Tissues and Histology

Definition and Importance

Tissues are groups of cells similar in structure and function, forming the basic building blocks of organs. Histology is the study of tissues, essential for understanding organ structure and function. Most organs contain several tissue types, and their arrangement determines the organ’s capabilities.

• Epithelial tissue: Lining, covering, and glandular tissue

• Connective tissue: Supportive and nutritive

• Muscular tissue: Contracts to produce movement

• Nervous tissue: Integration and control

Epithelial Tissue

Characteristics of Epithelial Tissue

Epithelial tissue consists of closely packed cells forming continuous sheets, which may be single or multiple layers. The apical layer is exposed to the body’s exterior or internal organ cavities, and may have modifications such as microvilli or cilia. The basal layer is attached to a basement membrane composed of a basal lamina made of glycoprotein.

• Functions: Protection, absorption, filtration, secretion

• Avascular: No blood supply; relies on diffusion from capillaries

• Innervated: Supplied by nerve fibers

• High regeneration: Well-nourished cells can reproduce rapidly

• Cell junctions: Tight junctions, desmosomes, gap junctions

Naming and Classification of Epithelium

Epithelial tissues are named based on cell shape and arrangement:

• Shapes: Squamous (thin, flat), Cuboidal (cube-shaped), Columnar (column-shaped)

• Arrangements: Simple (single layer), Stratified (multiple layers), Pseudostratified (appears stratified, but is not), Transitional (stretchable), Ciliated (possess cilia)

How to Name Epithelial Tissue

• First name: Number of layers (simple or stratified)

• Second name: Shape of cells (squamous, cuboidal, columnar)

Types of Epithelial Tissue

Simple Squamous Epithelium

This is the thinnest tissue in the body, forming semipermeable membranes for transport. Found in alveolar membranes (lungs), capillary walls, and kidneys.

• Function: Allows transport of gases, nutrients, wastes

• Basement membrane: Important filter for substances

Endothelium and Mesothelium

• Endothelium: Lines lymphatic vessels and cardiovascular system

• Mesothelium: Lines serous membranes of ventral body cavity and organs

Simple Cuboidal Epithelium

Single layer of cube-shaped cells forming ducts of glands, kidney tubules, and ovary surface.

Simple Columnar Epithelium

Single layer of column-shaped cells, with nuclei near the basal layer. Apical surface may have microvilli or cilia. Non-ciliated type is found in the GI tract; ciliated type in respiratory and reproductive tracts.

• Microvilli: Increase surface area for absorption (small intestine)

• Goblet cells: Secrete mucus for protection and lubrication

Stratified Epithelia

Contain two or more cell layers, regenerating from below. Function is protection; identified by the shape of the upper layer.

Stratified Squamous Epithelium

• Most widespread stratified epithelium

• Surface cells are squamous; deeper cells are cuboidal or columnar

• Forms epidermis of skin and lines body openings (mouth, anus, vagina)

Stratified Cuboidal Epithelium

• Typically two layers of cuboidal cells

• Rare; found in ducts of large glands (mammary, sweat glands)

Stratified Columnar Epithelium

• Limited distribution; found in pharynx, male urethra, glandular ducts

• Occurs at transition areas between other epithelia

• Only apical layer is columnar

Pseudostratified Ciliated Columnar Epithelium

Single layer of columnar cells of uneven height; nuclei at different levels. Ciliated type lines respiratory tract; non-ciliated found in large glands and male urethra.

Transitional Epithelium

Highly modified stratified squamous epithelium, found in bladder, ureters, and urethra. Allows stretchability to prevent excessive pressure during urination. Basal cells are cuboidal or columnar; apical cells change shape depending on organ fullness.

Glandular Epithelium

Definition and Types

A gland consists of one or more cells producing a secretion, usually a protein in aqueous solution or lipid-rich (oil glands). Two main types:

• Exocrine glands: Have ducts; secrete onto epithelial surfaces

• Endocrine glands: Ductless; secrete hormones directly into blood or lymph

Endocrine Glands

• Ductless glands

• Secrete hormones into blood or lymph

• Examples: Thyroid, pituitary

Exocrine Glands

• Have ducts

• May be unicellular (goblet cells) or multicellular (sweat, oil glands)

• Secretions empty through ducts onto surfaces

• Goblet cells secrete mucin, forming mucus for protection and lubrication

Multicellular Exocrine Glands

Complex structure with two basic parts: epithelium-derived duct and secretory unit. Supported by connective tissue, which supplies blood vessels and nerves, and forms a fibrous capsule dividing the gland into lobes.

Classification of Multicellular Exocrine Glands

Based on duct type and secretory unit:

• Simple glands: Unbranched ducts

• Compound glands: Branched ducts

• Tubular: Secretory cells form tubes

• Alveolar: Secretory cells form flask-like sacs

• Tubuloalveolar: Both types present

Mode of Secretion – Functional Classification

• Merocrine glands: Secrete products by exocytosis; cells remain intact (e.g., pancreas, sweat, salivary glands)

• Holocrine glands: Accumulate products until rupture; secretion includes cell fragments (e.g., sebaceous glands)

Summary Table: Types of Multicellular Exocrine Glands

Key Terms

• Epithelium: Tissue forming linings and coverings

• Basement membrane: Glycoprotein-rich layer anchoring epithelium

• Goblet cell: Unicellular gland secreting mucus

• Merocrine: Secretion by exocytosis

• Holocrine: Secretion by cell rupture

Additional info:

• Transitional epithelium is unique for its ability to stretch and change shape, crucial for organs like the bladder.

• Cell junctions (tight, desmosomes, gap) are essential for maintaining tissue integrity and communication.