The stratum spinosum is a layer of the epidermis whose name is due to the fact that it has a large number of tonofilaments that radiate from the cytoplasm to the desmosomes, which are proteins that connect adjacent cells.
The spiny layer originates from cell division of the basal layer, which is the deepest layer of the epidermis. The cells of the stratum spinosum have the function of differentiating and forming the granular and corneal layers. The presence of Langerhans cells and melanocytes in the spinous stratum provides, respectively, protect against pathogens and sunlight.
Source: Skinlayers_ (Italian).png: Adertderivative work: Fulvio314
General characteristics
The epidermis is made up of cells called keratinocytes, named for their keratin biosynthesis capacity. The spinous stratum, in addition to having keratinocytes, has scattered melanin granules and Lanhergans cells.
When the keratinocytes of the stratum spinosum migrate to the outermost part of the epidermis, they begin to produce keratohyalin granules and lamellar bodies.
Keratohyalin granules contain proteins, such as involucrin, loricrin, and profilaggrin. The latter is cut and turned into filaggrin.
Lamellar bodies, also known as membrane-covered granules, Odland's bodies, or keratinosomes, derive from the Golgi apparatus. They are 0.2–0.3 µm in size. They are main actors in the formation of intercellular components. They contain hydrolytic enzymes, polysaccharides and lipids.
The content of the lamellar bodies is discharged to the intercellular spaces of the granular layer through exocytosis. In this layer, lipids become precursors of other lipids that form part of the intercellular space of the stratum corneum.
The composition of the lamellar bodies changes as the cells migrate to the more superficial layers of the epidermis. For example, phospholipids are abundant in the basal layer, but decreased in the spiny layer.
Histology
The skin consists of two main layers: dermis and epidermis. The latter is a stratified squamous epithelium composed of keratinocytes, which are cells with the ability to synthesize keratin.
From the deepest to the most superficial, the layers that make up the epidermis are: stratum basal or germinative, stratum spinosum, stratum granulosa, stratum lucid and stratum corneum. Keratinocytes divide by mitosis and move up the epidermis to form the stratum spinosum.
The stratum spinosum consists of four to six levels of cells. Histological preparations generally cause cells to shrink. Consequently, in the intercellular spaces, numerous extensions of the cytoplasm, or spines, appear to form, projecting from its surface.
Spines are desmosomes anchored to tonofilaments, which are bundles of intermediate keratin filaments called tonofibrils that connect neighboring cells. Desmosomes are called Bizzorero nodes.
As keratinocytes mature and move to the surface, they increase in size, flatten, and are oriented parallel to the surface. Meanwhile, the nuclei of these cells undergo elongation, and the keratinocytes begin to produce keratohyalin granules and lamellar bodies.
Features
In the stratum spinosum, the lamellar bodies participate in the formation of the intercellular water barrier of the epidermis. This barrier is established during the differentiation of keratinocytes.
The elements of the water barrier of the epidermis are the cell envelope (EC) and the lipid envelope. The cell envelope is formed by the deposition of insoluble proteins on the inner surface of the plasma membrane. The lipid envelope is formed by the binding of lipids to the outer surface of the plasma membrane.
The thickness of the nuclear envelope increases in the epithelium. This makes the skin more resistant to mechanical stress. An example of this are the lips, the palms of the hands and the soles of the feet. The proteins present in CD are cystatin, desmoplakin, elafin, filaggrin, involucrin, loricrin, and various types of keratins.
The lipid envelope is formed by binding of the cell surface to lipids via ester bonds. The main lipid components of this envelope are sphingolipids, cholesterol and free fatty acids.
Ceramides participate in signaling. They are partly responsible for the induction of cell differentiation, apoptosis and reduction of cell proliferation.
Langerhans cells
Langerhans cells, present in the stratum spinosum, are derived from CD34 stem cells in the bone marrow. These cells are responsible for finding and presenting antigens that enter through the skin.
Langerhans cells, similar to macrophages, express major histocompatibility complexes I and II, as well as immunoglobulin G (IgG) receptors and C3b complement receptors.
Analysis of skin biopsy from HIV patients reveals that Langerhans cells contain HIV in their cytoplasm. Because Langerhans cells are more resistant than T cells, the former serve as a reservoir for the HIV virus.
Melanocytes
Melanocytes are dendritic cells found in the basal layer. They extend the tonofilaments between the keratinocytes of the stratum spinosum. Their function is the biosynthesis of melanin, which protects against the actions of UV light and sunlight. The ratio of melanocytes to keratinocytes varies between 1: 4 and 1:10.
Throughout their lives, melanocytes maintain their ability to replicate. However, its rate of division is slower than that of keratocytes. In this way, the epidermis-melanin unit is maintained.
Melanin is produced by the oxidation of tyrosine to 3,4-dihydro-phenylalanine (DOPA) with the intervention of a tyrosinase and the transformation of DOPA into melanin. These transformations take place in a membrane-bound structure, called premelanosomes, that comes from the Golgi apparatus.
The absence of pigmentation in the skin, such as in albinism, is due to the absence of tyrosinase. On the other hand, skin pigmentation is related to the amount of melanin present in keratocytes.
Differences in melanin content produce a wide spectrum of colors in human skin, characteristics of different races.
In humans there are two types of melanins: eumelamines, which are brown to black in color; pheomelanins, yellow to reddish-brown in color.
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