Anatomy & Physiology: Tissues, Membranes, and Skin Study Guide

Major Tissue Types in the Human Body

Epithelial, Connective, Muscle, and Nervous Tissues

The human body is composed of four primary tissue types, each with distinct structures and functions. Understanding their characteristics is essential for grasping how organs and systems operate.

• Epithelial tissue: Covers body surfaces, lines cavities, and forms glands. Functions include protection, absorption, filtration, and secretion.

• Connective tissue: Supports, connects, and protects other tissues and organs. Includes bone, cartilage, blood, and adipose tissue.

• Muscle tissue: Responsible for movement. Types include skeletal, cardiac, and smooth muscle.

• Nervous tissue: Conducts electrical impulses and processes information. Composed of neurons and neuroglial cells.

Cell Junctions in Epithelial Tissue

Types and Functions

Cell junctions are specialized structures that connect adjacent cells, providing structural integrity and regulating the passage of substances.

• Tight junctions: Form a seal between cells, preventing free passage of molecules (including ions) between epithelial cells.

• Desmosomes: Provide strong cell-to-cell adhesion using cytoskeletal connections, common in tissues experiencing intense mechanical stress (e.g., cardiac muscle, bladder tissue, gastrointestinal mucosa).

• Gap junctions: Allow exchange of ions, secondary messengers, and small metabolites between adjacent cells, facilitating communication (e.g., cardiac muscle, smooth muscle).

Comparison of Epithelial and Connective Tissue

Structural and Functional Differences

Epithelial and connective tissues differ in their location, structure, and function. The following table summarizes key differences:

Epithelial	Connective
Lines cavities & covers surfaces of organs	Supports, connects, and separates different body tissues
Closely packed cells, cells similar in type	Varied cell types scattered throughout extracellular matrix that contains fibers
Avascular – no blood capillaries	Blood capillaries; 2 exceptions
Good regeneration	Regeneration varies; can be due to fibroblast and fiber variation
Above basement membrane; see composition below	Below basement membrane

Classification of Epithelial Tissue

Layering and Cell Shape

Epithelial tissues are classified by the number of layers and the shape of their cells:

• Simple: Single layer of cells

• Stratified: Multiple cell layers

• Shapes: Squamous (flat), cuboidal (cube), columnar (column)

• Special types: Transitional (stretches and flattens), pseudostratified (appears layered but is not)

Examples:

• Simple squamous: Blood vessel lining, alveoli of lungs

• Stratified squamous: Skin, mouth, esophagus

• Pseudostratified: Trachea, upper respiratory tract

• Transitional: Urinary bladder, ureters

• Simple columnar: Intestines, stomach

Basement Membrane

Structure and Function

The basement membrane separates tissues and protects them from mechanical stress. It consists of:

• Basal lamina: Extracellular matrix layer found beneath epithelial cells, provides support and filtration.

• Reticular lamina: Contains collagen fibers, adds strength and support.

Glands and Modes of Secretion

Types of Glands and Secretion Mechanisms

Glands are made of epithelial tissue and classified by their mode of secretion:

• Endocrine: Secrete hormones into the blood stream

• Exocrine: Secrete products onto surfaces or into ducts

Unicellular gland example: Goblet cell (secretes mucus)

Modes of secretion:

Mode	Description	Examples
Apocrine	Secretion involves loss of part of the cell	Mammary glands
Holocrine	Entire cell disintegrates to release secretion	Sebaceous (oil) glands
Merocrine	Secretion by exocytosis, no cell loss	Sudoriferous (sweat) glands, salivary glands

Connective Tissue

Cells, Matrix, and Fiber Types

Connective tissue is derived from mesenchyme tissue in the mesoderm. It consists of various cell types and an extracellular matrix composed of ground substance and fibers.

• Cells: Fibroblasts, adipocytes, chondrocytes, osteocytes, osteoblasts, RBCs, WBCs, macrophages, mast cells

• Matrix components: Ground substance (e.g., hyaluronic acid, plasma) and fibers

• Fiber types:

  • Collagen: Tough, high tensile strength

  • Elastic: Stretch and recoil, thin and branched

  • Reticular: Support, branching, found in organs

Loose Connective Tissue Types

Loose connective tissue has scattered fibers and a less dense ground substance. The main types are areolar, adipose, and reticular.

