Introduction

Vernix caseosa is a physiological, viscous biofilm that is produced by desquamated fetal skin and sebaceous glands covering the fetus at the third trimester in-utero. The substance's gross morphology in post-partum is described by the etymology, as vernix means varnish, and caseosa means cheesy-like matter. Although an observer would think of it as an unfunctional byproduct of parturition, it has shown developmental effects on the fetal skin and other visceral systems' biochemical adaptation to the extrauterine environment. Also, it forms a mechanical barrier and vaginal lubrication, facilitating parturition, and protecting the fetus from vertical transmission of any microbes.[1][2][3][4]

Cellular Level

Vernix caseosa is composed of 80.5% (water), 10.3% (lipids) and 9.1% (proteins).[5]

However, since the fetal skin epidermal layer (stratum corneum, corneocytes) is still in its immature form (nonlamellar, no intracorneal desmosomes, and highly hydrated), which is desquamated within the vernix and mixed with sebaceous gland's sebum. Hence, the gross caseous morphology, while having approximately 80% water.

Water

The corneocytes in-utero engulf most of the vernix water, which helps in the epidermal development and the efficient connection with the local sebaceous glands. In elaboration, the immature corneocytes are devoid of nuclei, having networks of keratin microfilaments and a thick, nonlamellar layer of amorphous lipids, which indicates a way in which water contributes to the process of cornification (maturation) and the sebaceous glands forming an epidermal hydrophobic barrier in physiological assistance.[6]

Lipids

The lipids are produced by stratum corneum and the sebaceous glands.

Stratum corneum (ceramides):

• Cholesterol esters
• Wax esters, etc.

Sebaceous glands (sebum):

• Cholesterol
• Triglycerides
• Free fatty acids
• Phospholipids, etc

The non-polar lipids (e.g., triglycerides) are predominating, with carbon chains up to 32 atoms. Although the vernix lipids resemble the adult stratum corneum, it has a decreased level of stratification order, in comparison.[7]

Additionally, three different groups of lipid mediators are present within the vernix:

• Eicosanoids and related oxylipin analogs
• Endocannabinoids
• Sphingolipids

Demonstrating gestational phase, gender-specific, and maternal lifestyle-related differences, such as:

• Gestational phases: as the gestation progresses, there is a noted increase in the ceramide to sphingomyelin ratio, endocannabinoids-anandamide, and 2-arachidonoylglycerol.[8]
• Gender-specific: an increase in sebaceous glands' proportions of wax esters and triacylglycerols with long hydrocarbon-chains in female fetuses.[9]
• Maternal lifestyle: an anthroposophical maternal lifestyle (is an integrative lifestyle with philosophical basis, aiming to balance body, mind, and spirituality with a tendency to restrict the use of modern therapies, such as antibiotics or vaccines) shows an increase in the levels of linoleic acid oxidation products 9(10)-epoxy-12Z-octadecenoic and 12(13)-epoxy-9Z-octadecenoic acid (EpOMEs) and 12,13-dihydroxy-9Z-octadecenoic acid (DiHOME).[8]

Proteins

• Cathelicidins
• Defensins
• Cystatin A
• UGRP-1
• Calgranulin A, B and C, etc.

The resultant of many conducted studies is the presence of 41 proteins, of which 25 are novel (e.g., cystatin A); moreover, 39% of identified proteins are components of the fetal innate immunity, and 29% have direct antimicrobial properties.[10][1]

Development

In the fourth week of gestation, the fetal skin starts developing from the ectoderm forming a basal cuboidal layer, which later matures into the dermis. Sequentially, in the fifth week, a layer of squamous/cuboidal, non-keratinized cells layer is developed on the top of the basal layer forming the periderm or epitrichium that works as an adherent mechanical isolator for the underlaying epidermal strata of the skin to develop (stratification).

As the epidermal strata are forming and this peridermal layer is going through cyclic keratinization and desquamation, until the third trimester in which this peridermal layer will be replaced by the stratum corneum (cornification). Simultaneously, sebaceous glands will be forming from buds at the epidermal root sheath of hair follicles, working at the production of sebum. The anatomical and physiological combination of the stratum corneum (corneocytes) and sebaceous glands (sebum) forms the fetal vernix caseosa reaching maximal production and formation in the third trimester.[11][12][13]

Function

Vernix caseosa carries out various physiological functions to assist the fetal extra-uterine adaptation to the environment.  

Development of viscera in-utero

The vernix is in direct communication with the amniotic fluid, many of its contents are detached and mixed, which works as a supply to the fetus, as the fetus swallows/breathes the amniotic fluid. Mainly, glutamine (>20% of the amino acids in vernix) functions as a trophic factor for high mitotic-rate cells (e.g., the gastrointestinal tract epithelium, lymphocytes, etc.) as it is needed.[14] Additionally, there is an interaction with the lung's surfactants to ensure their functionality in maintaining alveolar vacancy.[15]

Skin formation

There are embryological processes for the skin to be cornified (matured) from which desquamation and dehydration of the stratum corneum are facilitated by the decreased pH, as an increased enzymatic activity will initiate.[16] Therefore, the vernix works as a moisturizing and hydrating biofilm in which these processes can be optimized, leading to a softer, healthier fetal skin (i.e., the vernix is approximately 80% water, slowly released).[17]

