Milk lipid globules and their surrounding membrane: a brief history and perspectives for future research

Most of the lipids in milk are triacylglycerols that occur in globules surrounded by a membrane derived from cellular membranes. This membrane, the milk-fat or milk-lipid globule membrane (MLGM), surrounds globules during the process of their secretion from the cell. The nature and cellular origin of the milk lipid globule membrane has been the subject of a considerable amount of research. Milk lipid globules originate as very small lipid droplets formed on or in the endoplasmic reticulum followed by release into the cytosol. These droplets consist of a triacylglycerol-rich core coated with a layer of proteins and polar lipids. How these droplets are formed, how they can grow in volume, how they move through the cell, and how they are secreted are questions that have been the basis for a number of investigations. While the general outlines of droplet formation, growth, movement, and secretion are known, virtually no molecular details of any of these processes have been elucidated. In this article I have presented a brief historical account of research on milk fat globules, their surrounding membrane, and on aspects of the intracellular origin, growth, and secretion of milk lipid globules. I have also attempted to call attention to those areas where further research is needed to gain a better understanding of the processes involved.