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TISSUES

Study of “cells” is known as “cytology”

Study of “tissues” is known as “Histology”

Tissue: a tissue is a collection of cells having similar origin, structure, and function.

These tissues are classified according to the size, shape and functions of these cells. They

include:

I. Epithelial tissue (covering)

II. Connective tissue (support)

III .Muscular tissue (movement)

IV .Nervous tissue (control)

I.EPITHELIAL TISSUE:

This group of tissues is found covering the body and lining cavities, hallows organs and

tubes. It is also found in glands and it consists of cells that are situated very near to each other with

the least inter cellular space (matrix). These are supported by connective tissue. Although epithelium

is innervated (supplied by nerve fibers), it is avascular (contains no blood vessels). Epithelial cells

are nourished by substances diffusing from blood vessels in the underlying connective tissue.

Epithelium has a high regenerative capacity. Epithelial tissues help in

Protection

Absorption

Excretion

Secretion.

Epithelial tissues are classified into two types, simple and stratified epithelium based on

arrangement of cells in number of layers.

Simple epithelium: This type of epithelial tissues having single layer of cells on a basement

membrane. It is usually found on absorptive or secretory surfaces. Different types of simple

epithelial tissues are:

1. Squamous epithelium

2. Cuboidal epithelium

3. Columnar epithelium

4. Ciliated epithelium.

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1. Squamous epithelium: (pavement epithelium)

Description: It is situated on the basement membrane and it consists of single layer of flat cells.

Nucleus is usually placed in the centre. The cells appear like the tiles on the floor hence, it is also

called as pavement epithelium.

Function: Allows passage of materials by diffusion and filtration in sites where protection is not

important.

Location: Lungs, bowman’s capsule, heart, blood vessels and lymph vessels etc... as an endothelium

(endo means inside).

2. Cuboidal epithelium: (Glandular epithelium)

Description: This type of tissue consists of cube-shaped cells with large, spherical central nuclei

fitting closely together lying on a basement membrane.

Functions: It actively involved in secretion, absorption and excretion.

Location: Bronchioles, salivary glands, thyroid gland, kidney tubule etc...

3. Columnar epithelium:

Description: It is composed of single layer of cells, the height of which is more than its breadth.

These are having round to oval nuclei.

Functions: Protection, absorption and secretion.

Location: Most of the digestive tract (stomach to anal canal)

4. Ciliated epithelium:

Description: Cells of ciliated epithelium may be cuboidal or columnar in shape they consists of

about15-25 hair like structures on each cell, these hair like structures are known as cilia.

Functions: Secretion of mucus, enzymes and other substances. Ciliated type cells propel mucus by

ciliary action.

Location: Small bronchi, uterine tubes (fallopian tubes) and some areas of uterus.

2. Stratified epithelium

It is the type of epithelial tissue having more than one layer of epithelial cells of various

shapes. The superficial layers grownup from below. Basement membranes are usually absent. The

main function of stratified epithelium is to protect underlying structures from mechanical wear and

tear. Stratified epithelium is mainly divided into two types:

1. Stratified squamous epithelium 2.

Transitional epithelium

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1. Stratified squamous epithelium: This is again divided into two types based on presence and

absence of keratin. Those are non-keratinized stratified squamous epithelium and keratinized

stratified squamous epithelium.

i. Non-keratinized stratified squamous epithelium:

Description: Cells are arranged in multilayers and are not filled with keratin.

Function: This is found on wet surfaces subjected to wear and tear, provide protection from

drying.

Location: Esophagus, mouth, vagina, conjunctiva of eyes, pharynx.

ii. Keratinized stratified squamous epithelium:

Description: Cells are arranged in multilayers and cells in superficial layers filled with

keratin. These cells are generally dead cells.

Function: These form a tough, relatively waterproof protective layer that prevents drying of

the live cells underneath. The surface layer of skin is rubbed off and is replaced from below layers.

Location: skin, hair and nails.

