HDL particles play a central role in the reverse transport of cholesterol, the process by which cholesterol in tissues other than the liver is returned to the liver for conversion to bile salts and excretion from the body and provided to tissues such as the adrenals and gonads for steroid hormone synthesis (Tall et al. 2008).
HDL particles are heterogeneous and can be fractionated into sub-populations based on their electrophoretic mobility, their density, or their content of various apolipoproteins (Kontush and Chapman 2006). All HDL particles share two key features: they are assembled on a protein scaffold provided by apolipoprotein A-I (apoA-I), and they are recycled to allow a net flow of lipids from peripheral tissues to the liver and steroidogenic tissues while allowing apoA-I molecules to be re-used.
Here, the assembly of nascent (discoidal) HDL particles on newly synthesized apoA-I, a process that in the body occurs primarily in the liver, and the loading of discoidal HDL with additional lipid through interaction with cells carrying excess cholesterol (transformation to spherical HDL) are annotated.