Megakaryocyte size and concentration in the bone marrow of thrombocytopenic and nonthrombocytopenic neonates.

Thrombocytopenia is frequent among sick neonates, but little is known about its underlying mechanisms. It is known, however, that neonatal megakaryocytes are smaller and of lower ploidy than their adult counterparts and that smaller megakaryocytes produce fewer platelets than larger, more polyploid, megakaryocytes. We hypothesized that neonatal megakaryocytes would not increase their size in response to thrombocytopenia, thus limiting the ability of neonates to mount a response. To test this, we obtained marrow specimens from thrombocytopenic and nonthrombocytopenic neonates and adults. Megakaryocytes were immunohistochemically stained, quantified using an eyepiece reticle, and measured using an image analysis system with incorporated electronic micrometer. We found that, after adjusting for differences in cellularity, neonates and adults had similar megakaryocyte concentrations. When samples from the same sources were compared (tibial clot and vertebral body sections in neonates, iliac crest biopsies in adults), there were also no differences in megakaryocyte concentration between thrombocytopenic and nonthrombocytopenic subjects. The megakaryocyte diameter, however, was greater in adults than in neonates (19.4 +/- 3.0 versus 15.3 +/- 1.7 microm, p<0.0001). Thrombocytopenic adults also had a higher proportion of large megakaryocytes than nonthrombocytopenic adults (p<0.001). This was not observed among thrombocytopenic neonates, suggesting a developmental limitation in their ability to increase megakaryocyte size.