Generative cells of the epidermis, follicles, and nail units all mature to become dead cornified cells that contain large quantities of the protein, keratin. The filament of cornified cells that takes the form of hair is akin to the layer of cornified cells that makes up the stratum corneum and the plate of corneocytes that constitutes the nail. All epithelial cells in the skin that contain keratin and that cornify ultimately are designated keratocytes (erroneously called “keratinocytes,” which is analogous, in its wrongness, to “melaninocytes”).
Cornification is the raison d’être of epidermis, that most superficial epithelium which consists of surface and infundibular parts in continuity with one another. That specific form of cellular differentiation results in formation of the outermost dead layer of the epidermis, namely, the stratum corneum or cornified layer. The structure of infundibular epidermis is indistinguishable from that of surface epidermis, both consisting of a basal, spinous, granular, and cornified layer, the latter typified by corneocytes arrayed in basket-weave pattern. Corneocytes on the surface of the skin are products of maturation both of epidermal and intraepidermal adnexal (acrosyringeal) keratocytes (Fig. 1.17). The process of cornification involves (1) synthesis of lamellar granules and distinctive proteins, e.g., differentiation-specific keratins, filaggrin, loricrin, involucrin, and desmosomal proteins that are cross-linked by transglutaminases, and (2) changes progressively of nuclei, cytoplasmic organelles, plasma membranes, and desmosomes. To understand the mechanisms of cornification and the function of the cornified layer, it is useful to define keratocytes, to characterize them cytologically, and to consider how they are organized, as assessed histologically, in the epidermis.
Keratocytes of the acrosyringium (intraepidermal segment of an eccrine duct) exist in intimate association with the predominant keratocytes of surface epidermis, they passing through that epithelium en route to the skin surface.
Keratocytes are cells that make keratins. They represent a site for synthesis of a class of soluble molecules, namely, cytokines that are important in the regulation of contiguous epithelial cells of the epidermis, as well as of cells in the dermis. Keratins consist of more than 40 highly insoluble proteins that serve as units for the formation of intermediate filament polymers, the latter constituting a major network in the cytoplasm of keratocytes.
Epidermal keratocytes undergo characteristic alterations during a short trip of about 2 weeks in which they are transformed from undifferentiated basal cells to fully differentiated cornified cells. Four continuous cellular “layers,” namely, basal, spinous, granular, and cornified, each recognizable readily histologically, represent expressions morphologic of successive stages of maturation of germinative keratocytes to fully cornified ones (Fig. 1.18). The basal row of keratocytes consists of cuboid or low columnar cells that contain larger oval nuclei and more basophilic cytoplasm than the more mature keratocytes above them. Suprabasal keratocytes are polygonal and are named “spinous cells” because of the distinct, but delicate, spine-like appearance of processes that, with conventional microscopy, are seen to transverse intercellular spaces and form contacts between adjacent keratocytes; with electron microscopy, those “spines” are seen to be desmosomes (Figs. 1.19 and 1.20). The spinous zone merges with horizontally oriented diamond-shaped cells filled with coarse, basophilic keratohyaline granules, i.e., cells of the granular zone (Fig. 1.21). Keratohyaline granules are made up of profilaggrin, an electron-dense protein, loricrin, and keratin intermediate filaments. The distal part of the epidermis, composed entirely of flat, anuclear, eosinophilic corneocytes, is the cornified layer which, of the four so-called layers, is the only one that qualifies truly as a layer, that is, a sheet of material covering a surface.
Stages in maturation of a basal keratocyte to a cornified one. In the course of approximately 13 days, a columnar basal cell matures to become a somewhat polygonal spinous cell, then a rather diamond-shaped granular cell, and, last, a flat cornified cell that, in actuality, covers approximately 25 basal cells.
Cytoplasm of a spinous cell filled nearly entirely by tonofibrils, except for a small number of mitochondria and scattered ribosomes. Tonofibrils surround the nucleus and radiate toward the periphery, where they converge on desmosomes. (x9500) (Courtesy of Ken Hashimoto, M.D.)
Loops of tonofibrils at a desmosome situated between adjacent keratocytes. (x132,000) (Courtesy of Douglas E. Kelly, Ph.D.)
Cytoplasm of a granular cell contains keratohyaline granules, as well as tonofibrils and a few ribosomes. (x11,500) (Courtesy of Ken Hashimoto, M.D.)
In sum, as cornification proceeds, vertically oriented, columnar basal keratocytes become transformed into pancake-shaped cornified cells aligned parallel to the skin surface. At the conclusion of this sequence of changes, each elongated waferlike corneocyte covers an area occupied by about 25 basal keratocytes. The corneocytes themselves are stacked in orderly columns that resemble pie plates, an arrangement that varies somewhat on different anatomic sites.
