This unit is about tissue which are highly organized units and membrane which are thin pliable layers of epithelial and/or connective tissue.



Cells are highly organized units. But in multicultural organisms, they do not function in isolation. They work together in-group of similar cells called tissue. Tissue is a group of similar cell and their intercellular substance that have a similar embryological origin and function together to perform a specialized activity. A science that deals with the study of a tissue is Histology.The various tissues of the body are classified in to four principal parts according to their function & structure. These are epithelial, connective, muscular, and Nervous tissue.

Epithelial tissue

Epithelial tissues covers body surface, lines body cavity & ducts and form glands. They are subdivided in to:

Covering & lining epithelium

Glandular epithelium

Covering and lining epithelium: it forms the outer covering of external body surface and outer covering of some internal organs. It lines body cavity, interior of respiratory & gastro intestinal tracts, blood vessels & ducts and make up along with the nervous tissue (the parts of sense organs for smell hearing, vision and touch). It is a tissue from which gametes (egg & sperm) develops.

Covering and lining epithelium are classified based on the arrangement of layers and cell shape.

According to the arrangement of layers covering and lining epithelium is grouped in to:

Simple epithelium: it is specialized for absorption, and filtration with minimal wear & tear. It is a single layered

Stratified epithelium, it is many layered and found in an area with high degree of wear & tear.

Pseudo-stratified, is a single layered but seam to have many layer.

Based on the cell shape covering and lining epithelium is grouped in to:

Squamous: – flattened & scale like

Cuboidal: – cube shaped

Columnar: – tall & cylindrical

Transitional: – combination of cell shape found where there is a great degree of distention or expansion, these may be cuboidal to columnar, cuboidal to polyhydral and cuboidal to Squamous, Therefore considering the number of layers and cell shape we can classify covering and lining epithelium in to the following groups:

Simple epithelium

Simple – Squamous epithelium, contain single layer of flat, scale like resemble tiled floor. It is highly adapted to diffusion, osmosis & filtration. Thus, it lines the air sacs of lung, in kidneys, blood vessels and lymph vessels.

Simple – cuboidal epithelium, Flat polygon that covers the surface of ovary, lines the anterior surface of lens of the eye, retina & tubules of kidney.

Simple – columnar epithelium, Similar to simple cuboidal. It is modified in several ways depending on location & function. It lines the gastro-intestinal tract gall bladder, excretory ducts of many glands. It functions in secretions, absorption, protection & lubrication.

Stratified epithelium

It is more durable, protects underlying tissues form external environment and from wear & tear.

Stratified Squamous epithelium: In this type of epithelium, the outer cells are flat. Stratified squamous epithelium is subdivided in to two based on presence of keratin. These are Non-Keratnized and Keratinized stratified squamous epithelium. Non-Keratnized stratified squamous epithelium is found in wet surface that are subjected to considerable wear and tear. Example: – Mouth, tongue and vagina. In Keratinized, stratified squamous epithelium the surface cell of this type forms a tough layer of material containing keratin. Example: skin. Keratin, is a waterproof protein, resists friction and bacterial invasion.

Stratified cuboidal epithelium, rare type of epithelium. It is found in seat glands duct, conjunctiva of eye, and cavernous urethra of the male urogenital system, pharynx & epiglottis. Its main function is secretion.

Stratified columnar epithelium, uncommon to the body. Stratified columnar epithelium is found in milk duct of mammary gland & anus layers. It functions in protection and secretion.

Transitional epithelium

The distinction is that cells of the outer layer in transitional epithelium tend to be large and rounded rather than flat. The feature allows the tissue to be stretched with out breakage. It is found in Urinary bladder, part of Ureters & urethra.

Pseudo stratified epithelium

Lines the larger excretory ducts of many glands, epididymis, parts of male urethra and auditory tubes. Its main function is protection & secretion

Glandular Epithelium

Their main function is secretion. A gland may consist of one cell or a group of highly specialized epithelial cell. Glands can be classified into exocrine and endocrine according to where they release their secretion.

