Epithelium & Glands

What are the four main tissue types?

Nerve
Connective tissue
Epithelium
Muscle

*Use the acronym NCEM

What is unique about connective tissue compared to other tissue types?

Connective tissue consists of extracellular matrix + cells

What is epithelium?

Sheets of cells that line the external and internal surfaces of the body

What are two key characteristics of epithelium?

1. Avascular

2. Polarized

What does "avascular" mean for epithelium, and what is its implication?

• Separated from other tissues by basement membrane

• No blood supply

• Implication: Relies on diffusion for nutrients and waste removal

What does "polarized" mean for epithelial cells?

• Top of cell is different from bottom

• Divided into three domains: apical, lateral, and basal

To help remember the key characteristics and structure of epithelium, you can use the acronym "SLAVE":

S - Sheets of cells

L - Line surfaces

A - Avascular

V - Polarized

E - External and internal

Name the three domains of polarized epithelial cells (Think "LAB")

L - Lateral (sides)

A - Apical (top)

B - Basal (bottom)

What are the five main functions of epithelium?

S - Sensation (receptors)

P - Protection (barrier)

A - Absorption

T - Transport (permeability)

S - Secretion

*Think "SPATS"

How does epithelium provide protection?

Acts as a barrier

What aspect of epithelium allows for sensation?

Presence of receptors

How does epithelium facilitate transport?

Through its permeability

What are the three types of simple epithelium?

Squamous
Cuboidal
Columnar

How many cell layers does simple epithelium have?

One layer

Describe simple squamous epithelium

• Polygonal cells

• Flattened shape

• Flat nuclei

Describe simple cuboidal epithelium

• Cells as tall as they are wide

• Circular nuclei

Describe simple columnar epithelium

• Cells taller than they are wide

• Long nuclei

What is the basic structure of Simple Squamous Epithelium?

Single layer of flat cells

Cells resemble fried eggs when viewed from the top

Each cell contains a single nucleus

What are the main functions of Simple Squamous Epithelium?

Lines blood vessels and body cavities

Regulates passage of substances into underlying tissue

In which body cavities is Simple Squamous Epithelium found?

Pericardial cavity

Pleural cavity

Peritoneal cavity

What is the basic structure of Simple Cuboidal Epithelium?

Consists of a single layer of cube-shaped cells

Cells are arranged like a row of dice

What type of tissue commonly contains Simple Cuboidal Epithelium?

Glandular (secreting) tissue

What might the cube shape of Simple Cuboidal Epithelium cells suggest about their function?

The cube shape may provide more internal space for organelles involved in secretion or absorption processes

What is the basic structure of Simple Columnar Epithelium?

Consists of a single layer of cells

Cells resemble a row of pillars

Each cell is tall and columnar-shaped

What is a characteristic feature of the nucleus in Simple Columnar Epithelial cells?

The nucleus may be elongated

How do the cells in Simple Columnar Epithelium relate to each other?

The cells form a single row where neighboring cells attach to each other

Define Stratified Epithelium.

Epithelium made up of more than one layer of cells

Describe the cell arrangement in Stratified Squamous Epithelium.

Consists of multiple layers of cells

Cells closest to the free surface are very flat

Arrangement resembles a tall pile of very flat cookies

What are the two main types of Stratified Squamous Epithelium?

a. Keratinized

b. Non-keratinized

List the characteristics of Keratinized Stratified Squamous Epithelium.

Has dark bands

Contains dead cells in the outermost layer

Example: skin

List the characteristics of Non-keratinized Stratified Squamous Epithelium.

Contains nucleated cells throughout all layers

Tends to be protective

Cells remain alive in all layers

Define Stratified Cuboidal Epithelium.

Composed of multiple layers of cells

Only the most superficial layer is made up of cuboidal cells

The other layers can be other types of cells

In Stratified Cuboidal Epithelium, what shape are the cells in the superficial layer?

Cuboidal (cube-shaped)

Define Stratified Columnar Epithelium.

Composed of column-shaped cells arranged in multiple layers

What is Pseudostratified Columnar Epithelium?

A type of epithelium that appears stratified but is actually a single layer where:

• All cells contact the basement membrane

• Not all cells reach the surface

• Has a lot of cilia

Where is Pseudostratified Columnar Epithelium commonly found?

It lines the upper respiratory tract

What are two surface features of Pseudostratified Columnar Epithelium in the respiratory tract?

Cilia

Mucus

What is Transitional Epithelium (Urothelium)?

Made up of several layers of cells that become flattened when stretched

Lines most of the urinary tract

Allows the bladder to expand

Where is Transitional Epithelium (Urothelium) found?

It lines most of the urinary tract

Why is the ability of Transitional Epithelium to flatten when stretched important?

It allows the bladder to accommodate varying volumes of urine without damage to the epithelial lining

What is Complex (Seminiferous) Epithelium?

Epithelium found in the testes, physically divided into two compartments

Into what two compartments is the Complex (Seminiferous) Epithelium divided?

Basal and apical compartments

What structure divides the basal and apical compartments in Complex (Seminiferous) Epithelium?

The blood-testis barrier / Sertoli cell epithelial barrier

Where is Complex (Seminiferous) Epithelium found?

