Histology of Epithelial Tissue

Epithelial Tissue Types

• Simple squamous: Single layer of flat cells; found in blood vessels, heart, lymphatic vessels, alveoli, kidney tubules, and serous membranes.
• Simple cuboidal: Layer of cube-shaped cells; located in kidney tubules, glands, choroid plexuses, terminal bronchioles, and ovaries.
• Simple columnar: Narrow cells; found in bronchioles, auditory tubes, uterus, fallopian tubes, stomach, intestines, gallbladder, bile ducts, and brain ventricles.
• Pseudostratified columnar: Appears stratified but is a single layer of ciliated tall cells; located in nasal cavity, sinuses, auditory tubes, pharynx, trachea, and bronchi.
• Stratified squamous: Multiple layers of flat cells; nonkeratinized forms found in the esophagus, vagina, and pharynx; keratinized forms found in the mouth.
• Stratified cuboidal: Rare type comprised of multiple cube-shaped layers; found in sweat, salivary glands, and ovarian follicles.
• Stratified columnar: Rare, with surface columnar cells and irregular or cuboidal deeper cells; located in mammary gland ducts, larynx, and parts of the male urethra.
• Transitional: Specialized cells that change shape; squamous when stretched and cuboidal when not; found in urinary bladder, ureters, and superior urethra.

Structural Units of Epithelial Tissue

• Microvilli: Increase surface area, especially in the small intestine.
• Cilia: Move materials across cell surfaces, found in the trachea.
• Goblet cells: Produce mucus, found in the stomach.

Cell Connections

• Tight Junctions: Bind adjacent cells together.
• Desmosomes: Provide mechanical links between cells.
• Hemidesmosomes: Anchor cells to the basement membrane.
• Gap Junctions: Channels allowing molecule passage between cells.

Glands

• Composed primarily of epithelium supported by connective tissue.
• Exocrine glands: Connected by ducts; examples include sweat and oil glands.
• Endocrine glands: Secretions directly into blood without ducts.

Nervous Tissue

• Specialized for conducting electrical signals (action potentials).
• Found in the brain, spinal cord, and ganglia.

Neurons

• Transmit information as action potentials and process data.
• Composed of:
  • Cell Body: Contains the nucleus, performs general cell functions.
  • Dendrites: Receive stimuli leading to electrical changes.
  • Axon: Transmits action potentials to other neurons.

Glial Cells (Neuroglia)

• Support neurons; protect, nourish, and form sheaths around axons.

Neuroglia Types

• CNS (Central Nervous System):

  • Astrocytes: Maintain blood-brain barrier, recycle ions and neurotransmitters.
  • Oligodendrocytes: Myelinate CNS axons.
  • Ependymal Cells: Line ventricles and produce cerebrospinal fluid.
  • Microglia: Remove debris and pathogens through phagocytosis.

• PNS (Peripheral Nervous System):

  • Schwann Cells: Myelinate peripheral axons.
  • Satellite Cells: Regulate nutrient and neurotransmitter levels around neurons in ganglia.

Integumentary System

• Comprises skin and accessory structures (hair, nails, glands); serves as the body's first line of defense.

Skin Functions

• Protection: Prevents water loss, UV damage, abrasion, and foreign invaders.
• Sensation: Contains sensory receptors for various stimuli.
• Vitamin D production: Facilitates calcium and phosphate absorption.
• Temperature regulation: Maintains homeostasis around 37°C (98.6°F).
• Excretion: Skin glands can secrete sweat.

Skin Structure

• Composed of two main layers: epidermis (outer layer) and dermis (inner layer).
• Epidermis: Made of stratified squamous epithelium, prevents water loss and abrasion.
• Dermis: Contains dense connective tissue, blood vessels, nerves, and other structures.

Epidermis Layers

• Stratum Basale: Deepest, mitotically active layer.
• Stratum Spinosum: Contains spiny keratinocytes.
• Stratum Granulosum: Contains keratin-rich cells, appearing grainy.
• Stratum Lucidum: Found only in thick skin, translucent layer.
• Stratum Corneum: Outermost layer with dead, keratin-filled cells.

Dermis Layers

• Papillary Layer: Thin connective tissue with blood vessels; forms fingerprints.
• Reticular Layer: Deepest layer, accounts for 80% of dermis.

Skin Color Factors

• Melanocytes: Produce melanin, contributing to skin color; varying melanin amount affects pigmentation.
• Melanosomes: Vesicles containing melanin.
• Protect against UV light: Melanin serves as a natural sunscreen.

Hypodermis (Subcutaneous Layer)

• Attaches skin to bone and muscle, contains loose connective tissue and adipose tissue.
• Serves as padding, insulator, and reservoir for lipids.

Accessory Structures of Skin

• Hair: Found almost everywhere except palms, soles, and various other areas.
• Nails: Composed of dead keratinized cells, offering protection.
• Glands: Include sebaceous (oil-producing) and sweat glands (eccrine and apocrine).

Nail Structure

• Nail Body: Visible part.
• Nail Root: Covered part by skin.
• Cuticle: Skin extending over nail body.
• Nail Matrix: Produces most nail tissue.
• Lunula: Whitish crescent shape at the base of the nail.

Burn Definition

• Injury to tissue caused by heat, cold, chemicals, electricity, or radiation.