Chylomicrons are lipoprotein particles that consist of triglycerides (85–92%), phospholipids (6–12%), cholesterol (1–3%), and proteins (1–2%).

Also known as (ULDL) ultra low-density lipoprotein relative to surrounding water.

Fatty acids, monoglycerides, and some diglycerides are absorbed by the duodenum, once the triglycerides have been broken down.

In the intestine, lipases and bile split triglycerides into monoacylglycerol and free fatty acids in a process called lipolysis. 

Theses substances are subsequently moved to absorptive enterocyte cells lining the intestines. 

The triglycerides are rebuilt in the enterocytes from their fragments and packaged together with cholesterol and proteins to form chylomicrons. 

Chylomicrons are excreted from the cells and collected by the lymph system and transported to the large vessels near the heart before being mixed into the blood. 

They are formed in the endoplasmic reticulum in the absorptive cells, known as enterocytes, of the small intestine.

The villi of the brush border of the small intestine provide a large surface area for absorption of chylomicrons.

The newly formed chylomicrons are secreted through the basolateral membrane of the small intestine into the lacteals, where they join lymph to become chyle.

Lymphatic vessels carry the chyle to the venous return of the systemic circulation.

Chylomicrons supply the tissue with fat absorbed from the diet.

Chylomicrons transport lipids absorbed from the intestine to adipose, cardiac, and skeletal muscle tissue

At these sites their triglyceride components are hydrolyzed by lipoprotein lipase, allowing the released free fatty acids to be absorbed.

Lipids from the diet bypass the hepatic portal system.

When a large portion of the triacylglycerol core have been hydrolyzed, chylomicron remnants are formed and are taken up by the liver, thereby also transf2242ing dietary fat to the liver.

There are three stages to chylomicron lifecycle:

Nascent chylomicron

Mature chylomicron

Chylomicron remnant

Triglycerides are emulsified by bile and hydrolyzed by lipase, resulting in a mixture of fatty acids and monoglycerides.

The above pass from the intestinal lumen into the enterocyte, where they are re-esterified to form triglycerides.

The triglycerides are then combined with phospholipids, cholesteryl esters, and apolipoprotein B to form a nascent chylomicron.

These are then released by exocytosis from the enterocytes into the, lymphatic vessels 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 composed primarily of triglycerides (85%) and contain some cholesterol and cholesteryl esters.

The main apolipoprotein component is apolipoprotein B.

Circulating chylomicrons exchange 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 to a mature chylomicron.

APOC2 is the coenzyme for lipoprotein lipase (LPL) activity.

Once triglyceride stores are distributed, the chylomicron returns APOC2 to the HDL, keeping APOE, and, becomes a chylomicron remnant, now only 30–50 nm.

Chylomicrons are one of the five major groups of lipoproteins: chylomicrons, very low-density lipoprotein, intermediate-density lipoprotein, low-density lipoprotein, high-density lipoprotein, that enable fats and cholesterol to move within the water-based solution of the bloodstream.

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