Electron microscopic characterization of neonatal platelet ultrastructure: effects of sampling techniques

Neonatal platelets: mediators of primary hemostasis in the developing hemostatic system

The human hemostatic system is developmentally regulated, resulting in qualitative and quantitative differences in the mediators of primary and secondary hemostasis as well as fibrinolysis in neonates and infants. Although gestational age-related differences in coagulation factor levels occur, the existence of a unique neonatal platelet phenotype remains controversial. Complicated by difficulties in obtaining adequate neonatal blood volumes with which to perform functional assays, ambiguity surrounds the characterization of neonatal platelets. Thus, much of the current knowledge of neonatal platelet function has been based on studies from cord blood samples. Studies suggest that cord blood-derived platelets, as a surrogate for neonatal platelets, are hypofunctional when compared with adult platelets. This relative platelet dysfunction, combined with a propensity toward thrombocytopenia in the neonatal intensive care unit population, creates a clinical conundrum regarding the appropriate administration of platelet transfusions. This review provides an appraisal of the distinct functional phenotype of neonatal platelets. Neonatal platelet transfusion practices and the impact of the relatively hypofunctional neonatal platelet on those practices will be considered.

Imparied platelet--dense granule release in neonates

PURPOSE

The aim of this study is to examine differences in platelet dense granule (PDG) uptake and release between preterm infants, term infants, and adults.

METHODS

PDG uptake and release was examined by flow cytometry using mepacrine fluorescent staining in phycoerythrin-anti-GPIIb/IIIa bound platelets taken from cord blood of eight term infants and seven preterm infants and venous blood from eight adults.

RESULTS

Analysis of the baseline fluorescence in the untreated versus thrombin-treated samples revealed significant differences in the way infant PDGs responded to thrombin stimulation when compared with adults. Initial uptake of mepacrine in both term and preterm platelets was similar to that in adult platelets. Statistically significant differences between adults and both term and preterm infants, at two concentrations of mepacrine, were observed after stimulation with thrombin.

CONCLUSION

Persistent mepacrine staining of infant PDGs, when compared with those of adults, after thrombin stimulation implies defective infant PDG release. This may partially explain why infants have impaired response to agonists requiring ATP release from PDGs.

Increased neonatal platelet deposition on subendothelium under flow conditions: the role of plasma von Willebrand factor

In vitro platelet function of umbilical cord blood and neonatal peripheral vein blood from full-term newborns was compared with that of adults. Citrated whole blood was subjected to shear stress (1300 s(-1)) on subendothelial extracellular matrix (ECM)-coated wells in a cone and plate(let) analyzer. Adhered platelets on the ECM were quantitated by image analyzer. Both umbilical cord and neonatal peripheral blood platelets demonstrated more extensive adhesion than adult platelets, and similar aggregate formation on ECM. The ability of neonatal platelets to form aggregates on ECM was confirmed by scanning electron microscopy. Similar activation of neonatal and adult platelets after subjection to shear stress, in the suspension phase, was established by flow cytometry, which showed an increase in fibrinogen binding and a decrease in glycoprotein Ib expression on platelet membrane. The difference in adhesion rates between neonatal and adult platelets was preserved even when the hematocrit level of the neonatal blood was adjusted to that of adults. Reconstitution of neonatal or adult platelet-rich plasma with autologous or heterologous red packed cells yielded no change in adhesion and aggregation. When von Willebrand factor-covered plates were used to prevent deposition of plasma von Willebrand factor on the surface, no difference in platelet adhesion was seen between neonatal and adult blood. In conventional aggregometry assay, the response to ristocetin of washed platelets of either neonatal or adult source was higher on addition of plasma from neonates than from adults. Our data suggest that the extensive neonatal platelet deposition on ECM is mediated by plasma von Willebrand factor, which is known to be more multimerized and, therefore, more active in neonates than in adults. This mechanism may provide balanced primary hemostasis in neonates despite the platelet hyporeactivity to agonists without application of shear stress.

Platelet hyporeactivity in young infants during cardiopulmonary bypass

Platelet dysfunction is one of the most important factors contributing to a postoperative hemorrhagic diathesis in children with congenital heart disease undergoing operations requiring cardiopulmonary bypass (CPB). However, very little is known about the influence of CPB on platelets in neonates and young infants. We studied 16 patients--8 young infants (<2 mo old) and 8 children (>12 mo old)--with congenital heart disease undergoing CPB. Surface density of an important platelet adhesive receptor, glycoprotein Ib, and degree of platelet activation, indicated by p-selectin positivity, were measured by whole blood flow cytometry in samples obtained at seven time points during the operations. We found that the percentage of p-selectin-positive platelets increased significantly in children, but not in young infants, during CPB. The young infant group exhibited a significantly smaller reduction of glycoprotein Ib than the child group during CPB (21.0% +/- 12.0% vs 32.7% +/- 18.1%; P < 0.05). Lack of CPB-induced increase of p-selectin and a smaller decrease of glycoprotein Ib in young infants in the current study suggest reduced platelet reactivity in young infants and neonates during CPB. The clinical significance of the reduced platelet reactivity in young infants and neonates remains to be determined.

IMPLICATIONS

Platelets of young infants are less reactive than those of children during cardiopulmonary bypass, as determined by the cardiopulmonary bypass-induced alterations in platelet membrane adhesive receptors.

