Histology: Structure and Function of Tissues

Histology

Introduction to Tissues

Histology is the study of tissues, which are groups of similar cells that share a common embryological origin and are specialized for particular functions. Understanding tissue structure and function is fundamental in Anatomy & Physiology.

• Tissue: A group of similar cells with a common function and origin.

• Histology: The scientific study of tissues.

• Organs: Structures composed of multiple tissue types, usually all four primary types.

Arrangement of tissues within organs determines organ function.

Primary Tissue Types

There are four primary tissue types in the human body, each with distinct roles and numerous subclasses:

• Epithelial tissue: Covers body surfaces, lines hollow organs, body cavities, and ducts; forms glands.

• Connective tissue: Protects, supports, and binds body organs together.

• Muscle tissue: Responsible for movement and generation of force.

• Nervous tissue: Transmits action potentials (nerve impulses) to control body activities.

Epithelial Tissue

Functions of Epithelial Tissue

Epithelial tissue serves as a protective barrier, forms boundaries between different environments, and is involved in absorption, secretion, filtration, and sensation.

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

• Absorption: Uptake of substances such as nutrients and gases.

• Secretion: Release of substances like enzymes, hormones, and mucus.

• Filtration: Selective movement of substances across membranes.

• Sensation: Contains sensory nerve endings for detecting stimuli.

Characteristics of Epithelial Tissue

• Cellularity: Composed of closely packed cells with minimal extracellular material.

• Specialized Contacts: Cells are bound together by tight junctions and desmosomes.

• Attachment/Support: Adheres to underlying connective tissue via the basement membrane, which resists stretching and tearing.

• Polarity: Has an apical (free) surface and a basal (attached) surface, each with distinct structures and functions.

• Avascular but Innervated: Lacks blood vessels but contains nerve fibers.

• High Regeneration Capacity: Rapidly replaces lost or damaged cells.

Naming of Epithelial Tissue

Epithelial tissues are named based on the number of cell layers and the shape of the cells.

• Number of Layers:

  • Simple: Single layer of cells.

  • Stratified: Two or more layers of cells.

  • Pseudostratified: Appears to have multiple layers but all cells touch the basement membrane.

• Cell Shape:

  • Squamous: Flat, scale-like cells.

  • Cuboidal: Cube-shaped cells.

  • Columnar: Tall, rectangular cells.

  • Transitional: Cells that change shape, found in areas subject to stretching.

Types of Epithelial Tissue

Type	Appearance	Function	Location
Simple Squamous	Single layer, flattened cells	Diffusion, filtration	Blood vessels, alveoli, kidney glomeruli, serous membranes
Simple Cuboidal	Single layer, cube-shaped cells	Secretion, absorption	Ducts of glands, kidney tubules, thyroid gland
Simple Columnar	Single layer, tall cells	Secretion, absorption	Digestive tract, small bronchi, uterine tubes
Pseudostratified Columnar	Single layer, varying heights, nuclei at different levels	Move material across surface	Respiratory tract, male sperm ducts
Stratified Squamous	Multiple layers, apical cells squamous	Protection from abrasion	Skin, esophagus, mouth, vagina, anus
Transitional	Multiple layers, apical cells dome-shaped	Stretching	Urinary bladder, ureters, urethra

Glandular Epithelium

Glandular epithelia consist of cells specialized to produce and secrete substances.

• Endocrine glands: Ductless glands that secrete hormones directly into blood or lymph (e.g., thyroid, pituitary).

• Exocrine glands: Glands that secrete products onto surfaces or into body cavities via ducts (e.g., sweat, digestive enzymes, milk).

Connective Tissue

Functions of Connective Tissue

Connective tissue is the most abundant and widely distributed tissue type, providing binding, support, protection, insulation, and transportation.

• Binding and Support: Connects and supports other tissues and organs.

• Protection: Shields organs from damage.

• Insulation: Maintains body temperature.

• Transportation: Moves substances throughout the body (e.g., blood).

Characteristics of Connective Tissue

• Origin: All connective tissues arise from embryonic mesenchyme.

