Vernix caseosa

Vernix caseosa, also known as vernix, is a waxy gray substance covering the skin of newborn humans, produced by dedicated cells during fetal development and serving protective roles both in utero and shortly after birth. The term "vernix" comes from Latin, meaning varnish, while "caseosa" means cheesy, first published in 1846. Vernix forms around the 21st week of gestation when periderm cells shed and mix with sebaceous gland secretions, gradually covering the body. It aids in stratum corneum formation and is partially consumed by the fetus later in development, increasing amniotic fluid turbidity.

Composed primarily of water (80%), lipids (10%), and proteins (10%), vernix includes ceramides, cholesterol, fatty acids, and uncommon fatty acids like omega-7. Term infants have higher squalene levels than preterm infants. Morphologically, it consists of mobile corneocytes in a lipid matrix, with cells lacking nuclei and desmosomal attachments, forming a water-storage scaffold.

Vernix is white and viscous, non-polar, and more vapor-permeable than stratum corneum. Its functions include waterproofing skin during gestation, lubricating for birth, preventing infections via antimicrobial components, moisturizing the stratum corneum, aiding wound healing, and contributing to gut development when consumed. It also provides thermoregulation postnatally, though evidence is mixed.

Medically, vernix measures cocaine exposure in pregnant women and is proposed for diagnosing uterine rupture and amniotic fluid embolism. Disorders include granuloma and peritonitis from C-sections and Neonatal Aspiration Syndrome from high volumes. Vernix is unique to humans, though a 2018 study found similar material in California sea lion pups.