Traditional 2D monolayer culture flattens cells onto plastic, altering their morphology, signaling, and drug responses. 3D culture systems better recapitulate the in vivo environment by allowing cell–cell and cell–matrix interactions in all dimensions.

Why 3D?

Monolayer culture forces cells to adhere to a rigid, flat surface. This changes their cytoskeletal organization, nuclear shape, and gene expression. 3D culture restores:

• Natural cell–cell junctions and polarity.
• Diffusion gradients of oxygen, nutrients, and drugs.
• Extracellular matrix (ECM) interactions that regulate proliferation and survival.
• Resistance to chemotherapeutic agents that are effective against 2D monolayers.

Spheroid Culture

Spheroids are simple 3D aggregates of cells, typically 200–500 µm in diameter. They are formed by:

• Hanging drop: cells suspended in a droplet on the lid of a culture dish aggregate at the bottom of the drop by gravity.
• Ultra-low attachment (ULA) plates: round-bottom plates coated with a hydrophilic polymer prevent cell attachment, forcing cells to aggregate into a single spheroid per well.
• Magnetic levitation: cells are incubated with magnetic nanoparticles and assembled into spheroids using a magnetic field.

Spheroids develop a necrotic core when they exceed 400–500 µm, mimicking the hypoxic, nutrient-depleted cores of solid tumors. This makes them useful for studying cancer drug penetration and resistance.

Organoids

Organoids are self-organizing 3D structures derived from stem cells that recapitulate the architecture and function of an organ. They are more complex than spheroids, containing multiple differentiated cell types organized in a tissue-like pattern.

Organoids are grown in a basement membrane extract (Matrigel, BME) supplemented with a defined cocktail of growth factors. Common organoid types include:

• Intestinal organoids: contain all intestinal epithelial cell types (enterocytes, goblet, Paneth, enteroendocrine) in crypt-villus structures.
• Brain organoids: self-organizing neural tissue with cortical layering.
• Tumor organoids (tumoroids): derived from patient tumors, used for drug sensitivity testing and personalized medicine.
• Liver, kidney, pancreatic, retinal organoids: established for specific organs.

Scaffolds and Hydrogels

Natural ECM components (collagen I, Matrigel, fibrin) or synthetic hydrogels (PEG, alginate) provide a 3D scaffold for cell growth. The matrix stiffness, porosity, and ligand density influence cell behavior. Synthetic hydrogels offer better defined and more reproducible matrices than Matrigel, which is animal-derived and batch variable.

Assays in 3D

Standard cell assays require adaptation for 3D:

• Viability: ATP-based assays (CellTiter-Glo 3D) have improved lysis and detection for spheroids.
• Imaging: confocal or multiphoton microscopy is required to image through the thickness of the structure.
• Drug penetration: fluorescence-tagged drugs can be visualized as they diffuse through the spheroid.
• Invasion: spheroids embedded in collagen or Matrigel can be monitored for invasive outgrowth.