The electron microscopic structure of human umbilical cord blood lipoproteins

Neonatal VLDL, LDL, HDL2 and HDL3 were isolated from umbilical cord blood by preparative ultracentrifugation and analyzed by electron microscopy. Cord blood VLDL were round particles that were heterogeneous in size, mean diameter 49.5 +/- 10.3 nm. This size was very similar to that of the normal adult population. Cord blood LDL had a mean diameter of 25.9 +/- 3.4 nm. Most LDL particles were round in profile, but there was always a small fraction of particles which had flattened sides and formed short, linear aggregates. These latter structures may represent a metabolically distinct subpopulation of particles although their origin is unknown. Cord blood HDL3 were homogeneous round particles indistinguishable from those of the adult, and they had a mean diameter of 8.8 +/- 1.4 nm. HDL2 from cord blood had a mean diameter of 11.5 +/- 1.7 nm and are larger than the adult population. The HDL2 were characterized by the presence of small amounts of rectangular-shaped structures, 14.0 by 10.0 nm in size. These latter particles are enriched in the density fraction d 1.095 g/ml and are unique to the cord blood HDL. The presence of these unusual particles suggests that cord blood HDL may transport lipids in a somewhat different fashion from that of normal adult HDL.