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.

Platelet disorders in newborn infants: diagnosis and management

Platelets are small, disc-shaped, anucleated cells formed by fragmentation of megakaryocytes in the bone marrow. They circulate in blood with a lifespan of 7 to 10 days and, together with fibrin, form hemostatic plugs at sites of vessel injury. Abnormalities of platelets, either quantitative or qualitative, may cause clinically significant bleeding with resultant morbidity and, on occasion, mortality. This review will focus on platelet disorders in neonates, defined as infants of up to 4 months of age. Special emphasis will be given to the physiology of platelet function in healthy and sick newborn infants. The review will be divided into sections as follows: role of platelets in hemostasis, platelet function in newborn infants, quantitative platelet disorders, qualitative platelet disorders, and platelet transfusion therapy.

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

Fetal dermal wound healing is unique because of its rapidity, minimal inflammation, and lack of scarring. Cytokines such as transforming growth factor beta (TGF-beta) and platelet-derived growth factor (PDGF) evoke an inflammatory response and scarring when applied to fetal wounds. Because adult and fetal platelet counts are comparable, the aim of this study was to test the hypothesis that the minimal inflammatory response seen in the fetus is attributable to differences in the serum content of cytokines released by fetal platelets. Using Yorkshire swine, blood was collected from 10 adults and 10 fetuses at day 60 of gestation (fullterm, 114 days). Platelets were isolated from anticoagulated blood and examined by transmission electron microscopy. Serum was analyzed for PDGF-AB and TGF-beta 2 by enzyme-linked immunosorbent assay (ELISA), and TGF-beta 1 by 125I radioimmunoassay. TGF-beta samples were assayed with and without prior acid activation to determine the total TGF-beta and the biologically active form of the cytokine. Electron microscopy of adult and fetal platelets showed no gross structural differences. Alpha granules, which contain cytokines as well as procoagulant factors, were present in similar quantities and with the same degree of homogeneity. The cytokines analyzed were present in all the adult and fetal sera tested. However, PDGF-AB was present in significantly lower concentrations in the fetus (383 +/- 72 pg/mL v 972 +/- 185 pg/mL in the adult; P<.05). In addition, the fetal samples contained lower amounts of TGF-beta 1 (13,895 +/- 1,770 v 29,864 +/- 5,050 pg/mL; P < .05) and TGF-beta 2 (6,758 +/- 734 v 13,407 +/- 1,395 pg/mL; P < .05). The majority of TGF-beta was in latent form; the adult sera contained significantly more active TGF-beta 1 and active TGF-beta 2 than the fetal sera. The ratios of active TGF-beta 1 to active TGF-beta 2 were similar for the adult (22.3) and fetus (18.5). However the ratio of total TGF-beta 1 to total TGF-beta 2 was significantly lower for the fetus (2.26 v 7.69). The authors conclude that although no gross differences in platelet ultrastructure were noted, fetal porcine platelets release lower quantities of cytokines into serum. This lower serum cytokine content and the relative concentrations of TGF-beta 1 of TGF-beta 2 may explain, in part, the minimal inflammation and sparse fibrosis characteristic of fetal wounds. These observations provide further insight into the unique fetal response to wounding and may offer alternative avenues to modulate the postnatal wound healing response.

Platelet ultrastructure of high-risk premature infants

Hemorrhagic and thrombotic complications are common in the term and preterm infant. Limited information is currently available about neonatal platelet structure and function, and how these may predispose infants to bleeding problems. This study comparing platelet ultrastructure of 71 different term and preterm infants with that of 15 adult control subjects revealed certain platelet morphological differences. Specifically, the adult platelets had more pseudopods, larger glycogen deposits, more visible microtubular structure, markedly fewer alpha granules, and smaller areas/perimeters than the infant platelets. Also, in infants greater than 31 weeks gestation, the platelets of vaginally-delivered infants were larger than those of both infants delivered by C-section and normal adults. These differences in platelet size and morphology may be related to developmental differences and/or the stress of delivery. These findings provide a framework for further exploration of neonatal platelet structure and function.

Ultrastructural morphometric analysis of human blood platelets exposed to minimal handling procedures

A detailed morphometric study of normal human blood platelets is described. The purpose has been to evaluate the morphological characteristics of platelets exposed to minimal handling procedures in order to obtain an optimal basis for the appraisal of platelets in disease. Blood from 10 healthy volunteers was collected directly into buffered glutaraldehyde and processed for electron microscopy. This platelet fixation procedure resulted in excellent preservation of resting platelet ultrastructure with one exception: the dense bodies. Compared to platelets fixed following washing procedures, our directly fixed platelets comprised fewer pseudopods and contained more glycogen. An unexpected feature of the open canalicular system was the apparent release of blisters interpreted as microvesicles. Employing a computerized image analyzer, 300 of the platelets were examined morphologically. The morphometric data thus obtained were analyzed statistically, resulting in a set of standard values for morphological characteristics of human platelets which we have found useful in subsequent evaluations of platelet morphology in disease. Significant inter-individual variance was, however, detected in two instances, in the section area of the alpha granules, as well as the area fraction of platelet sections occupied by channels of the open canalicular system (OCS). This should be taken into consideration when appraising platelet ultrastructure in health and disease.