Tissue	Cells	Fibers	Matrix Characteristics	Location	Function
Areolar	Fibroblast, macrophage	Collagen, elastic, reticular	Gelatinous ground substance	Dermis of skin, surrounds capillaries	Supports organs, cushions
Adipose	Adipocytes	Reticular, collagen	Packed cells, gelatinous ground	Abdomen, subcutaneous, around organs	Reserve fuel source, insulation
Reticular	Reticular	Reticular	Loose fibers, gelatinous ground substance	Lymph organs (spleen & lymph nodes)	Internal skeleton for certain organs, supports cells

Dense Connective Tissue Types

Tissue	Cells	Fibers	Matrix Characteristics	Location	Function
Dense Irregular	Fibroblasts	Collagen and some elastic	Irregular bundles, little ground	Fibrous capsule of joints, skin, dermis	Structural strength & withstand tension
Dense Regular	Fibroblasts	Collagen	Parallel bundles, little ground	Tendons, ligaments	Connect muscle to bone or bone to bone
Hyaline (cartilage)	Chondrocytes	Collagen, some elastic	Gel matrix of chondroitin sulfate	Ends of bones, joints	Support & cushioning
Fibrocartilage	Chondrocytes	Collagen	Little ground, fibers visible	Intervertebral discs, meniscus	Compressible strength & absorbs shock
Elastic cartilage	Chondrocytes	Elastin	Little ground	External ear, epiglottis	Shape with flexibility
Osseous (bone)	Osteoblasts & osteocytes	Collagen	Rigid Ca salts	Bones	Support, protection, mineral storage
Blood	Erythrocytes, leukocytes, platelets	Fibers are soluble proteins that form during clotting	Liquid matrix (plasma)	In blood vessels	Transport

Additional info: Cartilage is unique for its lack of blood supply and gel matrix; bone is nourished from the central canal of each osteon; blood is unique for its liquid ground substance and soluble fibers.

Muscle Tissue

Types and Functions

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

• Skeletal muscle: Striated, voluntary, multinucleated, attached to bones

• Cardiac muscle: Striated, involuntary, branched, uninucleated, intercalated discs, found in heart

• Smooth muscle: Non-striated, involuntary, spindle-shaped, uninucleated, found in walls of hollow organs and blood vessels

Intercalated discs are specialized junctions in cardiac muscle that allow rapid electrical transmission and mechanical connection.

Nervous Tissue

Neurons and Neuroglia

Nervous tissue consists of neurons (conduct action potentials) and neuroglial cells (support cells). Neurons do not undergo mitosis; neuroglia do not conduct impulses but provide structural and metabolic support.

Between neurons and neuroglia is a fluid-filled matrix called the extracellular space.

Membranes

Types and Functions

Membranes are sheets of tissue that cover surfaces, line cavities, and divide spaces. Types include:

• Mucous membranes: Line body cavities open to the exterior; secrete mucus

• Serous membranes: Line closed cavities; secrete serous fluid

• Synovial membranes: Line joint cavities; produce synovial fluid for lubrication

• Cutaneous membrane: Skin; protects body surface

Serous membranes have two layers: parietal (lines cavity) and visceral (covers organ).

Tissue Repair

Steps and Inflammation

Tissue repair involves three steps:

1. Inflammation – redness, heat, swelling

2. Granulation – tissue repair and regeneration

3. Maturation/Regeneration/Fibrosis – scarring, irregular tissue

During inflammation, injured cells release chemicals (histamine, serotonin, prostaglandins) that increase blood flow and permeability, allowing WBCs and plasma proteins to enter the area.

Skin Structure and Function

Layers and Functions

The skin is the largest organ and serves multiple functions:

• Thermoregulation

• Protection

• Production of Vitamin D

• Excretion

• Blood reservoir

Vitamin D is absorbed and retained in the skin, aiding calcium and phosphorus metabolism.

The dermis contains many blood capillaries, holding 8-10% of total blood volume when the body is at rest.

The epidermis is made of stratified squamous epithelium. The most superficial layer is the stratum corneum (dead cells).

Keratin is a protein produced by keratinocytes in the stratum spinosum and granulosum, making the skin abrasion-resistant and waterproof.

Equations and Scientific Terms

• Osmosis: Movement of water across a semipermeable membrane from low solute concentration to high solute concentration.

• Diffusion: Movement of molecules from high to low concentration.

• LaTeX equation for osmosis: $J = -D \frac{dC}{dx}$ where J is flux, D is diffusion coefficient, C is concentration, x is position.

Additional Info

• Parenchyma is the functional tissue of an organ; stroma is the supportive tissue.

• Cardiac fibrosis (scarring) results in excess collagen and reduced flexibility.