B. Epidermis - derm-hokudai.jp

1 The Epidermis is on average about mm thick, and 95% ofthe cells composing it are epidermal keratinocytes that proliferateand divide in the epidermal basal layer and move up to the upperlayers as they mature (to form cornified cells). In the Epidermis ,epidermal keratinocytes at different stages of maturation arearranged in layers that can be divided into four levels (Figs. ). The period between the production of daughter epider-mal cells and their exfoliation from the outer surface of the epi-dermis is called the turnover time, which is approximately 28days in normal basal cell layer is a single layer consisting of basal cellsincluding the epidermal stem cell subpopulation.

2 Basal cellsvary in shape from cubic to columnar. They contain basophilic(or darkly staining) cytoplasm and an elliptical nucleus that isrich in chromatin. The basal cells have desmosomes (for cell-cell attachment), gap junctions (for cell communication), and1. Basal cell layera. structure and cells of the dermis1B. Epidermisepidermal basement membraneepidermal basement membrane100mm100mmgranular cell layer (2-3 layers)horny cell layer (10-20 layers)basal cell layer (1 layer)suprabasal cell layer (5-10 layers)granular cell layer (2-3 layers)horny cell layer (10-20 layers)basal cell layer (1 layer)suprabasal cell layer (5-10 layers)Fig.

3 The four layers of the Epidermis :basal cell layer, suprabasal cell layer,granular cell layer and horny cell cell layerdermissuprabasal cell layerdermis epidermal basement membrane epidermal basement membranecell nucleuscell nucleusbasal cell layerbasal cell layerFig. Ultrastructural anatomy of the junctionbasement membranehemidesmosomeFig. Intracellular positions of desmosomes, gap junction Back to the TopTo Order, Visit the Purchasing Page for Detailshemidesmosomes (for connection with the extracellular matrixand underlying basal membrane) (Fig.)

4 Cell cytokeratins(keratin filaments, tonofilaments) are abundant in the cytoplasmof many epidermal keratinocytes and are distributed in bundles atthe periphery of the nucleus, from where they distally connectwith hemidesmosomes and desmosomes to form a rigid androbust cellular suprabasal cell layer is composed of five to ten layers thatappear connected to each other by prickle-like (prickle) cells are polygonal in the lower layer andflattened in the upper layers. They are larger than basal cells andcontain a small amount of chromatin in their circular part that gives the appearance of a prickle corresponds to thedesmosome (a form of intercellular bridge).

5 The granular cell layer is composed of two or three layers ofcells containing basophilic keratohyalin granules. The cells andnuclei in the granular cell layers are even flatter than those in thesuprabasal layer. Spherical lamellar granules, each with a diame-ter of approximately 300 nm (also known as Odland bodies orketatosomes), can be observed in the granular cell layers by elec-tron microscopy. The main component of lamellar granules isreleased into the intercellular space of horny cells as stratumcorneum horny cell layer, also called the stratum corneum, is com-posed of about ten sub-layers.

6 Enucleated dead keratinocytesbecome membranous and multilayered, resembling fallen leaves,and exfoliate sequentially, beginning with the outer layer, in whatis commonly called grime. The horny cell layer is very thick inthe palms and soles. Horny cells are flat, and their cytoplasm isfilled with aggregated keratin fibers. Directly above the granularcell layer, the horny cell structure appears as an eosinophiliclayer. The horny cells gradually change into membranous struc-tures in the upper layers. By electron microscopy, the contrastbetween electron-dense interfibrous substance and the low-elec-tron-dense keratin fibers is clear; this contrast is called the keratinpattern.

7 The cell membranes are thicker in the horny cell layer than inthe other layers. The lining structure is called a cornified cellenvelope or a marginal band (Figs. ). The protein,component of the cornified cell envelope, is extremely stableagainst physicochemical degradation. 4. Horny cell layer3. Granular cell layer2. Suprabasal cell layer41 structure and Function of the Skin1horny cell layercornified cell envelopehorny cell layergranular cell layerkeratin patternFig. Horny cell cellshorny cellsFig. Ultrastructural image of the epider-mis between the granular cell layer andthe horny cell layer, with cornified cellenvelopes and lamellar granules (arrows).

8 Keratinocytes account for 95% of the cells within the epider-mis. The remaining 5% are melanocytes, Langerhans cells, a-dendritic cells and Merkel cells, which are involved in melaninformation, antigen presentation, and sensation, epidermal basement membrane plays a key role in dermal-epidermal adhesion. Even when normal skin is rubbed, the epi-dermal basement membrane keeps the Epidermis and dermisfrom separating. The epidermal basement membrane zone, which lies immedi-ately under the Epidermis , stains by periodic acid Schiff (PAS)under the light microscope.

9 The complicated structure of thebasal membrane includes the lamina densa (LD) and the laminalucida (LL), which are observed by electron microscopy ( ). The lamina densa is 60 to 80 nm thick and consists mainly offibronectins, heparan sulphate proteoglycan, type IV collagenand laminin 5 (including laminin 332). Under the electron micro-scope, it appears to be an electron-dense lattice network struc-ture. Hemidesmosomes play an important role in adhesionbetween the basal cells and the lamina densa. Although thehemidesmosome resembles a desmosome that has been cut inhalf, the molecular components of hemidesmosomes and desmo-somes are very different.

10 Keratin fibers within basal cells linkhemidesmosomes, to maintain and anchor the structure of the cell(Fig. ).The lamina lucida includes the component laminin 332. TypeXVII collagen (BP180; 180kDa bullous pemphigoid antigen)bridges the lamina lucida to connect hemidesmosomes directlywith the lamina densa. Anchoring fibrils, which form semi-circular loop structures,form firmly connections around type I and III collagens in thedermis linking it to the lamina densa. Epidermal keratinocytes adhere to each other by desmosomesand structures such as adherens junctions, gap junctions and desmosome is composed of an attachment plaque (com-prising inner and outer plaques), a structure that penetrates mem-branes to connect cells.