Key features of neonatal megakaryopoiesis. (A) Hematopoietic progenitors from full-term CB or adult PB were cultured in a collagen-based semisolid culture medium (Megacult; StemCell Technologies, Burnaby BC, Canada) in the presence of TPO only. MK colonies generated from CB progenitors were significantly larger than those generated from adult PB. Photomicrographs were taken at a magnification of 200x. (B) CD34⁺ cells from CB and PB were cultured in a serum-free liquid culture medium with 50 ng/mL of recombinant human TPO as the only growth factor. MKs were evaluated after a 14-day culture period. Representative photomicrographs and ploidy levels by flow cytometry of CB- and PB-MKs demonstrated the smaller size and lower ploidy levels of CB-MKs. Both pictures were taken at a magnification of ×600. (C) Despite their small size, cultured CB-MKs exhibit abundant α-granules containing von Willebrand factor and P-selectin. (D) Flow sorted 2N/4N CB-MKs (left) contain abundant granules (Gr) and a well-developed demarcation membrane system (DMS), consistent with mature MKs and similar to flow-sorted PB MKs with ploidy ≥8N (right). Adapted from Liu et al²⁸ and Davenport et al.¹¹⁴