Chylomicrons are large lipoprotein particles that transport dietary lipids from the intestines to other locations in the body. Chylomicrons are one of the 5 major groups of lipoproteins (chylomicrons, VLDL, IDL, LDL, HDL) which enable fats and cholesterol to move within the water based solution of the blood stream.
Chylomicrons transport exogenous lipids to liver, adipose, cardiac and skeletal muscle tissue where their triglyceride components are unloaded by the activity of lipoprotein lipase. Consequently chylomicron remnants are left over which are taken up by the liver.
There are three stages in the chylomicron's "life cycle":
- Nascent chylomicron
- Mature chylomicron
- Chylomicron remnant
Chylomicrons are created by the absorptive cells of the small intestine
. They are relatively large, having a diameter of 75 to 1,200nm
. These nascent chylomicrons are released by exocytosis
, lymphatic vessels
originating in the villi of the small intestine, and are then secreted into the bloodstream
at the thoracic duct's
connection with the left subclavian vein
Nascent chylomicrons are primarily composed of triglycerides (85%) and contain some cholesterol and cholesteryl esters. The main apolipoprotein component is apolipoprotein B-48 (APOB48).
While circulating in lymph and blood, chylomicron exchanges components with high-density lipoproteins
(HDL). The HDL donates apolipoprotein C-II
(APOC2) and apolipoprotein E
(APOE) to the nascent chylomicron and thus converts it into a mature chylomicron (often referred to simply as "chylomicron"). APOC2 is the cofactor for lipoprotein lipase
Once triglyceride stores are distributed, the chylomicron returns APOC2 (but keeps APOE) back to the HDL
and thus becomes a chylomicron remnant, now only 30 - 50 nm. APOB48 and APOE are important to identify the chylomicron remnant in the liver for endocytosis and breakdown.