Claim: Corneocytes play a role as the natural moisturizing factor within human skin, taking in water from their surroundings so they can hydrate the skin's innermost lipid barrier.
Evidence: Corneocytes are terminally differentiated keratinocytes and compose most if not all of the stratum corneum, the outermost part of the epidermis. They are regularly replaced through desquamation and renewal from lower epidermal layers, making them an essential part of the skin barrier property. They contain a highly insoluble cornified envelope within the plasma membrane, and lipids (fatty acids, sterols and ceramides) released from lamellar bodies within the epidermis. Those junctions disintegrate as corneocytes migrate toward the surface of the skin and result in desquamation. Desquamation, commonly called skin peeling, is the shedding of the outermost membrane or layer of a tissue, such as the skin. At the same time, as those loosened junctions encounter more hydration, they will expand and connect together, forming potential entry pores for microorganisms. Corneocytes contain small molecules called natural moisturizing factors, which absorb small amounts of water into the corneocytes thereby hydrating the skin. The natural moisturizing factor is a collection of water-soluble compounds produced from the degradation of histidine-rich proteins called filaggrin, which are responsible for aggregating keratin filaments to form keratin bundles that maintain the rigid structure of the cells in stratum corneum. When filaggrin is degraded, urea, pyrrolidone carboxylic acid (1,2), glutamic acid and other amino acids are produced. These are collectively referred to as the 'natural moisturizing factor' of the skin. The components of the natural moisturizing factor absorb water from the atmosphere to ensure that the superficial layers of the stratum corneum stay hydrated. The intercellular lipid layer helps prevent the loss of natural moisturizing factor by sealing the outside of each corneocyte. 
Filaggrin (filament aggregating protein) is a filament-associated protein that binds to keratin fibers in epithelial cells. Ten to twelve filaggrin units are post-translationally hydrolyzed from a large profilaggrin precursor protein during terminal differentiation of epidermal cells. Urea, also known as carbamide, is an organic compound with chemical formula CO(NH2)2. Urea serves an important role in the metabolism of nitrogen-containing compounds by animals and is the main nitrogen-containing substance in the urine of mammals. It is a colorless, odorless solid, highly soluble in water, and practically non-toxic (LD50 is 15 g/kg for rats). Glutamic acid (symbol Glu or E; the ionic form is known as glutamate) is an α-amino acid that is used by almost all living beings in the biosynthesis of proteins. It is non-essential in humans, meaning the body can synthesize it. It serves as the precursor for the synthesis of the inhibitory gamma-aminobutyric acid (GABA) in GABA-ergic neurons. Amino acids are organic compounds that contain amine (–NH2) and carboxyl (–COOH) functional groups, along with a side chain (R group) specific to each amino acid. When filaggrin is degraded, urea, pyrrolidone carboxylic acid (1,2), glutamic acid and other amino acids are produced. A carboxylic acid is an organic acid that contains a carboxyl group (C(=O)OH) attached to an R-group.