified epithelia cover surface of body; cover and line
abrasion and injury interior of organs
absorption
simple columnar (absorptive) cells lining small intestine
and stomach absorb variety of products
selective
permeability
nephrothelium, enterocyes, et al., control movement of
materials via selective permeability of intercellular
junctions between epitheliocytes
secretion of
mucus, hormones,
enzymes etc from
various glands
goblet cells (unicellular mucous gland cells) in epithelial
sheets; mucous and serous glands derived from
epithelium; other epithelial cells in oviducts, bronchioles,
type II pneumocytes, etc, secrete various products.
excretion, transcellular transport
gallbladder and kidney tubule epithelial cells actively
transport various ions, nitrogenous wastes, water, etc
contractility
myoepithelial cells around secretory units & smallest
ducts of glands (salivary glands, mammary glands).
sensory reception
taste buds, an example of neuroepithelium on papillae in
the tongue
immunologic
function
Langerhans cells in epidermis of integument, a type of
antigen-presenting cell (macrophage)
CLASSIFICATION OF THE EPITHELIAL TUSSUE
The epithelium is classified basing on the number of cell layers
and the morphology of the surface cells.
1. Simple epithelium
2. Stratified epithelium
Simple epithelium consists of a single layer of cells all cells of
which are attached to the basement. The stratified epithelium has
several layers of cells while only cells of the basal layer contact
the basement membrane.
2
Recto-Anal
Junction (*),
H & E.
*
1
The epithelium is classified basing on the number of cell layers and the
morphology of the surface cells.
EPITHELIUM
• Simple epithelium
Stratified epithelium
nonkeratinized
squamous epithelium
-squamous
cuboidal epithelium
-cuboidal
columnar epithelium
- columnar
pseudostratified:
-cuboidal
-columnar
Simple epithelium consists of a single layer of
cells all cells of which are attached to the
basement. The stratified epithelium has several
layers of cells while only cells of the basal layer
contact the basement membrane.
keratinized
-squamous
transitional
simple
Types of Epithelia
squamous
cuboidal
columnar
pseudostratified
pseudostratified
columnar
transitional
stratified
squamous
non-keratinized
cuboidal
keratinized
columnar
transitional relaxed
transitional (distended)
Classification of Simple Epithelia
Type of epithelium
Location
Simple squamous
endothelium, mesothelium, parietal layer of
Bowman’s capsule, thin segment of loop of
Henle, rete testis, pulmonary alveoli
Simple cuboidal
thyroid, choroid plexus, ducts of many
glands, inner surface of capsule of lens,
covering surface of ovary
Simple columnar
surface epithelium of stomach,
small & large intestine,
proximal convoluted tubules
& distal convoluted tubules of kidney,
gallbladder, excretory ducts of glands, uterus
oviduct
small bronchi of lungs
some paranasal sinuses
Pseudostratified
columnar
large excretory ducts of glands, portions of
male urethra,
epididymis
trachea, bronchi,
Eustachian tube
portions of tympanic cavity
Specialization
striated border
brush border
cilia
cilia
cilia
stereocilia
cilia
cilia
cilia
Classification of Stratified Epithelia
Type of
epithelium
Location
Specialization
stratified
squamous
nonkeratinized
buccal surface, esophagus,
epiglottis, conjunctiva, cornea,
vagina
gingiva, hard palate
epidermis of skin
keratin
keratin
keratinized
stratified
cuboidal
ducts of sweat glands
stratified
columnar
cavernous urethra, fornix of
conjuctiva, large excretory ducts
of glands
transitional
urinary system: renal calyces to
urethra
germinal
seminiferous tubules of adult
testes
Types of Simple epithelium
A.Simple squamous epithelium lines the external surfaces of the digestive
organs, lungs and heart. The lining of the pleural, abdominal and
pericardial cavities is called mesothelium.
Similarly a simple squamous epithelium that lines the lumen of
heat, blood vessels and lymphatic vessels is called endothelium.
B. Simple cuboidal epithelium lines small excretory ducts in
different organs, like salivary glands, liver, etc. For example,
proximal convoluted tubules of kidney are lined with simple
cuboidal epithelium, the apical surfaces of the epithelial cells
contain a brush border (composed of microvilli).
