Simple Epithelium

Object: Examination of Simple Epithelium

General Classification

Epithelium is classified by: (1) number of cell layers — simple (one layer) or stratified (multiple layers); and (2) shape of surface cells — squamous (flat), cuboidal, columnar, or transitional. Simple epithelium consists of a single layer of cells all directly attached to the basement membrane.

A — Simple Squamous Epithelium

Structure

Extremely thin, flat cells with a central disc-shaped or ovoid nucleus that bulges into the lumen. The cytoplasm is barely visible except around the nucleus. Cells fit together like floor tiles — irregular polygonal shapes with interlocking borders.

Locations and Function

• Mesothelium: lines all serous cavities (pleura, pericardium, peritoneum); reduces friction between organs
• Endothelium: lines all blood and lymphatic vessels; regulates exchange, haemostasis
• Alveoli (Type I pneumocytes): extremely thin for rapid gas diffusion
• Bowman's capsule parietal layer: kidney
• Loop of Henle (thin limbs): passive water/ion movement

B — Simple Cuboidal Epithelium

Structure

Cells appear square in cross-section with a large, centrally placed spherical nucleus. Height approximately equals width. Mitochondria abundant in actively transporting cells.

Locations and Function

• Thyroid follicles: secrete thyroid hormone; height increases with TSH stimulation
• Kidney collecting tubules and distal convoluted tubule: reabsorption
• Small ducts of exocrine glands: pancreas, salivary glands
• Ovarian surface epithelium
• Choroid plexus of brain: produces CSF

C — Simple Columnar Epithelium

Structure

Cells taller than they are wide; oval nuclei aligned in a row near the base. May possess: (a) microvilli (brush border) — in intestinal absorptive cells and PCT of kidney; (b) goblet cells — mucus-secreting unicellular glands interspersed among columnar cells; (c) cilia — in fallopian tube, uterus.

Locations and Function

• Stomach: surface mucous cells (no goblet cells, no brush border)
• Small intestine: enterocytes with brush border + goblet cells; absorption
• Large intestine: abundant goblet cells, fewer absorptive cells
• Gall bladder: simple tall columnar; no goblet cells; concentration of bile
• Fallopian tube: ciliated columnar — transport of ovum
• Endometrium

D — Pseudostratified Columnar Epithelium

Structure

Appears stratified because nuclei lie at different levels — but all cells contact the basement membrane (confirmed by EM). It is technically simple epithelium. Most commonly ciliated with goblet cells (respiratory epithelium).

• Trachea, bronchi, nasal cavity, paranasal sinuses: pseudostratified ciliated columnar with goblet cells (respiratory epithelium). Cilia beat synchronously to move the mucus blanket (mucociliary escalator) upward.
• Epididymis: pseudostratified with stereocilia (non-motile, long microvilli) — for reabsorption
• Male urethra (part)

Key Distinguishing Feature

All nuclei seem to be at different levels but no free surface cells are uninucleated — every nucleus is connected to the basement membrane if traced.