Introduction

• The human body has many levels of structural organization. Tissue comes under the category of structural organization.
• Tissues consist of large numbers of the same type of cells and are classified according to the size, shape and functions of their constituent cells.
• The word tissue comes from a form of an old French verb meaning “to weave”. A nonliving material, called the intercellular matrix, fills the spaces between the cells.

Epithelial tissue

• Epithelial tissue form the covering the free surface of the body or lines cavities, hollow organ and tubes.
• They perform vital functions like protection, excretion, glandular secretion and absorption.
• They are arranged on a basement membrane.
• It also found in glands.
• It can be ectodermal, mesodermal or endodermal in origin.

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Epithelial tissue are further classified as:- Simple Epithelium and stratified epithelium

1. Simple epithelium :- It consists of a single layer of identical cells and is divided into three main types- Squamous, Cuboidal and Columnar.
1. It is usually found on absorptive or secretory surfaces, where the single layer enhances these processes and seldom on surfaces subject to stress.
2. The types are named according to the shape of the cells which differs according to their function.

1. Squamous epithelium:-

• Also known as pavement epithelium.
• Consists of single layer of flat cells arrangement edge to edge as in a mosaic.
• The cells fit closely together like paving stones, forming a thin and very smooth membrane across which diffusion occurs easily.
• It forms the lining of the following structures:-
• heart – where it is known as endocardium
• Blood vessels and lymph vessels - where it is known as endothelium.
• Alveoli of the lungs and collecting ducts of nephrons in the kidneys.

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b) Cuboidal epithelium:-

• This consists of cube shaped cells fitting closely together and lying on a basement membrane.
• It forms the walls of kidney tubules and is found in some glands such as the thyroid.
• Cuboidal epithelium is actively involved in secretion, absorption or/ and excretion.

c) Columnar epithelium

• This is formed by a single layer of tall, thin cells on a basement membrane.
• It lines many organs and often has adaptations that make it well suited to a specific function.
• The lining of the stomach is formed from simple columnar epithelium without surface structure.
• The free surface of the columnar epithelium lining the small intestine is covered with microvilli.

d) Ciliated Epithelial tissue

• Cilia or brush like border are present in columnar and Cuboidal epithelium they form a ciliated epithelium.
• Found in your buccal cavity that helps to swallow the food.
• Forms the inner lining of bronchioles also.

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d) Glandular epithelial tissue�

• It is a special type of epithelium makes up the gland in body.
• It secrets enzyme, hormones, fats or mucous.
• The glandular tissue are a mixture of both endocrine (ductless, hormones are secreted into the blood) and exocrine (have ducts, hormones are secreted onto surfaces) glands.

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2) Stratified Epithelium

• Consist of several layer of cells of various shape.
• Locations :- Body surfaces Internal cavities and tubes, Parenchyma of glands.
• Absence of Basement membrane.
• Function:- To protect underlying structures from mechanical wear and tear.
• TYPES:- Stratified Squamous epithelium and transitional epithelium

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Stratified epithelium tissue

1. Stratified Squamous epithelium

• Composed of several layer.
• In the deepest layers, the cells are mainly columnar and as they grow towards the surface, they become flattened and are then shed.
• TYPES:-
• Keratinized stratified epithelium
• Non-Keratinized stratified epithelium

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Keratinized stratified epithelium�

• Found on dry surfaces.
• Example: skin, hair, nail.
• The surface consist of dead epithelial cells that have lost their nuclei and contain the protein keratin
• Examples of keratinized stratified squamous epithelium include epidermis of the palm of the hand and sole of the foot, and the masticatory mucosa.

Non- Keratinized stratified epithelium�

• Protects moist surfaces and prevent them from drying out.
• Example : conjunctiva of eyes, the oral cavity, esophagus, external female genitalia, and vagina.
• Non-keratinized surfaces must be kept moist by bodily secretions to prevent them from drying out.

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b) Transitional Epithelium

• Transitional epithelium is a stratified tissue made of multiple cell layers, where the cells constituting the tissue can change shape depending on the distention in the organ.
• When the organ is filled with fluid, cells on the topmost layer of this epithelium can stretch and appear flattened.
• Alternately, they can also appear cuboidal with a rounded shape when the fluid pressure is low.

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Function of Transitional Epithelium�

• Due to its location in the excretory system, especially in the ureters and urinary bladder, one of the primary functions of this tissue is to be an extremely effective permeability barrier, impenetrable to water and small molecules. 
• The second important function of these cells is to allow the organ to stretch and increase its volume depending on fluid pressure. 

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Connective Tissue�

• Most diverse and abundant of all tissues
• Connective tissue holds cells together and supports the body.
• Made up of cells suspended in a non-cellular matrix.
• The matrix can be liquid, gel-like or solid, all depending on the type of connective tissue.
• For example, the extracellular matrix of bone is a rigid mineral framework. The extracellular matrix of blood is liquid plasma.
• Connective tissues such as bone and cartilage generally form the body's structure.

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