Tissues of the Human Body

TISSUES OF THE HUMAN BODY

TISSUE

A tissue is a group of cells that have similar structure and work together to perform a specific function.
Different tissues may be:

• Hard (e.g., bone)
• Semisolid (e.g., fat)
• Liquid (e.g., blood)

Tissues combine to form organs such as the stomach, heart, lungs, and brain.

The study of tissues is called Histology.

Types of Tissue

The human body contains four main types of tissues, each performing unique roles:

These four tissues form the foundation of all organs in the body.

I. EPITHELIAL TISSUE

Epithelial tissue covers:

• Body surfaces
• Internal organs
• Body cavities
• Blood vessels
• Glandular structures

Thus, epithelial tissue acts as a protective covering and also forms the functional units of glands.

Characteristics of Epithelial Tissue

1. Cells form continuous sheets
   – Tightly packed like tiles, with very little space between them.
2. Apical Surface
   – The top (free) surface faces a cavity or the outside of the body.
3. Basement Membrane
   – The lower surface attaches to underlying connective tissue.
4. Avascular
   – Contains no blood vessels; nutrients diffuse from nearby connective tissue.
5. Rapid regeneration
   – Repairs and replaces cells quickly due to constant wear and tear.

Functions of Epithelial Tissue

• Protects the body from friction, dehydration, and injury
• Allows selective exchange of chemicals
• Secretes hormones into the bloodstream
• Produces sweat, mucus, enzymes, and other secretions

TYPES OF EPITHELIAL TISSUE

Epithelial tissues are classified based on:

1. Based on Arrangement of Layers

                   i)        Simple epithelium

                   ii)      Stratified epithelium

i)                  Simple Epithelium – Types

• Simple squamous epithelium
• Simple cuboidal epithelium
• Simple columnar epithelium
• Simple ciliated epithelium
• Glandular epithelium

Glandular Epithelium – Types

·         Unicellular glands

·         Multicellular glands

o    Exocrine glands

o    Endocrine glands

ii)               Stratified Epithelium – Types

• Stratified squamous epithelium
• Keratinised stratified squamous epithelium
• Non-keratinised stratified squamous epithelium
• Stratified cuboidal epithelium
• Stratified columnar epithelium
• Transitional epithelium
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2. Based on Shape of Cells

• Squamous epithelium (flat)
• Cuboidal epithelium (cube-like)
• Columnar epithelium (tall)
• Ciliated epithelium
• Glandular epithelium

SIMPLE EPITHELIUM

Cells are arranged in a single layer.
Their thinness makes them ideal for absorption, filtration, secretion, and diffusion.

Types of Simple Epithelium

1. Simple Squamous Epithelium

• Single layer of flat, thin cells
• Allows easy diffusion of gases and nutrients

Location:

• Air sacs of lungs
• Lining of blood & lymph vessels
• Heart lining

Function:

• Lubrication
• Diffusion
• Filtration

2. Simple Cuboidal Epithelium

• Single layer of cube-shaped cells
• Often involved in secretion and absorption

Location:

• Kidney tubules
• Small gland ducts

Function:

• Secretion
• Absorption

3. Simple Columnar Epithelium

• Tall, pillar-shaped cells
• May contain microvilli or goblet cells (mucus-secreting)

Location:

• Digestive tract lining (non-ciliated)
• Uterine tubes, uterus, bronchi (ciliated)

Function:

• Absorption
• Secretion of mucus and enzymes

4. Simple Ciliated Epithelium

• Columnar cells with cilia on the free surface
• Cilia move substances like mucus or ovum

Location:

• Trachea, upper respiratory tract
• Fallopian tubes
• Spinal cord canal

Function:

• Moves mucus or reproductive cells

5. Glandular Epithelium

• Specialized for secretion
• Made of cuboidal or columnar cells containing secretory granules

Types of Glands:

Exocrine glands: Secrete enzymes, sweat, saliva
Endocrine glands: Secrete hormones directly into blood

STRATIFIED EPITHELIUM

Also called compound epithelium.
It has many layers of cells and mainly provides protection.

It usually lacks a distinct basement membrane due to multiple layers.

