The Tissue Level of Organization

Overview of Animal Tissues

Animal tissues are groups of cells with similar structure and function, organized to perform specific roles in the body. There are four primary tissue types in animals, each with distinct characteristics and functions.

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

• Connective tissue: Provides structural support, connects tissues, stores energy, and defends against pathogens.

• Muscle tissue: Responsible for movement, maintaining posture, and producing heat.

• Nervous tissue: Specialized for communication via electrical impulses, controlling and coordinating body activities.

With advancing age, tissue regeneration decreases, and cancer rates increase.

Epithelial Tissue

Classification and Functions

Epithelial tissue is classified based on cell shape and the number of layers. It serves essential functions such as protection, secretion, absorption, and sensation.

• Cell shape: Squamous (flat), cuboidal (cube-shaped), columnar (tall).

• Number of layers: Simple (one layer), stratified (multiple layers).

Epithelial tissue forms physical barriers and lines both external and internal surfaces.

Cell Secretion & Glands

Some epithelial cells are specialized for secretion, forming glands. Glands are classified as either endocrine or exocrine based on their method of secretion.

• Endocrine glands: Release hormones directly into the bloodstream; they do not have ducts. Examples include the pituitary, adrenal, and thyroid glands.

• Exocrine glands: Discharge exocrine secretions through ducts onto epithelial surfaces. Examples include sweat, saliva, mucus, earwax, and milk glands.

Example: The skin of the blue poison dart frog (Dendrobates azureus) contains both mucous and poison-secreting exocrine glands.

Classification of Exocrine Glands

Exocrine glands are further classified by their structure and method of secretion.

• Structure of the duct: Simple (undivided) or compound (divided).

• Shape of secretory portion: Tubular (tube-shaped), alveolar/acinar (blind pockets).

• Relationship between ducts and glandular areas: Branched (several secretory areas sharing one duct).

Methods of Secretion

• Merocrine secretion: Released by secretory vesicles via exocytosis. Example: Merocrine sweat glands.

• Apocrine secretion: Released by shedding cytoplasm. Example: Mammary glands.

• Holocrine secretion: Released by cells bursting, killing gland cells. Gland cells are replaced by stem cells. Example: Sebaceous glands.

Surface Specializations: Cilia & Microvilli

Some epithelial cells possess surface specializations to enhance their function.

• Cilia: Motile projections that move fluids across the epithelial surface (e.g., oviducts).

• Microvilli: Fingerlike extensions of the plasma membrane that increase surface area for absorption or secretion (e.g., lining of the small intestine).

Connective Tissue

Functions of Connective Tissue

Connective tissues have diverse roles, reflecting the properties of their components.

• Establishing a structural framework for the body

• Transporting fluids and dissolved materials

• Protecting delicate organs

• Supporting, surrounding, and interconnecting other tissue types

• Storing energy reserves (especially triglycerides)

• Defending the body from invading microorganisms

Components of Connective Tissue

• Specialized cells: Fibroblasts, fibrocytes, adipocytes, mesenchymal cells, melanocytes, macrophages, mast cells, lymphocytes, microphages.

• Extracellular protein fibers: Collagen, reticular, and elastic fibers.

• Ground substance: Proteins, carbohydrates, and water forming the matrix.

Connective Tissue Fibers

• Collagen fibers: Most common, strong, flexible, resist force in one direction. Abundant in tendons and ligaments.

• Reticular fibers: Form networks (stroma), strong and flexible, resist forces in many directions, stabilize cells and structures (e.g., sheaths around organs).

• Elastic fibers: Contain elastin, branched and wavy, return to original length after stretching (e.g., elastic ligaments of vertebrae).

Categories of Connective Tissue

• Connective tissue proper: Loose and dense types; connects and protects.

• Fluid connective tissues: Blood and lymph; transport cells and dissolved materials.

• Supporting connective tissues: Cartilage and bone; provide structural strength and protection.