C.Simple columnar epithelium lines the inner surfaces the
digestive organs (stomach, small intestine, large intestine and gall
bladder).
In the small and large intestines, the surfaces of absorptive
epithelial cells that line the villi and/or crypts exhibit a striated
border (also formed by the microvill)
Pseudostratified Columnar Epithelium
• The cells are arranged in single layer of cells on a
basement membrane.
• But they appear as if they are arranged in two or
three different layers as the epithelial cells are of
different shape and their nuclei form two to three
rows.
• Hence the name is pseudostratified (columnar)
epithelium.
• It lines the air-conducting ways, the lumen of the
epididymis and vas deferens.
• in trachea, bronchi and larger bronchioles and
ductuli efferentes of the testes the surface cells
contain motile cilia.
• In epididymis and in vas deferens, the surface cells
are lined by immotile sterocilia.
Pseudostraified Columnar Epithelium of the Nasal Mucosa.
G
The lining epithelial cells are of different shape and their nuclei are located at
different levels forming three rows, but all the epithelial cells are attached to the
basement membrane (arrow) either by a thin or a thick base hence epithelium is
of a simple type. G-goblet cell.
Stratified Epithelium
• The cells are arranged in more than one layer.
Different layers are placed one above the other.
There are the following types of the stratified
epithelium.
• A. TRANSITIONAL EPITHELIUM
• Many layers of cells when collapsed.
• The surface cells are dome shaped when contracted
and become flattened like squamous epithelial cells
when stretched.
• It lines the excretory system (ureters, urinary
bladder, urethra)
• B. STRATIFIED SQUAMOUS EPITHELIUM
• Contains multiple cell layers.
• The basal cells are cuboidal to columnar.
• These give rise to cells that migrate toward the
surface and become squamous.
• It may keratinized and non-keratinized.
•
•
•
•
•
•
•
TYPES OF THE EPITHELIAL TISSUE
1b.The keratinized epithelium covers the skin
(epidermis). It contains nonliving, kerainized cells
that are filled with keratin. Thick epidermis is
present on the surface of the palms and soles.
2b.The non- keratinized epithelium contains
multiple cell layers.
It exhibits live surface cells with nuclei.
It lines the oral cavity, pharynx, cornea,
esophagus, exocervix of the uterus, vagina and
anal canal.
C. Stratified cuboidal epithelium (always nonkeratinized) and
D. Stratified columnar epithelium (always nonkeratinized) have a limited distribution.
Both types line the excretory ducts salivary
glands and sweat glands.
Stratified Squamous Non-keratinized Epithelium of
of the Esophagus
squampous
cells
epithelium
polyhedral
cells
basal cells
connective
tissue
capillary
2b.The non- keratinized epithelium contains multiple cell layers.
It exhibits live surface cells with nuclei.
It lines the oral cavity, pharynx, cornea, esophagus, exocervix of the uterus,
vagina and anal canal.
Stratified Squamous Keratinized Epithelium of the Skin
excretory ducts
of the sweat glands
stratum
corneum
epidermis
stratum
granulosum
stratum
spinosum
papillae
stratum
basale
connective
tissue
The keratinized epithelium covers the skin (epidermis). It
contains nonliving, kerainized cells that are filled with keratin. Thick
epidermis is present on the surface of the palms and soles.
CLINICAL CORRELATIONS
Each epithelium within the body has its own unique
characteristics, location, cell morphology and so on, all of which
are related to function. In certain pathological conditions, the
cell population of an epithelium may undergo metaplasia,
transforming into another epithelial type.
Pseudostratified ciliated columnar epithelium of the
bronchi of heavy smokers may undergo squamous metaplasia,
transforming into stratified squamous epithelium. This change
impairs function, but the process may be reversed when the
pathological insult is removed.
Tumors that arise from epithelial cells may be benign or
malignant. Malignant tumors arising from epithelia are called
carcinomas; those arising from the glandular epithelial cells are
called adenocarcinomas.