Types of Stratified Epithelium

1. Stratified Squamous Epithelium

Has many layers:

• Bottom layers → cuboidal/columnar
• Top layers → flattened (squamous)

Two forms:

a) Keratinised Stratified Squamous Epithelium

• Contains keratin, a tough waterproof protein
• Found on dry surfaces exposed to friction

Location: Skin, hair, nails

b) Non-keratinised Stratified Squamous Epithelium

• Moist surfaces
• Protects from friction and prevents drying out

Location:

• Mouth
• Pharynx
• Esophagus
• Vagina
• Conjunctiva

Function (both forms):

• Protection from wear and tear

2. Stratified Cuboidal Epithelium

• Two or more layers
• Apical layer contains cuboidal cells

Location:

• Sweat gland ducts
• Male urethra
• Uterus and anus

Function:

• Protection
• Secretion
• Absorption

3. Stratified Columnar Epithelium

• Several layers
• Apical layer contains columnar cells

Location:

• Large excretory gland ducts
• Conjunctiva of the eye
• Parts of urethra

Function:

• Protection
• Secretion

4. Transitional Epithelium

• Cells are pear-shaped and stretchable
• Looks cuboidal when relaxed but squamous when stretched

Location:

• Urinary bladder
• Ureters

Function:

• Allows stretching
• Protects underlying tissues

II. CONNECTIVE TISSUE

Connective tissue is the most abundant and widely distributed tissue in the human body.
It supports, connects, and binds various organs and tissues together.

Examples include: cartilage, bone, adipose tissue, areolar tissue, and blood.

Connective tissue contains:

• Cells
• Fibres
• Ground Substance (Matrix)

These three components give strength, elasticity, and support to the tissue.

Functions of Connective Tissue

1. Provides mechanical support to organs
2. Connects and binds tissues
3. Acts as a medium for exchange of nutrients and waste
4. Stores energy (fat tissue)
5. Provides insulation
6. Offers defence – barriers, antibodies, phagocytosis

Composition of Connective Tissue

Connective tissue has fibres, cells, and ground substance.

FIBRES

1. Collagen Fibres

• Thick & strong
• Provide tensile strength
• Most abundant fibre in connective tissue

2. Elastic Fibres

• Made of elastin
• Stretch and recoil easily
• Found in skin, lungs, arteries

3. Reticular Fibres

• Thin, branched fibres
• Form delicate networks
• Provide support in soft organs (spleen, lymph nodes)

CELLS

1. Fibroblasts

• Most common cell
• Produce fibres and ground substance

2. Macrophages

• “Big eaters”
• Perform phagocytosis of foreign bodies and microbes
• Two types:
• Wandering macrophages – move to infection sites
• Fixed macrophages – remain in organs (lungs, spleen)

3. Plasma Cells

• Derived from B-lymphocytes
• Produce antibodies
• Found in respiratory and digestive tract, lymph nodes

4. Mast Cells

• Secrete histamine, important during inflammation
• Dilate blood vessels

TYPES OF CONNECTIVE TISSUE

Connective tissue is broadly classified into:

1. Loose Connective Tissue
2. Dense Connective Tissue
3. Cartilage Tissue
4. Bone Tissue
5. Blood (Fluid Connective Tissue)

1. LOOSE CONNECTIVE TISSUE

Cells and fibres are loosely arranged in a semi-fluid matrix.

a) Areolar Tissue

• Most widely distributed
• Contains all types of fibres and cells
• Fills spaces between organs

Location:
Under the skin, between muscles, around blood vessels and nerves

Functions:

• Strength
• Flexibility
• Support

b) Adipose Tissue

• Made of adipocytes (fat cells)
• Stores fat as a single large droplet

Location:
Under the skin, around heart and kidneys, yellow bone marrow

Functions:

1. Prevents heat loss (insulation)
2. Energy storage
3. Protection of organs
4. Gives shape to body

c) Reticular Tissue

• Contains reticular fibres and reticular cells
• Forms a supportive network (stroma)

Location:
Liver, spleen, lymph nodes, red bone marrow

Functions:

• Forms structural framework
• Filters worn-out blood cells (spleen)
• Removes microbes (lymph nodes)

2. DENSE CONNECTIVE TISSUE

Contains closely packed fibres and fewer cells.

