What Happens After the Injection?
What Does “New Collagen” Actually Mean?
30 Aug, 2026
We hear the phrase constantly in aesthetics: new collagen, collagen stimulation, collagen remodelling.
It sounds as though collagen is something the skin can simply be instructed to produce, with more of it automatically meaning better skin. But collagen is a little more complicated than that. The amount produced matters, but so do the steps that follow.
That made me wonder what we actually mean when we say that a treatment “stimulates collagen”.
Collagen starts inside the fibroblast
Collagen is made by fibroblasts, but the molecule they produce initially isn't the finished structure that eventually gives skin its tensile strength.
The process begins with long protein chains called pro-α chains. Inside the endoplasmic reticulum, selected prolines and lysines are modified, including the formation of hydroxyproline and hydroxylysine. Three pro-α chains then wind together into the characteristic triple-stranded structure of procollagen. Alberts et al., Molecular Biology of the Cell.
Procollagen is secreted into the extracellular space, where its terminal sections are removed. The resulting collagen molecules can then assemble into fibrils.
So when a paper reports increased collagen production, it may be describing an earlier part of a much longer process.
Making collagen is only the beginning
Once collagen molecules are outside the cell, they assemble side by side and end to end into fibrils. Those fibrils can then form larger collagen fibres.
The next step is cross-linking. Enzymes including lysyl oxidase modify particular lysine and hydroxylysine residues, allowing covalent bonds to form between neighbouring collagen molecules. These cross-links greatly increase the strength and stability of the fibrils.
This is not a minor finishing touch.
Wound Healing: Methods and Protocols notes that collagen maturation and cross-linking contribute substantially to tensile strength, and that measurements of collagen content alone don't necessarily tell us how mature or mechanically useful that collagen is.
That gives us a useful distinction: more newly synthesised collagen is not necessarily the same thing as more mature, functional collagen.
The arrangement matters too
Collagen fibres aren't simply packed into the dermis at random.
The cells themselves help regulate the size and arrangement of collagen fibrils, while other extracellular-matrix proteins help organise them. Fibronectin participates in fibril assembly, and other collagen types associate with fibrils and help influence their organisation. Alberts et al., Molecular Biology of the Cell.
The result is a tissue with a particular architecture rather than a pile of collagen molecules.
That matters because the mechanical properties of connective tissue depend on how the collagen is assembled and organised, not simply how much of it is present.
Which makes “collagen stimulation” a slightly incomplete description
An injectable can increase fibroblast activity or markers of collagen production, but those measurements don't necessarily tell us what happens to the collagen afterwards. The newly produced molecules still have to be processed, assembled into fibrils, cross-linked and incorporated into the surrounding extracellular matrix before they become part of mature connective tissue.
Those later stages are important because the organisation and cross-linking of collagen contribute to the mechanical properties of the tissue. Yet they are much harder to measure than collagen synthesis itself, which may explain why studies so often stop at production rather than following the whole process.
What might injectables actually be changing?
Different materials can influence this process in different ways.
With cross-linked hyaluronic acid, human studies suggest that the physical environment created by the filler may influence fibroblast behaviour. In a Wang et al. (2007), Archives of Dermatology study, fibroblasts around injected HA appeared stretched and biosynthetically active, with increased markers of newly synthesised collagen. The authors proposed that mechanical stretching of the surrounding dermis could contribute to the response.
A Turlier et al. (2013), Journal of Dermatological Science study also found a relationship between the physical behaviour of the filler, mechanical stress and collagen-related changes in treated skin.
PLLA gives us another example. In a Stein et al. (2015), Journal of Dermatological Science study, macrophages and fibroblasts were found around PLLA particles, with collagen III concentrated close to the particles and collagen I more peripherally. The response therefore involved more than fibroblast collagen production alone; the material, immune cells and surrounding matrix were all part of the process.
PDRN offers yet another mechanism. In a Thellung et al. (1999), Life Sciences study using human skin fibroblasts, PDRN increased fibroblast proliferation and produced evidence of A2-purinergic signalling. That provides a possible cellular pathway, although it does not establish what a modern PDRN or PN injectable does in intact human skin.
These treatments are often grouped together as “collagen stimulators”, but the biology underneath that label can be quite different.
So what does “new collagen” actually mean?
Perhaps it means less than we sometimes assume.
A rise in collagen synthesis tells us that cells are producing more collagen-related material. It doesn't automatically tell us whether that collagen has reached maturity, how it has been cross-linked or how well it has been organised within the surrounding matrix.
That doesn't make collagen production unimportant. It is essential to tissue repair and maintenance. It simply means that the final quality of the tissue depends on what happens after the collagen is made.
For aesthetic medicine, I think that distinction is worth keeping in mind.
When we hear that an injectable “stimulates collagen”, the more useful question may be what evidence we have for the whole process, rather than one measurement along the way.
Because skin doesn't just need collagen.
It needs collagen that has been made, assembled, matured and organised into tissue.
And that is a considerably more interesting thing to study.
C·🌷
Research notes
Collagen biosynthesis, post-translational modification, extracellular fibril assembly and covalent cross-linking are described in Molecular Biology of the Cell and Junqueira's Basic Histology. The wound-healing literature also emphasises the contribution of collagen maturation and cross-linking to tissue strength.
The injectable examples are included to show that “collagen stimulation” can arise within very different biological contexts. The HA and PLLA sections are supported by human tissue studies; the PDRN example is primarily mechanistic and should not be extrapolated directly to every commercial PN or PDRN product.
References
Mescher AL. Junqueira's Basic Histology: Text and Atlas, 16th ed. McGraw Hill; 2021.
Alberts B, Heald R, Johnson A, et al. Molecular Biology of the Cell, 7th ed. Garland Science; 2022.
Frantz et al. (2010), Journal of Cell Science. The extracellular matrix at a glance. 123(24):4195-4200. doi:10.1242/jcs.023820.
Wang et al. (2007), Archives of Dermatology. In vivo stimulation of de novo collagen production caused by cross-linked hyaluronic acid dermal filler injections in photodamaged human skin. 143(2):155-163. doi:10.1001/archderm.143.2.155.
Turlier et al. (2013), Journal of Dermatological Science. Association between collagen production and mechanical stretching in dermal extracellular matrix: In vivo effect of cross-linked hyaluronic acid filler. A randomised, placebo-controlled study. 69(3):187-194. doi:10.1016/j.jdermsci.2012.12.006.
Goldberg et al. (2013), Dermatologic Surgery. Single-arm study for the characterization of human tissue response to injectable poly-L-lactic acid. 39(6):915-922. doi:10.1111/dsu.12164.
Stein et al. (2015), Journal of Dermatological Science. The biological basis for poly-L-lactic acid-induced augmentation. 78(1):26-33. doi:10.1016/j.jdermsci.2015.01.012.
Oh et al. (2023), Cells. Poly-L-lactic acid fillers improved dermal collagen synthesis by modulating M2 macrophage polarization in aged animal skin. 12(9):1320. doi:10.3390/cells12091320.
Thellung et al. (1999), Life Sciences. Polydeoxyribonucleotides enhance the proliferation of human skin fibroblasts: involvement of A2 purinergic receptor subtypes. 64(18):1661-1674. doi:10.1016/S0024-3205(99)00104-6.
Wound Healing: Methods and Protocols. Humana Press.