Due to continuous mitotic cell division, genetically and
phenotypically atypical cells form cauliflower-like tumors which
invade the normal tissues, change the contour of the original
structure penetrate into undelying tissues and give metastasis
to the distant organs through the blood and lymph vessels.
Polarity and Cell-Surface Specializations
1.Epithelial cells possess apical domain facing the
lumen and representing the free surface of cells, and
basolateral domain including basal and lateral aspects
of cells facing basement membrane and lateral surface
of adjacent cells accordingly.
2.The apical domain displays surface specializations,
like microvilli, cilia, stereocilia, flagella.
3.The apical domains is rich in ion channels, carrier
proteins, ATPase and aqueporins (channel-forming
proteins regulating water balance).
4.Basolateral domain exhibits many types of junctional
specializations.
Surface Modifications of the Apical Domain
•
1.Cilia are motile structures which project in parallel rows from
certain epithelial surfaces. They cleans the inhaled air and
conduct mucus and particulate material across the cell surfaces
to the exterior of the body (respiratory tract, uterine tubes of the
female reproductive tract; in the efferent ductuli of the male
reproductive tract they transport sperm out of testis into
epididymis).
•
2.Sterocilia are immotile, they absorb fluids produced by the
testis (epididymis and vas deferens in the male reproductive
tract) or function as sensors of cochlear hair cells in the inner
ear.
•
3.Flagella - long processes - sensor and transportation organ (in
humans only with spermatozoa).
•
4.Microvilli (in brush border or striated border): the function is to
absorb the nutrients and fluid from the filtrate/contents that
passes through these tubules/organs (renal tubules, small
intestine, etc).
STRIATED BORDER IN THE EPITHELIAL CELLS OF THE SMALL INTESTINE
villi
columnar
epithelium
central
lacteal
smooth muscle
basement
membrane
lymphocyte
oblique
section of
epityhelium:
apical and
basal parts
of cells
goblet cell
capillary
connective
tissue (lamina
propria)
smooth muscle
fibers
goblet cell
striated border
In the bowel the lining simple columnar epithelium displays striated border at
the apical ends of the epithelial cells. The striated border in the lining epithelial
cells are formed by the microvilli (visible under EM) increasing the surface of
absorption.
M
AF
MICROVILLI of
the BRUSH
BORDER of the
ILEUM, TEM
AC
Electron microphotograph showing the surface of a cell lining the
small bowel. Microvilli (M) form finger-like projections, each having an actin
filament (AF) core that enters the cell and merges with with the actin cortex
(AC) which is also known as terminal web.
MICROVILLUS
lateral
anchoring
protein
(myosin)
cell
membrane
linkage to cell
membrane
actin cortex linked
by spectrin
amorphous
tip region
(villin)
actin
filaments
actin-binding
proteins
(fimbrin &
fascin)
spectrin
cytokeratin filaments
Each microvillus is a finger-like
extension of cell membrane
which is stabilized by a bundle
of actin filaments held rigidly 10
nm apart by actin-binding
proteins. The actin bundle is
bound to the lateral surface of
the microvillus by a helical
arrangement of myosin molecules, which bind on one side to
the actin and on the other to the
inner surface of cell membrane.
The bundle is also adherent to
the apex of the microvillus in an
amorphous area of anchoring
proteins which may represent
capping proteins for the actin
filaments to prevent their
depolymerization. At the base of
the microvillus the entering
actin bundle is stabilized by
actin/spectrin cell cortex, under
which are cytokeratin intermediate filaments.
C
Cilia (C) form a hair-like layer
at the apical cell surface. They are
motile, hair-like projections that
emanate from the surface of certain
epithelial cells. In the epithelia of the
respiratory tract and oviduct there are
hundreds of cilia in orderly arrays on
the
luminal
surface
of
the
epitheliocytes. Other epithelial cells as
the hair cells of the vestibular
apparatus in the inner ear, possess
only one cilium (stereocilia) which
functions as a sensory mechanism.
The internal strucrute is
consistently conserved throughout
the plant and animal kingdom.
Cilia of the epithelial cells of the respiratory mucosa, H & E.
Microtubular arrangement
the axoneme in the cilia
of
plasma
membrane
The core of the cilium contains a
complex
of
uniformely
arranged
microtubules
called
axoneme.
