Neutrophil recruitment is usually initiated by platelet degranulation at the wound bed during initial hemostasis after wounding. It is, therefore, not surprising that the diminished inflammatory response in fetal wound healing is partly attributed to differences in platelet activity. Platelet morphology is consistent across fetal and adult tissues, but fetal platelets have been shown to produce lower levels of transforming growth factor β1 (TGF-β1) and platelet-derived growth factor.¹⁴ They also demonstrate suboptimal aggregation in response to collagen prior to fetal transition from scarless healing to scar formation.¹⁵ Tumor necrosis factor α (TNF-α) and interleukin-1 (IL-1) are also released during platelet degranulation. Together these molecules act as chemoattractants, directing circulating neutrophils to wounded dermal tissue. TNF-α and IL-1 also play a role in upregulating neutrophil adhesion molecules, which have been found to be much less abundant among fetal neutrophils.¹⁶ Neutrophil migration to the fetal wound bed is limited by lower neutrophil-endothelial cell interactions secondary to attenuated adhesion molecule expression.¹⁷,¹⁸ The absence of subpopulations of proinflammatory and anti-inflammatory neutrophils further differentiates the early stages of the fetal wound healing response from that found in adults.¹⁹

Macrophage localization to the wound bed follows neutrophil infiltration.¹⁸ Persistent macrophage activity in wounds is thought to cause excess scar formation.²⁰ Studies of embryonic and fetal murine models demonstrate that macrophages are not recruited to wounds prior to
gestational day 14 (E14.5), except in cases of substantial tissue damage.²¹ Interestingly, fetal transition to scar formation occurs later at around E19 (term = E22).⁸ Despite the potential for macrophage recruitment during this window period between E14.5 and E19, their activation remains diminished because of noticeably lower TGF-β1 levels.²² This growth factor is in part responsible for the transition of circulating monocytes to activated macrophages. Conversely, TGF-β3 acts as a stop signal for terminal differentiation, and is at its peak concentration during the period of scarless fetal wound healing.²³,²⁴ This favorable ratio of TGF-β1:β3 contributes to the privileged scarless wound healing phenotype.

Mast cells are the third and final population to localize to the wound environment. The mast cell response is approximately halved in healing fetal tissues, and those that are recruited to the wound bed were noted to be more immature and less able to undergo degranulation. Fetal mast cells also release lower levels of inflammatory mediators including histamine, TGF-β, TNF-α, and vascular endothelial growth factor (VEGF), reducing neutrophil chemotaxis and extravasation.²⁵ Although mast cells were once thought to contribute to scar formation during fetal wound healing,²⁵ subsequent work has shown that they are not required for healing of cutaneous full-thickness excisional wounds in adult mice.²⁶

Three isoforms of TGF-β exist¹,²,²⁷ whose patterns of expression differ between adult and fetal wounds. Adult wounds demonstrate a greater quantity of TGF-β1 and TGF-β2, whereas TGF-β3 predominates in fetal wounds.²⁸ The finding that fetal wounds are relatively hypoxic compared to adult wounds suggests a potential role for hypoxia-inducible factor 1α in preferential TGF-β3 regulation.²⁹ In fact, elevated oxygen tensions have been shown to impede fetal fibroblasts.³⁰ TGF-β1 plays a major role in adult wound inflammation, stimulating chemotaxis in neutrophils, monocytes, and fibroblasts. The positive feedback loop generated by these infiltrating cells releasing further TGF-β1 is thus inhibited in fetal wounds, where there is a relative dearth of this proinflammatory cytokine. As mentioned previously, the TGF-β3 dominating the fetal wound response acts as a stop signal for terminal differentiation of tissue. Fetal cells at the wound bed may be held at a more immature state secondary to TGF-β3, with their less fully developed inflammatory machinery implicated in scarless healing in the fetus.

Similar to the TGF-β cytokine family, subtypes of ILs play antagonistic roles in the inflammatory response to acute wounding. Levels of IL-6 and IL-8, known proinflammatory signaling molecules highly expressed during adult wound healing, are diminished in the fetal wound.¹⁸,³¹ IL-10, however, acts as an anti-inflammatory cytokine. Fetal wounds have been found to overexpress IL-10,³² whereas its absence in fetal IL-10 knockout mice predisposes to scar formation that would otherwise not occur.³³ IL-10 inhibits proinflammatory expression of IL-1, IL-6, IL-8, and TNF-α³⁴; it also inhibits inflammatory cell migration while allowing for normal collagen deposition and dermal architecture.³²

VEGF is fundamental to angiogenesis in adult wound healing, but its upregulation has also been observed in association with keloid and hypertrophic scar development.³⁵,³⁶,³⁷,³⁸,³⁹ VEGF expression in fetal wound healing has been noted in some studies to be relatively tempered, resulting in fetal wounds that are less vascular than their fibrotic counterparts.⁴⁰ Our understanding of the exact role of VEGF in fetal wound healing is incomplete as results are not consistent across all studies. Colwell et al.⁴¹ noted significant VEGF upregulation in scarless wound repair occurring in E16 fetal mice compared to scarring repair in E18. However, despite this finding, there was little difference in terms of neovascularization based on histologic findings. Beyond stimulation of angiogenesis, VEGF does act in a proinflammatory manner by increasing vascular permeability and promoting inflammatory cell infiltration of the wound bed. Antibody-mediated neutralization of VEGF resulted in up to a 75% reduction in scar width.⁴⁰ Taken together, these findings suggest that VEGF plays a multifactorial role in modulating wound healing, whether up- or downregulating scar-forming processes.