Lower cytokine release by fetal porcine platelets: a possible explanation for reduced inflammation after fetal wounding

Reduced hyaluronan in keloid tissue and cultured keloid fibroblasts

Extracellular matrix hyaluronan is prominent during wound healing, appearing at elevated levels early in the repair process. It is prevalent throughout the course of fetal wound healing, which is scar-free, but decreases late in adult wound repair, that is often marked by scarring. To determine whether aberrant hyaluronan metabolism is associated with the excessive scarring that characterizes keloids, cultured fibroblasts derived from keloids and from the dermis of normal human skin and scar were compared. Levels of hyaluronan in 48 h conditioned media of keloid-derived cultures were significantly lower than in cultures of normal skin and scar fibroblasts. Profiles of hyaluronan polymer size were comparable in these two cell types, suggesting that excessive hyaluronan degradation was not involved. Hydrocortisone decreased hyaluronan levels approximately 70% in the conditioned media of both keloid and normal fibroblasts. Diminished hyaluronan accumulation in keloid-derived cells compared with normal fibroblasts was also observed in an in vitro wound healing model. Histolocalization of hyaluronan in keloids, normal skin, and scar samples confirmed the biochemical observations that the dermis of keloids, which comprises most of the scar tissue, contained markedly diminished levels of hyaluronan. Alterations in hyaluronan in the epidermis overlying keloids, however, were also observed. A modest increase in hyaluronan staining intensity was observed in the epidermis of keloids, as well as changes in the patterns of distribution within the epidermis, compared with that in normal skin and scar. Increased hyaluronan was present in the granular and spinous layers of the keloid epidermis Abnormalities are present apparently in both the overlying epidermis as well as in the dermis of keloids. Aberrations in signaling between keloid stroma and keloid epidermis may underlie abnormalities that contribute to the excessive fibrosis characteristic of these lesions.

Fetal wound repair: where do we go from here?

In contrast to adult wound healing, early-gestation fetal skin wound healing occurs rapidly, in a regenerative fashion, and without scar formation. The accelerated rate of healing, relative lack of an acute inflammatory response, and an absence of neovascularization distinguishes fetal from adult wound healing. However, this remarkable ability of the fetus to heal without scarring still remains poorly understood. The uncertainties include the role of cytokines, extracellular matrix components, homeobox genes, and certain cell types in the scarless wound repair process. Nevertheless, some strides have been made within the last two decades. This report, discusses the current knowledge of the mechanisms and characteristics of scarless fetal wound healing. Furthermore, to shy away from being just another all inclusive review, the authors point out deficiencies in the knowledge base on this important topic. Last, the future direction of research is discussed that may elucidate the mechanisms regulating the scarless repair phenomena.

Scar wars: implications of fetal wound healing for the pediatric burn patient

Scar formation and fibrosis often cause devastating disabilities in children suffering severe burn injury. In contrast to the child, the fetus has the ability to heal skin injury without scar formation, and instead with regeneration of epithelial and mesenchymal tissues and restoration of normal skin architecture. In this paper we review those unique features of the fetus and fetal wound healing that may contribute to the scarless repair process. It is hoped that an understanding of these remarkable reparative capabilities may lead to the development of new wound healing therapies that reduce or prevent scar formation and fibrosis in the management of children with burns.

Hyaluronic acid inhibits fetal platelet function: implications in scarless healing

Platelets are important for the initiation of inflammation in adults, but the role of fetal platelets in fetal wound healing is unclear because fetal dermal wounds heal with a minimal inflammatory response and lack of excessive scarring. Because fetal tissue is abundant in glycosaminoglycans (GAGs), predominantly hyaluronic acid (HA), this study was designed to test the hypothesis that HA inhibits the reactivity of platelets and thus contributes to the minimal scarring characteristic of fetal tissue repair. Platelets were isolated from 10 fetal pigs at day 80 of gestation (term, 115 days) and exposed to 0.5 mg/mL of arachidonic acid, an agent shown in prior studies to evoke maximal aggregation and degranulation of fetal platelets. The ability of HA at 0.1 and 0.5 mg/mL to inhibit this response was determined. The presence of HA resulted in a dose-dependent reduction in platelet aggregation at 180 seconds (control, 99.7 +/- 0.3%; HA [0.1 mg/mL] 91.7 +/- 3.8%; and HA [0.5 mg/mL] 48.5 +/- 9.0%; P < .005 v control). The onset of aggregation was also significantly delayed by 0.5 mg/mL of HA (13.5 +/- 2.5 seconds) compared to control (2.9 +/- 0.7 seconds), P < .05. No significant diminution of platelet aggregation could be achieved by the addition of other GAGs at similar concentrations. HA also significantly impaired the release of platelet-derived growth factor (PDGF)-AB from fetal platelets. The authors conclude that HA, the predominant GAG in fetal dermal matrix, inhibits platelet aggregation and cytokine release. This inhibition of platelet aggregation and resultant inflammatory response may explain, in part, the minimal inflammation and scarless healing characteristic of fetal dermal repair.

