The skin is composed of two major layers: a superficial epidermis and a deeper dermis. The epidermis consists of several layers beginning with the innermost (deepest) stratum basale (germinatum), followed by the stratum spinosum, stratum granulosum, stratum lucidum (when present), and ending with the outermost layer, the stratum corneum. The topmost layer, the stratum corneum, consists of dead cells that shed periodically and is progressively replaced by cells formed from the basal layer. The stratum basale also contains melanocytes, cells that produce melanin, the pigment primarily responsible for giving skin its color. Melanin is transferred to keratinocytes in the stratum spinosum to protect cells from UV rays.
The dermis connects the epidermis to the hypodermis, and provides strength and elasticity due to the presence of collagen and elastin fibers. It has only two layers: the papillary layer with papillae that extend into the epidermis and the lower, reticular layer composed of loose connective tissue. The hypodermis, deep to the dermis of skin, is the connective tissue that connects the dermis to underlying structures; it also harbors adipose tissue for fat storage and protection.
Comprises:
skin: waterproof; protecting against ultraviolet radiation, and through sweating it dissipates water and helps regulate body temperature.
accessory structures: hair, nails, sebaceous glands, ceruminous or wax glands in the ear canal, and sweat glands.
The epidermis is composed of stratified, squamous, keratinized epithelium.
As cells move up to the surface of the epidermis they lose water, and their nuclei change chemically, a process called keratinization.
3. There are five layers of the epidermis, from outermost to deep:
Stratum Corneum: dead cells converted to protein or keratinized cells that constantly are being shed; barrier to light and heat waves, most chemicals, and microorganisms.
Stratum Lucidum: one or two flat and transparent layers of cells thick; difficult to see.
Stratum Granulosum: two or three layers of cells thick and is very active in keratinization.
Stratum Spinosum: several layers of spiny-shaped polyhedron-like cells; has interlocking cellular bridges or desmosomes
Stratum Basale or Germinativum: rests on the basement membrane; produces new epidermal cells by mitosis. Contains melanocytes that produce melanin; responsible for skin color and protection from the harmful ultraviolet rays of the sun. Dark-skinned individuals have more active melanocytes. Albinism is a genetic condition that results from the absence of melanin.
provides strength and elasticity due to collagen and elastin fibers; also called corium or true skin; composed of dense connective tissue.
Contains blood and lymph vessels, nerves, muscles, glands, and hair follicles
It is divided into two portions:
papillary portion with papillae that extend into the epidermis
reticular portion comprising loose connective tissue
Contains fat (adipose) for energy storage, protection and insulation.
albinism
genetic disorder that affects the skin, in which there is no melanin production
basal cell
type of stem cell found in the stratum basale and in the hair matrix that continually undergoes cell division, producing the keratinocytes of the epidermis
dermal papilla
(plural = dermal papillae) extension of the papillary layer of the dermis that increases surface contact between the epidermis and dermis
dermis
layer of skin between the epidermis and hypodermis, composed mainly of connective tissue and containing blood vessels, hair follicles, sweat glands, and other structures
desmosome
structure that forms an impermeable junction between cells
elastin fibers
fibers made of the protein elastin that increase the elasticity of the dermis
eleiden
clear protein-bound lipid found in the stratum lucidum that is derived from keratohyalin and helps to prevent water loss
epidermis
outermost tissue layer of the skin
hypodermis
connective tissue connecting the integument to the underlying bone and muscle
integumentary system
skin and its accessory structures
keratin
type of structural protein that gives skin, hair, and nails its hard, water-resistant properties
keratinocyte
cell that produces keratin and is the most predominant type of cell found in the epidermis
keratohyalin
granulated protein found in the stratum granulosum
Langerhans cell
specialized dendritic cell found in the stratum spinosum that functions as a macrophage
melanin
pigment that determines the color of hair and skin
melanocyte
cell found in the stratum basale of the epidermis that produces the pigment melanin
melanosome
intercellular vesicle that transfers melanin from melanocytes into keratinocytes of the epidermis
Merkel cell
receptor cell in the stratum basale of the epidermis that responds to the sense of touch
papillary layer
superficial layer of the dermis, made of loose, areolar connective tissue
reticular layer
deeper layer of the dermis; it has a reticulated appearance due to the presence of abundant collagen and elastin fibers
stratum basale
deepest layer of the epidermis, made of epidermal stem cells
stratum corneum
most superficial layer of the epidermis
stratum granulosum
layer of the epidermis superficial to the stratum spinosum
stratum lucidum
layer of the epidermis between the stratum granulosum and stratum corneum, found only in thick skin covering the palms, soles of the feet, and digits
stratum spinosum
layer of the epidermis superficial to the stratum basale, characterized by the presence of desmosomes
vitiligo
skin condition in which melanocytes in certain areas lose the ability to produce melanin, possibly due an autoimmune reaction that leads to loss of color in patches
The skin consists of two layers and a closely associated layer. View this animation to learn more about layers of the skin. What are the basic functions of each of these layers?
The epidermis provides protection, the dermis provides support and flexibility, and the hypodermis (fat layer) provides insulation and padding.
[link] If you zoom on the cells at the outermost layer of this section of skin, what do you notice about the cells?
[link] These cells do not have nuclei, so you can deduce that they are dead. They appear to be sloughing off.
[link] If you zoom on the cells of the stratum spinosum, what is distinctive about them?
[link] These cells have desmosomes, which give the cells their spiny appearance.
This ABC video follows the story of a pair of fraternal African-American twins, one of whom is albino. Watch this video to learn about the challenges these children and their family face. Which ethnicities do you think are exempt from the possibility of albinism?
There are none.
1. The papillary layer of the dermis is most closely associated with which layer of the epidermis?
A) stratum spinosum
B) stratum corneum
C) stratum granulosum
D) stratum basale
D
2. Langerhans cells are commonly found in the ________.
A) stratum spinosum
B) stratum corneum
C) stratum granulosum
D) stratum basale
A
3. The papillary and reticular layers of the dermis are composed mainly of ________.
A) melanocytes
B) keratinocytes
C) connective tissue
D) adipose tissue
C
4. Collagen lends ________ to the skin.
A) elasticity
B) structure
C) color
D) UV protection
B
5. Which of the following is not a function of the hypodermis?
A) protects underlying organs
B) helps maintain body temperature
C) source of blood vessels in the epidermis
D) a site to long-term energy storage
C
1. What determines the color of skin, and what is the process that darkens skin when it is exposed to UV light?
The pigment melanin, produced by melanocytes, is primarily responsible for skin color. Melanin comes in different shades of brown and black. Individuals with darker skin have darker, more abundant melanin, whereas fair-skinned individuals have a lighter shade of skin and less melanin. Exposure to UV irradiation stimulates the melanocytes to produce and secrete more melanin.
2. Cells of the epidermis derive from stem cells of the stratum basale. Describe how the cells change as they become integrated into the different layers of the epidermis.
As the cells move into the stratum spinosum, they begin the synthesis of keratin and extend cell processes, desmosomes, which link the cells. As the stratum basale continues to produce new cells, the keratinocytes of the stratum spinosum are pushed into the stratum granulosum. The cells become flatter, their cell membranes thicken, and they generate large amounts of the proteins keratin and keratohyalin. The nuclei and other cell organelles disintegrate as the cells die, leaving behind the keratin, keratohyalin, and cell membranes that form the stratum lucidum and the stratum corneum. The keratinocytes in these layers are mostly dead and flattened. Cells in the stratum corneum are periodically shed.