a) Dense Regular Connective Tissue

• Fibres arranged in a parallel pattern
• Very strong

Location:

• Tendons (muscle → bone)
• Ligaments (bone → bone)

Function:

• Provides strong attachments

b) Dense Irregular Connective Tissue

• Fibres arranged in irregular patterns
• Can withstand tension from multiple directions

Location:

• Dermis of skin
• Organ capsules

c) Elastic Connective Tissue

• Contains freely branching elastic fibres
• Yellowish in appearance

Location:

• Lungs
• Elastic arteries
• Vocal cords
• Bronchial tubes

Function:

• Allows stretching and elasticity of organs

3. CARTILAGE TISSUE

Cartilage is firmer than other connective tissues but more flexible than bone.
Cells called chondrocytes lie in spaces called lacunae.

Types of Cartilage

a) Hyaline Cartilage

• Most common type
• Glassy, bluish-white appearance
• Chondrocytes arranged in small groups

Location:
Nose, trachea, bronchi, rib ends, fetal skeleton

b) Fibrocartilage

• Many collagen fibres
• Very strong and shock-absorbing

Location:
Intervertebral discs, pubic symphysis

c) Elastic Cartilage

• Yellow elastic fibres
• Most flexible type

Location:
External ear, epiglottis

4. BONE TISSUE

Bone is the hardest connective tissue.
Its strength comes from:

• Calcium salts (calcium phosphate & carbonate)
• Collagen fibres

Components of Bone

Bone consists of:

• Periosteum
• Compact bone
• Cancellous bone
• Bone marrow

Microscopic Structure

Functions of Bone

1. Provides structural support
2. Protects organs
3. Stores calcium
4. Produces blood cells in red bone marrow

5. BLOOD (FLUID CONNECTIVE TISSUE)

Blood is considered a connective tissue because it:

• Has cells suspended in a matrix (plasma)
• Transports materials throughout the body

Components of Blood

• RBCs (Erythrocytes)
• WBCs (Leukocytes)
• Platelets
• Plasma (matrix)

Functions

1. Transport of gases, nutrients, hormones
2. Defence against infection
3. Clotting
4. Regulation of body temperature

III. MUSCLE TISSUE

Muscle tissue contains long fibres called myofibrils.
They contract and relax in response to stimulation.

Functions

• Produce movement
• Maintain posture
• Allow locomotion
• Provide protection
• Help adjust body position

TYPES OF MUSCLE TISSUE

1. Skeletal Muscle Tissue

• Long, cylindrical, striated fibres
• Multinucleated
• Under voluntary control

Location: Attached to bones
Function: Movement, posture, facial expressions

2. Cardiac Muscle Tissue

• Striated, branched cells
• Connected by intercalated discs
• Involuntary control

Location: Walls of the heart
Function: Pumps blood throughout the body

3. Smooth Muscle Tissue

• Spindle-shaped cells
• No striations
• Single central nucleus
• Involuntary

Location:

• Walls of blood vessels
• Digestive tract
• Uterus
• Bladder
• Respiratory tract
• Iris of eye

Functions:

• Digestion and movement of food
• Urine flow regulation
• Control of diameter of blood vessels
• Movement of reproductive cells

IV. NERVOUS TISSUE

Nervous tissue is responsible for control and communication in the body.

The basic structural and functional unit is the neuron.

A neuron has three main parts:

1. Cell Body (Soma)

• Contains nucleus
• Controls metabolic activities
• Gives rise to axon and dendrites

2. Axon

• Longest part of neuron
• Carries impulses away from the cell body
• May be covered by:
• Myelin sheath (fatty insulation)
• Neurilemma (outer covering)

Breaks in the myelin sheath are called Nodes of Ranvier.

3. Dendrites

• Short, branched projections
• Carry impulses towards the cell body

Synapse

A synapse is the junction between the axon of one neuron and the dendrite of another.

Location

Brain, spinal cord, peripheral nerves

Functions of Nervous Tissue

1. Generates and transmits nerve impulses
2. Responds to internal and external stimuli
3. Communicates information throughout the body
4. Coordinates body activities
5. Produces neurotransmitters
6. Provides insulation and support