The
axoneme is composed of a constant
number of longitudinal tubules arranged
in a consistent 9+2 organization. Two
centrally placed microtubules (singlets)
are evenly surrounded by nine doublets
of microtubules.
The two central microtubules are
separated from each other, each
displaying a circular profile in crosssection, composed of 13 protofilaments.
Each of the 9 doublets are composed of
the 2 subunits. Subunit A also displays
circular
profile
composed
of
13
protofilaments while subunit B possesses
plasma
only
10
protofilaments
exhibiting
memincomplete circular profile and sharing
brane
3 protofilaments with subunit A.
central
microtubular
pair
peripheral microtubule douplet
microtubular
triplet
basal body
dynein arms
(every 24 nm)
nexin linking
protein, every
86 nm
central
sheath
projections
(every 14
nm)
DIAGRAM OF A CILIUM
radial spoke
(every 29 nm)
cell
membrane
central pair of
microtubules
0.25 mcm
outer doublet
tubulin
microtubule
The nine outer doublet tubules
are made of tubulin while arms
initiating
movement
are
composed of protein dynein
(with ATPase activity). Arms
occur every 24 nm down the
length of the cilium and
interact with adjacent doublets
as a “molecular motor” to
produce
bending.
Links
between
neighbouring
douplets are composed of
another protein, nexin, and
they are more widely spaced
(every 86 nm) and hold the
microtubules
in
position.
Radial spokes extend from
each of nine outer doublets
toward the central pair of
tubules at 29 nm intervals,
while the central sheath
projections are present every
14 nm.
Respiratory epithelium
cell with cilia, TEM
C
The base of each cilium
(C) is seen to arise as a
specialized derivative of
the centriole (the basal
body, BB, with triplets of
mucrotubules
and
without a central pair of
microtubules). Here the
outer doublets of the
cilium arise directly from
the outer triplet of the
centriole. CM – cell
membrane;
Cy
–
cytoplasm.
CM
BB
Cy
CELL MODIFICATION OF THE
BASOLATERAL DOMAIN
1.Basolateral foldings.
2. Specialized structures which link the
individual cells together into a
functional unit are called cell junctions.
Basolateral Folding of Epitheliocytes
Striated ducts of the parotis gland, H & E.
Proximal convoluted tubule of kidney, TEM
Basolateral
folds
are
deep
invaginations of the lateral or
basal surface of cells. They are
particularly
evident
in
cells
involved in fluid and ion transport,
and are commionly associated
with a high concentration of
mitochondria which provide the
energy for ion and fliud transport.
The presence of the basal folds
and
mitochondria imparts a
stripped appearance to the basal
cytoplasm of such cells giving rise
to the descriptive term “striated
epithelial cells”. Basal folds are
seen in the renal tubular cells and
in the ducts of many glands. Cell
surface may be increased by
folding
of
the
lateral
cell
membrane which can be seen in
some
epithelial
cells,
like
absorptive cells lining the gut.
Cell junctions
• Epithelial cells form special contacts with each other
called cell junctions. There are three major types of
cell junctions:
• I. Occluding junctions (zonula occludens and fascia
occludens). They link cells to form impermeable
barrier. The tight or occluding junctions are present
only in the epithelium.The contiguous cell
membranes are fused.
• 2. Adhesive junctions (zonula adherens, desmosome
= macula adherens, fascia adherens). They link the
cells to provide mechanical strength.
• 3. Gap junctions allow movement of molecules
between the cells. Gap junction = nexus, it is a
narrow intercellular gap between the cells bridged
by tiny tubular bonds for communicating junction.
OCCLUDING JUNCTIONS
1. They bind cells together and maintain integrity of epithelial cells
as a barrier. They separate apical and basolateral domains of the
cells.
2. They have two main functions:
- prevention of diffusion of molecules between adjacent cells,
thereby contributing to the barrier function of the epithelial cells in
which they are present,
- prevention of lateral migration of specialized cell membrane
proteins, thereby deliniating and maintaining speciailized cell
membrane domains.
