Cell Biology and Tissue Culture: Cell Culture Types and Evolutions

Learning Objectives for Cell Biology and Tissue Culture

Prokaryotic vs. Eukaryotic Cells: Students must be able to explain the structural differences between these two cell types and relate those differences to basic cellular functions.
Organelle Roles: Analyze the roles of major organelles and evaluate how their specific functions influence cell behavior in an in vitro (artificial) environment.
Adherent vs. Suspension Cultures: Compare these two cell types and assess how their physical growth characteristics affect laboratory techniques and general workflow.
Evolution of Cell Lines: Describe the progression of cultures from primary culture to continuous cell lines, including the implications this has for experimental use.
Culture Maintenance and Decision-Making: Apply decision-making skills to determine when cells require media changes or passaging (sub-culturing) based on observed morphology and growth patterns.
Trypsinization: Explain the specific purpose of the enzyme trypsin in animal cell culture and justify its use during the processes of cell detachment and passaging.

The Hierarchy of Cell Culture Types

Cell culture can be derived and categorized based on the complexity of the starting material and the resulting growth structure. The progression typically follows this path:

Organ Culture: * Definition: The process of cultivating all or part of an organ outside of a living organism, typically in an artificial environment. * Goal: To maintain the organ's native structure and function as closely as possible. * Applications: Researching organ development, disease pathology, and testing the effects of substances (toxins or drugs) on specific organ systems.
Tissue (Explant) Culture: * Definition: Removing living tissue (an explant) from an organism and placing it in a laboratory setting. * Mechanism: The tissue retains some of its original architecture. A cell line can be created through the "outgrowth" of cells from the edges of the explant.
Primary Culture: * Definition: The initial, direct culturing of cells obtained from a living organism's tissue or organ. * Key Characteristic: These are cells that have not yet been sub-cultured or passaged.
Cell Line: * Definition: Established when a primary culture is successful and is subsequently split (passaged) to allow for continued growth.
3D Culture: * Organoids: In vitro generated, three-dimensional ( $3D$ ) mini-clusters of cells that highly simulate the structure and function of the corresponding organ in vivo. * Spheroids: 3D clusters of cells that lack the complex internal organization of organoids.

Advanced 3D Culture and Bioengineering

Bioprinting: * A specialized type of $3D$ printing utilizing biocompatible materials, known as "bioinks," and living cells. * Purpose: To create complex, three-dimensional tissue structures by combining tissue engineering principles with 3D printing technology to fabricate functional biological components.
3D Cell Culture Environments: * An artificially created environment where biological cells grow or interact with their surroundings in all three dimensions. * Scaffold-Based: Cells are grown within a support structure made of synthetic or natural materials. * Scaffold-Free: Cells are encouraged to aggregate into clusters without an external support framework.

Methodology: Establishing a Primary Culture

A primary culture is established directly from living tissue using three primary methods, depending on the level of processing required:

Explant Culture: * The most gentle method with minimal processing. * Maintains some of the original tissue structure. * Cells migrate out from the tissue piece onto the culture surface.
Mechanical Dissociation: * Involves physical cutting or mincing of tissue. * Produces small cell clusters rather than a single-cell suspension. * This is a simple method but is less precise and is often combined with enzymatic methods to improve yield.
Enzymatic Dissociation: * Utilizes enzymes like trypsin to break down the extracellular matrix and cell-to-cell junctions. * Common Applications: Frequently used for epithelial and fibroblast cell types. * Outcome: Produces a single-cell suspension with a high cell yield. * Warning: Requires extremely careful timing to avoid proteolytic damage to the cells.

Case Study: Tissue Processing Examples

Zebrafish Hearts: * Explant culture involves placing adult zebrafish hearts on fibrin gels. * Specific focus is often on the apex of the ventricle and the behavior of epicardial cells as they interact with the cover glass and fibrin gel.
Fallopian Tube Processing: * Step 1: Intact fallopian tube with prominent fimbria is immersed in Phosphate-Buffered Saline ( $PBS$ ). * Step 2: Transfer to a Petri dish for manipulation. * Step 3: Mincing of the fallopian tube tissue into small fragments. * Step 4: Suspension of the minced tissue in a dissociation medium to prepare for primary culture.

Evolution and Transformation of Cell Lines

When a primary culture is sub-cultured, it moves into the "Secondary Culture" phase and may become a cell strain or a cell line.

Finite Cell Line: * Consists of "normal" cells with a limited life span. * Growth Curve: Initially, there is a decline in cell number due to selection, followed by exponential growth during the replicative phase, and finally growth arrest and deterioration following senescence.
Continuous Cell Line: * Cells that have the ability to grow indefinitely. * This state is achieved through Transformation, where cells undergo genetic changes—either spontaneous or induced—that allow them to bypass senescence. * Continuous lines often proliferate at an enhanced rate compared to finite lines.
Cell Strain: * Derived from a primary culture or a cell line through the selection or cloning of cells that possess specific properties or markers. * A cell strain is distinct from its parent cell line due to these specific, selected characteristics.

Methods of Cellular Transformation

Transformation is required to create a continuous cell line. Common laboratory protocols include:

Calcium Phosphate Transfection: Chemicals used to introduce DNA into cells.
Liposome Mediated Transfection: Using lipid vesicles to deliver genetic material.
Electroporation Protocol: Using electrical pulses to create temporary pores in the cell membrane for DNA entry.
Virus Infection: Using viral vectors to integrate new genetic material into the host cell genome.

Comparative Analysis: 2D vs. 3D Cell Culture

Feature	2D Cell Culture (Monolayer)	3D Cell Culture (Scaffold/Spheroid)
Advantages	Cost-effective and easy to use. Highly convenient with easy downstream processing.	More accurate representation of the in vivo (natural) scenario. More applicable drug resistance models.
Signaling	Reduced cell-to-cell interactions.	Increased relevant cell-to-cell and cell-to-Extracellular Matrix ( $ECM$ ) signaling.
Disadvantages	Less biologically relevant models.	Added expense and a more complex culture system.
Processing	Standard downstream processing.	Further complex downstream processing; endpoint assays depend heavily on the culture method used.

Microscopy Notes: Imaging at $25\,KV$ with magnifications of $500\times$ (for scaffold views) and $1.00\,KX$ (for detailed cell views) reveals the structural complexity of $3D$ environments compared to the flat landscape of $2D$ monolayers.

Selecting and Acquiring a Cell Line

Selection Criteria

When choosing a cell line for research, several factors must be considered:

Experiment Type: What is the ultimate goal of the study?
Species: Does the research require human, murine, or other animal cells?
Functional Characteristics: Do the cells need to perform specific biological functions?
Growth Life: Do you need a finite (normal) or continuous (transformed) line?
Culture Conditions: Available lab setup, specific media requirements, and necessary growth environments.
Logistics: Availability of frozen stocks.

Acquisition Sources

Establish Your Own: This involves creating a primary culture from specific tissue samples. This provides highly specific cells but is labor-intensive.
Commercial/Governmental Cell Banks: * ATCC (American Type Culture Collection): A primary global source for standardized cell lines. * Cell Bank Australia: A regional resource for authenticated lines.
Other Laboratories: * Risk: Transitioning cells from one lab to another is considered risky and is generally not recommended due to high potential for contamination and lack of authentication.

Characterization

Cell lines must be characterized to ensure they are what they are claimed to be. Key metrics include:

Growth Rate: Establishing the doubling time of the population.
Karyotyping: Analyzing the chromosomal makeup of the cells to ensure genetic stability or to identify transformation-related changes.