Structure and Function of Stratum Granulosum Layer

The cells lying in the outermost layer of epidermis are sloughed off daily as dead cells. This is a continuous process, leading to formation of new layer of epidermis every 35 to 45 days.

The epidermis is the topmost layer of the skin, which is exposed to the environment. It is this layer that we scratch to relieve an itchy sensation. It is this layer that is affected with minor wounds. Compared to other layers in the skin, the epidermis is relatively thin, and just 5% of the skin is made up of the epidermis.

The epidermis is composed of 5 sub-layers. The outermost layer is the stratum corneum and the innermost layer is the stratum basale, through which blood vessels run to supply nutrition. The third layer of epidermis is the stratum granulosum.

It is a 3- to 5-cell layers thick and essentially made up of flattened polygonal or rectangular-shaped cells. It is a thin layer of epidermis and considered a transitional layer sandwiched between the metabolically active layers beneath and the non-viable layer (as it contains dead cells) above.

It is also referred to as the granular layer, as the cells contain irregularly shaped granules. There are two types of granules formed in this layer of the epidermis―the basophilic keratohyalin and the lamellar granules.

The basophilic keratohyalin granules secrete proteins like tonofilaments and filaggrin. These are keratin intermediate filaments that bundle together to form tonofibrils, which are nothing but prekeratin structures. Thus, the process of keratinization (formation of keratin) actually begins in the middle layer.

So, as the cells from the previous layer enter the granulosum layer, they become flat, their nuclei break up and keratin begins to form. Due to loss of nuclei, the cells die, which leads to the formation of dark cytoplasmic material within the layer.

So, when these cells reach the above layer (stratum corneum), they are fully packed with keratin and dead. It is the keratin that makes the skin flexible and strong.

The lamellar granules secrete a lipid-rich substance that coats the membrane lying between the cells of stratum granulosum. In short, it accumulates in the extracellular space. This lipid-rich material ensures that the cells are firmly connected together and also forms a waterproof barrier.

In other words, the lipids by acting as water sealant, make the layer waterproof, and moreover, also reduce its permeability. Due to the presence of these waterproofing lipids, stratum granulosum prevents water and water-soluble substances from passing through and entering the lower layers. Penetration of any foreign material or microbes is also not possible

This also ensures that the active cells of the lower epidermis do not combine with the dead cells lying above the stratum granulosum. The waterproofing ability of stratum granulosum is also effective at keeping the moisture trapped in the deeper layers of epidermis. It helps retain moisture and does not allow them to dehydrate.

Above the stratum granulosum usually lies the stratum corneum, which is impermeable and mainly composed of dead cells of keratin.