3. They are particulaly well developed in the small bowel, where they:
- prevent digested molecules form passing between the cells,
- confine specialized areas of the cell membrane involved in
absorption or secretion to the luminal site of the cell.
4. They are important in cells that actively transport substances
(esp. ions) against concentration gradient where they prevent
back diffusion of the transported substances.
OCCLUDING JUNCTIONS
Intercellular
sealing apical
space
strands membrane
CM
CM
600 nm
a)
intermembranous proteins
b)
Occluding junctions are particularly evident between epithelial cells
that have secretory or absorptive roles. A) A collar of occluding junction is
present between each cells, sealing individual cells into a tight barrier. The
intramembanous proteins that form these junctions are arranged as
serpentine intertwining lines (sealing strands) which stitch the membrane of
adjacent cells together. At the fusion sites, claudins or occludins,
transmembrane junctional proteins, of the two membranes bind to each
other, thus forming a seal occluding the intercellular space.
On EM (B) it is seen as an area of close apposition of adjacent areas of cell.
O
A
D
D
A junctional complex between the
enterocytes (Stevens)
In the occluding junctions (O) the
membranes of the adjoining cells
approximate each other, their outer leaflets
fuse, then diverge, then fuse again several
times within a distance of 0.1- 0.3 mcm.
The tightness of the occluding junction is
related to the number of sealing strands
present between the cells.
A junctional complex is commonly
seen toward the apex of cuboidal and
columnar cells. Immediately below the cell
apex are occluding junction (O) is followed
by an adherent junction (A) and below this
by desmosomes (D). Such complexes are
well developed in the intestine, in other
place they are not common.
The classic junctional complex = O + A + D
ANCHORING (ADHESIVE)
JUNCTION
Cytoskeletal
filaments
of
adjacent
cells are joined through
intercellular link proteins,
which
attach
the
filaments
to
transmembrane
link
proteins
(namely,
cadherins). These can
then interact with similar
proteins
on
adjacent
cells. The extracellular
interaction
may
be
mediated by additional
extracellular proteins or
ions, such as Ca++.
Different link proteins and
transmembrane proteins
operate for the different
classes of junction.
cell membrane
intracellular
link
protein
cytoskeletal
filament
extracellular
accessory
link proteins
or ions
transmembrane
link proteins
intercellular
space
cytosol
ANCHORING
JUNCTIONS:
adherens, fascia adherens,
adherens = desmosome
Zonula
macula
1. They link the cytoskeleton of cells both
to each other and to underlying tissues.
2 They provide mechanical stability to
groups of epithelial cells so that they can
function as cohesive unit.
3. They are most common toward the
apex of adjacent columnar and cuboidal
epithelial
cells,
where
they
link
submembranous actin bundles into a so
called actin belt. They are visible by LM as
an eosinophilic band (the terminal bar).
4. Actin fibers in the adjacent cells are
linked by actin-binding proteins (alphaactinin and vinculin) to a transmembrane
protein, which is one of a group of cell
surface glycoproteins mediating cell
adhesion (cadherins). The type in
adjacent junctions is E-cadherins, which
links cells in the presence of Ca++.
zonula
occludens
zonula
adherens
macula
adherens
Junctional complex, TEM, large
magnification (Fawcett, 1981).
1)
2)
3)
1)
2)
Zonulae adherentes:
are belt-like junctions that assist adjoining cells to adhere to one
another,
they are located just basal to the zonulae occludentes and also
encircle the cell,
the intercellular space of 15-20 nm between the outer leaflets of the
two adjacent cell membranes is occupied by the extracellular
molecules of cadherins. These Ca++-dependent integral proteins of
the cell membrane are transmembrane linker proteins.Their
intracytoplasmic aspect binds to a specialized region of the cell
web, specially a bundle of actin filaments that run parallel to and
along the cytoplasmic aspect of the cell membrane. The actin
filaments are attached to each other and to the cell membrane by
vinculin and alpha-actinin. The extracellular region of the cadherins
of one cell forms bonds with those of the adjoining cells
participating in the formation of zonula adherens. Then this junction
not only joins the cell membranes, but also links the cytoskeleton of
the two cells via the transmembrane linker protein.
