the irregular junction between the dermis and epidermis, projections called
dermal papillae
dermal papillae interdigitate with invaginating with this ridges to strengthen adhesion of the two layers.
epidermal ridges
Epidermal derivatives
hairs, nails, and sebaceous and sweat glands
a loose connective tissue layer usually containing pads of adipocytes.
subcutaneous tissue or hypodermis
The specific functions of the skin fall into several broad categories.
Protective, Sensory, Thermoregulatory, Metabolic, and Sexual signaling.
Protective
It provides a physical barrier against thermal and mechanical insults, such as friction, and against most potential pathogens and other material.
Sensory
Many types of sensory receptors allow skin to constantly monitor the environment, and various skin mechanoreceptors help regulate the body’s interactions with physical objects.
Thermoregulatory
A constant body temperature is normally easily maintained; thanks to the skin’s insulat ing components (eg, the fatty layer and hair on the head) and its mechanisms for accelerating heat loss (sweat pro duction and a dense superficial microvasculature).
Metabolic
Cells of skin synthesize vitamin D3 , needed in calcium metabolism and proper bone formation, through the local action of UV light on the vitamin’s precursor.
Sexual signaling
Many features of skin, such as pigmentation and hair, are visual indicators of health involved in attraction between the sexes in all vertebrate species, including humans.
These ridges and the intervening sulci form distinctive patterns unique for each individual, appearing as combinations of loops, arches, and whorls - also known as fingerprints and footprints
dermatoglyphs
The epidermis consists mainly of a stratified squamous kera tinized epithelium composed of cells called
keratinocytes
There are also three much less abundant epidermal cell types
pigment-producing melanocytes, antigen-presenting Lang erhans cells, and tactile epithelial cells called Merkel cells
thick skin
found on the palms and soles
thin skin
found elsewhere on the body
is a single layer of basophilic cuboidal or columnar cells on the basement membrane at the dermal-epidermal junction
basal layer (stratum basale)
characterized by intense mitotic activity and contains, along with the deepest part of the next layer, progenitor cells for all the epidermal layers.
stratum basale
intermediate filaments about 10 nm in diameter, An important feature of all keratinocytes in the stratum basale is the cytoskeletal
keratins
are lymph-filled spaces created between the epidermis and dermis of thick skin by excessive rubbing, as with ill-fitting shoes or hard use of the hands.
Frictions blisters
Friction blisters, If continued, such activity produces protective thickening and hardening of the outer cornified epidermal layers, seen as
corns and calluses
is normally the thickest layer, especially in the epidermal ridges, and consists of generally polyhedral cells having central nuclei with nucleoli and cytoplasm actively synthesizing keratins.
spinous layer (stratum spinosum)
Just above the basal layer, some cells may still divide and this combined zone is sometimes called the
stratum germinativum
The keratin filaments assemble here into microscopically visible bundles called
tonofibrils
consists of three to five layers of flattened cells, now undergoing the terminal differentiation
granular layer (stratum granulosum)
a terminal differentiation process wherein, cytoplasm is filled with intensely basophilic masses.
keratinization
name for the intensely basophilic masses in keratinization
keratohyaline granules
found only in thick skin, consists of a thin, translucent layer of flattened eosinophilic keratinocytes held together by desmosomes
stratum lucidum,
con sists of 15-20 layers of squamous, keratinized cells filled with birefringent filamentous keratins.
stratum corneum
the cells that have lost nuclei and cytoplasm, consisting only of flattened, keratinized structures are called
squames
a chronic skin condition wherein keratocytes are typically produced and differentiate at accelerated rates, causing at least slight thickening of the epidermal layers and increased keratinization and desquamation.
psoriasis
The color of the skin is the result of several factors, the most important of which are the keratinocytes’ content of
melanin and carotene
are brown or black pigments produced by the melanocyte
Eumelanins
melanocyte
a specialized cell of the epidermis found among the cells of the basal layer and in hair follicles.
The similar pigment found in red hair is called
pheomelanin
The first step in melanin synthesis is catalyzed by a transmembrane enzyme in Golgi-derived vesicles called