Stratum granulosum definition - Stratum granulosum layer function

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Stratum granulosum is a well defined region of epidermis found under the lucidum layer in the palms and soles and under corneum in the other areas of the skin.

As granulosum has visible granules, it is also known as granular layer. Among the five sublayers of epidermis of skin, granulosum is in the middle having spinosum  and lucidum layers on the inner and outer sides.

Definition of stratum granulosum

Stratum granulosum is defined as "the layer of epidermis just under the stratum corneum or (on the palms and soles) just under the stratum lucidum; contains cells (with visible granules) that die and move to the surface." by  wordnetweb.princeton.edu/perl/webwn.

Structure and function of stratum granulosum

Skin structure showing stratum granulosum

In the regions of the body, where the skin is thin, there is no clear cut differentiation between granulosum and lucidum tiers. Under the microscope, this tier appears grainy and consists of 2-5 cell thickness. These grainy, granular or granulosum cells are the keratinocytes, which have migrated from the lower spinosum. These are the last layer of living cells which are in the process of losing their nuclei and dying.

These squamous cells have many basophilic granules called keratohyalin granules which are small and are not bound by membrane. Filaggrin, a type of protein, is found in large quantities in theses cells and is believed to bundle keratin. Lipid containing lamellar granules are present which are bound by membrane. These membrane-bound granules secrete the contents to extracellular space by exocytosis (peni-cytosis) and give rise to sheets of waterproof fatty barrier. This barrier function also prevent nutrients diffusion from the lower cells, leading to death of cells.

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Reference:
1.Harding, C. R. (2004), The st. corneum: structure and function in health and disease. Dermatologic Therapy, 17: 6–15. doi: 10.1111/j.1396-0296.2004.04S1001.x
Image source: http://en.wikipedia.org/wiki/File:Skinlayers.png
Author: Serephine
License: Public domain
Current topic in dynamic skin care: Stratum granulosum layer - Definition and function