Fascia adherens:
is similar to zonula adherens but does not go around the entire
circumference of the cell. Instead of being belt-like it is “ribbon-like”.
Cardiac muscle cells are attached to each other at their longitudinal
terminals via the fascia adherens.
Desmosome. TEM, large magnification (Fawcett, 1981).
Desmosome (= macula
adherens) (arrows) is
the last of the three
components
of
the
junctional complex.
They
are
weld-like
junctions
along
the
lateral cell membrane
that help to resist
shearing forces. They
are presnt in both
simple and stratified
epithelia (epidermis).
intercellular
space
CM
CM
P
transmembrane
proteins
(desmogleins)
intracellular
CF
plaque of
desmoplakin
intermediate filaments
anchored to plaque
a)
cell membrane
X
CF
CM
CM
b)
a) Each desmosome consists of an intracellular plaque composed of several
link proteins (the main types being desmoplakins), into which cytokeratin
intermediate filaments (tonofilaments) are inserted. The cell adhesion is
mediated by transmembrane protein called desmogleins (Stevens).
b) The disc-shaped adhesive plaques (P) in adjacent cells are seen as electrondense areas into which cytokeratin filaments (CF) are inserted. The cell
membranes (CM) between adhesion plaques are about 30 nm apart and there
may be an electron-dense band between cells in some desmosomes (X).
Desmosome. TEM, large
magnification. (Fawcett, 1981).
The dense accumulation of the
intracellular
intermediate
filaments (arrow) inserting into
the attachment plaque of each
cell is visible.
Disk-shaped attachment plaques
are located opposite each other on the
cytoplasmic
aspect
of
the
plasma
membranes of adjacent cells. Each plaque
is composed of a series of attachment
proteins, the best characterized of which are
desmoplakins and pakoglobins.Cytokeratin
filaments are observed to be insert into the
plaque, where they make a hairpin turn, then
extend back out into cytoplasm. These
filaments are responsible for dispersing of
shearing forces on the cell.
In the region of the opposing
attachment plaques, the intercellular space
is up to 30 nm in width and contain
filamentous material with a thin dense
vertical line located in the middle of the
intercellular space. It is formed of
desmoglein, the extracellular component of
the Ca-dependent transmembrane linker
protein of the cadherin family. The
cytoplasmic aspects of transmembrane
linker proteins bind to desmoplakins and
plakoglobins constituting the plaque.
2-4 nm
adjacent cell
membrane
connexon
pore 1.5 mcm
diameter
GAP JUNCTION (Stevens)
Communication junction allow
direct cell-cell communication.
Communication
(gap)
junctions allow selective diffusion
of molecules between adjacent
cells and facilitate direct cell to
cell communication.
Gap junctions are usually
present at relatively low density in
most adult epithelia, but are found
in in large number during
embryogenesis,
when
they
probably have a role in the spatial
organization of developing cells.
Gap junctions are also important
in cardiac and smooth muscle
cells where they pass signals
involved in contraction from one
cell to another.
Each of the gap junctions is a circular patch studded with several
hundred pores, formed by the protein subunits traversing the cell membranes and
termed a connexon. Pores on adjacent cells are aligned allowing small molecules
to move between cells.
Gap junctions:
1) also called nexus of communicating junctions are
regions of intercellular communication,
2) they are widespread in the epithelia, as well as in
cardiac muscle, smooth muscle and neurons. The
intercellular cleft at the gap junction is only 2-3 nm,
3) are built by six closely packed transmembrame
proteins (connexins) that assemble to form a structure
called connexons, aqueous pores, through the plasma
membrane extending into the intercellular space. The
two connexons fuse, forming the junctional
intercellular communication channel. With a diameter
of 1.5-2 nm the hydrophilic channels allow the passage
of ions, amino acids, cAMP, molecules smaller than 1
kD in weight, and certain hormones,
4) are controlled by pH and Ca++ concentration. They
close in low pH and high Ca++ concentration, and visa
versa.
Exocrine Glands
• When the cells release their secretary products, the
membranes of secretory granules fuse with the cell
membrane and the granular contents spill out of the cell
in the process called exocytosis.
