Human Tissues

Tissues are groups of similar cells that work together to perform a specific function. The proper functioning of every organ in the body depends on the highly organized arrangement and specific roles of these tissues. Based on their structure and function, all human and animal tissues are classified into four major categories: Epithelial, Connective, Muscle, and Nervous tissue.

1. Epithelial Tissue (Epithelium)

Epithelial tissue acts as a covering or a lining. It covers all outer surfaces of the body (like the skin) and lines internal surfaces (like the digestive tract, blood vessels, and organs).

Functions of Epithelium:

Protection: Shields underlying structures from mechanical, chemical, and dehydration damage.
Secretion: Forms glands or glandular epithelium (e.g., sweat glands, digestive glands).
Absorption: Takes in nutrients (e.g., in the intestines).
Filtration: Found in structures like the kidney tubules.

Epithelial cells are closely packed with minimal intercellular substance (matrix). They lack their own blood supply (are avascular) and receive oxygen and nutrients from the underlying connective tissue via diffusion across a thin layer called the basement membrane.

Classification of Epithelium

Epithelial tissues are classified based on two criteria: the shape of the cells and the number of cell layers.

Criteria	Simple Epithelium (One Layer)	Stratified Epithelium (Multiple Layers)
Simple Squamous	Flat, thin cells; ideal for absorption, secretion, and filtration.	Stratified Squamous
Simple Cuboidal	Cube-shaped cells; found on surfaces that absorb or secrete.	Stratified Cuboidal
Simple Columnar	Tall, narrow cells; found in high-activity tissues (e.g., digestive tract).	Stratified Columnar
Specialized		Transitional Epithelium (allows stretching, e.g., bladder)

Stratified Epithelia function primarily in protection against mechanical damage. Cells continuously divide in the lower (basal) layer and are pushed toward the surface, where they are eventually shed.

2. Connective Tissue

Connective tissue is the most abundant tissue type in the body and acts to bind, support, protect, insulate, and transport.

Structure: Connective tissue cells are generally separated by a large amount of intercellular substance called the matrix.
Matrix: The matrix is non-living material that gives the tissue its structure and function. It can range in consistency from a semisolid jelly-like substance (like in cartilage) to a dense, rigid structure (like in bone) or a liquid (like in blood). The matrix often contains fibers (collagen, elastic) that provide support.
Blood Supply: Most connective tissues, unlike epithelium, have an adequate blood supply.
Types: Examples include areolar tissue, adipose tissue (fat), fibrous tissue, elastic tissue, cartilage, bone, and blood.
Cells Present: Key cells include fibroblasts (which produce the matrix and fibers), fat cells, macrophages (for defense), leukocytes (white blood cells), and mast cells.

3. Muscle Tissue

Muscle tissue is specialized for contraction and relaxation, enabling movement throughout the body.

Requirement: Muscle fibers require a rich blood supply to deliver oxygen, nutrients (like glucose and calcium), and to remove waste products.
Types of Muscle Tissue:

4. Nervous Tissue

Nervous tissue is responsible for generating, receiving, transmitting, and maintaining electrochemical impulses, allowing the body to react to stimuli and process information.

Location: Found in the Central Nervous System (CNS) (brain and spinal cord) and the Peripheral Nervous System (PNS) (all nerves extending from the CNS).
Primary Cells:

Neurons: The excitable nerve cells that produce and transmit electrical impulses. All neurons share a basic structure:
Cell Body: Contains the nucleus.
Dendrites: Processes that carry impulses toward the cell body.
Axon: A process that carries impulses away from the cell body (most neurons have one).

Neuroglia (Glial Cells): Non-excitable supporting cells. Examples include Schwann cells in the $\text{PNS}$ , which form the myelin sheath to electrically insulate the axons, speeding up impulse transmission.
Synapse: Nerve impulses are electrical along the cell membrane, but they cannot cross the small gap between neurons called the synapse. Impulse transmission across the synapse relies on neurotransmitters—chemicals that travel from the axon of the sending neuron to the dendrite of the receiving neuron.