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Notes on skin anatomy
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Skin anatomy, skin layers,Ati Teas science
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CHAPTER 1Anatomy and Physiology of the Skin
Paul A.J. Kolarsick, BS, Maria Ann Kolarsick, MSN, ARNP-C,
and Carolyn Goodwin, APRN-BC, FNP
Introduction cells known as keratinocytes, which function to synthesize
keratin, a long, threadlike protein with a protective role.
The skin is the largest organ of the body, accounting for The middle layer, the dermis, is fundamentally made up of
about 15% of the total adult body weight. It performs many the fibrillar structural protein known as collagen. The der-
vital functions, including protection against external physical, mis lies on the subcutaneous tissue, or panniculus, which
chemical, and biologic assailants, as well as prevention of ex- contains small lobes of fat cells known as lipocytes. The
cess water loss from the body and a role in thermoregulation. thickness of these layers varies considerably, depending on
The skin is continuous, with the mucous membranes lining the geographic location on the anatomy of the body. The
the body’s surface (Kanitakis, 2002). eyelid, for example, has the thinnest layer of the epidermis,
The integumentary system is formed by the skin and measuring less than 0.1 mm, whereas the palms and soles
its derivative structures (see Figure 1-1). The skin is of the feet have the thickest epidermal layer, measuring
composed of three layers: the epidermis, the dermis, and approximately 1.5 mm. The dermis is thickest on the back,
subcutaneous tissue (Kanitakis, 2002). The outermost where it is 30–40 times as thick as the overlying epidermis
level, the epidermis, consists of a specific constellation of (James, Berger, & Elston, 2006).
Figure 1-1. Cross-Section of Skin and Panniculus
Apocrine
unit
Straight duct
Epidermis
Meissner nerve Coiled gland
ending
papillary
Eccrine
Dermis sweat unit
Straight duct
reticular Straight duct
Sebaceous gland Coiled duct
Arrector pili muscle Eccrine gland
Hair shaft
Dermal
Pacini nerve ending vasculature
Subcutaneous tissue
Superficial plexus
Deep plexus
Note. From Andrews’ Diseases of the Skin: Clinical Dermatology (10th ed., p. 1), by W.D. James, T.G. Berger, and D.M. Elston, 2006, Phila-
delphia: Elsevier Saunders. Copyright 2006 by Elsevier Saunders. Reprinted with permission.
1
, SKIN CANCER
Epidermis individual cell populations pass not only one another but also
melanocytes and Langerhans cells as they move toward the
The epidermis is a stratified, squamous epithelium layer surface of the skin (Chu, 2008).
that is composed primarily of two types of cells: keratinocytes
and dendritic cells. The keratinocytes differ from the “clear” Keratinocytes
dendritic cells by possessing intercellular bridges and ample
amounts of stainable cytoplasm (Murphy, 1997). The epidermis At least 80% of cells in the epidermis are the ectodermally
harbors a number of other cell populations, such as melanocytes, derived keratinocytes. The differentiation process that occurs
Langerhans cells, and Merkel cells, but the keratinocyte cell as the cells migrate from the basal layer to the surface of the
type comprises the majority of the cells by far. The epidermis skin results in keratinization, a process in which the kerati-
commonly is divided into four layers according to keratino- nocyte first passes through a synthetic and then a degradative
cyte morphology and position as they differentiate into horny phase (Chu, 2008). In the synthetic phase, the cell builds up a
cells, including the basal cell layer (stratum germinativum), cytoplasmic supply of keratin, a fibrous intermediate filament
the squamous cell layer (stratum spinosum), the granular cell arranged in an alpha-helical coil pattern that serves as part
layer (stratum granulosum), and the cornified or horny cell layer of the cell’s cytoskeleton. Bundles of these keratin filaments
(stratum corneum) (James et al., 2006; Murphy) (see Figure converge on and terminate at the plasma membrane forming
1-2). The lower three layers that constitute the living, nucleated the intercellular attachment plates known as desmosomes.
cells of the epidermis are sometimes referred to as the stratum During the degradative phase of keratinization, cellular
malpighii and rete malpighii (Murphy). organelles are lost, the contents of the cell are consolidated
The epidermis is a continually renewing layer and gives rise into a mixture of filaments and amorphous cell envelopes,
to derivative structures, such as pilosebaceous apparatuses, and the cell finally is known as a horny cell or corneocyte.
nails, and sweat glands. The basal cells of the epidermis un- The process of maturation resulting in cell death is known as
dergo proliferation cycles that provide for the renewal of the terminal differentiation (James et al., 2006).
outer epidermis. The epidermis is a dynamic tissue in which
cells are constantly in unsynchronized motion, as differing Basal Layer
The basal layer, also known as the stratum germinativum,
Figure 1-2. Three Basic Cell Types in the Epidermis contains column-shaped keratinocytes that attach to the base-
ment membrane zone with their long axis perpendicular to
the dermis. These basal cells form a single layer and adhere
to one another as well as to more superficial squamous cells
through desmosomal junctions (Murphy, 1997). Other dis-
tinguishing features of the basal cells are their dark-staining
K oval or elongated nuclei and the presence of melanin pigment
L
transferred from adjoining melanocytes (Murphy).
The basal layer is the primary location of mitotically active
cells in the epidermis that give rise to cells of the outer epidermal
L layers. However, not all basal cells have the potential to divide
(Jones, 1996; Lavker & Sun, 1982). Epidermal stem cells in the
M
basal layer are clonogenic cells with a long lifespan that progress
K through the cell cycle very slowly under normal conditions. Hy-
M
perplasiogenic conditions, such as wounding, can increase the
number of cycling cells in the epidermis by stimulating division
of stem cells. DNA damage caused by carcinogenic agents may
D
mutate cell proliferation machinery and can also affect the rate
of cellular division. Migration of a basal cell from the basal layer
The three basic cell types in the epidermis include keratinocytes
(some labeled K ) and Langerhans cells (L) in the Malpighian to the cornified layer in humans takes at least 14 days, and the
layer and melanocytes (M ) in the basal layer. Arrows point to transit through the cornified layer to the outermost epidermis
the basement membrane zone, which separates the basal layer requires another 14 days (Chu, 2008).
of the epidermis from the underlying dermis (D).
Note. From Andrews’ Diseases of the Skin: Clinical Dermatology Squamous Cell Layer
(10th ed., p. 4), by W.D. James, T.G. Berger, and D.M. Elston,
2006, Philadelphia: Elsevier Saunders. Copyright 2006 by El- Overlying the basal cell layer is a layer of the epidermis
sevier Saunders. Reprinted with permission. that is 5–10 cells thick and known as the squamous cell layer
2
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