Tissue level of organization
BY: Dr. Tanvi H. Desai
• A tissue is a group of similar cells that usually have a common embryonic
origin and function together to carry out specialized activities.
• Tissues may be hard (bone), semisolid (fat) or even liquid (blood) in their
• Histology is the science that deals with the study of tissues.
• Pathologist is a physician who specializes in laboratory studies of cells and
tissues to help other physicians to make accurate diagnosis.
• He examines tissues for any changes that might indicate disease.
Types of tissues and their origins
• On the basis of structure and function, body tissues can be classified into four
• 1. Epithelial tissue: It covers body surfaces and lines hollow organs, body
cavities, and ducts. Glands are also formed by it.
• 2. Connective tissue: It protects and supports the body and its organs. Various
types of connective tissue bind organs together, store energy reserves as fat,
and help provide immunity to pathogens.
• 3. Muscular tissue: It generates the physical force needed to make body
• 4. Nervous tissue: It detects changes in a variety of conditions inside and
outside the body and responds by generating action potentials (nerve
impulses) that help maintain homeostasis.
• All tissues of the body develop from three primary germ layers that
differentiate in the human embryo called the ectoderm, endoderm and
• Epithelial tissues develop from all three primary germ layers.
• All connective tissue and most muscle tissues derive from mesoderm.
• Nervous tissue develops from ectoderm.
• The subtypes of epithelia include covering and lining epithelia and glandular
• An epithelium consists mostly of cells with little extracellular material
between adjacent plasma membranes.
• The apical, lateral and basal surfaces of epithelial cells are modified various
ways to carry out specific functions.
• Epithelium is arranged in sheets and attached basement membrane.
• Although it is avascular, it has a nerve supply.
• Epithelia are derived from all three primary germ layers and have a high
capacity for renewal.
• Epithelial layers can be simple (one layer) or stratified (several layers).
• The cell shapes may be squamous (flat), cuboidal (cube like), columnar
(rectangular), or transitional (variable).
• Epithelial tissue may be divided into two types.
(1) Covering and lining epithelium forms the outer covering of the skin and
some internal organs.
• It also forms the inner lining of blood vessels, ducts, and body cavities, and
the interior of the respiratory, digestive, urinary, and reproductive systems.
(2) Glandular epithelium makes up the secreting portion of glands such as the
thyroid gland, adrenal glands, and sweat glands.
1. Covering and Lining Epithelium
• According to arrangement of cells into layers and
shapes of the cells, the types of covering and lining
epithelia are as follows:
• A Simple Epithelium
• 1. Simple Squamous Epithelium: It consists of single
layer of flat cells having centrally located nucleus.
• Location: It lines heart, blood vessels, lymphatic
vessels, air sacs of lungs, glomerular (Bowman's)
capsule of kidneys, and inner surface of the
tympanic membrane (eardrum), forms epithelial
layer of serous (lubricating) membranes, such as the
peritoneum (tissue of abdomen wall).
• Function: Filtration (eg., kidneys), diffusion (eg.,
oxygen in lungs) osmosis, and secretion in serous
• 2. Simple Cuboidal Epithelium: It consists of single layer of cube-shaped cells
having centrally located nucleus.
• Location: It covers surface of ovary, lines kidney tubules and smaller duct of
many glands, and make up the secreting portion of some glands like thyroid
gland and the ducts of some glands, eg., the pancreas.
• Function: It performs the function of secretion and absorption.
3. Simple columnar epithelium:
• The cells of simple columnar epithelium appear like columns (taller than they are
wide), with oval nuclei near the base.
• Simple columnar epithelium exists in two forms:
(I) Non ciliatedsimple columnar epithelium
(II) Ciliated simple columnar epithelium.
• I: Nonciliated Simple Columnar Epithelium: It consists of single layer of nonciliated
column like cells with nuclei near base of cells.
• It contains goblet cells (the primary site for nutrient digestion and mucosal
absorption) and cells with microvilli in some locations.
• Location: It lines the gastrointestinal tract, ducts of many glands, and gall bladder.
• Function: Secretion and absorption.
• II. Ciliated Simple Columnar Epithelium: It consists of single layer of ciliated
column like cells with nuclei near base; contains goblet cells in some locations.
