Epithelial Tissue and Glands

Epithelium and Glands

• Epithelial tissue covers the body's exterior surfaces and lines internal closed cavities.
• It forms the secretory portion of glands and their ducts.
• Epithelial tissue also makes up receptors in certain sensory organs.
• Epithelial tissue is avascular, meaning it lacks blood vessels.

Learning Outcomes

• Students will understand the general and specific characteristics of various types of epithelia.
• They will be able to differentiate and compare the functions and locations of 8+ different types of epithelia.
• Students will understand that glands are collections of epithelial cells performing specific functions.
• Differences between mucous and serous glands, and modes of exocrine secretion (merocrine, apocrine, holocrine) will be discussed.

Four Basic Types of Tissues

• The four basic tissue types are epithelial, connective, muscle, and nervous tissue.
• Images of each tissue type are shown.

Epithelial Tissue

• Epithelial tissue is classified by cell arrangement (simple or stratified) and cell shape (squamous, cuboidal, columnar).
• Cell characteristics include arrangement in sheets, intercellular junctions, and distinct surface domains (apical, lateral, basal).
• Epithelial cells rest on a basal lamina (non-cellular, protein-polysaccharide-rich layer).

Epithelium-Function

• Transport: Motile cilia move particles/mucus (trachea, bronchi).
• Sensory Stimulation: Neuroepithelia (taste buds, retina, smell, hearing).
• Lubrication: Mucous and serous fluids (lubrication).
• Excretion: Filtering blood (urine, sweat).
• Reproduction: Sex cells.

Epithelium-Classification

• Epithelial cells are classified by their arrangement (simple or stratified) and shape (squamous, cuboidal, columnar).
• Simple epithelium is one cell thick.
• Stratified epithelium consists of two or more layers.

Epithelium-Classification of Cells (Squamous)

• Squamous cells are flattened, with their width exceeding their height.
• Simple squamous epithelium lines blood vessels, Bowman's capsule in the kidney, and lines respiratory spaces in the lung.
• Function of squamous cells involves exchange, barrier in the central nervous system, and exchange/lubrication between compartments.

Epithelium-Classification of Cells (Cuboidal)

• Cuboidal cells are approximately the same width, depth, and height.
• Simple cuboidal is one cell thick and is found in follicle walls of the thyroid, kidney tubules (especially collecting ducts), and the surface of the ovary.
• Simple cuboidal epithelium is also found in small ducts of exocrine glands.

Epithelium-Classification of Cells (Columnar)

• Columnar cells are taller than they are wide.
• Simple columnar epithelium lines the intestinal tract (from stomach to rectum), uterus, and cervix, and the gallbladder.
• Functions of columnar epithelium include absorption, secretion, and act as a barrier.

Epithelium-Classification of Cells (Pseudostratified)

• Pseudostratified epithelium appears stratified but all cells rest on the basal lamina.
• It lines the respiratory tract (trachea, bronchi), epididymis (stereocilia), and ductus deferens.

Epithelium-Classification of Cells (Transitional)

• Transitional epithelium is stratified and can accommodate distension.
• It lines the urinary bladder, parts of the urethra, ureter and renal calyces.

Epithelium-Classification of Cells (Stratified Squamous)

• Stratified squamous epithelium is multiple-layered, with superficial layers being squamous.
• It functions as a barrier and protects the body.
• Locations include the epidermis, lining of the oral cavity, and lining of the vagina.
• Can be keratinized (like skin) or non-keratinized (like the esophagus).

Glands

• Glands are composed of epithelial cells adapted for secretion.
• They secrete into ducts (exocrine), the bloodstream (endocrine), or the extracellular space (paracrine).
• Two main types are exocrine and endocrine.

Exocrine Glands

• They secrete substances into ducts that lead to the outside or surface.
• They can be unicellular (e.g., goblet cells) or multicellular (e.g, sweat glands, salivary glands).
• Exocrine glands are further classified according to their structure (tubular, alveolar, acinar).

Methods of Glandular Secretion

• Merocrine: Most common, secretion released through vesicles (e.g., salivary glands, digestive enzymes).
• Apocrine: Cell membrane pinches off to release secretion (e.g., sweat gland).
• Holocrine: Cells rupture to release products (e.g., sebaceous glands).

Epithelial Regeneration

• Certain epithelia (epidermis, intestinal epithelium) continually regenerate.
• Inappropriate stimulation of epithelial cell division can lead to tumor formation.
• Cancers of epithelial origin are called carcinomas; cancerous glands are adenocarcinomas. Mesenchymal origin cancers are sarcomas.

MCQ

• Transitional epithelium lines the urinary bladder.
• A lesion in the inner most layer of trachea impacts pseudostratified columnar epithelium.

This quiz focuses on the characteristics and functions of epithelial tissue, including its types and the role of glands. Students will explore the differences between mucous and serous glands and their secretion modes. By the end, participants will differentiate and compare over eight types of epithelia.