Histology

Study of cells, tissues and organs as seen with the help of microscope

Histology

• Often called Microscopic Anatomy

• Greek word – Histos= tissueLogia=science/study

• Cells work together in functionally related groups called tissues

4 basic type of tissues:• Epithelial – lining

and covering• Connective –

support • Muscle –

movement• Nervous –

control

Epithelial Tissue or Epithelium

• Consist of sheets of cells• Covers a external surface of the body• May line the internal cavities and the organs• Forms most organs & glands

Special Characteristics of Epithelia

• Cellularity–cells are in close contact with each

other with little or no intercellular space between them

• Specialized contacts: Junctional Complexes–may have junctions for both

attachment and communication

Special Characteristics of Epithelia

• Polarity–epithelial tissues always have an apical

and basal surface• Support by connective tissue –at the basal surface, both the epithelial

tissue and the connective tissue contribute to the basement membrane

• Avascular..No blood vessel, no lymphatics–nutrients must diffuse

Special Characteristics of Epit• Innervated

• Regeneration – epithelial tissues have a high capacity for

regeneration

• Invaginates and grows in the underlying CT..specializing as glands

• Apical surface shows modification: presence of cilia or microvilli

• Nuclear shape corresponds to the cell shape: oval in columnar,round in cuboidal or polyhedral and flat in squamous cells

Functions

• Functions of epithelium– Protection– Absorption, – May secrete material and ion transport– Filtration– Forms slippery surfaces– Function as sensory surfaces

Embryologically

• Epithelia are derived from all the 3 germ layers:

• Ectoderm Epithelium of skin• Endoderm Epithelium of gut• MesodermEpithelium of pericardial,

peritoneal and pleural cavities

Basement Membrane

• All cells rest on it.• Thin, non-cellular• Separates epithelium from underlying connective

tissue• Easily seen with light microscope• Made up of:

Basal Lamina-Amorphous, product of epitheliumReticular Lamina-Reticular fibres, product of CT

Basement Membrane • The epithelial cells lie

on the reticular lamina (collagen – CT)

• Reticular lamina is bound to another CT called areolar CT.

• Together this structure is called the "basement membrane”

Basal Feature: The Basal Lamina• Noncellular supporting sheet between the epithelium

and the connective tissue deep to it • Consists of proteins secreted by the epithelial cells • Functions:– Acts as a selective filter, determining which molecules from

capillaries enter the epithelium – Acts as scaffolding along which regenerating epithelial cells

can migrate • Basal lamina and reticular layers of the underlying

connective tissue deep to it form the basement membrane

Intercellular Junctions ORJunctional Complexes

• Zonula occludens (Tight Junctions)• Zonula adherens• Macula adherens (Desmosome) and

hemidesmosome• Gap junction (Nexus)

Lateral Surface Features

• Factors holding epithelial cells together– Adhesion proteins link

plasma membranes of adjacent cells

– Contours of adjacent cell membranes

– Special cell junctions

Tight junctions (zona occludens)

• – close off intercellular space – Found at apical region of most epithelial types– Some proteins in plasma membrane of adjacent

cells are fused– Prevent molecules from passing between cells of

epithelial tissue…..Serves as a SELECTIVE BARRIER, giving it a sealing effect.

– Example- Intestine and urinary bladder

Adherens junctions (zonula adherens) – anchoring junction

– Transmembrane linker proteins attach to actin microfilaments of the cytoskeleton and bind adjacent cells

– Along with tight junctions, form the tight junctional complex around apical lateral borders of epithelial tissues

• Present just below the tight junctions

• Provides Rigidity to the apex of the cell.

• Presence of dense plaque – like material on the cytoplasmic surface of the plasma membranes of the junction.

Desmosomes (Macula Adherens) Hemidesmosomes

• Gap of 30nm• Transmembrane

Proteins• Electron dense

plaque• Attachment to

Intermediate Filaments

FIRM ADHESION between cellsSubjected to friction,Epidermis of skin.

Desmosomes

• Desmosomes – two disc-like plaques connected across intercellular space– Plaques of adjoining cells

are joined by proteins called cadherins

– Proteins interdigitate into extracellular space

– Intermediate filaments insert into plaques from cytoplasmic side

Desmosomes

Gap junctions (Nexus)

• passageway between two adjacent cells– Let small molecules

move directly between neighboring cells

– Cells are connected by hollow cylinders of protein

– Passage of inorganic ions– Exchange of chemical

messengers in cell recognition and differentiation.

Gap Junction

Tight Junctions

• In the apical • Band or belt• Barrier device

Surface Modifications

• Glycocalyx-rich in polysaccharides

• Concentrates ions prior to absorption

• Act as receptor sites for hormones and enzymes.