• Location: Lines a few portions of upper respiratory tract, uterine (fallopian)
tubes, uterus, some paranasal sinuses, central canal of spinal cord, and
ventricles of the brain.
• Function: Moves mucus and other substances by ciliary action.
• 4. Pseudo-stratified Columnar
Epithelium: It is not a true stratified
tissue, nuclei of cells are at different
levels; all cells are attached to basement
membrane, but not all reach the apical
• Location: Pseudostratified ciliated
columnar epithelium lines the airways of
most of upper respiratory tract;
pseudostratified nonciliated columnar
epithelium lines larger ducts of many
glands, epididymis, and part of male
• Function: Secretion and movement of
mucus by ciliary action.
B. Stratified Epithelium
• 1. Stratified Squamous Epithelium : It consists of several layers of cells; layers
possess cuboidal to columnar cells.
• Squamous cells form the apical layer and several layers deep to it.
• Cells from the basal layer replace surface cells as they are lost regularly.
• Location: Keratinized variety forms superficial layer of skin; nonkeratinized variety
lines wet surfaces, such as lining of the mouth, esophagus, part of the epiglottis,
part of pharynx and vagina, and covers the tongue.
• Function: Protection and limited secretion.
• 2. Stratified Cuboidal Epithelium : It consists of two or more layers of cells in
which the cells in the apical layer are cube shaped.
• Location: Ducts of adult sweat glands and esophageal glands and part of male
• Function: Protection and limited secretion and absorption.
• 3. Stratified Columnar Epithelium : It consists of several layers of irregularly
shaped cells; only the apical layer has columnar cells.
• Location: Lines part of urethra, large excretory ducts of some glands, such as
esophageal glands, small areas in anal mucous membrane, and part of the
conjunctiva of the eye.
• Function: Protection and secretion.
• 4. Transitional Epithelium: Its appearance is variable (transitional); shape of
cells in apical layer ranges from squamous (when stretched) to cuboidal
• Location: Lines urinary bladder and portions of ureters and urethra.
• Function: Permits distention (expansion).
• 1. Functions of Connective Tissue: Connective tissue is one of the most abundant
and widely distributed tissues in the body.
• In its various forms, connective tissue have a variety of functions :-
• It binds together, supports and strengthens other body tissues.
• It protects the internal organs.
• It compartmentalizes structures such as skeletal muscles.
• It serves as the major transport system within the body eg., blood and lymph.
• It is the primary location of stored energy reserves (adipose or fat tissue).
• It is the main source of immune responses.
• 2. General Features of ConnectiveTissue
• Connective tissue consists of two basic elements: extracellular matrix and cells.
• Connective tissue consists of relatively few cells and an abundant extracellular
matrix (ECM) secreted by the widely spaced cells.
• The ECM consists of protein fibres and ground substance (gel-like structure), and
determines the tissue's qualities, eg., stable and flexible in cartilage but hard and
inflexible in bone.
• Connective tissue does not usually occur on free (body) surfaces.
• It has a nerve supply (except for cartilage), and is highly vascular (except for
cartilage, tendons, and ligaments).
3. Classification of Connective Tissues:
Connective tissues may be classified as
I. Embryonic connective tissue
B. Mucous connective tissue
II. Mature connective tissues
A. Loose connective tissue
1. Areolar connective tissue
1. Adipose tissue
1. Reticular connective tissue
B. Dense connective tissue
1. Dense regular connective tissue
1. Dense irregular connective tissue
1. Elastic connective tissue
1. Hyaline cartilage
1. Elastic cartilage
D. Bone tissue
E. Liquid connective tissue
1. Blood tissue
• I. Embryonic connective tissue
A. Mesenchyme: It consists of irregularly shaped mesenchymal (proliferate) cells
embedded in a semi fluid ground substance that contains reticular fibers.
Location: skin and along developing bones of embryo; some mesenchymal cells occur in
adult connective tissue, especially along blood vessels.
Function: Forms all other types of connective tissue.
B. Mucous Connective Tissue (Wharton's jelly): It consists of widely dispersed fibroblasts
(having collagen and fibers) embedded in a viscous, jelly like ground substance that contains
the collagen fibers.
