Histo Epithelium 8/7

What are the defining structural features of epithelial tissue?

Epithelia are highly cellular, tightly adherent, avascular, and polarized. They rest on a basement membrane and have minimal extracellular matrix.

Where are epithelial tissues located?

They line all internal and external body surfaces, organs, ducts, and form the secretory portion of glands.

What is the regeneration capacity of epithelia?

Very high, due to basal stem or progenitor cell populations that constantly renew cells.

What are the main functions of epithelial tissue?

Protection, secretion, absorption, excretion, transport, lubrication, sensory reception, and reproduction.

Why do epithelia need high turnover?

They're exposed to stress, abrasion, dehydration, and pathogens. Constant renewal maintains the barrier and functions.

How does epithelial tissue reduce friction?

By secreting lubricating fluids (e.g., serous fluid in body cavities).

What criteria are used to classify epithelial tissue?

Microscopic structure, location, function, and pathological behavior.

What are the basic cell shapes in epithelia?

Squamous (flat), cuboidal (cube-like), and columnar (tall, oval nuclei).

What are the layer types in epithelial classification?

Simple: one layer

Stratified: multiple layers

Pseudostratified: all cells touch the basement membrane but appear layered

Transitional: variable layers and shapes, for stretching

How is stratified epithelium named?

By the shape of the apical (topmost) layer.

What does epithelial polarity refer to?

Functional and structural differences across apical, lateral, and basal domains.

What happens at the apical surface?

It faces the lumen or environment; may contain microvilli, cilia, or stereocilia to increase surface area, absorb, or move fluids.

What are microvilli and what is their function?

Actin-based membrane projections that increase surface area for absorption (e.g., intestines, kidneys).

What is the role of the basal surface?

Anchors the cell to the basal lamina and can have basal infoldings to accommodate mitochondria and ion pumps.

How does polarity support metabolic function?

It creates metabolic gradients, directs secretion, and organizes receptors and enzymes spatially.

What are the main types of cell junctions?

Tight junctions (zonula occludens), adherens junctions (zonula adherens), desmosomes (macula adherens), and gap junctions.

What proteins make up tight junctions?

Occludin, claudin, and JAMs. Linked intracellularly by ZO proteins to actin.

What adhesion molecule is calcium-dependent?

Cadherins, used in adherens junctions and desmosomes.

What is the E-cadherin/catenin complex involved in?

Adhesion, polarity, division, and differentiation.

What are focal adhesions?

Junctions linking actin microfilaments to ECM components (e.g., laminin, fibronectin) via integrins.

What are hemidesmosomes?

Anchor intermediate filaments to the basal lamina using integrins and type VII collagen.

What role do tight junctions play in barrier function?

Prevent paracellular diffusion; regulate permeability.

How are cell junctions regulated?

By Ca²⁺, phosphorylation, and G-protein signaling.

What pathology results from damaged hemidesmosomes?

Autoimmune blistering diseases (e.g., bullous pemphigoid) due to epithelium detaching from connective tissue.

What is a desmosome?

A spot-like anchoring junction using cadherins (desmoglein, desmocollin) to attach intermediate filaments (cytokeratin) between adjacent cells.

What is the function of desmosomes?

Provide mechanical strength and resist shearing forces.

How are hemidesmosomes different from desmosomes?

Hemidesmosomes anchor cells to the basal lamina (not another cell) using integrins, while desmosomes anchor two adjacent cells using cadherins.

What are gap junctions made of?

Connexons composed of connexin proteins form channels between cells.

Are gap junctions bidirectional and regulated?

Yes; they allow ions and small molecules to pass and are regulated by Ca²⁺ and pH.

What is the basal lamina?

A 40-75 nm thick layer made by epithelial cells, composed of type IV collagen, laminin, proteoglycans.

What is the basement membrane?

The basal lamina plus the reticular lamina (made by underlying connective tissue), visible by light microscopy.

What is the function of the basement membrane?

Structural support, selective permeability, filtration, cell signaling, and anchoring.

What is the embryonic origin of epithelial tissue?

Ectoderm, mesoderm, or endoderm, depending on location.

Why do many tumors originate in epithelia?

High turnover rates and exposure to environmental stress increase mutation risk; these cancers are called carcinomas.

What is the parenchyma of a gland?

The secretory epithelial portion responsible for function.

What is the stroma of a gland?

The connective tissue support framework.

How do exocrine and endocrine glands differ structurally?

Exocrine glands remain connected to surface via ducts; endocrine glands lose that connection and secrete directly into blood.

What do endocrine glands secrete?

Hormones that act in autocrine, paracrine, or endocrine fashion.

How can exocrine glands be classified by duct structure?

Simple: unbranched duct

Compound: branched duct

How can they be classified by secretory shape?

Tubular: tube-like

Acinar/alveolar: flask-like

Tubuloacinar: mixed type

What's the difference between secretory and ductile portions?

Secretory cells produce product; ductile cells modify and transport it.

What are characteristics of serous secretions?

Watery, enzyme-rich; cells stain basophilic with round nuclei and zymogen granules.

What are characteristics of mucous secretions?

Viscous glycoproteins; cells appear foamy with flat basal nuclei.

What is a mixed gland?

Contains both serous and mucous cells (e.g., submandibular gland).

What are myoepithelial cells and where are they found?

Contractile cells found around glandular acini (e.g., salivary, mammary, sweat glands) that help expel secretions.

Where are epithelial stem cells located?

Basal layer of stratified epithelia or base of glands.

Why are they important?

Support constant renewal of epithelial lining, especially in high-turnover areas like skin and GI tract.