spinosum

Spinosum, from Latin spinosus meaning thorny or spiny, commonly refers to the stratum spinosum, a major layer of the epidermis in human and other mammalian skin. The stratum spinosum lies above the stratum basale and below the stratum granulosum; its thickness varies by body site, typically containing about eight to ten cell layers in thin skin and more in thick skin such as the palms and soles.

Histology and cells: keratinocytes in the stratum spinosum develop cytoplasmic processes that extend to neighboring cells and are bound together by desmosomes, giving a prickly or spiny appearance on microscopic examination. The layer also contains Langerhans cells, dendritic immune cells that contribute to skin defense, and occasional melanocytes derived from the basal layer.

Development and function: keratinocytes originate in the basal layer and migrate outward through the spinous layer, where they begin to accumulate keratin. They progress toward the granulosum, where keratohyalin granules appear as part of the keratinization process. The primary roles of the stratum spinosum are to provide mechanical strength to the epidermis and to participate in immune surveillance.

Clinical relevance: thickening of the stratum spinosum, known as acanthosis, can occur in various skin conditions such as psoriasis and chronic dermatitis. Conversely, thinning or disorganization of this layer can be associated with aging, nutritional deficiencies, or systemic disease affecting the skin.

See also: epidermis, stratum basale, stratum granulosum, keratinocytes, Langerhans cells.