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Keratinization begins in the stratum spinosum, [5] although the actual keratinocytes begin in the stratum basale. [4] They have large pale-staining nuclei as they are active in synthesizing fibrillar proteins, known as cytokeratin , which build up within the cells aggregating together forming tonofibrils .
It does this through connections to desmosomes, cell–cell junctional plaques, and hemidesmosomes, cell-basement membrane adhesive structures. Cells in the epidermis contain a structural matrix of keratin, which makes this outermost layer of the skin almost waterproof, and along with collagen and elastin gives skin its strength.
Keratohyalin is a protein structure found in cytoplasmic granules of the keratinocytes in the stratum granulosum of the epidermis.Keratohyalin granules (KHG) mainly consist of keratin, profilaggrin, [1] loricrin [2] and trichohyalin proteins which contribute to cornification or keratinization, the process of the formation of epidermal cornified cell envelope.
Micrograph showing low molecular weight cytokeratin (LMWCK) staining of intermediate trophoblast (placental tissue) and endometrial glands.. There are two categories of cytokeratins: the acidic type I cytokeratins and the basic or neutral type II cytokeratins.
Keratinization is part of the physical barrier formation (cornification), in which the keratinocytes produce more and more keratin and undergo terminal differentiation. The fully cornified keratinocytes that form the outermost layer are constantly shed off and replaced by new cells.
A stratified squamous epithelium consists of squamous (flattened) epithelial cells arranged in layers upon a basal membrane.Only one layer is in contact with the basement membrane; the other layers adhere to one another to maintain structural integrity.
Histologic image showing a section of epidermis. Stratum granulosum labeled near center. The stratum granulosum (or granular layer) is a thin layer of cells in the epidermis lying above the stratum spinosum and below the stratum corneum (stratum lucidum on the soles and palms). [1]
This fundamental cytoskeletal function extends beyond individual cell levels. Typically, keratin filaments are integrated into desmosomes (see Fig. 1b, d) and hemidesmosomes, contributing not only to cell-to-cell stability but also to the attachment to the basement membrane and the connective tissue within a particular epithelium. [4]