Lecture #5: Chapter 4a (Tissue Level of Organization - Epithelial Cells

What is a tissue?

A group of cells that have descended from a common progenitor and work together to perform a coordinated function.

What are the four types of human tissue?

Epithelial, connective, muscular, and nervous tissue.

What are the main functions of epithelial tissue?

Covers and protects the body, lines hollow organs, forms glands, and specializes in exchange with the environment.

What are the main functions of connective tissue?

Protects and supports the body and internal organs, connects organs, stores energy, and aids immunity.

What are the functions of muscular tissue?

Made of specialized cells that contract to generate force and produce body heat.

What are the functions of nervous tissue?

Detects and responds to stimuli and generates electrical signals called action potentials to stimulate changes in muscle or gland activity.

What are cell junctions?

Sites of contact between adjacent cells.

What are the 5 main types of cell junctions?

1. Tight junctions

2. Adhering junctions

3. Desmosomes

4. Hemidesmosomes

5. Gap junctions

What is the function of tight junctions?

Form leakproof tissues by binding plasma membranes together with interlocking transmembrane proteins.

Give examples of where tight junctions are found.

Stomach lining, intestinal epithelium, lining of the urinary bladder.

What is the function of adhering junctions?

Connect adjacent cells with transmembrane proteins, plaque, and microfilaments to resist pulling forces.

Give an example of where adhering junctions are found.

Contractions of the small intestine.

What is the function of desmosomes?

Connect adjacent cells like adhering junctions but using intermediate filaments, providing stronger connections than adhering junctions to prevent separation.

Give an example of where desmosomes are found.

Epidermis during stretching, heart muscle during contractions.

What is the function of hemidesmosomes?

½ a desmosome; Anchor cells to the basement membrane and resist abrasion.

Give an example of where hemidesmosomes are found.

Anchoring skin to underlying connective tissue.

What is the function of gap junctions?

Permit the flow of signals between cells through protein channels called connexons, allowing passage of ions and small molecules.

Give an example of where gap junctions are found.

Cardiac muscles beating in sync, electrical synapses in the nervous system.

Compare and contrast epithelial and connective tissues.

• epithelial

  • many cells tightly packed

  • avascular; NO blood vessels

  • function: covers surfaces & controls substance exchange

• connective

  • fewer cells in extracellular matrix

  • well-vascularized; blood vessels

  • function: provide structural support & transport substances

What are the key features of epithelial tissue?

High rate of cell turnover, nerve supply, avascular, and may contain microvilli or cilia on the apical surface.

What are the structural classifications of epithelial tissue layers?

Simple (1 layer), stratified (multiple layers), pseudostratified (1 layer that looks stratified).

What are the different shapes of epithelial cells?

Squamous (flat), cuboidal (cube-shaped), columnar (column-like), transitional (variable shape).

What is the difference between exocrine and endocrine glands?

Exocrine glands secrete into ducts onto epithelial surfaces, while endocrine glands secrete hormones into the bloodstream.

What are the structural classifications of exocrine glands?

Unicellular (single cell) and multicellular (distinct structures).

What are the 3 functional classifications of exocrine glands?

Merocrine (exocytosis), apocrine (decapitation), and holocrine (rupture).

Describe merocrine glands

• discharge secretory products by EXOCYTOSIS

• most of body’s glands = merocrine

Describe Apocrine glands.

• accumulate secretory products at the apical surface

• protrusion → DECAPITATION by exocytosis

  • e.g. mammary glands secreting breastmilk

Describe holocrine glands.

• accumulate secretory products in the cytosol

  • mature cells RUPTURE & leaked contents are released

  • e.g. sebaceoous glands

What is the scientific study of tissues?

Histology

Describe the 2 structural components and functions of basement membrane.

2 layers of protein:

1. Basal Lamina (epithelial cells)

2. Reticular Lamina (connective tissue)

functions:

• separates overlying epithelial tissue from underlying connective tissue

• provides surface for epithia to anchor, migrate, and grow

List function & location of simple squamous epithelium.

function:

• filter body fluids

• diffusion of nutrients & gases

where:

• endothelium → lines blood vessels

• mesothelium → makes SEROUS MEMBRANES

List function & location of simple cuboidal epithelium

function:

• secretion of hormones & other substances

• absorption of water/solutes

where:

• lining kidney tubules

• bronchioles

• thyroid, pancreas

List function & location of simple non-ciliated columnar epithelium

function:

• secretion of mucus; contain goblet cells

• absorption of nutrients

• microvilli at apical surface inc. surface area

where:

• stomach

  • mucus protects the lining from hydrochloric acid

List function & location of simple ciliated columnar epithelium

function:

• cilia move mucus from goblet cells

• protection from invasion (e.g. air pollutants)

where:

• bronchioles; pollutants stuck in mucus and cilia “sweeps” it out

• uterine tubes: cilia sweeps egg cells to destination

List function & location of pseudostratified non-ciliated columnar epithelium

function:

• no cilia or goblet cells

• absorption & secretion of mucus

where:

• epididymis

• ducts for larger glands

• male urethra

List function & location of pseudostratified ciliated columnar epithelium.

function:

• contains goblet cells & cilia

• protection from invasion

  • filtering of air

where:

• upper resp. tract

  • trachea, bronchi

List function & location of keratinized stratified squamous epithelium

function:

• superficial layer can be shed & replaced

• keratin = insoluble

  • mechanical srength

  • protects skin from abrasion, fluid loss, UV radiation, invasion

where:

• skin

• mouth

• esophagus

• rectum

List function & location of non-keratinized stratified squamous epithelium

function:

• no large amounts of keratin

• provides protection from abrasion & invasion

where:

• most surfaces

  • mouth, esophagus, vagina, tongue

    • e.g. allows mouth to not bleed after eating chips; lots of layers of cells to shed/protect

List function & location of stratified cuboidal epithelium

function:

• protects ducts and tubes from abrasion

where:

• lining sweat/sudoriferous glands

• parts of male urethra

List function & location of stratified columnar epithelium

function:

• protection from invasion

• secretion of mucus for lubrication

where:

• urethra

• esophageal & salivary gland ducts

• conjunctiva of eye (secrete mucus on surface of eye)

List function & location of transition epithelium (urothelium)

function:

• lines hollow organs that expand to store fluids

where:

• urinary bladder, ureters, portions of urethra