Week 5 - Epithelium & Connective Tissue Series 2: Epithelium 1

Review of epithelium characteristics:

Highly cellular with little intercellular material
Forms sheets/layers
Multiple functions - Exchange, Absorption + Secretion, Protection for Tissues
Cells attached to and supported by basal laminar
Avascular - No blood vessels
Dynamic - Cells have high turnover rate, good regenerative ability
Epithelium is found in all three germ layers > Ectoderm, Mesoderm, Endoderm

Basal Lamina:

Basal Lamina is the boundary layer which sits at the base of epithelial cells. Always lies at the interface between epithelial cells and adjacent connective tissue.

Can be present between:

Muscle cells + CT
Nervous tissue + CT
Forms boundary of a group of glandular epithelium

It can create cell polarity. As it is found at the basal of the epithelium, different regions of cells can receive different signals. Organelles will therefore be organised accordingly into region specific functions.

Characteristics of Basal Lamina:

Thickness: 20 - 100 nm.

Has the components

Fine granular + Fibrous proteins + Glycans
Collagen Type IV
Laminin (Large glycoprotein)
Nidogen
Perlecan (Proteoglycan linker)
Anchoring Fibrils

Origin of Basal Lamina:

Most components are secreted from the basal surfaces of EPITHELIAL CELLS, which produce the Type IV Collagen, Laminin, Nidogen, Perlecan.

But, reticular laminar, which sits beneath the basal laminar, also forms part of the basement membrane. It has collagen type fibrils which are synthesised + secreted by connective tissue, not epithelial cells.

Basement Membrane:

Made up of a basal lamina and reticular lamination. Or can be made from 2 fused basal laminae.

Function:

Filter - Selectively permeable, allows passage of small molecules, not large molecules or cells.

Fence - Marks boundaries between tissues, limiting tissue region growth. Acts as a migration surface for cell growth and repair.

Frame - Supports epithelium and attaches to underlying CT.

Glandular Epithelium:

Known as a secretory epithelium, cells produce secretion.
Occurs as:
- Single cells in surface epithelia, meaning it is known as glandular surface epithelium
- Cluster of cells (Acini) located deep to surface epithelium within CT.
- Discrete glandular organs
  Exocrine Glands (Salivary Glands) - Saliva
  Endocrine Glands (Adrenal Glands) - Release hormones such as adrenaline

Some organs can act as both exoskeleton and endocrine glands, I.E. Liver

Surface Epithelium:

Cells which form sheets to cover all natural surfaces + line all natural body cavities. Forms protective barrier layer which:

Selectively controls passage of substances
Modify the substances
Secretion (Inner to Cavity)
Absorption (Cavity/Outer to Inner)
Bidirectional Transportation (Both Direction)

Surface epithelium contains cell to cell adhesion. It has

Adhering Junctions (Attach Cells)
Tight Junctions (Control of substances passing through)
Communicating Junctions (Co-ordination)

Cell polarity is also needed. The asymmetrical internal organisation allows for defined surfaces + membrane domains, so organelles have specialised functions in specific locations.

Surface Epithelia:

Cells which form sheets to cover all natural surfaces + line all natural body cavities. Forms protective barrier layer which:

Types of surface epithelia:

Different appearances of surface epithelia reflect exposure to different environments and hence, different functional demands. Structure + Function is interrelated. I.E. If epithelium has several different functions, could be several cell types, each contributing to one component of the overall function

Appearances vary based on:

Number of cell layers

1 = Simple Epithelium

> 1 = Stratified Epithelium

Appears to be > 1, but is actually just 1 = Pseudostratified

Pseudostratified can be determined by there being cells forming different layers, but it is still only one layer.

Transitional Epithelium = The cells are different shapes in the surface layer, not just one consistent shape.

Shape of cells in contact with free surface
Squamous: Height < Width
Cuboidal: Height = Width
Columnar: Height > Width

Present surface modifications to surface of cells in contact with cavity/space
Microvilli, Cilia, Stereocilia, Keratinisation (Are cells alive or dead)

Hence, when examining surface epithelia and identifying, we ask three questions:

How many cell layers are present?
What is the shape of the cells in the apical layer?
Are there any surface modifications to cells in the apical cell layer?

Functional Correlations:

Individual structural features can indicate function.

Simple (Single Cell Layer) - Selective transport between environments (Absorption, Secretion, Transport)
Cuboidal + Columnar shapes support absorptive and secretory functions. Material are modified as they pass through epithelium. These shapes allow for enough space in cytoplasm for organelles to transport molecules
Squamous - Minimal distance between 2 environments. Allows for rapid diffusion of particles.

Stratified (Multi Layered) - Protective barrier function). Majority of Stratified epithelium are squamous cells. Creates more layers for less thickness.

More than 1 cell type indicates epithelium cells have > 1 function.

Cell turnover of surface epithelia:
Damaged/sub-optimal cells are replaced by new cells. These new cells arrive at the basal lamina edge of epithelium via mitosis.

Turnover Rates:

It is location specific. Most frequent turnover occurs in location which undergoes most stress.

Varies from days to months

I.E. GIT - Rapid Turnover

Skin - Slower Turnover

Large Glands - Very slow turnover rates.

Metaplasia:

A protective function. It is the transformation of a epithelium type into another type. Happens via replacement of a cell type with another which is not normally present in tissue. It happens as a it is adapting to injurious agents in a changed environment.

I.E. Cigarette Smoke, Alcohol, Bacteria

It is reversible if the original environment is restored. But persistent stress can lead to dysplasia or carcinoma.

For example, respiratory epithelium is replaced by a stratified squamous non-keratinised epithelium. This lessens the chance for toxins in smoke to gain entry into deeper tissue layers and blood. However, the original filtering function of the respiratory epithelium is lost, meaning air cannot be filtered, which can lead to other problems sometimes.

Learning Outcomes:

Review the characteristics that distinguish epithelial tissue from other tissue types

Epithelial Tissue Characteristics:

Highly cellular with little intercellular material
Forms sheets/layers
Multiple functions - Exchange, Absorption + Secretion, Protection for Tissues
Cells attached to and supported by basal laminar
Avascular - No blood vessels
Dynamic - Cells have high turnover rate, good regenerative ability
Epithelium is found in all three germ layers > Ectoderm, Mesoderm, Endoderm

Distinguish between surface & glandular epithelia according to locations, structural features & functions

Surface Epithelium:

Location: Found on the top of all natural surfaces and lines all natural body cavities.

Structural Features: Has

Adhering Junctions (Attach Cells)
Tight Junctions (Control of substances passing through)
Communicating Junctions (Co-ordination)

Functions:

Modifies substances and selectively controls their passage.
Secretion (Inner to Cavity)
Absorption (Cavity/Outer to Inner)
Bidirectional Transportation (Both Direction)

Glandular Epithelium:

Location: Lines certain internal organs

Structural Features:

Occurs as:
- Single cells in surface epithelia, meaning it is known as glandular surface epithelium
- Cluster of cells (Acini) located deep to surface epithelium within CT.
- Discrete glandular organs
  Exocrine Glands (Salivary Glands) - Saliva
  Endocrine Glands (Adrenal Glands) - Release hormones such as adrenaline

Some organs can act as both exocrine and endocrine glands, I.E. Liver

Functions: Secretion and Absorption

Describe the origins, properties & functions of the basal lamina

Basal Lamina is the boundary layer which sits at the base of epithelial cells. Always lies at the interface between epithelial cells and adjacent connective tissue. Therefore, it can be present between:

Muscle cells + CT
Nervous tissue + CT
Forms boundary of a group of glandular epithelium