Vernix has a highly variable makeup but is primarily composed of sebum, cells that have sloughed off the fetus's skin and shed lanugo hair. 12% of the dry weight of vernix is branched-chain fatty acid-containing lipids, cholesterol and ceramide. Vernix of term infants has more squalene and a higher wax ester to sterol ester ratio than preterm infants.
Vernix is theorized to serve several purposes, including moisturizing the infant's skin, and facilitating passage through the birth canal. Vernix is also thought to have an antibacterial effect; though there is little to support a chemical role of vernix in protecting the infant from infection, it may form a physical barrier to the passage of bacteria.
The sebum in vernix is produced in utero by the sebaceous glands around the 20th week of gestation. Vernix appears primarily in full term infants, while premature and postmature births generally lack any.
In Latin vernix means varnish and caseosa means cheesy.
The vernix is sometimes offered as supporting evidence for the aquatic ape hypothesis (AAH) that human evolution involved a period of adaptation to a semi-aquatic environment. No other land mammal, including other apes, produces vernix-coated pre-term infants. In contrast, some sea mammals, including the harbour seal, do.