Tissue: The Living Fabric

Introduction to Tissues

Tissues are groups of cells that are similar in structure and function, forming the basic fabric of the human body. There are four primary tissue types: epithelial, connective, muscle, and nervous tissue. Each type plays a distinct role in maintaining the body's structure and function.

Epithelial Tissue

Characteristics of Epithelial Tissue

Epithelial tissue covers body surfaces, lines cavities, and forms glands. It is characterized by:

• Cellularity: Composed almost entirely of closely packed cells.

• Special contacts: Cells are joined by specialized junctions (e.g., tight junctions, desmosomes).

• Polarity: Has an apical (free) surface and a basal surface attached to a basement membrane.

• Supported by connective tissue: The basal surface is attached to underlying connective tissue.

• Avascular but innervated: Contains no blood vessels but is supplied by nerve fibers.

• Regenerative: High capacity for renewal and repair.

Locations and Functions of Epithelia

Epithelial tissues are found covering surfaces, lining internal cavities, and composing glands. Their main functions include:

• Protection: Shields underlying tissues from mechanical and chemical stress.

• Absorption: Uptake of substances (e.g., nutrients in the intestine).

• Filtration: Selective movement of substances (e.g., in kidneys).

• Excretion: Removal of waste products.

• Secretion: Production and release of substances (e.g., mucus, hormones).

• Sensory reception: Detects changes in the environment (e.g., skin receptors).

Classification of Epithelia

Epithelia are classified by the number of cell layers and the shape of the cells:

• Simple: One cell layer thick.

• Stratified: Multiple layers of cells.

• Squamous: Flat, scale-like cells.

• Cuboidal: Cube-shaped cells.

• Columnar: Tall, column-like cells.

• Special types: Pseudostratified (appears layered but is not), Transitional (changes shape).

Simple Squamous Epithelium

Simple squamous epithelium consists of a single layer of flat cells, resembling a row of fried eggs. It is found in areas where diffusion and filtration occur, such as the inner lining of blood vessels and the alveoli of the lungs.

• Functions: Diffusion, filtration

Stratified Squamous Epithelium

This tissue type is composed of many layers of flat cells, providing protection in areas subject to abrasion. It is found in the epidermis, vagina, mouth, and esophagus.

• Functions: Protection against mechanical stress and pathogens

Simple Cuboidal Epithelium

Simple cuboidal epithelium consists of a single layer of cube-shaped cells with central nuclei. It is commonly found in glands, ducts, and kidney tubules.

• Functions: Secretion and absorption

Transitional Epithelium

Transitional epithelium is composed of poufy, dome-shaped cells that can stretch and change shape. It is found in the urinary system, particularly the bladder, allowing for expansion as the bladder fills.

• Function: Expansion and recoil

Simple Columnar Epithelium

Simple columnar epithelium consists of tall cells with oval nuclei aligned near the basal surface. It lines much of the digestive tract and is specialized for secretion and absorption.

• Functions: Secretion and absorption

Pseudostratified Epithelium

Pseudostratified epithelium appears to have multiple layers due to varying cell heights, but all cells touch the basement membrane. It often contains cilia and goblet cells and is found in the trachea, where it functions in protection and secretion.

• Functions: Protection, secretion

Epithelia: Glandular

Glandular epithelia form glands, which are one or more cells that make and secrete an aqueous fluid. Glands are classified by:

• Site of product release: Endocrine (into blood) or exocrine (onto surfaces or into ducts)

• Number of cells: Unicellular or multicellular

Multicellular Exocrine Glands

These glands are classified by duct type (simple or compound), secretory unit shape (alveolar or tubular), and mode of secretion:

• Merocrine: Secretion by exocytosis

• Holocrine: Secretion by cell rupture

Epithelial Membranes

Epithelial membranes cover surfaces, line body cavities, and form sheets around organs. They serve to protect and lubricate. There are three main types:

• Cutaneous membrane: The skin; a dry membrane containing keratin

• Mucous membranes (mucosae): Line body cavities open to the exterior; secrete mucus; found in the digestive, respiratory, and urogenital tracts

• Serous membranes (serosae): Line closed ventral body cavities; secrete serous fluid; include pleura, pericardium, and peritoneum

Connective Tissue

Types of Connective Tissue

Connective tissue is the most abundant and widely distributed tissue type. It includes:

• Connective tissue proper: Fibroblasts

• Cartilage: Chondroblasts

• Bone (osseous): Osteoblasts

• Blood (liquid): Hematopoietic stem cells

The suffix "-blast" indicates an immature, actively secreting cell; "-cyte" indicates a mature cell.

Characteristics of Connective Tissue

• Origin: All connective tissues arise from mesenchyme (embryonic tissue).

• Degree of vascularity: Varies from avascular (cartilage) to highly vascular (bone).

• Matrix: Extracellular matrix composed of ground substance and fibers, which separates the living cells.

Matrix Components

• Ground substance: Interstitial fluid, adhesion proteins, proteoglycans

• Fibers: Collagen (strength), elastic (flexibility), reticular (support)

Functions of Connective Tissue

• Binds structures together

• Supports and protects organs

• Hematopoiesis (blood cell formation)

• Cushions and insulates

• Transports substances (e.g., blood)

Examples of Connective Tissue Types

• Areolar: Wraps and cushions organs; found under epithelial tissues

• Adipose: Stores fuel, insulates, supports; found under skin, around organs

• Hyaline cartilage: Supports, reinforces, resists compression; found in nose, trachea, costal cartilage

• Elastic cartilage: Maintains shape with flexibility; found in ear, epiglottis

• Fibrocartilage: Absorbs shock; found in intervertebral discs, pubic symphysis, menisci

• Bone: Supports, protects, stores minerals; skeleton

• Blood: Transports substances; within blood vessels

Connective Tissue Membranes

These membranes are composed solely of connective tissue and include synovial membranes (joints), meninges (brain and spinal cord), fascia, periosteum (bone), perichondrium (cartilage), and pericardium (heart).

Nervous Tissue

Types and Functions

Nervous tissue consists of neurons and neuroglia. Neurons are specialized for transmitting electrical signals, while neuroglia support and protect neurons. Nervous tissue is found in the brain, spinal cord, and nerves, and is responsible for communication and control within the body.

Muscle Tissue

Characteristics and Types

Muscle tissue is specialized for contraction, enabling movement. It is characterized by contractility, extensibility, elasticity, and is well vascularized. Muscle cells are often referred to as "fibers." There are three types:

• Skeletal muscle: Voluntary, striated, multinucleated; attached to skeleton; produces movement

• Cardiac muscle: Involuntary, striated, branching cells with intercalated discs; found in heart wall; moves blood

• Smooth muscle: Involuntary, non-striated; found in walls of hollow organs; moves substances through the body

Tissue Trauma and Repair

Inflammation

Tissue trauma triggers inflammation, characterized by capillary dilation and increased vessel permeability. The classic signs are redness, heat, swelling, and pain.

Steps in Tissue Repair

1. Inflammation and clot formation

2. Organization and restoration of blood supply

3. Regeneration and fibrosis (scar formation)