In adult epidermis, as basal cells migrate outward, keratins of increasing molecular weight are synthesized and, in that process, corneocytes come into being. Keratins (Types 5 and 14) of 50 and 58 KDa are expressed by basal keratocytes, and keratins (Types 1 and 10) of 56.6 and 67 KDa by keratocytes above the basal layer. Certain combinations of types of keratins correlate with stratification and extent of maturation of the epidermis. Processes pathologic in skin often are typified by changes in the expression of keratins in the epidermis. As but one example, Type 1 and 10 keratins are decreased in the skin of healing wounds and of psoriasis, whereas type 6 and 16 (48 and 56 KDa) are increased.
Near the top of the spinous zone, lamellar granules (also referred to as keratosomes, cementosomes, membrane-coating granules, and Odland bodies) appear near the Golgi apparatus, navigate the cytoplasm, and fuse with the plasma membrane, whence their contents are discharged from keratocytes into intercellular spaces. Those granules contain free sterols, polar lipids, e.g., phospholipids and glucosylceramides (the precursors of ceramides), and several hydrolytic enzymes, e.g., lipases, glycosidases, and acid phosphatase. After contents of lamellar granules are released into intercellular spaces, they are restructured into lamellae that provide the basis structurally for an effective epidermal barrier to permeability.
Concurrent with the appearance of keratohyaline and lamellar granules, nuclei and most of the cytoplasmic organelles of keratocytes disappear, signs that herald formation imminently of a cornified layer.
Throughout the process of cornification, keratocytes are fastened to one another by the specialized contact zones just referred to, i.e., desmosomes, which are intercellular attachments (colloquially called “intercellular bridges”) that break and re-form continuously as keratocytes move outward and mature. Cleavage between desmosomes in the cornified layer results in shedding of corneocytes (Fig. 1.22), referred to in times past as the stratum disjunctum, in contrast to the stratum compactum situated immediately beneath it.
Cornified cells attached to one another by vestigial desmosomes. A cornified cell is a package of tonofibrils encased in a protein matrix. The nucleus and the organelles within the cytoplasm have been lost during maturation. Melanosomes are found within keratocytes at all levels of the epidermis, including the cornified layer. (x75,000) (Courtesy of Ken Hashimoto, M.D.)
Desmosomes consist of a number of proteins, among them being desmoglein 1 and 3, desmoplakin, plakoglobin, and desmosomal cadherins. When the function of desmosomes is impaired, keratocytes tend to become detached from one another and, in the process, assume a round shape. The phenomenon occurs in conditions as disparate as pemphigus vulgaris, where it develops as a consequence of the effect of autoantibodies against desmoglein 3, pemphigus foliaceus in which the autoantibodies are directed against desmoglein 1, and staphylococcal scalded skin syndrome in which exfoliatins produced by the Gram-positive cocci compromise desmosomes severely. The assembly of desmosomes is dependent on calcium and phosphorylation, calcium being that which acts as the major signal for maturation of keratocytes and a requirement for formation of desmosomes and activation of enzymes like transglutaminase. Both extracellular calcium and 1,25 dihydroxyvitamin D are necessary for enabling keratocytes to mature. Within hours of the “calcium switch” having been turned on, keratocytes shift from making basal keratins, namely, Keratin 5 and Keratin 14, and begin producing Keratin 1 and Keratin 10, followed soon thereafter by an increase in levels of profilaggrin, involucrin, and loricrin. Molecular defects in transportation of calcium are thought to be responsible for skin diseases in which the structural integrity of keratocytes is impaired, as is the case in conditions characterized by acantholysis, e.g., Hailey-Hailey disease and Darier’s disease.
Cholesterol sulfate has been implicated as a substance integral for “cementing” keratocytes one to another and hydrolysis of it to free cholesterol coincides with desquamation of corneocytes. Patients with X-linked ichthyosis lack steroid sulfatase, that absence preventing removal of cholesterol sulfate and, thereby, limiting shedding of corneocytes, resulting in hyperkeratosis that is reflected clinically in plate-like scales.
Small molecules, metabolites, and ions are able to pass between keratocytes by way of intercellular communications known as gap junctions.
The unique property of permeability possessed by the cornified layer is crucial to its role, throughout life, in contributing to maintenance of fluid and electrolyte balance of the body. The extent to which molecules diffuse through the cornified layer accounts, too, for the ability of allergenic substances to enter the viable epidermis, promote sensitization, and elicit reactions of allergic contact dermatitis on one hand and the efficacy of medicaments applied topically on the other.
The major proliferative population of keratocytes is housed in the lowest part of the viable epidermis. That proliferative compartment, i.e., the two lower rows of keratocytes in a normal epidermis, has a cell cycle of 13 days, compared with that of psoriatic epidermis, which is only 1.5 days. The renewal time of normal epidermis has been estimated to be about 26 days, divided approximately as 13 days for the time it takes viable keratocytes to travel from the base of the epidermis to the cornified layer, and another 13 days for the time it takes dead keratocytes (corneocytes) to be shed at last.