• Vascularity: Highly vascular (except cartilage and tendons).

• Innervation: Highly innervated (except cartilage).

• Extracellular Matrix: Composed of fibers and ground substance, providing structural support.

Structural Elements of Connective Tissue

• Fibers:

  • Collagen: Strong, resists force in one direction.

  • Elastic: Allows stretch and recoil.

  • Reticular: Fine, branching networks.

• Ground Substance: Fluid-like material composed of interstitial fluid, cell adhesion proteins, and proteoglycans (e.g., glycosaminoglycans).

• Cells: Fundamental cell types include fibroblasts (fibers), chondroblasts (cartilage), osteoblasts (bone), and their mature forms (fibrocytes, chondrocytes, osteocytes).

Types of Connective Tissue

Type	Appearance	Function	Location
Loose (Areolar)	Gel-like matrix, all fiber types	Binds body parts, reservoir of water	Wraps blood vessels, links organs
Reticular	Network of fine, branching fibers	Internal framework for blood cells	Lymph nodes, spleen
Adipose	90% adipocytes, scant matrix	Energy storage, protection, insulation	Hypodermis, abdomen, breasts, kidneys
Dense Regular	Parallel collagen fibers	Withstands stress in one direction	Tendons, ligaments
Dense Irregular	Irregularly arranged collagen fibers	Withstands tension in many directions	Dermis, digestive tract
Elastic	High proportion of elastic fibers	Stretch and recoil	Large arteries, bronchial tubes
Hyaline Cartilage	Fine collagen fibers, gel matrix	Firm support, flexibility, reduces friction	Ends of bones, ribs, nose, trachea, larynx
Elastic Cartilage	Network of elastic fibers	Strength, elasticity, maintains structure	Ear, epiglottis
Fibrocartilage	Bundles of wavy collagen fibers	Absorbs compressive shock	Intervertebral discs, pubic symphysis, knee
Bone (Osseous)	Hard, calcified matrix	Support, protection, mineral storage	Bones
Blood	Liquid matrix (plasma), cells	Transport gases, nutrients, wastes	Blood vessels

Lining Membranes

Types of Lining Membranes

Lining membranes are flat sheets of pliable tissue that cover or line parts of the body, incorporating both connective and epithelial tissues.

• Cutaneous membrane: The skin; consists of keratinized stratified squamous epithelium attached to dense irregular connective tissue.

• Mucous membranes (mucosae): Line body cavities that open to the exterior (e.g., digestive, respiratory tracts); consist of either stratified squamous or simple columnar epithelium attached to areolar connective tissue.

• Serous membranes (serosa): Line body cavities that do not open to the exterior and cover organs within these cavities (e.g., pleura, pericardium, peritoneum); consist of simple squamous epithelium and areolar connective tissue.

Muscle Tissue

Overview of Muscle Tissue

Muscle tissue is highly cellular and vascularized, responsible for movement. There are three subtypes, each differing in appearance, function, and location.

• Skeletal muscle: Long, cylindrical, multinucleate cells; striated; voluntary movement; attached to skeleton.

• Cardiac muscle: Branched cells, intercalated discs, uninucleate; striated; involuntary; found in heart.

• Smooth muscle: Spindle-shaped, single central nucleus; no striations; involuntary; found in walls of hollow organs (e.g., digestive tract, blood vessels).

Functions and Locations of Muscle Tissue

Type	Function	Location
Skeletal	Voluntary movement, locomotion	Attached to skeleton
Cardiac	Involuntary, circulation of blood	Heart
Smooth	Involuntary, propelling substances	Walls of hollow organs

Thought Question: Which muscle tissue lacks striations? Answer: Smooth muscle.

Nervous Tissue

Overview of Nervous Tissue

Nervous tissue is the main component of the nervous system, regulating and controlling body functions. It consists of two major cell types:

• Neurons: Cells with branching extensions that generate and conduct electrical impulses (action potentials).

• Neuroglia: Non-conductive cells that support, insulate, and protect neurons.

Additional info: Neurons are responsible for rapid communication throughout the body, while neuroglia maintain the environment necessary for neuron function.