Exocrine: Those glands that empties their secretion in to ducts/tubes that empty at the surface of covering. Their main products are mucous, oil, wax, perspiration and digestive enzyme. Sweat & salivary glands are exocrine glands.

Endocrine: They ultimately secret their products into the blood system. The secretions of endocrine glands are always hormones. Hormones are chemicals that regulate various physiological activities. Pituitary, thyroid & adrenal glands are endocrine.


Simple Squamous Epithelium

Stratified Squamous Epithelium

Cuboidal Epithelium

Columnar Epithelium

Simple Columnar Epithelium

Pseudostratified Columnar Epithelium

Transitional Epithelium

Classification of exocrine glands

They are classified by their structure and shape of the secretary portion. According to structural classification they are grouped into:

Unicellular gland: Single celled. The best examples are goblet cell in Respiratory, Gastrointestinal & Genitourinary system.

Multicultural gland: Found in several different forms

By looking into the secretary portion exocrine glands are grouped into

Tubular gland: If the secretary portion of a gland is

Acinar gland: If the secretary portion is flask like.

Tubulo-acinar: if it contains both tubular & flask shaped secretary portion

Further more if the duct does not branch it is referred as a simple gland and if it branch’s it is compound gland. By combining the shape of the secretary portion with the degree of branching of the duct of exocrine glands are classified into



Simple tubular Branched tubular Coiled tubular Acinar Branched Acinar





 Connective tissue

Connective tissues of the body are classified into embryonic connective tissue and adult connective tissue.

Embryonic connective tissue

Embrayonic connective tissue contains mesenchyme & mucous connective tissue. Mesenchyme is the tissue from which all other connective tissue eventually arises. It is located beneath the skin and along the developing bone of the embryo. Mucous (Wharton’s Jelly) connective tissue is found primarily in the fetus and located in the umbilical cord of the fetus where it supports the cord.

Adult connective tissue

It is differentiated from mesenchyme and does not change after birth. Adult connective tissue composes connective tissue proper, cartilage, osseous (bone) & vascular (blood) tissue Connective tissue proper, connective tissue proper has a more or less fluid intercellular martial and fibroblast. The various forms of connective tissue proper are:

Loose (areolar) connectives tissue, which are widely distributed and consists collagenic, elastic & reticular fibers and several cells embedded in semi fluid intercellular substances. It supports tissues, organ blood vessels & nerves. It also forms subcutaneous layer/superficial fascia/hypodermis.

Adipose tissue: It is the subcutaneous layer below the skin, specialized for fat storage. Found where there is loose connective tissue. It is common around the kidney, at the base and on the surface of the heart, in the marrow of long bone, as a padding around joints and behind the eye ball. It is poor conductor of heat, so it decrease heat loss from the body Dense (Collagenous) connective tissue: Fibers are closely packed than in loose connective tissue. Exists in areas where tensions are exerted in various directions. In areas where fibers are interwoven with out regular orientation the forces exerted are in many directions. This occurs in most fascia like deeper region of dermis, periosteum of bone and membrane capsules. In other areas dense connective tissue adapted tension in one direction and fibers have parallel arrangement. Examples are tendons and ligaments. Dense connective tissues provide support.

protection and connect muscle to bone.

Elastic connective tissue: Posses freely branching elastic fibers. They stretch and snap back in to original shape. They are components of wall of arteries, trachea, bronchial tubes & lungs. It also forms vocal cord. Elastic connective tissue allows stretching, and provides support & suspension.

Reticular connective tissue: Lattice of fine, interwoven threads that branch freely, forming connecting and supporting framework. It helps to form a delicate supporting storma for many organs including liver, spleen and lymph nodes. It also helps to bind together the fibers (cells) of smooth muscle tissue.


