# INTRODUCTION:-

-> Tissue is a group of similar cells that work together to perform a specific function. The study of tissues is called Histology. They 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 also covers every internal and external surface. Your skin, the inside of your gut, the lining of your lungs.

> Key characteristics:-

- Cells are tightly packed with very little space between them. They always rest on a basement membrane (a thin layer of proteins secreted by the cells). It has a free surface (apical surface) facing the outside lumen, and a basal surface attached to the basement membrane. It has no blood vessels (avascular) - gets nutrients by diffusion from below. It has high regeneration capacity - cells divide rapidly to replace worn-out cells. Cells are held together by special junctions: tight junctions, desmosomes, and 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, and mucus.

iv) Filtration- Kidney tubule epithelium filters blood.

v) Sensation- Specialised sensory epithelium detects touch, smell, and 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 the bottom layer touches the basement membrane. Thick - provides protection.

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

(B) Based on the shape of Cells:

i. Squamous: It is one in which cells are flattened, with scales and 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 the 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 the thyroid gland, kidney tubules.

iii. Columnar: The one wherein the height of the cells is greater than the width of the cells. The nuclei are 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 a distensible organ It can also change shape( stretch). e.g. urinary bladder.

> Special type:-

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

2) Connective Tissue: 

-> As the name suggests, 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 examples of connective tissue cells. Unlike epithelium, connective tissue has lots of space between cells, filled with a material called the extracellular matrix (ECM). Matrix is 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 the 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 a blood supply) - exception: cartilage is avascular. ECM determines the properties - e.g., hard matrix in bone, fluid in blood, and flexible in cartilage. It 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 the stomach.

c. Cardiac muscles- it is seen only in the heart. Contraction of this muscle helps in the pumping of blood throughout our body.

4) Nervous Tissue: 

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

> Organisation

-> Brain and spinal cord.Central Nervous System (CNS) — 

-> All nerves outside the brain and spinal cord.Peripheral Nervous System (PNS) 

-> Controls involuntary functions like heart rate, digestion, and breathing.Autonomic Nervous System (ANS).

> Key Characteristics

-> Composed of two main cell types: neurons and neuroglia (supporting cells

1. Neurons are the functional units — they generate and transmit electrical signals (nerve impulses). Neurons are largely non-dividing after maturity — damage to nervous tissue is often permanent. Extremely high metabolic demand — the brain alone consumes about 20% of the body's energy.

2. Neuroglia (glial cells) support, protect, and nourish the neurons; they do not transmit impulses themselves.

# The Neuron:-

 -> A neuron has three main parts: the cell body (soma), which contains the nucleus and organelles; dendrites, which are short branching extensions that receive incoming signals; and an axon, a long projection that carries the signal away from the cell body to the next neuron or target organ. Axons are often insulated by a myelin sheath, which greatly increases the speed of signal transmission.

> Functions:

A) Receives sensory information from the environment and the body.

B) Processes and integrates that information in the brain and spinal cord.

C) Sends motor commands to muscles and glands, generating a response.

D) Coordinates and regulates virtually all body functions.

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