• Exocrine glands release their secretions on the surface
of the body (mammary glands, sweat glands, sebaceous
glands) or in the lumen of the hollow organs (uterus,
stomach, trachea, etc). They contain secretory portions
and excretory ducts.
• On the contrary the endocrine glands secrete their
products directly in bloodstream and contain no
excretory ducts (thyroid gland, adrenal glands, etc).
• In many glands (sweat, lacrimal, mammary glands)
discharge of the secretions is facilitated by the
contraction of the myoepithelial cells.
• Secretions may be serous (parotid gland), mucous
(goblet cells), mixed (submandibular gland) and
sebaceous.
Glands
PANCREAS, H & E
originate from epithelial
cells that leave the
surface
where
they
C
developed
and
A
penetrate
into
the
underlying
tissue,
manufacturing a basal
lamina
around
themselves.
The
D
I
secretory units, along
with their ducts, are the
parenchyma
of
the
gland,
whereas
the
stroma of the gland
represents the elements
of the connective tissue
that invade and support
the parenchyma and
divides parenchyma into
lobules.
A – acini, D – duct, C – stroma, I – pancreatic islet (endocrine part of the gland).
SUBLINGUAL GLAND, H. & E.
*
*
The two large excretory ducts (asterisks) run in the stroma of the
gland. They are lined by stratified epithelium indicating the ectodermal
origin of the gland.
Surface epithelium
single
secretory
cells
secretory cells
Coiled tubular gland
Straight tubular gland
surface epithelium
surface epithelium
single
central
lumen
ionpump
ing
cells
secretory cells
Simple Unbranched Glands
secretory cells
in distal part of
gland
Exocrine Glands may be
classified by the:
1.Number
of
cells
(unicellularmulticellular).
2.Shape of the secretory
portion
(tubular
or
acinar or tubulo-acinar).
3.Structure
of
the
secretory
portion
(branched
or
nonbranched).
4.Structure
of
the
excretory duct (simplecompound. Unbranched
glands
are
always
simple).
5.Mode
of
secretion
(merocrine or apocrine
or holocrine).
6.Nature of secretion
(serous or mucous or
mixed or sebaceous).
Unicellular exocrine
glands
represent
the
simplest form of exocrine
gland.
Goblet cells are
dispersed individually in the
epithelia lining the digestive
and respiratory tract. The
secretions released by these
mucous glands protect the
linings of these tracts. When
stained by H & E, they
remain unstained as mucus
is
stained
neither
by
hematoxylin nor by eosin.
Unicellular glands
are always intraepithelial
while multicellular glands
may be intracellular (urethral
Littre’s
glands)
or
extraepithelial
(most
of
them).
*
*
Goblet cells (*) in the epithelial lining
of the ileum, H & E.
Schematic diagram of the classification of multicellular exocrine glands by
structure. Green reperesents secretory portion, lavender represents the duct
portion of the gland.
secretory
portion
simple tubular
simple branched
tubular
simple coiled
tubular
simple
acinar
simple branched
acinar
duct
compound
tubular
compound
acinar
compound
tubuloacinar
Branched Compound Glands
Branched Compound Gland
minor ducts
lined by ionpumping cells
add fluid &
electrolytes
surface epithelium
main excretory duct
lined by
columnar
epithelial
cells
secretory epithelial cells
around small central
lumen – acinus
myoepithe
-lial cells
expel
secretions
Compound glands are always
branched while branched
glands may be both simple
and compound.
CLASSIFICATION OF GLANDS
Type
Example
Unicellular glands
exocrine
goblet cells
endocrine
APUD-cells of the GIT
Multicellular exocrine glands
secretory sheet
surface epithelium of gastric mucosa
intraepithelial glands
urethral (Littre’s) glands
simple tubular glands
intestinal glands
simple coiled tubular glands
apocrine sweat glands
simple branched tubular glands
esophageal glands
simple branched acinar glands
Meibomian gland
compound tubular glands
glands of gastric cardia
compound tubuloacinar glands
pancreas
compound saccular glands
prostate
MODE OF SECRETION OF THE EXOCRINE GLANDS
exocrine
merocrine
apocrine
endocrine
holocrine
Secretion of cell produce may occur by:
- exocytosis from the cell apex into a lumen (merocrine secretion),
- pinching off of apical cell cytoplasm containing cell product (apocrine
secretion),
-shedding of the whole cell containing the cell product (holocrine
secretion); or
-endocytosis from the cell base into the blood stream (endocrine
secretion).
