Tissues: Structure, Function, and Repair in Human Anatomy & Physiology

Tissues: Groups of Cells with Similar Function

Introduction to Tissues

Tissues are groups of cells that work together to perform specific functions in the body. The study of tissues is known as histology. There are four basic tissue types, each with distinct structures and functions.

• Epithelial Tissue: Covers body and organ surfaces, lines cavities, forms glands.

• Connective Tissue: Binds, supports, and protects body structures.

• Muscle Tissue: Contracts to produce movement.

• Nervous Tissue: Sends messages (impulses).

Four Basic Tissue Types

Epithelial Tissue

Epithelial tissue forms the covering and lining of surfaces and cavities throughout the body, and also comprises glandular tissue. It is specialized for protection, absorption, filtration, secretion, and sensory reception.

• Functions:

  • Covers surfaces, lines cavities and tubes, forms secretory glands.

  • Protects, absorbs, filters, secretes, and senses.

• Characteristics:

  • Cells are closely joined together to form continuous sheets.

  • Supported by connective tissue.

  • Regenerates continuously if surface is damaged.

  • Has no own blood supply; nourished from underlying tissues.

• Classification by Shape:

  1. Squamous – flat

  2. Cuboidal – cube-shaped

  3. Columnar – taller than wide, ciliated and non-ciliated

• Classification by Layers:

  1. Simple – single layer

  2. Stratified – two or more layers

  3. Pseudostratified – appears stratified, is not

  4. Transitional – switches from one to other

• Glandular Epithelium:

  • Exocrine glands: Secrete products into ducts (e.g., sweat, salivary, liver, pancreas).

  • Endocrine glands: Ductless glands; secrete hormones directly into the blood (e.g., pituitary, thyroid, pancreas).

  • Apocrine glands: Release product by pinching off part of the cell (e.g., mammary glands).

Connective Tissue (CT)

Overview and Functions

Connective tissue is the most abundant and widely distributed tissue type in the body. It provides support, binds other tissues together, and plays a role in protection and repair.

• Functions:

  • Support, protection, binding, storage, and transport.

  • Wound healing.

• Origin: Derived from embryonic mesenchyme.

• Vascularity: Varies from avascular (cartilage) to highly vascular (bone).

• Components:

  • Cells: Fibroblasts, chondroblasts, osteoblasts, blood cells.

  • Matrix: Ground substance and fibers (collagen, elastic, reticular).

Major Types of Connective Tissue

Type	Main Cell Type	Matrix	Vascularity
Connective tissue proper	Fibroblast	Collagen, elastic, reticular fibers	Varies
Cartilage	Chondroblast	Firm, flexible	Poorly vascularized
Bone	Osteoblast	Rigid, calcified	Well vascularized
Blood	Hematopoietic cells	Fluid (plasma)	Well vascularized

Special Connective Tissue Structures

• Aponeuroses: Flat sheet-like tendons connecting muscle to muscle or bone.

• Fascia: Fibrous membrane wrapping around groups of muscles, nerves, and blood vessels.

Muscle Tissue

Types and Functions

Muscle tissue is specialized for contraction and movement. There are three main types:

• Skeletal muscle: Voluntary, long cylindrical multinucleated cells, striations.

• Cardiac muscle: Involuntary, branching cells, striations, intercalated discs.

• Smooth muscle: Involuntary, non-striated, spindle-shaped cells.

Nervous Tissue

Structure and Function

Nervous tissue is responsible for communication and control within the body. It consists of two main cell types:

• Neuroglia: Support neurons, vary in size, shape, and location.

• Neurons: Communicate via impulses; comprised of a cell body, dendrites, and axon.

Epithelial Membranes

Types of Membranes

Epithelial membranes are continuous multicellular sheets composed of epithelial tissue bound to an underlying layer of connective tissue.

• Cutaneous membrane: Skin; keratinized stratified squamous epithelium attached to a thick layer of dense irregular connective tissue.

• Mucous membrane: Lines body cavities open to the exterior (digestive, respiratory, urinary, reproductive tracts); usually stratified or simple columnar epithelium over loose connective tissue.

• Serous membrane: Lines body cavities closed to the exterior; simple squamous epithelium on areolar connective tissue; secretes serous fluid to lubricate surfaces.

Tissue Repair

Phases of Tissue Repair

Tissue repair is initiated by growth factors released by injured cells and involves several phases:

1. Inflammation: Injured cells release histamine and other chemicals, causing blood vessels to dilate and become permeable. White blood cells migrate to the area, and clotting proteins form a clot.

2. Organization: Blood clot is replaced by granulation tissue, which restores blood supply. Fibroblasts synthesize new collagen fibers to bridge the gap.

3. Regeneration and/or Fibrosis: Surface epithelium regenerates and thickens. Scar tissue may form depending on the severity of the wound.

Developmental Aspects of Tissues

Embryonic Germ Layers

During embryonic development, three primary germ layers give rise to the four primary tissue types:

• Ectoderm: Gives rise to epithelial and nervous tissues.

• Mesoderm: Gives rise to epithelial, muscle, and connective tissues.

• Endoderm: Gives rise to epithelial tissues.

Key Terms and Definitions

• Histology: The study of tissues.

• Matrix: The extracellular material in connective tissue, consisting of ground substance and fibers.

• Fibroblast: A cell that produces collagen and other fibers in connective tissue.

• Chondroblast: A cell that produces cartilage matrix.

• Osteoblast: A cell that produces bone matrix.

• Hematopoietic cell: A cell that forms blood cells.

Important Equations

• Rate of Diffusion (Fick's Law): $J = -D \frac{dC}{dx}$ Where J is the rate of diffusion, D is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

Summary Table: Tissue Types and Functions

Tissue Type	Main Function	Location Example
Epithelial	Protection, absorption, secretion	Skin, lining of GI tract, glands
Connective	Support, binding, transport	Tendons, bone, blood
Muscle	Movement	Skeletal muscles, heart, walls of hollow organs
Nervous	Communication, control	Brain, spinal cord, nerves

Example: The skin is composed of epithelial tissue (epidermis) and connective tissue (dermis), providing both protection and support.