Stratum spinosum - Prickle cell layer - Functions of stratum spinosum

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Stratum spinosum or prickle cell layer of the epidermis has the vital function of Keratinization. Prickle cell layer is the fourth tier from the exterior and is situated between stratum granulosum and germinativum (basale).

The basale and spinosum keratinocytes are together denoted as Malpighian layer. This prickle tier ranges between 5-10 keratinocytes in thickness and the keratinocytes are modified basal keratinocytes having large nuclei which contain very prominent nucleoli.

Keratinization process begins in stratum spinosum. The large pale staining nuclei of prickle layer, function actively to synthesize cytokeratin, a type of fibrillar protein. Cytokeratin builds up within the keratinocytes and aggregate together to form tonofibrils. These tonofibrils form desmosomes which radiate from the keratinocytes forming points of intracellular connections between adjacent prickle keratinocytes. However it is to be noted that there is no continuity of cytoplasm through the 'prickles' of adjacent spinosum keratinocytes.

The name spinosum denotes the prickle or spiny appearance of the keratinocytes due to the radiating desmosomes. This net-like prickle tier provides protection to the underlying tissues. This net-like spinosum also gives some elasticity and flexibility to the epidermis. The prickle cells tier is thicker in areas of skin subject to frequent abrasion such as palms and soles.

One of the important function of the spinosum keratinocytes is the production of lamellar bodies having polar lipids, free sterols, phospholipids and enzymes. Bipolar lipids are produced and retained by these prickle keratinocytes, whose function is prevention of loss of moisture from the skin.

This spinosum layer contains Langerhans cells which are macrophages with the function of protection against pathogens. Langerhans macrophages contain Birbeck granules also known as Birbeck bodies, which are rod shaped with a striated appearance. Birbeck granules function in receptor-mediated endocytosis and phagocytosis. The origin of Langerhans cells is in the bone marrow and they migrate to this prickle region of epidermis. As these polyhedral spinosum keratinocytes approach the granulosum tier they get gradually flattened. Mitosis (cell division function), which occurs continuously in the basale epidermis is nearly absent in these spinosum keratinocytes.

Illustration of Prickle cell layer (stratum spinosum)

Layers of epidermal skin
stratum spinosum (Prickle cell layer)

Like the keratinocytes of basale region, spinosum keratinocytes are also susceptible to mutations caused by sunlight and can become malignant. Prickle cell carcinoma is found in fair-skinned individuals who have a history of longer term sun exposure.

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Reference:

• Harding, C. R. (2004), The stratum corneum: structure and function in health and disease. Dermatologic Therapy, 17: 6–15. doi: 10.1111/j.1396-0296.2004.04S1001.x

1.Image source:
http://en.wikipedia.org/wiki/File:Epidermal_layers.png
Author: Mikael Häggström, based on work by Wbensmith
License: CC BY-SA 3.0
2.Image source:
http://en.wikipedia.org/wiki/File:Prickle_cell.JPG
Author: Sandy G Phillips-Long
License: CC BY-SA 3.0
Current topic: Functions of stratum spinosum (Prickle cell layer).