Tissues of the Human Body

The Four Major Types of Tissues

The human body is composed of four primary tissue types, each with distinct functions and characteristics. These tissues combine to form organs and organ systems.

• Connective tissue: Provides structural support, binds tissues together, and stores energy.

• Epithelial tissue: Covers body surfaces, lines cavities, and forms glands; involved in protection, absorption, filtration, secretion, and sensory reception.

• Nervous tissue: Specialized for communication via electrical and chemical signals.

• Muscle tissue: Responsible for movement and force generation.

Main functions of epithelial tissue: Protection, absorption, filtration, excretion/secretion, and sensory reception.

Special Characteristics of Epithelial Tissue

1. Cellularity

Epithelial tissues are composed of closely packed cells with minimal extracellular material. Cells are tightly joined by specialized junctions such as tight junctions, desmosomes, and gap junctions.

• Cellularity: Densely packed cells, little extracellular matrix.

2. Polarity

Epithelial cells are polar, meaning they have distinct structural and functional regions:

• Apical surface: Faces the body surface or external environment. Example: Microvilli on the apical surface increase absorption in the respiratory tract.

• Basal surface: Attached to underlying connective tissue; regulates molecule exchange.

• Lateral surface: Between neighboring cells; contains junctions for cell adhesion and communication.

3. Attachment to the Basement Membrane

The basal surface of epithelial cells rests on a basement membrane, which consists of:

• Basal lamina: Derived from epithelial cells.

• Reticular lamina: Derived from connective tissue.

The basement membrane provides support, filtration, and regeneration cues.

4. Avascularity

Epithelial tissues lack blood vessels. They receive nutrients and remove waste by diffusion from capillaries in the underlying connective tissue.

5. Innervation

Most epithelial tissues are supplied by nerves, allowing them to sense stimuli such as touch and pain.

6. High Regeneration Capacity

Epithelial cells have a high mitotic rate. Stem cells in the basal layer continuously divide to replace damaged or lost cells, which is crucial for tissues exposed to friction or chemical stress.

7. Specialized Modifications

Depending on location and function, epithelial cells may show:

• Microvilli: Increase surface area for absorption.

• Cilia: Move substances over the surface.

• Keratinization: Provides protection (e.g., skin).

• Goblet cells: Secrete mucus for lubrication and protection.

Summary Table: Epithelial Tissue Characteristics

Cell Membrane Composition

The cell membrane of epithelial cells is supported by the basal lamina (from epithelial cells) and the reticular lamina (from connective tissue).

Classification of Epithelial Cells

2 Ways of Classifying Epithelial Cells

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

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

Mesothelium and Endothelium

• Mesothelium: The epithelium that lines body cavities (e.g., pleura, peritoneum).

• Endothelium: The epithelium that lines blood vessels and the heart.

Pseudostratified Epithelium

Cells appear layered because nuclei are at different levels, but all cells touch the basement membrane.

Keratin: Function and Location

• Function: Provides protection and waterproofing; found in the epidermis.

• Additional info: Keratin also provides stretching and recoil in the bladder.

Glandular Epithelium

Definition and Function

Glandular epithelial cells produce and secrete substances, including water, proteins, and lipids. Secretion is both a process and a product.

Categories of Glands

• Endocrine glands: Ductless; secrete hormones into blood and lymph.

• Exocrine glands: Have ducts; secrete substances onto surfaces or into ducts.

• Unicellular glands: Single cell found among epithelial cells (e.g., goblet cells, mucous cells).

• Multicellular glands: More than one cell; involve both epithelial and connective tissue layers (e.g., sweat, sebaceous, salivary glands).

Classification of Exocrine Glands

• Structure: Simple (unbranched) or compound (branched) ducts.

• Secretion type: Merocrine, apocrine, holocrine.

Merocrine Secretion

• Cells release their product by exocytosis; cell packages the substance in vesicles, which fuse with the cell membrane and release the content outside.

• Cell stays intact and keeps working.

• Example glands: Sweat glands (eccrine type), salivary glands.

Apocrine Secretion

• The top (apical) portion of the cell pinches off, carrying the secretion with it.

• The cell loses part of itself, but then repairs and continues working.

• Example glands: Mammary glands (milk secretion), some sweat glands in the armpit and groin.

Holocrine Secretion

• The whole cell fills up with product and then bursts.

• The entire cell dies and its contents become the secretion.

• New cells form to replace the destroyed ones.

• Example glands: Sebaceous (oil) glands in the skin.

Membranes: Types and Functions

Four Types of Membranes

• Mucous membrane: Lines passages open to the outside (digestive, respiratory); usually stratified squamous or simple columnar epithelium with lamina propria.

• Serous membrane: Lines body cavities; secretes fluid to reduce friction; simple squamous epithelium with connective tissue.

• Cutaneous membrane: Skin; keratinized squamous epithelium and connective tissue; dry outer membrane.

• Synovial membrane: Lines joint cavities.

Primary Germ Layers

Overview

During early embryonic development, three primary germ layers form and give rise to all tissues and organs in the body.

Ectoderm

• Outermost layer; forms skin and appendages (epidermis, hair, nails, sweat and oil glands).

• Forms nervous system (brain, spinal cord, peripheral nerves).

• Forms sensory organs.

• Additional info: Also forms enamel of teeth and pituitary gland.

Mesoderm

• Middle layer; forms muscles, bones, blood, and connective tissues.

• Forms skeletal, smooth, and cardiac muscles.

• Develops into bone, cartilage, and connective tissue.

• Gives rise to heart, blood vessels, and blood cells.

• Forms kidneys, ureters, gonads, and associated ducts.

Endoderm

• Innermost layer; forms linings of the digestive tract, lungs, and other internal organs.