Chapter 4: The Tissues – Part 1 (Epithelial Tissue, Membranes, and Glands)

Chapter 4: The Tissues – Part 1

Introduction to Tissues

Tissues are groups of cells with similar structure and function, working together to perform specific activities in the body. The study of tissues is known as histology, which is a foundational discipline in anatomy and physiology.

• Histology: The scientific study of tissues.

• Tissue: A group of cells with similar structure and function.

• Main tissue types in the human body:

  • Epithelial tissue

  • Muscle tissue

  • Nervous tissue

  • Connective tissue

• Example: Epithelial tissue lines the gastrointestinal tract and covers the skin surface.

Epithelial Tissue

General Characteristics and Functions

Epithelial tissue is a type of tissue that covers body surfaces, lines internal cavities and tubes, and forms glands. It serves as a protective barrier and is involved in absorption, secretion, and sensation.

• Locations:

  • Covers the body surface (skin)

  • Lines body cavities, tubes, ducts, and organs

  • Covers organs inside body cavities

• Functions:

  • Protection from physical and chemical injury

  • Protection against microbial invasion

  • Contains receptors for sensory stimuli

  • Filters, secretes, and reabsorbs materials

  • Secretes serous fluids for lubrication

• Structural features:

  • Apical (free) surface exposed to the body exterior or cavity

  • Lateral surfaces connect to adjacent cells

  • Basement membrane anchors the tissue to underlying connective tissue

  • Avascular (no blood vessels)

  • High regenerative capacity (mitosis)

Classification of Epithelial Tissue by Shape and Arrangement

Epithelial tissues are classified based on the shape of their cells and the number of cell layers present.

• Cell shapes:

  • Squamous: Flat, scale-like cells

  • Cuboidal: Cube-shaped cells

  • Columnar: Tall, column-like cells

• Cell arrangements:

  • Simple: Single layer of cells

  • Stratified: Multiple layers of cells

  • Pseudostratified: Appears layered but all cells touch the basement membrane

  • Transitional: Multiple layers with cells that can change shape (found in urinary system)

Examples of Epithelial Tissue Types

• Simple Squamous Epithelium: Single layer of flat cells; found in air sacs of lungs, lining of blood vessels.

• Simple Cuboidal Epithelium: Single layer of cube-shaped cells; found in kidney tubules, glands.

• Simple Columnar Epithelium: Single layer of tall cells; lines digestive tract, gallbladder.

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

• Pseudostratified Ciliated Columnar Epithelium: Appears layered, has cilia and goblet cells; lines respiratory tract (trachea).

• Transitional Epithelium: Multiple layers, cells change shape; found in urinary bladder, ureters.

Modified Arrangements of Epithelial Tissue

Some epithelial tissues have specialized arrangements to perform unique functions.

• Pseudostratified Epithelium: All cells touch the basement membrane, but nuclei are at different levels, giving a stratified appearance.

• Ciliated Epithelium: Contains cilia to move substances (e.g., mucus in respiratory tract).

• Goblet Cells: Unicellular glands that secrete mucus, found among columnar cells.

Epithelial Membranes

Epithelial membranes are sheets of epithelial tissue combined with underlying connective tissue, serving as protective barriers and lining surfaces.

• Types of Epithelial Membranes:

  • Cutaneous membrane: The skin; covers the body surface.

  • Mucous membrane (mucosae): Lines body cavities open to the exterior (e.g., digestive, respiratory, urinary tracts); secretes mucus.

  • Serous membrane: Lines closed body cavities (e.g., peritoneum, pleura, pericardium); secretes serous fluid.

• Structure: Consists of epithelial layer and underlying connective tissue.

Glandular Epithelium

Glandular epithelium forms glands that produce and secrete substances. Glands are classified as endocrine or exocrine based on their method of secretion.

• Endocrine glands: Ductless glands that release hormones directly into the bloodstream (e.g., thyroid, adrenal glands).

• Exocrine glands: Glands that release their products onto body surfaces or into body cavities via ducts (e.g., sweat, salivary glands).

Types of Exocrine Glands

• Unicellular exocrine glands: Single cells, such as goblet cells, that secrete mucus.

• Multicellular exocrine glands: Composed of a duct and a secretory unit; classified by structure and method of secretion.

Methods of Secretion in Multicellular Exocrine Glands

Type	Mechanism	Example
Merocrine	Secrete products by exocytosis; cells remain intact	Salivary glands, pancreas
Apocrine	Portion of cell pinches off with secretion; cell repairs itself	Mammary glands
Holocrine	Entire cell ruptures to release product; cell replaced by mitosis	Sebaceous (oil) glands in skin

Summary Table: Epithelial Tissue Types and Locations

Type	Shape	Layers	Location	Function
Simple Squamous	Flat	Single	Alveoli, blood vessels	Diffusion, filtration
Simple Cuboidal	Cube	Single	Kidney tubules, glands	Secretion, absorption
Simple Columnar	Tall	Single	Digestive tract, gallbladder	Absorption, secretion
Stratified Squamous	Flat	Multiple	Skin, mouth, esophagus	Protection
Pseudostratified Ciliated Columnar	Tall	Appears layered	Trachea, respiratory tract	Secretion, movement of mucus
Transitional	Variable	Multiple	Urinary bladder, ureters	Stretching, protection

Additional info: The notes reference the Pap smear (Papanicolaou test), which is a clinical application of epithelial tissue study, used to detect cancerous changes in cervical epithelial cells.