What is the Skin Barrier?

The Stratum Corneum is the outermost layer of the skin and is responsible for permeability barrier function aka skin barrier.

A healthy barrier and hydrated skin = a happy skin

The Stratum Corneum has two main components, namely corneocytes and intercellular lipids, both structures originate from the terminal differentiation of keratinocytes. We can use the “brick-and-mortar” model (Elias 1988) as an illustration of the relationship of these two structures. The integrity of both determines the strength of the skin's barrier.

Formation of Corneocytes and intercellular lipids: Terminal differentiation of Keratinocytes

Stratum Corneum

Corneocytes have formed. Precursor lipids are released into Stratum
Corneum where they are converted into fatty acids and ceramides, making
up the intercellular lipids (mortar). Corneocytes act as the “bricks”, forming
the outermost layer of the skin.

Stratum Granulosum

Keratinocyte have died and converted profillagrin to filaggrin. Filaggrin is broken down to form the NMF, ensuring hydration of cells.

Stratum Lucidum

Keratinocyte to Corneocyte transition: Keratinocytes flatten.
Keratohyaline granules, made up of profilaggrin are present. The
cornified cell envelope begins to form with the conversion of profilaggrin
to filaggrin. Lamellar bodies produce precursor lipids, certain enzymes
and anti microbial peptides.

Stratum Spinosum

Formation of lamellar bodies and synthesis of precursor lipids (glucosyl
ceramides, phospholipids, sphingolipids, cholesterol and sulfate) +
formation of precursor proteins of Corneocyte cell envelope.

Stratum Basale

The "birth" of the skin's primary cell type keratinocytes from stem cells also known as basal cells or mother cells that attach to the basement membrane that forms the physical boundary of the dermis.