Which is the best definition of chylomicrons?
a. enzymes synthesized in the intestines that transport cholesterol and fat‑soluble vitamins into epithelial cells
b ater‑soluble lipoproteins synthesized from triglycerides in epithelial cells and transported to the lymphatic system
c. fatty droplets containing lecithin that transport cholesterol and fat‑soluble vitamins into epithelial cells
d. fatty droplets removed from large globules of triglycerides by bile salts and transported to the lymphatic system

D. fatty droplets removed from large globules of triglycerides by bile salts and transported to the lymphatic system

Explanation:

The word ''chylomicron originates from two Greek words: ‘’chylos’’ means milky, and ‘’mikros’’ means small. Thus, chylomicrones means small milky (globules). It is a large plasma lipoprotein, occurring as a droplet consisting primarily of triglycerides and functioning in the transport of neutral lipids from intestine to the tissues by way of the lymph.  

Chylomicrones are considered large lipoproteins because of lipid, phospholipid and protein constituents. They consist of 85-92% triglycerides, 6% phospholipids, 1-3% of cholesterol and 1-2% proteins. The diameter of chylomicrones ranges from 75-1200 nm. The chylomicrone of 1200 nm is the largest and it is produced in the intestinal cells.

Chylomicrones are formed in the endoplasmic reticulum of the mucosal cells within the villi of the duodenum. They are secreted through the basolateral membrane into the lymphatic system. Then, from the lymphatic tissue they arrive into the blood system.

The life cycle of the chylomicrones consists of 3 stages that differ in the constituency of chylomicrones due to some of the protein in the form of apolipoprotein that will enter the chylomicron particles as it reaches its destination via the circulation.

The life cycle of chylomicrones has the following stages: the nascent chylomicrones, the mature chylomicrones and the remnant chylomicrones.

The nascent chylomicrones are in their initial form. They are formed in the intestinal epithelial cells or inside the enterocytes and released by exocytosis. They are in the central lacteal of vessels to enter the blood system through the thoracic duct. They contain mainly triglycerides, cholesterol, phospholipids and apolipoprotein B-48.

The mature chylomicrones origitnate when HDL donates apolipoprotein C2 and apolipoprotein E to the nascent chylomicron, and convert it to mature chylomicrone. Apolipoprotein C2 is the co-factor for lipoprotein lipase activity.

The remnant chylomicrones are formed once the store of triglycerides become low, the chylomicron donates its apolipoprotein 2 back to HDL. It is much smaller in size. Apolipoprotein B-48 and apolipoprotein are its main constituents and degrade by endocytosis.

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