Modes of Secretion
merocrine
holocrine
disintegrating cell
and its
contents
(secretion)
apocrine
secretion
intact cell
new cell
portion of
the cell
(secretion)
In apocrine glands (lactating mammary glands, some sweat glands) a small
portion of apical cytoplasm is released along with a secretory product. In
holocrine glands (e.g. sebaceous glands) as a secretory cell matures, it dies
and becomes the secretory product.
microvilli
Substances secreted by most of the exocrine
glands may be described as mucous, serous
and mixed (both) types.
1) Mucous glands secrete mucinogens, large
glycosylated proteins that upon hydration
swell to become a thick, viscous, gel-like theca
protective lubricant known as mucin, a major
component of mucus. Examples: goblet cells
mucino
-gen
(unicellular glands) and the minor salivary
dropglands of the tongue and palate.
lets
2) Serous glands, such as pancreas and
parotid gland, secrete an enzyme-rich watery
fluid.
3) Mixed glands contain acini (secretory
units) that produce mucous secretions as
nucleus
well as acini that produce serous secretion. stem
In addition, some of their mucous acini
possess serous demilunes, a group of cells
that secrete a serous fluid. Examples:
submandibular and sublingual salivary
A goblet cell containing
glands.
tightly packed secretory
4) Sebaceous glands secrete sebum (skin
granules in the theca.
sebaceous glands, meibomian glands). They
are always holocrine glands.
sG
BC
HF
Sebaceous glands
Sebaceous glands (sG) are branched,
acinar holocrine glands, which
produce an oily sebum. The secretion
of these glands is delivered into the
lumen of a hair follicle (HF) with
which
sebaceous
glands
are
associated. Basal cells (BC), located
at the periphery of the gland,
undergo mitotic activity to replenish
the dead cells, which, in holocrine
glands, become the secretory
product. Note that as these cells
accumulate
sebum
in
their
cytoplasm, they degenerate, as
evidenced by the gradual pyknosis of
their nuclei (arrow).
PAROTID (SEROUS) SALIVARY GLAND
intercalated
duct
serous
acini
striated
duct
Parotid gland is a purely serous gland with merocrine type of secretion. All the
cells of the secretory portions (acini) are serocytes, they are stained light
basophylic. Intercalated ducts are the smallest excretory ducts inserted into
the acini. They fuse together to form larger straied ducts.
SUBLINGUAL (MIXED) SALIVARY GLAND
mucus
mucous
acini
serous
acini
serous
demilunes
intercalated
duct
striated
duct
Sublingual gland is a compound merocrine gland with the two types of acini:
purely serous and mixed (with mucocytes surrounded by the serous
demilunes).
serous cell
myoepithelial cell
intercalated
duct cell
serous acinus
mucous acinus
intercalated duct
striated duct
.
serous
demilunes
mucous
cell
striated
duct cell
SALIVARY
(SUBMANDIBULAR)
GLAND
Acini of many
multicellular
exocrine glands
possess
myoepithelial
cells that share
the
basal
lamina of the
acinar
cells.
They are of
epithelial origin
though
have
some
characteristics
of the snooth
muscle
cells
(contractility).
ENDOCRINE GLANDS
1. Endocrine glands do not contain ducts, and their secretory
products are thus released directly into the bloodstream or
lymphatic system.
2. The endocrine system consists of several glands, composed of
islands of secretory cells of epithelial origin (like thyroid or
adrenal glands with follicular or cluster cell arrangement), as well
as of isolated groups of cells within certain organs (like pancreas
or gonades), and individual cells scattered among parenchymal
cells of the body (like APUD-cells of the gastro-intestinal tract).
Parathyroid gland with clusters of
endocrine cells around the blood
capillaries. C – chief endocrine cells,
O – oxyphil cells.