# INTRODUCTION:-

-> Tissue is a group of similar cells that work together to perform a specific functions..

-> The study of tissues is called Histology.

-> Tissues are the secondary level of organisation in the body - above cells but below organs.

# TYPES:-

-> In animals (including humans), there are four basic types of tissues;

1) Epithelial Tissue:

-> Covers and lines body surfaces, organs, and cavities. Acts like a protective sheet.

-> It covers every internal and external surface. Your skin, the inside of your gut, the lining of your lungs.

> Key characterstics :-

- Cells are tightly packed with very little space between them.

- Always rests on a basement membrane (a thin layer of proteins secreted by the cells).

- Has a free surface (apical surface) facing outside lumen, and a basal surface attached to basement membrane.

- Has no blood vessels (avascular) - gets nutrients by diffusion from below.

- Has high regeneration capacity - cells divide rapidly to replace worn-out cells.

- Cells are held together by special junctions: tight junctions, desmosomes, gap junctions.

> Functions :-

i) Protection- Skin epithelium protects against physical injury, pathogens, and water loss.

ii) Absorption- Small intestine epithelium absorbs digested nutrients.

iii) Secretion- Glandular epithelium secretes enzymes, hormones, mucus.

iv) FILTRATION- Kidney tubule epithelium filters blood.

v) Sensation- Specialised sensory epithelium detects touch, smell, taste.

vi) Gas Exchange- Lung alveolar epithelium allows O2 and CO2 to diffuse.

> Classification of Epithelial Tissue:-

(A) Based on Number of Layers:-

a. Simple Epithelium: Only one layer of cells. All cells touch the basement membrane. Thin - good for diffusion/filtration.

b. Stratified Epithelium: Multiple layers. Only bottom layer touches basement membrane. Thick - provides protection.

c. Pseudostratified: Pseudo=false , stratified=multilayered. As the name suggest there is false appearance that the epithelium is multilayered. In fact some cells are short while the others are tall but all the cells rest on basement membrane e.g. epithelial lining of trachea.

(B) Based on Shape of Cells:

i. Squamous: It is one in which cells are flattened , scales like with nuclei raising a bulge in the cell. It allows rapid exchange of substances through it. e.g. epithelial lining of alveoli , capillary walls.

ii. Cuboidal: The one in which cells are cube shaped, like a square where height of the cell is equal to the width of the cell. The nucleus is central and round. Its main function is absorption and secretion. e.g. epithelial lining of follicles of thyroid gland, kidney tubules.

iii. Columnar: The one wherein the height of the cells is greater than the width of the cells. The nuclei is basal and vertically elongated. It is column/ pillar shape. It functions as protection, absorption and secretion. e.g. epithelial lining of fallopian tubes , stomach lining, small intestine.

iv. Transitional: This kind of epithelium lines an organ which is distensible . It can also change shape( stretch).e.g. urinary bladder.

> Special type:-

- Gland is a group of epithelial cells that specialise in secretion. Glands are formed when epithelial cells fold inward during development.

2) Connective Tissue: 

-> As the name suggested it connects, supports different components of our body and binds other tissues together includes bone, blood etc.

-> It is made up of cells, fibres and matrix. Fibroblasts, fibrocytes, macrophages and adipose cells are the example of some connective tissue cells. 

->  Unlike epithelium, connective tissue has lots of space between cells, filled with a material called the extracellular matrix (ECM). Matrix s composed of ground substance and fibres.

> Functions:-

i. Providing support to different parts of our body.

ii. Connects different components of our body.

iii. It is a medium for exchange of nutrients, metabolites and oxygen between blood and cells.

> Key Characteristics:-

- Has cells scattered in an extracellular matrix (ECM) - matrix = ground substance + fibres.

- Usually well vascularised (has blood supply) - exception: cartilage is avascular.

- ECM determines the properties - e.g., hard matrix in bone, fluid in blood, flexible in cartilage.

- Contains a variety of specialised cells: fibroblasts, macrophages, mast cells, adipocytes, etc.

3) Muscle Tissue: 

->  It enables movement of the body and internal organs.

-> Muscle tissue is specialised for contraction. 

-> Muscle cells (also called muscle fibres or myocytes) contain large amounts of the proteins actin and myosin, which slide past each other to shorten the cells- this is how muscles contract.

- There are three types of muscles namely:

 a. Skeletal muscles- they help in locomotion e.g.biceps femoris.

b. Smooth muscles- they are present in the viscera of our body. e.g. muscle in the wall of stomach.

c. Cardiac muscles- it is seen only in the heart.

-> Contraction of this muscle helps in pumping of blood throughout our body.

4) Nervous Tissue: 

-> It consists of central nervous system, peripheral nervous system and autonomic nervous system. It helps in sending different kinds of information to brain and bringing its responses back from brain to the effector organ

-> Nervous tissue makes up the brain, spinal cord, and nerves. It is specialised for receiving, processing, and transmitting electrical signals.

-> It is the fastest-communicating tissue in the body - signals travel at up to 120 m/s.

Eg - Neurons.

"In conclusion, tissues are organized communities of specialized cells that work collaboratively to carry out specific, complex functions essential for the survival of multicellular organisms".