General principles of wound healing

Wound healing is a complex process involving different biologic and immunologic systems. Despite improvements in diagnostics and therapy, wound failures remain a clinical problem. The approach to a nonhealed wound is an interdisciplinary challenge that should not be underestimated. Better understanding of the complex wound-healing cascade helps our approach to wound healing and its possible failure. Manipulations of the involved immunologic features offer future therapeutic strategies.

Collagen induces cytokine release by fetal platelets: implications in scarless healing

In previous studies the authors demonstrated that unlike adult platelets, fetal platelets respond poorly to collagen. When platelets make contact with the exposed collagen at the site of injury, the result is activation, aggregation, and degranulation with the release of cytokines and growth factors. This sequence of events is well characterized in adult wounds, which heal with an acute inflammatory response and dense scar formation. In sharp contrast, fetal dermal wounds heal without an acute inflammatory response and minimal scar formation. Therefore, the aim of this study was to test the hypothesis that collagen, abundant at the site of dermal injury, is a poor inducer of cytokine release by fetal platelets. This could explain, in part, the minimal inflammation accompanying fetal dermal wound healing. Platelet suspensions from six fetal Yorkshire swine at day 80 of gestation (term, 114 days) were exposed to either arachidonic acid, 0.5 mg/mL, collagen, 0.19 mg/mL, or saline. The release into plasma of transforming growth factor-beta (TGF-beta 1), and platelet-derived growth factor (PDGF)-AB effected by these agents was determined by enzyme-linked immunosorbent assays. Transmission electron microscopy (TEM) was used to correlate the biochemical findings with ultrastructural changes and showed that arachidonate-treated platelets were aggregated and devoid of granules. In contrast, collagen-treated platelets had undergone conformational changes but showed only a moderate change in the quantity and homogeneity of their secretory granules compared with saline-treated controls. There was a significant increase in TGF-beta 1 release into plasma after treatment with collagen (6.64 +/- 0.36 ng/mL) and arachidonate (7.64 +/- 0.77 ng/mL) compared with saline (4.74 +/- 0.36 ng/mL), P < .05. Likewise, PDGF-AB release was significantly higher after collagen (0.22 +/- 0.02 ng/mL) and arachidonate treatment (0.44 +/- 0.04 ng/mL) compared with saline (0.09 +/- 0.02 ng/ mL), P < .05. The authors conclude that fetal platelets actually do release cytokines in response to contact with collagen despite poor aggregation. Therefore, impaired aggregation to collagen cannot solely explain the minimal inflammation after fetal wounding.

Intracutaneous injections of platelets cause acute pain and protracted hyperalgesia

Suspensions of autologous, washed platelets were intracutaneously injected at the volar forearms of healthy volunteers. Injections of serum and vehicle served as control. Subjects and experimenter were blind with respect to the sequence of injections. In contrast to serum and solvent solution, platelets induced graded burning pain lasting several minutes. Platelet but not serum or vehicle injections dose-dependently caused large axon-reflex flares. At the site of platelet injections an induration developed and in parallel delayed mechanical and heat hyperalgesia was observed. Hyperalgesia to pressure and impact stimulation reached a maximum after 6 h and subsided during the following 48 h. Also, the threshold to heat stimuli decreased moderately by about 1 degree C, on average, after 24 h. Neither indurations nor hyperalgesia could be detected at the injection sites of serum or vehicle. The pathophysiological significance of this new inflammatory model for the research of posttraumatic hyperalgesia is discussed.