Module 4 – Part I: Tissues and Membranes in Anatomy & Physiology

Origins of Tissues

Embryonic Germ Layers

During embryonic development, the formation of three primary germ layers is a foundational event. These layers give rise to all tissues and organs in the body.

• Ectoderm: The outermost layer; gives rise to nervous tissue and parts of the skin.

• Mesoderm: The middle layer; forms connective and muscle tissues.

• Endoderm: The innermost layer; develops into mucosal linings and organs such as the digestive tract.

Epithelial tissues are unique in that they can arise from all three germ layers.

Germ Layer	Major Derivatives
Ectoderm	Nervous system, skin, hair, nails
Mesoderm	Muscle, bone, connective tissue, circulatory system
Endoderm	Digestive tract lining, respiratory tract lining, mucosae

Cell Junctions

Mechanisms for Binding Cells

Cells, especially in epithelial tissues, are bound together by several mechanisms to maintain tissue integrity and function.

• Adhesive glycoproteins in the cell coat help cells stick together.

• Wavy contours of plasma membranes allow adjacent cells to fit together in a tongue-and-groove fashion.

• Specialized membrane junctions provide additional binding and communication.

Tight Junctions

• Protein molecules in adjacent plasma membranes fuse tightly, forming an impermeable barrier.

• Prevent passage of molecules through the intercellular space (e.g., between epithelial cells lining the digestive tract).

• Protect underlying tissues from digestive enzymes and microorganisms.

Desmosomes

• Mechanical couplings that prevent separation of tissue layers.

• Held together by glycoprotein filaments attached to thickened areas (plaques) inside the plasma membrane.

• Abundant in tissues subject to mechanical stress (e.g., skin, heart muscle, uterus).

• Types: Spot desmosomes (localized) and belt desmosomes (encircle the cell).

Gap Junctions

• Allow direct passage of ions and small molecules between adjacent cells via connexons (protein channels).

• Important for communication in electrically excitable tissues (e.g., heart, smooth muscle).

• Vital in embryonic development for distributing nutrients before the circulatory system is established.

Membranes

Structure and Function

Membranes are sheets of tissue that cover or line surfaces, or separate organs and parts of organs. They often provide specialized secretions and act as barriers to ions and solutes.

• At the cellular level: Membranes are lipid bilayers restricting passage of ions and solutes.

• At the tissue level: Basement membranes separate epithelia from connective tissue.

Categories of Membranes

• Epithelial membranes (serous, mucous, cutaneous)

• Connective tissue membranes (synovial)

Types of Membranes

Type	Location/Function
Mucous	Line cavities that open to the exterior (digestive, respiratory, urinary, reproductive tracts); kept moist; lamina propria is the connective tissue portion; functions in absorption/secretion.
Serous	Line sealed internal cavities (pleura, peritoneum, pericardium); have parietal (lining cavity) and visceral (covering organ) layers; secrete serous fluid to reduce friction.
Cutaneous	Covers the body surface (skin); thick, waterproof, and dry.
Synovial	Lines joint cavities; produces synovial fluid for lubrication and smooth movement.

Four Groups of Tissue: General Characteristics

1. Epithelial Tissue

• Examples: Outer layer of skin, outer layer of sweat glands

• Widespread throughout the body; covers organs and lines body surfaces.

• Anchored to a basement membrane; tightly packed cells; little intercellular material; generally lack blood vessels; replaced frequently.

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

2. Connective Tissue

• Examples: Blood, adipose tissue (fat), bone

• Major cell types: Fibroblasts (secrete fibers), macrophages (defense), mast cells (release heparin and histamine).

• Functions: Bind, support, protect, serve as frameworks, fill spaces, store fat, produce blood cells, protect against infection, repair tissue damage.

• Mesenchyme is the embryonic connective tissue derived from mesoderm; differentiates into all other connective tissues.

Bone as Connective Tissue

• Support: Provides a hard framework for the body and supports soft organs.

• Protection: Skull protects the brain; vertebrae protect the spinal cord; rib cage protects thoracic organs.

• Movement: Bones act as levers for muscles, enabling movement.

• Storage: Stores fat and minerals (calcium, phosphorus, etc.); minerals can be mobilized as needed.

• Blood cell formation (hematopoiesis): Occurs in red marrow of spongy bone; location varies with age (more widespread in infants, localized in adults).

3. Muscle Tissue

• Specialized to contract and produce movement.

• Three types:

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

  • Smooth muscle: Involuntary, non-striated, single nucleus, found in walls of internal organs and blood vessels.

  • Cardiac muscle: Involuntary, striated, single nucleus, found only in the heart, connected by intercalated discs.

• Comprises 40-50% of body mass.

• Functions:

  • Movement: Locomotion, manipulation, movement of substances through organs.

  • Maintains posture: Continuous adjustments to maintain body position.

  • Generates heat: Muscle metabolism produces heat, important for maintaining body temperature.

4. Nerve Tissue

• Examples: Brain, nerves, spinal cord

• Specialized to generate and transmit electrochemical impulses.

• Neurons: Conduct nerve impulses; supported by neuroglia (helper cells).

• Characteristics of neurons:

  • Extreme longevity (can live over 100 years with good nutrition).

  • Amitotic (lose ability to divide after differentiation).

  • High metabolic rate; require continuous oxygen and glucose.

  • Large, complex cells with a cell body and processes (axons, dendrites).

Summary Table: Four Types of Tissue

Tissue Type	Main Function	Examples
Epithelial	Protection, secretion, absorption	Skin, lining of GI tract
Connective	Support, binding, storage	Bone, blood, fat
Muscle	Movement	Skeletal muscle, heart, digestive tract
Nervous	Communication, control	Brain, spinal cord, nerves

Additional info: For further study, refer to the provided YouTube links on body membranes and tissues for visual reinforcement of these concepts.