2. Transitional epithelium:

Description: It consists of 3-4 layers of cells. It is known as transitional because the height of

cells varies with situation. The upper layers of cells are large and flat. Lower layer of cells

are polyhedral cells.

Location: Pelvis of kidney, uterus, urinary bladder and urethra.

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II.CONNECTIVE TISSUE:

Connective tissue is found everywhere in the body Large amount of matrix is present in this

& the cells are relatively widely separated from each other, most of the connective tissues have a

relatively large supply of blood. Its amount in particular organs varies for example skin consists of

primarily of connective tissue while the brain contains very little. The origin of all connective tissues

is mesenchyme (an embryonic tissue). There are four main classes of connective tissue & several sub

classes.

Functions:

1. Binding and support (connective tissue proper)

2. Protection (adipose tissue, blood, bone)

3. Transport (blood)

4. Insulation (Adipose tissue)

Because of its matrix connective tissue is able to bear weight, withstand great tension, and

endure abuses such as physical trauma and abrasion that no other tissue would be able to tolerate.

Structural elements of connective tissue:

1. Extracellular matrix (ground substance)

2. Cells

1. Extra cellular matrix: Ground substance and fibers makeup the extracellular matrix. Ground

substance is the unstructured material that fills the space between the cells and fibers. It is composed

of interstitial fluid (tissue fluid), cell adhesion proteins (fibronectin, laminin) serve mainly as a

connective tissue glue that allows connective tissue cells to attach themselves to matrix elements.

Fibrillin controls the fiber organization.

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The ground substance holds large amounts of fluid and functions as a molecular sieve, or

medium, through which nutrients and other dissolved substances can diffuse between the blood

capillaries and cells.

2. Cells: Cells are embryologically derived from mesoderm. Mesenchymal cells are stem cells in the

adult which retain the capability to differentiate into any cell type derived from mesoderm.

Differentiated cells of connective tissue include:

1. fibrocytes common in many types of connective tissue

2. Chondrocytes in cartilage

3. Osteocytes in bone

4. Adipocytes- fat storing and metabolizing cells

5. Blood cells.

1. Fibrocytes: These are large flat cells with irregular processes. They produce collagen fibers,

elastic fibers and a matrix of extracellular material. Very fine collagen fibers, sometimes called as

reticulin fibers, are found in very active tissues, such as liver, lymphoid tissue.

a) Collagen fibers: (white fibers) collagen is secreted by fibroblasts as the precursor

tropocollagen which polymerizes to form collagen. It occurs in many variant types.

Type I - Provides tensile strength in dense connective tissues of the skin dermis, tendons and

ligaments, also loose supportive tissue and bone.

Type II - Predominant form in hyaline and elastic cartilage.

Type III - Makes up reticular fibers.

b) Elastic fibers: (yellow fibers) Elastin is secreted by fibroblasts and chondroblasts. Elastin

along with glycoprotein fibrillin gives elasticity to supporting tissues. These are found in

arterial walls, dermis of the skin, elastic cartilage, and lung.

Fat cells: (adipocytes) these cells occur singly or in groups in many types of connective tissues.

These are abundant in adipose tissue. They vary size and shape according to the amount of fat they

contain. These acts as storage sites of energy.

Mast cells: Similar to basophils in the blood. These produce histamine and heparin when allergen

binds to it, mediate the inflammatory response.

Plasma cells: A type of lymphocyte, they secrete antibodies. These cells, along with other white

blood cells migrate to connective tissues from the circulation.

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Connective tissue proper

A) Loose connective tissue:

i) Areolar connective tissue: This is the most generalized type of connective tissue. The

matrix is semisolid with many fibroblasts and some fat cells, mast cells and macrophages

widely separated by elastic and collagen fibers. It is found in almost every part of the body.

