2 Tissues of the Human Body

The four primary tissue types

A tissue is an organized group of cells and their surrounding material that work together to perform particular functions. The four primary tissue types—epithelial, connective, muscle, and nervous—combine in different ways to form organs. Histology is the study of tissue structure under a microscope.

Epithelial tissue

Epithelium covers body surfaces, lines internal passageways and cavities, and forms glands. Its cells are closely packed, with little extracellular material between them. Most epithelial cells have an exposed apical surface and an attached basal surface anchored to a basement membrane. Epithelia are usually avascular, so nutrients diffuse to them from underlying tissues. Many can replace lost cells relatively quickly.

Classification by layers and shape

Epithelia are classified by both the number of cell layers and the shape of their cells. Simple epithelium has one layer, while stratified epithelium has several. Pseudostratified epithelium is a single layer that appears multilayered because its cells vary in height. Cell shapes are squamous (flat), cuboidal (roughly cube-shaped), or columnar (tall). In transitional epithelium, cell shape changes as the tissue stretches.

Common types and locations

• Simple squamous: A thin layer suited to rapid exchange; found in lung air sacs and the lining of blood vessels.

• Simple cuboidal: Supports secretion and absorption; found in kidney tubules and small gland ducts.

• Simple columnar: Supports absorption and secretion and may have cilia; found in the digestive tract and parts of the reproductive tract.

• Stratified squamous: Has multiple protective layers; found in the skin and mouth lining.

• Transitional: Changes cell shape as it stretches; found in the urinary bladder.

Glands are epithelial structures specialized for secretion. Exocrine glands release products through ducts onto a surface or into a passageway. Endocrine glands release hormones into the surrounding fluid and blood.

Connective tissue

Connective tissue supports, binds, protects, and connects other tissues. Unlike epithelium, its cells are typically dispersed through an extracellular matrix made of ground substance and protein fibers. The composition of the matrix helps determine the tissue’s properties. Collagen fibers resist pulling forces, elastic fibers stretch and recoil, and reticular fibers form fine supportive networks.

Major groups

• Connective tissue proper: Loose tissues, including areolar tissue and fat, cushion and bind structures. Dense tissues, including tendons and ligaments, provide tensile strength.

• Supportive connective tissue: Cartilage provides flexible support, while bone has a mineralized, rigid matrix that supports and protects the body.

• Fluid connective tissue: Blood and lymph transport cells and substances through the body.

Fibroblasts produce much of the matrix and its fibers. Other connective-tissue cells store fat, help defend against pathogens, or contribute to repair.

Muscle tissue

Muscle cells are excitable, meaning they respond to stimulation, and contractile, meaning they generate force by shortening. The three types of muscle tissue differ in appearance, control, and role.

• Skeletal muscle: Long, striated fibers with many nuclei; usually voluntary. It moves the skeleton, maintains posture, and produces heat.

• Cardiac muscle: Branched, striated cells, usually with one nucleus; involuntary. It contracts rhythmically in the heart to pump blood.

• Smooth muscle: Spindle-shaped cells without visible striations; involuntary. It moves contents through organs and adjusts the diameter of passageways and blood vessels.

Nervous tissue

Nervous tissue receives information and coordinates responses through electrical and chemical signaling. It consists of neurons, which transmit signals, and glial cells, which support and help regulate neuronal function.

A neuron’s cell body contains its nucleus. Dendrites usually receive input, while an axon carries signals away from the cell body. At a synapse, chemical messengers called neurotransmitters can convey a signal to another neuron, muscle cell, or gland cell.

Tissue injury and repair

After injury, inflammation helps limit damage, remove harmful material, and initiate repair. Common signs are redness, heat, swelling, and pain; loss of function may also occur. Blood vessels widen and become more permeable, allowing fluid and immune cells to enter the affected area.

How a skin wound repairs

Repair involves clearing damaged material and rebuilding the affected region. In a skin wound, a clot helps limit bleeding and cover the surface. New blood vessels and connective tissue develop beneath the wound, while epithelial cells grow across it. Fibroblasts deposit collagen; if the original tissue architecture is not fully restored, this collagen may form a scar. Small wounds with edges close together generally heal with less tissue filling than larger, gaping wounds.

Regeneration replaces damaged cells with the same cell type. Fibrosis replaces some lost tissue with collagen-rich scar tissue.

Repair capacity varies by tissue

Epithelia often replace surface cells readily. Skeletal muscle can repair some damage with help from satellite cells, but extensive injury can leave scar tissue. Cardiac muscle has limited regenerative capacity, while smooth muscle generally repairs more readily. Nervous tissue also has limited repair, and recovery after nerve injury depends on the location and extent of damage.

The four tissue types differ in structure and role: epithelium covers and lines, connective tissue supports and links, muscle produces force, and nervous tissue communicates and coordinates. Healing may restore the original tissue through regeneration or leave a collagen-rich scar when full replacement is not possible.