Location: Umbilical cord of fetus.
II. Mature Connective Tissue
• A. Loose Connective Tissue
• 1. Areolar Connective Tissue: It consists of
fibres (collagen, elastic and reticular) and
several kinds of cells (fibroblasts,
macrophages, plasma cells, adipocytes, and
mast cells) embedded in a semifluid ground
• Location: Subcutaneous layer deep to skin;
papillary (superficial) region of dermis of skin;
lamina propria of mucous membranes; and
around blood vessels, nerves, and body
• Function: Strength, elasticity, and support.
• 2. Adipose Tissue: It consists of adipocytes, with a large centrally located droplet off
triglycerides (fats); nucleus and cytoplasm are peripherally located.
• Location: Subcutaneous layer deep to skin, around heart and kidneys, yellow bane marrow,
and padding around joints and behind eyeball in eye socket.
• Function: Reduces heat loss through skin, serves as an energy reserve, supports, and
protects. In newborns, brown adipose tissue (BAT) generates considerable heat that helps
to maintain proper body temperature.
• 3. Reticular Connective Tissue: It is a network of interlacing reticular fibres
and reticular cells.
• Location: Stroma (supporting framework) of liver, spleen, lymph nodes;
red bone marrow, which gives rise to blood cells; reticular lamina of the
basement membrane; and around blood vessels and muscles.
• Function: Forms stroma (net like structure) of organs, binds together
smooth muscle tissue cells, filters and removes worn out blood cells in the
spleen and microbes in lymph nodes.
• B. Dense ConnectiveTissue
• 1. Dense Regular Connective Tissue: Its
extracellular matrix looks shiny white; consists
mainly of collagen fibres arranged in bundles;
fibroblasts present in rows between bundles.
• Location: Forms tendons (attach muscle to bone),
most ligaments (attach bone to bone),and
aponeuroses (sheet like tendons that attach
muscle to muscle or muscle to bone).
• Function: Provides strong attachment between
• 2. Dense Irregular Connective Tissue: It consists
mainly of randomly arranged collagen fibres and
a few fibroblasts.
• Location: Fasciae (tissue beneath skin and around
muscles and other organs), reticular (deeper)
region of dermis of skin, periosteum of bone,
perichondrium of cartilage, joint capsules,
membrane capsules around various organs
(kidneys, liver, testes, lymph nodes), pericardium of
the heart, and heart valves.
• Function: Provides strength.
• 3. Elastic Connective Tissue: It consists mainly of
freely branching elastic fibres; fibrobalsts are
present in spaces between fibres.
• Location: Lung tissue, walls of elastic arteries,
trachea, bronchial tubes, true vocal cords,
suspensory ligament of penis, and ligaments
• Function: Allows stretching of various organs.
• Cartilage consists of a dense (thick) network of collagen fibres and elastic fibers
firmly embedded in chondroitin sulphate, a gel-like component of the ground
• Cartilage can tolerate considerably more stress than loose and dense connective
• The strength of cartilage is due to its collagen fibres, and its resilience (ability to
assume its original shape after deformation) is due to chondroitin sulphate.
• The cells of mature cartilage, called chondrocytes, occur singly or in groups within
spaces called lacunae in the extracellular matrix.
• A covering of dense irregular connective tissue called the perichondrium surrounds
the surface of most cartilage.
• The perichondrium is composed of two layers:
• an outer fibrous layer that consists of collagen fibers, blood vessels, and fibroblasts,
• an inner cellular layer that consists of cells involved in the growth of cartilage.
• Unlike other connective tissues, cartilage has no blood vessels or nerves,
except in the perichondrium.
• Cartilage does not have a blood supply because it secretes an anti-
angiogenesis factor, a substance that prevents blood vessel growth.
• Because of this property, anti angiogenesis factor is being studied as a
possible cancer treatment to stop cancer cells from promoting new blood
vessel growth that supports their rapid rate of cell division and expansion.
• Since cartilage has no blood supply, it heals poorly following an injury.
• 1. Hyaline Cartilage : It consists of a bluish-white, shiny ground substance with
fine collagen fibres and many chondrocytes; most abundant type of cartilage.