Functions: 1. Elasticity

2. Tensile strength

3. It connects and supports other tissues, for example

Under the skin

Between muscles

Supporting blood vessels and nerves

In gland’s secretory cells.

ii) Adipose tissue: Adipose tissue is characterized by abundant adipocytes which store

and metabolize fat. It is supported by collagen type I fibers synthesized by fibroblasts and is

highly vascular. It is usually surrounded by collagenous connective tissue which supports and

may divide the adipose tissue into pads which absorb shock (soles of the feet, around

kidneys, in the orbit around the eye).

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There are two main types of adipose tissue:

White adipose tissue

o is the main fat storing tissue(20-25% of total body weight) in the adult where it acts as

an energy reserve

o acts as insulation (under the skin) and as a shock absorber (eg. around kidney)

o is made up of unilobular adipocytes (lipid fuses into 1 large droplet)

o has a widespread distribution

Brown adipose tissue

o contributes to thermoregulation of newborns by producing heat

o is made up of multilobular adipocytes (fat is stored in multiple, small droplets)

o is organized into lobules separated by fibrocollagenous septa

o has a restricted distribution in the newborn and is mostly lost by adulthood

iii) Reticular tissue: (Lymphoid tissue) It forms a delicate supporting framework of reticular

fibres (type III collagen). Reticular tissue is classified as a type of 'loose' connective tissue

due to the sparsity of its fibres. It is found in lymph nodes and all parts of lymphatic system.

B) Dense connective tissue: This contains more fibers and fewer cells than loose connective

tissue.

i) Fibrous tissue: It is found in locations where mechanical support and tensile strength is

required. It is characterized by a dense arrangement of collagen fibers and reduced ground

substance compared to areolar connective tissue. It has collagen bundles whose alignment is

dependent on the direction from which the structure must withstand tension.

o Dense regular connective tissues such as tendons and ligaments have collagen

bundles in ordered, parallel orientations that provide maximum tensile strength.

o Dense irregular connective tissues have interwoven collagen bundles to resist tension

from many directions and provide tough physical support and protection. The dermis

of the skin, as an outer protective covering of some organs like kidney, lymph nodes

and the brain.

ii) Elastic tissue: Elastic tissue is capable of considerable extension and recoil. There are few

cells and the matrix consists of mainly elastic fibers secreted by fibroblasts. It is found in

organs where stretching or alteration of shape is required.

Example: Large blood vessels, trachea, bronchi and lungs.

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1. Cartilage:

Cartilage is a type of connective tissue that is hard but elastic in nature. This tissue is firmer

than other connective tissues. The cells are called chondrocytes and are less numerous. It is

characterized by proteoglycan-containing ground substance complexed to a fine collagen (mainly

Type II) fiber latticework. It acts in a structural fashion in limited sites (eg. ear, trachea) and is a

precursor in bone formation. Varying proportions of collagen and elastic fibers define three main

types:

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i).Hyaline cartilage

ii).Fibro cartilage

iii).Elastic cartilage

i) Hyaline cartilage:

Hyaline cartilage is a smooth bluish-white tissue. Chondrocytes are arranged in small

groups in cell nests. Matrix is solid and smooth.

Functions: It provides:

flexibility

Support

Smooth surfaces for the movement at joints.

Location:

Ends of long bones that form joints.

Coastal cartilages, which attaches ribs to the sternum.

Forming part of pharynx, larynx and trachea.

ii) Fibro cartilage:

This consists of dense masses of white collagen fibers in a matrix similar to that of

hyaline cartilage. It is a tough, slightly flexible, supporting tissue.

Location:

as intervertebral discs

As ligaments joining bones

Between articulating surfaces of the bones of the knee joint (semilunar

cartilage).

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iii) Elastic cartilage: This flexible cartilage consists of yellow elastic fibers lying in a solid

matrix. The chondrocytes lie between the fibers.

Function: It provides support and maintains shape.

Location: Lobe of the ear, epiglottis and tunica media of blood vessel walls.

Bone:

Among all the connective tissues bone is the hardest connective tissue. It contains

extracellular matrix composed of 25% water, 50%mineral salts (mostly calcium and phosphates) and

25% of collagen fibers.

