Tissues of the Body

Overview of Tissue Types

The human body is composed of four primary tissue types: epithelial, connective, muscle, and nervous tissues. Each type has distinct functions, structures, and locations within the body.

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

• Connective Tissue: Supports, binds, and protects organs.

• Muscle Tissue: Responsible for movement.

• Nervous Tissue: Enables communication via electrical impulses.

Epithelial Tissue

• Functions: Protection, absorption, filtration, excretion, secretion, and sensory reception.

• General Characteristics:

  • Cells are closely packed with minimal extracellular matrix.

  • Arranged in continuous sheets (single or multiple layers).

  • Classified by cell shape (squamous, cuboidal, columnar) and number of layers (simple, stratified).

• Types of Epithelia:

  • Simple squamous epithelium: Single layer of flat cells; found in alveoli and blood vessels.

  • Simple cuboidal epithelium: Single layer of cube-shaped cells; found in kidney tubules.

  • Simple columnar epithelium: Single layer of tall cells; lines digestive tract.

  • Pseudostratified epithelium: Appears layered but all cells touch the basement membrane; found in respiratory tract.

  • Stratified squamous epithelium: Multiple layers of flat cells; found in skin, mouth, esophagus.

Connective Tissue

• Functions: Binding, support, protection, insulation, and transportation of substances.

• General Characteristics:

  • Cells are scattered within an abundant extracellular matrix (ECM).

  • ECM consists of ground substance and protein fibers (collagen, elastic, reticular).

  • Cell types include -blast (immature, matrix-producing) and -cyte (mature, maintenance) cells.

• Types of Connective Tissue Proper:

  • Loose connective tissue:

    • Areolar: Supports and binds other tissues; found under epithelia.

    • Adipose: Stores fat; insulates and cushions organs.

    • Reticular: Forms a soft internal skeleton for lymphoid organs.

  • Dense connective tissue:

    • Dense regular: Parallel collagen fibers; found in tendons and ligaments.

    • Dense irregular: Irregularly arranged fibers; found in dermis.

    • Elastic: Contains elastic fibers; found in walls of large arteries.

• Specialized Connective Tissues:

  • Cartilage: Hyaline, elastic, fibrocartilage (see table below).

  • Bone: Rigid support and protection; stores minerals.

  • Blood: Transports nutrients, gases, and wastes.

Table: Types of Cartilage

Muscle Tissue

• General Characteristics: Specialized for contraction; contains actin and myosin filaments.

• Types:

  • Skeletal muscle: Voluntary, striated, attached to bones.

  • Cardiac muscle: Involuntary, striated, found in heart.

  • Smooth muscle: Involuntary, non-striated, found in walls of hollow organs.

Nervous Tissue

• Location: Brain, spinal cord, nerves.

• Function: Transmits electrical signals; coordinates body functions.

• Neurons: Specialized cells for communication; consist of cell body, axon, and dendrites.

Integumentary System

Overview

The integumentary system includes the skin, hair, nails, and associated glands. It protects the body, regulates temperature, and provides sensory information.

Skin Structure

• Major Regions:

  • Epidermis: Outermost layer; composed of stratified squamous epithelium.

  • Dermis: Deeper layer; contains connective tissue, blood vessels, nerves.

Layers of the Epidermis

• Stratum corneum

• Stratum lucidum (only in thick skin)

• Stratum granulosum

• Stratum spinosum

• Stratum basale

Each layer has unique characteristics, such as cell type and function. The stratum basale contains stem cells; the stratum corneum is composed of dead, keratinized cells.

Cell Types in the Epidermis

• Keratinoctyes: Produce keratin, a protective protein.

• Melanocytes: Produce melanin pigment.

• Dendritic cells: Immune defense.

• Tactile cells: Sensory receptors for touch.

Dermis

• Contains two layers: papillary (superficial, forms fingerprints) and reticular (deep).

• Dermal papillae: Projections that create fingerprints.

• Contains blood vessels, nerves, and glands.

Skin Glands

• Sudoriferous (sweat) glands: Eccrine (secrete watery sweat, regulate temperature) and apocrine (secrete into hair follicles, found in axillary/genital areas).

• Sebaceous glands: Secrete sebum (oil) into hair follicles; lubricate skin.

Other Features

• Three pigments affect skin color: melanin, carotene, hemoglobin.

• Thick skin (palms, soles) vs. thin skin (rest of body).

• Burns: Classified as 1st, 2nd, or 3rd degree based on depth and severity.

Bones and Skeletal Tissues

Cartilage Types

Cartilage is a flexible connective tissue found in joints, ear, nose, and respiratory tract. Three types are associated with the skeleton:

Bone Structure and Classification

• Bones are classified by shape:

  • Long (e.g., femur)

  • Flat (e.g., sternum)

  • Short (e.g., carpals)

  • Irregular (e.g., vertebrae)

• Compact vs. spongy bone:

  • Compact: Dense, forms outer layer.

  • Spongy: Porous, contains trabeculae.

• Needlelike pieces in spongy bone are called trabeculae.

Long Bone Anatomy

• Diaphysis: Shaft of the bone.

• Epiphysis: Ends of the bone.

• Epiphyseal line/plate: Growth plate; plate in children, line in adults.

• Articular cartilage: Covers joint surfaces.

• Periosteum: Outer membrane; contains nerves and blood vessels.

• Endosteum: Lines internal bone surfaces.

• Medullary cavity: Central cavity; contains bone marrow.

Microscopic Anatomy of Bone

• Osteogenic cells: Stem cells that differentiate into osteoblasts.

• Osteoblasts: Bone-forming cells.

• Osteocytes: Mature bone cells; maintain bone matrix.

• Osteoclasts: Bone-resorbing cells.

Table: Compact Bone Structure

Bone Formation (Ossification)

• Intramembranous ossification: Bone develops from fibrous membranes (e.g., skull).

• Endochondral ossification: Bone replaces hyaline cartilage (most bones).

• Primary ossification center: Initial site of bone formation in cartilage model.

Hormonal Control of Bone

• Parathyroid hormone (PTH):

  • Secreted by parathyroid glands in response to low blood calcium.

  • Increases blood calcium by stimulating osteoclasts.

• Calcitonin:

  • Secreted by thyroid gland in response to high blood calcium.

  • Lowers blood calcium by inhibiting osteoclasts.

Key Equations

• Calcium homeostasis:

Example: When blood calcium levels drop, PTH is released, stimulating bone resorption and increasing calcium in the blood.

Additional info: Some details, such as the specific functions of cell types and the process of ossification, have been expanded for academic completeness.