• Microvilli- minute finger like projections

• Increase absorptive surface

• Stereocilia – Long thick Microvilli, Non motile, may show branching, Increase surface area( Epididimis), helps perception of stimuli (Internal Ear)

• Cilia- long, hair like projections of plasma membrane

Microvilli and Cilia

• Nonmotile• Contain Microfilaments• Function-Absorption• Intestinal epithelium,

proximal convoluted tubules of the kidney

• Motile• Contain 9+2 pattern of

microtubules• Driving the entangled

particles, transport in one dcirection

• Examples: Respirastory tract,uterine tube and ependyma

Epithelial Tissues

• According to the number of cell layers

• First name of tissue indicates number of layers– Simple – one layer of cells

– Stratified – more than one layer of cells

Classifications & Naming of Epithelia

Classification & Naming of Epithelia• Last name of tissue describes shape of cells– Squamous – cells wider

than tall (plate or “scale” like)

– Cuboidal – cells are as wide as tall, as in cubes

Columnar – cells are taller than they are wide, like columns

Naming Epithelia• Naming the epithelia includes both the layers (first)

and the shape of the cells (second)– i.e. stratified cuboidal epithelium

• The name may also include any accessory structures– Goblet cells– Cilia– Keratin

• Special epithelial tissues (don’t follow naming convention)– Psuedostratified– Transitional

Simple Squamous Epithelium

• Description – single layer of flat cells with disc-shaped nuclei

• Special types – Endothelium (inner covering) • slick lining of hollow organs

– Mesothelium (middle covering) • Lines peritoneal, pleural, and pericardial cavities • Covers visceral organs of those cavities

Simple Squamous Epithelium

• Function – Passage of materials by passive diffusion and

filtration– Secretes lubricating substances in serosae

• Location – Renal corpuscles– Alveoli of lungs – Lining of heart, blood and lymphatic vessels– Lining of ventral body cavity (serosae)

Simple Squamous Epithelium

Simple squamous lining the

walls of the capillary

Simple Cuboidal Epithelium

• Description– single layer of cube-like cells with large, spherical central

nuclei• Function – secretion and absorption

• Location – kidney tubules, secretory

portions of small glands, ovary & thyroid follicles

Simple Columnar Epithelium

• Description – single layer of column-shaped (rectangular) cells

with oval nuclei• Some bear cilia at their apical surface• May contain goblet cells

• Function – Absorption; secretion of mucus, enzymes, and

other substances– Ciliated type propels mucus or reproductive cells

by ciliary action

Simple Columnar Epithelium

• Location – Non-ciliated form • Lines digestive tract,

gallbladder, ducts of some glands

– Ciliated form • Lines small bronchi,

uterine tubes, uterus

Pseudostratified Columnar Epithelium

• Description– All cells originate at basement membrane– Only tall cells reach the apical surface– May contain goblet cells and bear cilia– Nuclei lie at varying heights within cells

• Gives false impression of stratification

• Function – secretion of mucus; propulsion of mucus by cilia

Pseudostratified Columnar Epithelium

• Locations– Non-ciliated type • Ducts of male

reproductive tubes • Ducts of large glands

– Ciliated variety • Lines trachea and most of

upper respiratory tract

Stratified Epithelia

• Contain two or more layers of cells• Regenerate from below• Major role is protection• Are named according to the shape of cells at

apical layer

Stratified Squamous Epithelium

• Description– Many layers of cells – squamous in shape– Deeper layers of cells appear cuboidal or columnar – Thickest epithelial tissue – adapted for protection

Stratified Squamous Epithelium• Specific types – Keratinized – contain the protective protein keratin

• Surface cells are dead and full of keratin

– Non-keratinized – forms moist lining of body openings

• Function– Protects underlying tissues in

areas subject to abrasion

• Location – Keratinized – forms epidermis– Non-keratinized – forms lining of

esophagus, mouth, and vagina

Transitional Epithelium• Description – Basal cells usually cuboidal

or columnar– Superficial cells dome-

shaped or squamous

• Function– stretches and permits

distension of urinary bladder

• Location – Lines ureters, urinary

bladder and part of urethra

Introduction • Histology

• There are (4) types of tissue:– 1. Epithelial – 2. Connective– 3. Muscle – 4. Nervous

• Similarities between tissue types: – 1. All contain cells – 2. Cells that make up tissues

have similar functions

Epithelial Structure

Apical

Basement Membrane

Apical

Basement Membrane • The epithelial cells lie

on the reticular lamina (collagen – CT)

• Reticular lamina is bound to another CT called areolar CT.

• Together this structure is called the "basement membrane”

Classification and Examples 1. Simple Epithelium – Single layer – All cells anchored to

basement membrane

2. Simple Squamous– Kidney – filtration

3. Simple Cuboidal– Kidney tubules– Filtration; secretion,

absorption

Simple Epithelia

4. Simple Columnar– Tall, thin cells – Absorptive cells (small

intestine)– Goblet Cells

5. Pseudostratified ‘Ciliated’ Columnar Epithelium – “Pseudostratified” ? – Trachea – Goblet Cells and Mucus

Stratified Epithelium 1. Characteristics– 2+ layers

2. Stratified Squamous– Skin – outer layer

hardened by ‘keratin’ – 4 to 5 layers thick

3. Stratified Cuboidal – Ducts of sweat glands – This type + stratified

columnar are rare!

MCQ

• Transitional epithelium is found in1. Uterus2. Ureter3. Gall bladder 4. vagina

MCQ

• Stomach is lined by1. Simple columnar epithelium2. Straified squamous epithelium3. Simple cuboidal epithelium4. Pseudostratified columnar epithelium

MCQ

• Simple Squamous epithelium is seen in1. Alveoli of lungs2. Stomach3. Urinary bladder 4. Tongue

MCG

• Glycocalyx coat present in the absorptive surface of small intestine

1.Increases the surface area2.Transports the absorbed material3.Concentrates ions prior to absorption4.Participates in the digestion of carbohydrates

MCQ

• Pseudostratified Epithelium is seen in1. Ureter2.Skin3.Trachea4.Kidney

• Simple squamous epithelium lining of serous membrane is called………..

• Basement membrane is made up of how many layers….

• What are Microvilli?• Desmosomes?