• Location: Ends of long bones, anterior ends of ribs, nose, parts of larynx,
trachea, bronchi, bronchial tubes, and embryonic and fetal skeleton.
• Function: Provides smooth surfaces for movement at joints, as well as
flexibility and support.
• 2. Fibrocartilage: It consists of chondrocytes
distributed among bundles of collagen fibers within
the extracellular matrix.
• Location: Pubic symphysis (where hip bones join
anteriorly), intervertebral discs, menisci (cartilage
pads) of knee, and portions of tendons that insert
• Function: Support and fusion.
• 3. Elastic cartilage: It consists of chondrocytes
located in a threadlike network of elastic fibres
within the ECM.
• Location: Lid on top of larynx (epiglottis), part of
external ear (auricle), and auditory (eustachian)
• Function: Gives support and maintains shape.
D. Bone Tissue
• Compact bone: Compact(dense, solid) bone tissue consists of osteons (cylindrical
vascular tunnels formed by an osteoclast-rich tissue- haversian systems) that
contain lamellae, lacunae, osteocytes, canaliculi, and central (haversian) canals.
• By contrast spongy bone an issue consists of thin columns called trabeculae; spaces
between trabeculae are filled with red bone marrow.
• Location: Both compact and spongy bone tissue make up the various parts of bones
of the body.
• Function: Support, protection, storage; houses blood forming tissue; serves as
levers that act with muscle tissue to enable movement.
• E. Liquid ConnectiveTissue:
• 1. Blood: It consists of blood plasma and formed elements: red blood cells
(erythroyctes), white blood cells (leukocytes), and platelets (thrombocytes).
• Location: Within blood vessels (arteries, arterioles, capillaries, venules and
veins) and within the chambers of the heart.
• Function: RBCs transport O2 and some CO2; WBCs carry on phagocytosis
and are involved in allergic reactions and immune system responses;
platelets are essential for the clotting of blood.
• 2. Lymph: It consists of plasma like liquid component and lymphocytes as
• Location: Within lymphatic vessels.
• Function: It is concerned with the defense of the body against infection. It
also transports fats in the form of chylomicrons from intestine to blood
• Muscular tissue consists of fibres that are specialized for contraction.
• It provides motion, maintenance of posture, heat production and protection.
• Based on its location and certain structural and functional features, muscular
tissue is classified into 3 types - skeletal, cardiac, and smooth.
• A. Skeletal Muscle Tissue: It consists of long, cylindrical, striated fibers with
many peripherally located nuclei; voluntary control.
• Location: Usually attached to bones by tendons.
• Function: Motion, posture, heat production and protection.
• B. Cardiac Muscle Tissue: It consists of branched, striated fibers with one or
two centrally located nuclei; contains inserted discs; involuntary control.
• Location: Heart wall.
• Function: Pumps blood to all parts of the body.
• C. Smooth Muscles Tissue: It consists of spindle shaped (thickest in middle
and tapering at both ends), nonstriated fibres with one centrally located
nucleus; involuntary control.
• Location: Iris of the eyes, walls of hollow internal structures such as blood
vessels, airways to the lungs, stomach, intestines, gall bladder, urinary
bladder, and uterus.
• Function: Motion (constriction of blood vessels and airways, propulsion of
food through GI tract, contraction of urinary bladder and gall bladder).
Comparison of different type of muscles:
Location: Usually attached
Walls of heart Walls of visceral
Nature: Under Control of
Under Control of
Not under Control
Cell type: Long cylindrical
fibres arranged in
fibres arranged in
Cell membrane: Distinct double
Fewer than in
Filament fill the
One or two
At centre of each
One oval or rod
At centre of each
Vascular supply Good blood
Fair supply of
Nerve supply Somatic autonomic Autonomic
NERVOUS TISSUE AND ITS FUNCTIONS
• It consist of the two principle kinds of cells: 1) neurons (nerve cells) and 2)
• 1) Neurons: The neurons consists of three basic portion :
• a) Cell body: Cell body contains a nucleolus surrounded by cytoplasm that
includes typical organelles such as lysosomes, mitochondria and Golgi complex.