In the testes

What is the significance of the blood-testis barrier in Complex (Seminiferous) Epithelium?

It separates the basal and apical compartments, providing a specialized environment for different stages of sperm development

What is the apical domain of an epithelial cell?

The part of the cell facing towards the lumen or outside

List four types of epithelial specializations found in the apical domain.

Microvilli

Stereocilia

Cilia

Flagella

Describe the structure of microvilli.

• Can be striated or brush border

• Filled with actin

• Attached to terminal web

• Covered with glycocalyx made of glycoproteins

What is the main function of microvilli?

They increase surface area

What is the composition of the glycocalyx on microvilli?

Made of glycoproteins – glycosylated with small branched carbohydrate side chains

Are microvilli motile?

No, they are non-motile

How do stereocilia differ from microvilli?

Stereocilia are long microvilli

What is the main function of stereocilia?

They increase surface area

Are stereocilia motile?

No, they are non-motile

Describe the internal structure of cilia.

Microtubule filled

Axoneme structure: 9 doublets + 2 singlets

What proteins are associated with cilia?

Nexin

Dynein

What structure does a cilium attach to at its base?

It attaches to a basal body (9 triplets + 0)

Are cilia motile?

Yes, cilia are motile

What is the main structural difference between microvilli and cilia?

Microvilli are filled with actin, while cilia are filled with microtubules

What is the role of dynein in cilia?

Dynein is a motor protein that enables the movement of cilia

Where are flagella commonly found in human cells?

In spermatozoa (sperm cells)

Compare the structure of a flagellum to that of a cilium.

They have a similar internal structure (9+2 axoneme), but flagella are typically longer than cilia

What is the main function of flagella in spermatozoa?

To provide motility, allowing the sperm to swim towards the egg

Are flagella motile?

Yes, flagella are motile

What is the lateral domain of an epithelial cell?

The part of the cell that is on the sides, adjacent to neighboring cells

What is the general term for the specialized junctions in the lateral domain?

Junctional Complexes (also known as terminal bars)

What are the Junctional Complexes in the lateral domain?

Tight Junctions

Adherent Junctions

Desmosomes

Gap Junctions

What type of junction is described as an "occluding junction"?

Tight junctions (zonula occludentes)

Describe the distribution of tight junctions around the cell.

They surround the whole cell in a "quilted" pattern

Name two types of proteins involved in tight junctions.

Occludins and ZO (Zonula Occludens) proteins

What is one function of tight junctions regarding plasma membrane proteins?

They block the movement of plasma membrane proteins
*Helps maintain cell polarity by keeping different proteins in the apical and basolateral domains separate

How do tight junctions affect the movement of hydrophilic compounds between cells?

They prevent the movement of hydrophilic compounds between cells
*They create a selective barrier

Why are tight junctions called "occluding" junctions?

Because they occlude (block) the space between cells, preventing the passage of substances

How does the "quilted" pattern of tight junctions contribute to their function?

It creates a continuous seal around the cell, effectively separating the apical and basolateral domains

What is the Latin term for adherent junctions?

Zonula adherentes

Describe the distribution of adherent junctions around the cell.

They surround the whole cell in a "belt-like" manner

List four proteins involved in adherent junctions.

Cadherins

Catenins

Vinculin

Actinin

What cellular structures do adherent junctions connect between adjacent cells?

Actin filaments

What is the specialized form of adherent junction found in cardiac cells?

Fascia adherens

What is the main function of adherent junctions?

They connect actin filaments in adjacent cells, providing mechanical strength to the epithelium

Why might cardiac cells need a specialized form of adherent junction (fascia adherens)?

Cardiac cells need very strong mechanical connections to withstand the constant contractions of the heart

What are desmosomes also known as?

Macula adherentes

Describe the distribution pattern of desmosomes in epithelial cells

Randomly distributed "spot welds"

What are the main components involved in desmosomes?

Attachment plaques, desmoplankins, and cadherins

What cellular structures do desmosomes connect between adjacent cells?

Intermediate filaments

What is the main function of desmosomes?

To provide strong adhesion between adjacent cells

In which types of tissue are desmosomes particularly important?

Tissues that experience mechanical stress, such as skin and heart muscle

What is another name for gap junctions?

Nexus

How are gap junctions distributed in the cell membrane?

Randomly distributed "communication sites"

What are the building blocks of gap junctions?

Connexins

How many connexins make up a connexon?

Six

What is the size of the pores formed by gap junctions?

2 nm wide

What is the molecular weight limit for molecules that can pass through gap junctions?

Less than 1000 MW

What is the primary function of gap junctions?

Intercellular communication

What are the five main structures/features of the basal domain in epithelial cells?

Hemidesmosomes

Focal adhesions

Basal infoldings

Basement membrane

Basal lamina

What is the colloquial term used to describe hemidesmosomes in relation to the basement membrane?

Basement membrane "spot welds"

What are the three main components involved in hemidesmosome structure and function?

Attachment plaque, intermediate filaments, and integrins

What do hemidesmosomes connect to the basement membrane?

Intermediate filaments

What is the main function of focal adhesions in the basal domain?

They serve as actin attachment points

Which protein is involved in both hemidesmosomes and focal adhesions?

Integrins