Skin And Its Appendages Flashcards
(15 cards)
What is the epidermis?
The epidermis is a self-renewing stratified epithelial tissue consisting mainly of keratinocytes. Other cells within the epidermis include melanocytes (pigment-forming cells from the embryonic neural crest), Langerhans cells (immature antigen-presenting dendritic cells derived from bone marrow), lymphocytes and Merkel cells.
Merkel cells may function as sensory mechanoreceptors or possibly as part of the dispersed neuroendocrine system and many are associated with nerve endings. Free sensory nerve endings are sparsely present
within the epidermis. In routine histological preparations, the nonkeratinocyte cells are almost indistinguishable, and appear as clear cells surrounded by a space produced by shrinkage during processing. Their cytoplasm lacks prominent filament bundles.
What is cornification?
The population of keratinocytes undergoes continuous renewal throughout life. This turnover of keratinocytes is mediated by stem cells that reside in the basal layer of the epidermis. These basal cells generate daughter cells, which undergo a series of biochemical and physical changes as they migrate towards the surface of the skin to form the various layers of the epidermis. They transform from polygonal living cells to non-viable flattened squames full of intermediate filament proteins (keratins) embedded in a dense matrix of cytoplasmic proteins to
form mature keratin. The process is known as cornification.
Layers of the epidermis
The epidermis consists of several layers:
the innermost layer is the basal layer (stratum basale)
and is succeeded by the spinous or prickle cell layer (stratum spinosum),
granular layer (stratum granulosum),
clear layer (stratum lucidum)
and cornified layer (stratum corneum), which is the most superficial.
Basal layer
The basal layer or stratum basale is the innermost layer of the epidermis and is adjacent to the dermis. It is the site of epidermal cellular proliferation. This layer is in contact with a basal lamina, which is a thin layer of specialized extracellular matrix. The basal lamina consists of a clear lamina lucida (adjacent to the basal cell
plasma membrane) and a darker electron-dense lamina densa.
Dermal papillae (rete ridges) project into the epidermis, interlocking with adjacent downward projections of the epidermis (rete pegs).
The cytoplasm contains variable numbers of melanosomes and keratin filament bundles. In the basal keratinocytes, these keratins consist mostly of keratin 5 (K5) and keratin 14 (K14) proteins. The plasma membranes of interconnecting cells are coupled by desmosomes while those of the basal keratinocytes are linked to the basal lamina by hemidesmosomes. Melanocytes, Langerhans cells and Merkel cells are interspersed.
Prickle cell layer
The prickle cell layer, also known as the stratum spinosum, consists of several layers of closely packed keratinocytes. These cells are connected to each other by desmosomes, specialized cell–cell
junctions that provide tensile strength and cohesion to the layer. Appearance gives these suprabasal keratinocytes their characteristic spiny appearance. The cytoplasm of the keratinocytes in the prickle cell
layer contains prominent bundles of cytokeratin filaments. The cytoplasm also contains melanosomes, either singly or aggregated within memrane-bound organelles (compound melanosomes). Langerhans cells and occasional lymphocytes are also present in the prickle cell layer.
Granular layer
Extensive changes in keratinocyte structure occur in the 3–4 layers of flattened cells in the granular layer, also known as the stratum granulosum. The nuclei ecome pyknotic and begin to disintegrate. Organelles such as ribosomes and memrane-bound mitochondria and Golgi bodies degenerate. Cytokeratin filament bundles become more compact and associated with irregular, densely staining keratohyalin granules. Small round granules appear in the cytoplasm. Keratohyalin granules contain a histidine-rich, sulphur-poor protein which
Becomes modified as the cell reaches the stratum corneum.
Clear layer
The clear layer is only found in thick palmar or plantar skin. It represents a poorly understood stage in keratinocyte differentiation. It stains more strongly than the cornified layer with acidic dyes. It is more optically refractile and often contains nuclear deris. Ultrastructurally, the cells contain compacted keratin filaments and resemble the incompletely keratinized cells that are occasionally seen in the innermost part of the cornified layer of thin skin.
Cornified layer
The cornified layer is the final product of epidermal differentiation. It consists of closely packed layers of flattened polyhedral squames or corneocytes. The plasma memrane of the corneocytes appears thicker than that of other keratinocytes, partly due to the crosslinking of a solule precursor, involucrin, at the cytoplasmic face of the
plasma memrane, in the complex insolule cornified envelope. The cells lack a nucleus and memranous organelles, and consist solely of a dense array of keratin filaments emedded in a cytoplasmic matrix. Under normal conditions, the production of epidermal keratinocytes in the basal layer is matched by the loss of corneocytes from the cornified layer. Desquamation of these cells when excessive, it may appear as dandruff on the scalp or the flaking or peeling skin that follows sunburn. In certain skin disorders such as psoriasis, there is extensive scaling. The thickness of the cornified layer can be influenced y local environmental factors including chronic
scratching, which can lead to a considerale thickening of the whole epidermis including the cornified layer. The soles of the feet become much thickened if an individual habitually walks barefoot, and calluses develop in areas of frequent pressure, e.g. on various parts of the feet
from wearing tight shoes; palmar calluses in manual workers; and digital calluses in guitar players.