• In the cytoplasm it also contains the Chromatophilic substance (Nissl bodies)
which is ordinary arrangement of endoplasmic reticulum, the site of protein
synthesis and it also contain neurofibrils which forms the cytoskeleton and
provide the support and shape of the cells.
• b) Dendrites: Dendrites are the receiving or input portion of the neurons. They
are usually short, tapering (narrowing, pointed) and highly branched.
• Usually dendrites are not myelinated. Their cytoplasm contains chromatophilic substance,
mitochondria and other organelles.
c) Axon: It is a long, thin and cylindrical in shape. It is joined with cell body by axon hillock.
• The first portion of axon is known as initial segment where the nerves impulse are arise.
• It also contains mitochondria, microtubules and neurofibrils but no rough endoplasmic
reticulum so it does not synthesize protein.
• Its cytoplasm known as axoplasm which is surrounded by membrane known as axolema.
• The side branch of axon is known as axon collaterals.
• At the end of axon it divides branch like structure known as axon terminals.
• The tip of some axon terminals swell in to bulb shaped known as synaptic end bulbs.
• Classification of neurons:
• According to functional classification it is divided in to:
• i) Sensory neurons or afferent neurons: It transmits nerve impulse from receptors of skin,
sense organ, muscles, and joints into the CNS.
• ii) Motor or Efferent Neurons: It conveys motor nerve impulse from the CNS to the
effectors which may be either muscles or glands.
• According to structural it can be classified in to :
• i) Multi polar neurons: It has several dendrites and one axon. Most neurons of brain and
spinal cord are of this type.
• ii) Bipolar neurons: It has one main dendrites and one axon. It is found in the eye, inner
ear and olfactory areas of the brain.
• iii) Unipolar neurons: It's originated as bipolar neurons in the embryo but during
the development axon and body get fuse into a single process that divides in to
two branch and consist one cell body.
• It is always sensory neurons.
• 2) Neuralgia: Neuroglia or glia fills about half of the CNS.
• Its have the glue like characteristics so it held nervous tissue together.
• Neuroglias are generally smaller than neurons.
• Neuroglia can multiply and divide in the mature nervous systems.
• Classification of Neuroglia: There are mainly six types of Neuroglia in which four
astrocytes, olegodendrocytes, microglia and ependymal cells are found in the CNS.
• While neurolemmocytes (schwann cells) and satellite cells found in peripheral nervous
• i) Astrocytes: They are star shaped. It produces the metabolism of neurotransmitters
maintain the proper balance of K+ for generation of nerves impulse, and participate in
• It forms the blood brain barrier which regulates entry of substance in to the brain.
• ii) Olegodendrocytes: It is the most common glial cells in the CNS. It is smaller
• They coil around neurons and produce supporting structure to the neurons. It
produces protein and lipid covering known as myelin sheath.
• iii) Microglia: It is the small and phagocytic Neuroglia derived from
monocytes. They protect the CNS from the disease by engulfing invading
microbes and clearing away debris from dead cells.
• iv) Ependymal: It is the epithelial cells. The cells have different shaped from
cuboidal to columnar and many are ciliated. Ependymal cells line the fluid
filled ventricles, cavity within the brain and central canal means a narrow
passage from spinal cord. It forms the fluid which is known as cerebrospinal
• b) Neuroglia found in peripheral nervous system:
• i) Neurolemmocytes (schwann cells): Each cell produces myelin sheath
around PNS Neurons.
• ii) Satellite cells: Which supports neurons in ganglia in PNS.
• Myelination: The axons of most mammalian neurons are surrounded by a
multilayered lipids and proteins of Neuroglia and this covering is known as
myelin sheath and the axon with such a covering are said to be a myelinated.
• Whereas those without covering are known as unmyelinated axon.
• The sheath electrically protects the axon of neurons and increases the speed
of nerve impulse conduction.
• Function of Nervous tissue:
• a) Sensory function: It sense certain changes both within body (the internal
environment) such as stretching of your stomach or increase the acidity
and out side the body (the external environment) such as rain drop landing
on your arm or the aroma of rose.
• b) Integrative Function: It analyzes the sensory information, store some
aspect and make some decision regarding appropriate behavior.
• c) Motor function: It may respond to stimuli by initiating muscular
contraction or glandular secretion.
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