Unlike other connective tissue, cartilages have no blood vessels and nerves. It consists of a dense network of collagenous fibers and elastic fibers firmly embedded in chondriotin sulfate. The strength is because of collagenous fibers. The cells of a matured cartilage are called chondrocyte. The surface of a cartilage is surrounded by irregularly arranged dense connective tissue called perichondrium. Cartilages are classified in to hyaline, fibro & elastic cartilage.

Hyaline cartilage is called gristle, most abundant, blue white in color & able to bear weight. Found at joints over long bones as articlar cartilage and forms costal cartilage (at ventral end of ribs). It also forms nose, larynx, trachea, bronchi and bronchial tubes. It forms embryonic skeleton, reinforce respiration, aids in free movement of joints and assists rib cage to move during breathing.

Fibro cartilage: they are found at the symphysis pubis, in the inter-vertebral discs and knee. It provides support and protection.

Elastic cartilage: in elastic cartilage the chondrocyte are located in thread like network of elastic fibers. Elastic cartilage provides strength and elasticity and maintains the shape of certain organs like epiglottis, larynx, external part of the ear and Eustachian tube.

Osseous tissue (Bone)

The matured bone cell osteocytes, embedded in the intercellular substance consisting mineral salts (calcium phosphate and calcium carbonate) with collagenous fibers. The osseous tissue together with cartilage and joints it comprises the skeletal system.

Vascular tissue (Blood tissue)

It is a liquid connective tissue. It contains intercellular substance plasma. Plasma is a straw colored liquid, consists water and dissolved material. The formed elements of the blood are erythrocytes, leukocytes and thrombocytes. The fibrous characteristics of a blood revealed when clotted.

Muscle tissue

Muscle tissue consists of highly specialized cells, which provides motion, maintenance of posture and heat production. Classification of muscles is made by structure and function. Muscle tissues are grouped in to skeletal, cardiac and smooth muscle tissue.

Skeletal muscle tissue are attached to bones, it is voluntary, cylindrical, multinucleated & striated

Cardiac muscle tissue: It forms the wall of the heart; it is involuntary, uni-nucleated and striated.

Smooth muscle tissue: located in the wall of hallow internal structure like Blood vessels, stomach, intestine, and urinary bladder. It is involuntary and non-striated.

Nervous tissue

Nervous tissue contains two principal cell types. These are the neurons and the neuroglia. Neurons are nerve cells, sensitive to various stimuli. It converts stimuli to nerve impulse. Neurons are the structural and functional unit of the nervous system. It contains 3 basic portions. These are cell body, axons and dendrites. Neuroglias are cells that protect, nourish and support neurons. Clinically they are important because they are potential to replicate and produce cancerous growths.


Membranes are thin pliable layers of epithelial and/or connective tissue. They line body cavities, cover surfaces, connect, or separate regions, structures and organs of the body. The three kinds of membranes are mucous, serous and synovial.

Mucous membranes (mucosa) lines body cavity that opens directly to the exterior. It is an epithelial layer. Mucous membranes line the entire gastro intestine, respiratory excretory and reproductive tracts and constitute a lining layer of epithelium. The connective tissue layer of mucous membrane is lamina propra. To prevent dry out and to trap particles mucous membranes secret mucous.

Serous membrane / serosa: contains loose connective tissue covered by a layer of mesothelium. It lines body cavity that does not open directly to the exterior. Covers the organs that lie with in the cavity. Serosa is composed of parietal layer (pertaining to be outer) and visceral layer (pertaining to be near to the organ). Pleura and pericardium are serous membrane that line thoracic and heart cavity respectively. The epithelial layer of a serious membrane secret a lubricating fluid called serious fluid. The fluid allows organs to glide one another easily.

Synovial membrane: Unlike to other membranes this membrane does not contain epithelium. Therefore, it is not epithelial membrane. It lines the cavities of the freely movable joints. Like serious membrane it lines structures that do not open to the exterior. Synovial membranes secret synovial fluid that lubricate articular cartilage at the ends of bones as they move at joints.



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