Keratins
Epidermal keratinization has historically been the term applied to the final stages of keratinocyte differentiation and maturation, during which cells are converted into tough cornified squames. However, this is now regarded as ambiguous because the term keratin is assumed to refer to the protein of epithelial intermediate filaments, rather than to the whole complement of proteins in the terminally differentiated cell of the stratum corneum.
Keratins are the intermediate filament proteins found in all epithelial cells.
Melanocytes
Melanocytes are melanin pigment-producing cells derived from the neural crest. They are present in the epidermis and its appendages, oral epithelium, some mucous memranes, uveal tract (choroid coat) of the eyeball, parts of the middle and internal ear, and in the pial and arachnoid meninges, principally over the ventrolateral
surfaces of the medulla olongata.
Melanocytes are dendritic cells and lack desmosomal contacts with apposed keratinocytes, though hemidesmosomal contacts with the basal lamina are present.
The melanosome is a memrane-bound structure that undergoes a sequence of developmental stages during which melanin is synthesized and deposited within it y a tyrosine–tyrosinase reaction. Mature
melanosomes move into the dendrites along the surfaces of microtubules and are transferred to keratinocytes through their phagocytic activity. Each melanocyte supplies melanin to neighbouring keratinocytes. Keratinocytes engulf and internalize the tip of the dendrite with the susequent pinching-off of melanosomes into the keratinocyte cytoplasm.
Langerhans cells
Langerhans cells are dendritic antigen-presenting cells that are distriuted throughout the basal and prickle cell layers of the epidermis and its appendages. They are also present in other stratified squamous epithelia, including the buccal, tonsillar and oesophageal epithelia, as well as the cervical and vaginal mucosae, and the transitional epithelium of the ladder. In routine haematoxylin and eosin histological preparations, they appear as clear cells.
When stimulated by antigen, Langerhans cells migrate out of the epidermis to lymphoid tissues. Their numbers are increased in chronic skin inflammatory disorders.
Merkel cells
Merkel cells are present as clear oval cells, singly or in groups, in the basal layer of the epidermis. Merkel cells are
thought to derive embryologically from the epidermis, although a neural crest origin has been considered. The plasma memrane of a Merkel cell has short, stiff processes that interdigitate with adjacent basal keratinocytes to which it is attached by small desmosomes.
Merkel cells can undergo malignant transformation, possily triggered by the Merkel cell polyomavirus, and give
rise to a rare and aggressive tumour called a Merkel cell carcinoma that typically presents with a painless, rapidly growing nodule on sunexposed sites.
Papillary layer of the dermis
The papillary layer is immediately sujacent to the epidermis. It provides mechanical anchorage, metabolic support and trophic maintenance to the overlying epidermis, as well as supplying sensory nerve endings and blood vessels. This arrangement provides a mechanically stale sustratum for the epidermis. The superficial surface of the dermis is shaped into numerous papillae or rete ridges, which interdigitate with rete pegs in the base of the
epidermis and form the dermal–epidermal junction at their interface. In thin skin, especially in regions with little mechanical stress and minimal sensitivity, papillae are few and very small, while in the thick skin of the palm and sole of the foot they are much larger, closely aggregated, and arranged in curved parallel lines following the
pattern of ridges and grooves on these surfaces. Each papilla contains densely interwoven, fine undles of types I and III collagen fires and some elastic firils.
Reticular layer
The reticular layer merges with the deep aspect of the papillary layer. Its bundles of collagen fires are thicker than those in the papillary layer and form a strong three-dimensional lattice that contains a variable number of elastic fibres. The predominant parallel orientation of the collagen fires may be related to the local mechanical
forces on the dermis and may be involved in the development of skin lines.
Granulation tissue formation
The term granulation tissue refers to the macroscopic appearance of wound connective tissue, which appears pink and granular. It contains numerous capillaries that invade the initial wound clot and become organized into a microvascular network (angiogenesis), together with the cells and molecules necessary to stimulate neo-matrix deposition.
Angiogenesis is a complex process, promoted by dynamic interactions between endothelial cells, angiogenic cytokines (including FGF-2, VEGF (mainly from keratinocytes), PDGF and TGF-β1,2)) and the extracellular
matrix environment.
Wounds that fail to granulate do not heal satisfactorily, suggesting that granulation tissue formation is a key aspect of wound repair. Excessive granulation is also associated with delayed re-epithelialization.
Activated fibrolasts in the healing wound proliferate in response to growth factors, notaly TGF-β1, IGF-1, PDGF, FGF and EGF. Within 72 hours of injury, these fibrolasts synthesize components of the new extracellular matrix, which are deposited in an orderly sequence. Fironectin acts as an
initiation site for collagen firillogenesis, and as anchorage for myofi-
rolasts to effect wound contraction. Hyaluronan forms a highly
hydrated matrix that is easily penetrated y migrating cells. Ultimately
this and other neo-matrix components are replaced, first y collagen
type III, and susequently y collagen type I, which imparts strength to
the mature scar (see remodelling, elow). Non-structural proteins such
as tenascin are also found in the neo-matrix of healing wounds and
provide signals that orchestrate the repair process.
Cellularity decreases during the evolution of granulation tissue into
a mature scar (and during other phases of wound healing), mainly as
a result of apoptosis.