Connective Tissue Proper

Loose Connective Tissue

• Areolar tissue: Least specialized, open framework, viscous ground substance, elastic fibers, holds capillary beds. Example: Subcutaneous layer under skin.

• Adipose tissue: Stores fat, absorbs shock, insulates, found in adults and children. Adipocytes do not divide but expand/shrink as fat is stored/released. Brown fat (in children) is more vascularized and contains many mitochondria, releasing energy and warming the body.

• Reticular tissue: Provides support, forms a complex stroma, supports functional cells of organs. Found in liver, kidney, spleen, lymph nodes, and bone marrow.

Dense Connective Tissue

• Dense regular connective tissue: Tightly packed, parallel collagen fibers. Forms tendons, ligaments, and aponeuroses.

• Dense irregular connective tissue: Interwoven collagen fibers, provides strength to dermis, forms sheaths around cartilage and bone, and capsules around organs.

• Elastic tissue: Made of elastic fibers, found in elastic ligaments of spinal vertebrae.

Fasciae

• Superficial fascia: Separates skin from underlying tissues.

• Deep fascia: Sheets of dense regular connective tissue.

• Subserous fascia: Lies between deep fascia and serous membranes lining body cavities.

Fluid Connective Tissues: Blood and Lymph

• Blood: Contains plasma (watery matrix), cells, and cell fragments (formed elements: erythrocytes, leukocytes, platelets).

• Lymph: Forms as interstitial fluid entering lymphatic vessels, monitored by the immune system, returned to veins near the heart.

Blood Cell Types

• Red blood cells (erythrocytes): Transport oxygen and carbon dioxide, lack nuclei, make up about half the volume of blood.

• White blood cells (leukocytes): Defend against infection and disease. Types include neutrophils, eosinophils, basophils, monocytes, and lymphocytes.

• Platelets: Cell fragments involved in blood clotting.

Supporting Connective Tissues: Cartilage and Bone

Cartilage

• Provides shock absorption and protection.

• Matrix is a firm gel containing chondroitin sulfates.

• Cells are chondrocytes, located in lacunae.

• Avascular; chondrocytes produce antiangiogenesis factor to discourage blood vessel formation.

• Surrounded by perichondrium (outer fibrous layer for support, inner cellular layer for growth).

Types of Cartilage

Bone (Osseous Tissue)

• For weight support; matrix is calcified (rigid due to calcium salts).

• Resists shattering due to flexible collagen fibers.

• Bone cells (osteocytes) lie in lacunae, arranged around central canals.

• Small channels (canaliculi) allow exchange of materials with blood.

• Covered by periosteum (outer fibrous and inner cellular layers).

Muscle Tissue

Types and Functions

Muscle tissue consists of cells specialized for contraction, producing movement, maintaining posture, and generating heat. Contractions are produced by the interaction of actin and myosin proteins.

• Skeletal muscle: Large muscles responsible for voluntary body movement. Composed of long, thin muscle fibers that do not divide; produced by myosatellite cell division.

• Cardiac muscle: Found only in the heart; cells form branching networks connected at intercalated discs, regulated by pacemaker cells, striated, involuntary control.

• Smooth muscle: Found in walls of hollow organs; cells are small, spindle-shaped, can divide and regenerate, non-striated, involuntary control.

Nervous Tissue

Structure and Function

Nervous tissue is specialized for conducting electrical impulses and is concentrated in the brain and spinal cord. It consists of two main cell types:

• Neurons: Primary cells for communication and control; consist of a cell body (with nucleus and nucleolus), dendrites (receive signals), and axon (carries outgoing signals).

• Neuroglia: Supporting cells that provide structural support, nutrients, and insulation for neurons; maintain the blood-brain barrier and defend against pathogens.

Additional info: Neurons transmit signals via action potentials, and neuroglia include astrocytes, oligodendrocytes, microglia, and ependymal cells.