What "Physiologically Relevant" Actually Means in Organoid Culture

Team Dynamic Matrices | 19 November 2025

Open almost any organoid paper, product page or grant and you will find "physiologically relevant" doing unearned work. It is applied to 3D culture in general, to matrices with wildly different properties, and to our own field's marketing more than we would like. We sell a matrix, so we have a stake in this. Here is the standard we try to hold ourselves to, and the numbers behind it.

It is not a synonym for "3D"

The most common misuse is treating "physiologically relevant" as interchangeable with "three-dimensional". A 3D culture in a gel with the wrong stiffness, the wrong degradation kinetics or an ECM composition that does not match the tissue is not more relevant than a well-characterised 2D system. It is just three-dimensional. Relevance is a property of how closely specific, measurable parameters match the tissue being represented. Dimensionality is one parameter.

Stiffness has to be tissue-specific and it is often narrow

The organoid mechanobiology literature has converged on something useful: the right stiffness is not a single "soft" target, it is tissue-specific and often surprisingly tight.

• Intestinal stem cells expand best at around 1.3 kPa, but organoid formation and differentiation require the gel to soften to roughly 190 Pa (Gjorevski et al., Nature, 2016).
• Human liver cholangiocyte organoids proliferate optimally around 1.3 kPa, matching native liver, with YAP signalling gating expansion (Sorrentino et al., Nature Communications, 2020).
• Brain tissue sits around 0.1 to 1 kPa, and cerebral organoid patterning is sensitive to stiffness in that range.
• Native tissues overall span from below 1 kPa (brain, fat) through 1 to 3 kPa (liver, kidney) to around 10 kPa (muscle) and beyond (Engler et al., Cell, 2006; Guimarães et al., Nature Reviews Materials, 2020).

Those windows differ by an order of magnitude, so a single "physiologically relevant" formulation used across organ systems is close to a contradiction. A matrix tuned for liver applied to intestinal work is not relevant to intestine, whatever the label says.

Stress relaxation matters as much as stiffness

Native ECM is viscoelastic: it dissipates stress on timescales of seconds to minutes. Chaudhuri et al. (Nature Materials, 2016) showed that at fixed stiffness, faster stress relaxation changes MSC spreading and lineage; Chaudhuri et al. (Nature, 2020) reviewed the same effect across cell types and organoids. A purely elastic gel, which describes most PEG systems, reports a stiffness that looks physiological while presenting a mechanical environment that does not exist in tissue. Any relevance claim should state a relaxation half-time alongside the modulus.

Degradability is not optional, and getting it wrong causes harm

Non-degradable gels are called physiologically relevant because they are gels rather than plastic, but real ECM is remodelled continuously by cell-secreted proteases. A matrix cells cannot remodel is missing a process native tissue depends on. And the coupling matters: Gjorevski showed that the timing of softening had to match the timing of differentiation. Degrading too early collapsed expansion; degrading too late blocked crypt formation. Stiffness and degradability are coupled design parameters, not independent knobs where any combination is equally valid.

Composition means specific motifs, not "biomimetic"

A hydrogel with a single RGD peptide is not compositionally equivalent to an ECM presenting laminin, collagen IV, fibronectin, proteoglycans and growth-factor-binding sites in tissue-specific ratios. Gjorevski found RGD sufficient for intestinal expansion but laminin-111 required for differentiation. Hernandez-Gordillo et al. (Biomaterials, 2020) enumerated the ligand set needed for enteroids and endometrial organoids in a fully synthetic gel. The point is that the required motifs can be listed, and a relevance claim should list them.

The four things a claim should specify

If "physiologically relevant" is to mean anything, a paper or product should state:

1. Target stiffness, with the native tissue value it was matched to and a citation
2. Stress relaxation behaviour, or an explicit statement that the gel is elastic
3. Degradation kinetics, and whether they are tunable independently of stiffness
4. Ligand identity and density, and why those ligands suit the tissue

A claim missing these is unsubstantiated, and reviewers, funders and purchasers should treat it the way they would any other unquantified assertion. We publish these four for DyNAtrix on the product page and on each certificate of analysis, and we would encourage you to ask every supplier, including us, for the same.

Precision is the whole point

None of this argues against 3D culture. It argues that the field's most important claim about why 3D matters deserves the rigour we already apply to reproducibility and mechanism. "Physiologically relevant" should describe a specific, falsifiable match between matrix properties and tissue biology. Matrices with independently tunable stiffness, relaxation, degradability and composition are what make that claim testable rather than aspirational.