Every bone has a dense outer layer that looks smooth and solid to the naked eye, which is

compact bone. Internal to this is spongy bone (cancellous bone) which contain honey comb of small

needle shaped or flat pieces called trabeculae. In living bones the open spaces between trabaculae

are filled with red or yellow bone marrow.

Diaphysis: (dia = throgh, physis = growth) a tubular diaphysis or shaft forms the long axis of

the bones. It is constructed of a relatively thick compact bone that surrounds a central medullary

cavity. In adults, the medullary cavity contains yellow bone marrow. A storage site for triglycerides.

Epiphysis : (epi = upon, physis = growth) these are the bone ends. Between the diaphysis and

each epiphysis of an adult long bone is an epiphyseal line. The external surface of the entire bone

except the joint surfaces is covered by a glistering white double-layered membrane called the

periosteum.

Microscopy of compact bone:

The structural unit of the compact bone is called osteon or haversian system. Functionally osteons

are tiny weight bearing pillars. Each haversian system consists of haversian canal at the center.

This haversian canal is useful for the entry of blood vessels, nerves etc…

Around the haversian canal concentric circles will present, called lacuna

On the lacuna osteocytes will present

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The space between the two concentric circles is known as lamella (i.e little plate; pleural is

lamellae)

Lamellae give bone its hardness, and collagen fibers, which give bone its strength. The

lamellae are responsible for the compact nature of bone tissue.

In lamella canals like structures will present called as canaleculi (means little canals;

canaliculus is singular).

Canaliculi provide routes for nutrients to reach osteocytes and for wastes to leave them.

Blood:

Blood is fluidized connective tissue. It provides communication between different parts of the body

and with the external environment. Blood involved in transportation, regulation and protection

functions. Blood is a very good suspension, blood cells are suspended in plasma. Different types of

blood cells are red blood cells (R.B.C’s), white blood cells (W.B.C’s) and platelets. Blood contains

red colored pigment called hemoglobin (Hb).

III.Muscular tissue:

Muscle tissue is able to contract

and relax, providing movement

within the body and of the body

itself. Muscle cells possess

myofilaments, elaborate versions of

the actin and myosin filaments that

bring about movement or contraction

in all cell types. Muscle contraction

requires an adequate blood supply to

provide sufficient oxygen, calcium,

nutrients and to remove waste

products. These are three kinds of

muscle tissue. Those are:

1. Skeletal muscle voluntary in nature (under the control of will)

2. Smooth muscle

3. Cardiac muscle involuntary in nature (under the control of A.N.S)

1. Skeletal muscle :

Description: long, cylindrical, unbranched, striated, multinucleated cells.

This muscle is described as ‘skeletal’ because it forms those muscles that

move the bones.

Striations can be seen on microscopic examination, by this it is called as

‘striated’.

These are under the control of will, so these are said to be ‘voluntary’.

Functions: voluntary movement, locomotion and facial expression.

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2. Smooth muscle (visceral muscle)

This is found in visceral organs. These work under the control of autonomic nervous

system (A.N.S) and are involuntary in nature. These cells are spindle shaped cells with

central nuclei and no striations. The nucleus is surrounded by small amount of cytoplasm and

the fibers are not surrounded by any membrane. But several of them are joined together by

loose connective tissue to form sheets.

Functions:

Regulation of diameter of blood vessels

Regulation of diameter of trachea, bronchi and alveoli

Propel substances or objects like food stuffs, urine, glands, baby along internal

passage ways.

Location : mostly in the walls of hallow organs.

3. Cardiac muscle:

Cardiac muscle is found only in walls of the heart. The heart serves as a pumping organ.

Cardiac muscle fibers (cardiac cells) are branched and having striations, involuntary in nature.

Cardiac fibers are cushioned by small amounts of soft connective tissue and arranged in spiral or

number ‘8’ shaped bundles. Branching of cardiac muscle fibers helps in rapid conduction the

exciting electrical impulse across the heart.

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