Thermoregulation  

It has been documented in pre-term neonates with underdeveloped epidermis and a high trans-epidermal-water-loss rate (TEWL) to have a subnormal temperature, which could be unsafe for further development (i.e., an increase in the risk of fatality would be present especially and pre-term neonates). However, better prognostic differences are in neonates with the developed hydrophobic layer formed by the vernix were documented. That solidifies an inverse relationship observed between the amount of the vernix and TEWL, which hints to a possible secondary mechanism in thermoregulation.[18]

Antimicrobial defenses

There are different immuno-peptides, enzymes, and lipids found within the vernix that work for the skin flora identification/suppression as the skin is developing, and opportunistic pathogens are trying to break these physical and innate barriers to reach a state of an eruption (disease).[19][20][1]

The immunological molecules include [10][21]:

• [Alpha]- Defensins [human neutrophil peptide (1-3)]
• Cathelicidins (LL-37)
• Lysozyme
• Lactoferrin
• Psoriasin
• Palate lung nasal epithelial clone (PLUNC)

• Annexin 1
• Secretory leukocyte protease inhibitors

• Neutrophil gelatinase-associated lipocalin (NGAL)
• Ubiquitin
• Ribonuclease-7, etc

These molecules work in correspondence with the development of the stratum corneum and the acquired immune system. Alongside, the developmental acidic change in the stratum corneum has been showing a supportive, secondary antimicrobial environment for skin flora and pathogens.[22] As a result, possible future research to utilize the vernix as a prophylactic skin agent against possible infections for those vulnerable, such as in atopic dermatitis patients and bacterial infections.[23] Hence, research implies that maintaining the vernix on infants might decrease the chances of nosocomial infections by an intact epidermal-barrier and the functional immuno-peptides within.[24]

Resolution of wounds and burns

The increase in water percentage, lipids, enzymes, and peptides (e.g., glutamine as a trophic agent) aids in the neonatal wound and burns healing. Therefore, researchers are testing the possibility of applying similar methods to adult wounds and skin-grafting of burns with associate layers of a similar structure as the vernix; moreover, vernix-based topical creams might demonstrate sufficient potency in treating epidermal wounds and augmenting stratum corneum repair and maturation in infants.[25][26]

Related Testing

In certain cases, a specimen could be taken from the neonate's vernix caseosa to check for maternal substance-use disorders and rule out related developmental abnormalities. The technique sees significant use for cocaine-misuse cases, in which traces and metabolites would exist in the vernix, but it could be useful in other cases such as in fetal alcohol spectrum disorder (FASD). However, there are limited quantities available for sampling, putting the laboratories in quantitative difficulties measuring the different molecules within the specimen.[27][28]

Additionally, in atopic eczema (AE) (a chronic, inflammatory skin disease, increasingly seen in children, affecting their quality of life adversely) a need for non-invasively extracted biomarkers that allow the investigation and early detection of AE to manage its development is being researched.[29][30] Although the pathogenesis is not fully understood, there is a significant correlation between defects in skin barrier function, immune dysregulation, and environmental/infectious agents with AE, knowing that the vernix plays a major role in the skin development, it puts it as a valid option for extraction.[31][32] Therefore, specimens of the vernix were tested to check for the correlation between the quantity of protein within and the future development of AE. The results were that the proteins increase in the vernix is a possible biomarker for the identification of newborns predisposed to developing AE (mainly polyubiquitin-C and calmodulin-like protein 5).[33]

In forensic cases of abandoned neonates, bloody vernix caseosa could be of benefit in identifying the putative mother as the vernix can be the carrier of maternal blood.[34]

Clinical Significance

Rarely, as a postoperative complication after a cesarean section or a vaginal delivery, there might be a trigger of an inflammatory response due to a leak of amniotic fluid into the peritoneal cavity leading to a condition called vernix caseosa peritonitis (VCP). The clinical presentation commonly presents as a history of recent parturition (increased in cesarean sections) with admission as an acute abdomen. The diagnosis is made after a laparotomy showing no uterine scars or rupture, with the cheesy biofilm over the viscera and taking a biopsy to the laboratory. The histopathological study shows acute fibrinous serositis with an increased neutrophilic infiltration and anucleate, fetal squamous cells confirming a diagnosis of VCP. The pathophysiology is multifactorial (e.g., immunological hypersensitivity, the premature rupture of the amniotic sac, etc.). The treatment is mainly by performing a peritoneal lavage and then starting a course of antibiotics until the patient shows no signs or symptoms of acute peritonitis.[35][36][37][38]

Nonetheless, uterine rupture cases, post-cesarean might present with different signs and symptoms (e.g., cessation of labor, fetal distress, vaginal bleeding, abdominal pain, etc.). However, a new sign might present called vernixuria (vernix caseosa in urine), as tears might extend till the bladder setting a pathway for urine and the vernix to mix.[39]

There are cases of neonatal respiratory distress syndrome due to vernix caseosa aspiration which must be transferred to secondary/tertiary healthcare facilities for the administration of inhaled nitric oxide and extracorporeal membrane oxygenation, as it could be fatal moreover, it could be complicated with persistent hypertension of the newborn (PPHN) which increases the mortality rate, exponentially.[40][41][42]

The research on producing a vernix caseosa biomimetic is still primitive but promising for barrier-deficient fetal skin. Although the biomimetic has a decreased viscosity and elasticity, it has demonstrated comparable benefits similar to the natural biofilm.[43]