Question

How thick is the epidermis, in number of cells? Its thickness is not uniform so give a range from minimum to maximum.

12. What kind of epithelial cells does the epidermis contain?

13. What is different about the cells that make up the outermost layer of the skin when compared to these epidermal cells?

14. Compare the structure of the dermis to the epidermis. What structures are found in the dermis?

15. What is the structure of the hypodermis like when compared to the dermis? What is a possible function of hypodermis fat cells in mammals? Do you think the thickness of this layer would vary depending upon where the mammals lives?

Answer 1

If by "upper epidermis" you mean the outer or most superficial layer of the epidermis, called "***** layer", the answer is that it varies considerably, because some areas of the body have heavily keratinized skin (the palms and soles are thickest), while the epidermis is extremely thin in others (thinnest in the eyelids).
In fact, the thickness of the ***** layers determines that of the entire epidermis; in contrast, thickness of the remaining layers is more stable.
From base to top, the layers are:
Basal cell layers (stratum basale): single cell layer
Prickle cell layer (stratum spinosum): 5-7 layers
Granular cell layer (stratum granulosum): 2-3 layers
Clear layer (stratum lucidum) : not well defined except in thick skin, several layers of flattened dead cells.
***** layer (stratum corneum): very dense layer of flattened cells called "squames" rich in keratin. It is very difficult to distinguish individual cells.

Take a look at the photos in the site below, you will see that the ***** layer seems to be at least 10 cells thick in *thin* skin, and much thicker in *thick* skin.

Answer 2

The epidermis is composed of the outermost layers of cells in the skin,^[1] "epi" in Greek meaning "over" or "upon", which together with the dermis forms the cutis.

The epidermis is a stratified squamous epithelium,^[2] composed of proliferating basal and differentiated suprabasal keratinocyteswhich acts as the body's major barrier against an inhospitable environment, by preventing pathogens from entering, making the skina natural barrier to infection.^[3] It also regulates the amount of water released from the body into the atmosphere throughtransepidermal water loss (TEWL).^[4]

In humans, it is thinnest on the eyelids at 0.05 mm (0.0020 in) and thickest on the palms and soles at 1.5 mm (0.059 in).^[5] It isectodermal in origin.

Cellular components[edit]

The epidermis is avascular, nourished almost exclusively by diffused oxygen from the surrounding air.^[6] It is 95% keratinocytes^[2] but also containing melanocytes, Langerhans cells, Merkel cells,^[1] and inflammatory cells. Rete ridges ("rete tips"^[7]) are epidermal thickenings that extend downward between dermal papillae.^[8] Blood capillaries are found beneath the epidermis, and are linked to an arteriole and a venule.

Layers[edit]

Stratum lucidum Stratum granulosum Stratum spinosum Straturm basale

Schematic image showing a section of epidermis with epidermal layers labeled.

The epidermis is composed of 4 or 5 layers depending on the region of skin being considered.^[9] Those layers in descending order are:^[3]

• cornified layer (stratum corneum)

Composed of 10 to 30 layers of polyhedral, anucleated corneocytes (final step of keratinocyte differentiation), with the palms andsoles having the most layers. Corneocytes are surrounded by a protein envelope (cornified envelope proteins), filled with water-retaining keratin proteins, attached together through corneodesmosomes and surrounded in the extracellular space by stacked layers of lipids.^[10] Most of the barrier functions of the epidermis localize to this layer.^[11]

• clear/translucent layer (stratum lucidum, only in palms and soles)

The skin found in the palms and soles is known as "thick skin" because it has 5 epidermal layers instead of 4.

• granular layer (stratum granulosum)

Keratinocytes lose their nuclei and their cytoplasm appears granular. Lipids, contained into those keratinocytes within lamellar bodies, are released into the extracellular space through exocytosis to form a lipid barrier. Those polar lipids are then converted into non-polar lipids and arranged parallel to the cell surface. For example glycosphingolipids become ceramides andphospholipids become free fatty acids.^[10]

• spinous layer (stratum spinosum)

Keratinocytes become connected through desmosomes and start produce lamellar bodies, from within the Golgi, enriched in polar lipids, glycosphingolipids, free sterols,phospholipids and catabolic enzymes.^[4] Langerhans cells, immunologically active cells, are located in the middle of this layer.^[10]

• basal/germinal layer (stratum basale/germinativum).

Composed mainly of proliferating and non-proliferating keratinocytes, attached to the basement membrane by hemidesmosomes. Melanocytes are present, connected to numerous keratinocytes in this and other strata through dendrites. Merkel cells are also found in the stratum basale with large numbers in touch-sensitive sites such as thefingertips and lips. They are closely associated with cutaneous nerves and seem to be involved in light touch sensation.^[10]

The term Malpighian layer (stratum malpighi) is usually defined as both the stratum basale and stratum spinosum.^[2]

The epidermis is separated from the dermis, its underlying tissue, by a basement membrane.

Cellular kinetics[edit]

Cell division[edit]

The stratified squamous epithelium is maintained by cell division within the stratum basale. Differentiating cell delaminate from the basement membrane and are displaced outwards through the epidermal layers, undergoing multiple stages of differentiation until, in the stratum corneum, losing their nucleus and fusing to squamous sheets, which are eventually shed from the surface (desquamation). Differentiated keratinocytes secrete keratin proteins which contribute to the formation of an extracellular matrix and is an integral part of the skin barrier function. In normal skin, the rate of keratinocyte production equals the rate of loss,^[2] taking about two weeks for a cell to journey from the stratum basale to the top of the stratum granulosum, and an additional four weeks to cross the stratum corneum.^[3] The entire epidermis is replaced by new cell growth over a period of about 48 days.^[12]

Calcium concentration[edit]

Keratinocyte differentiation throughout the epidermis is in part mediated by a calcium gradient, increasing from the stratum basale until the outer stratum granulosum, where it reaches its maximum, and decreasing in the stratum corneum. Calcium concentration in the stratum corneum is very low in part because those relatively dry cells are not able to dissolve the ions. This calcium gradient parallels keratinocyte differentiation and as such is considered a key regulator in the formation of the epidermal layers.^[4]

Elevation of extracellular calcium concentrations induces an increase in intracellular free calcium concentrations.^[13] Part of that intracellular increase comes from calciumreleased from intracellular stores^[14] and another part comes from transmembrane calcium influx,^[15] through both calcium-sensitive chloride channels^[16] and voltage-independent cation channels permeable to calcium.^[17] Moreover, it has been suggested that an extracellular calcium-sensing receptor (CaSR) also contributes to the rise in intracellularcalcium concentration.^[18]

Development[edit]

Epidermal organogenesis, the formation of the epidermis, begins in the cells covering the embryo after neurulation, the formation of the central nervous system. In mostvertebrates, this original one-layered structure quickly transforms into a two-layered tissue; a temporary outer layer, the periderm, which is disposed once the inner basal layer orstratum germinativum has formed. ^[19]

This inner layer is a germinal epithelium that give rise to all epidermal cells. It divides to form the outer spinous layer (stratum spinosum). The cells of these two layers, together called the Malpighian layer(s) after Marcello Malpighi, divide to form the superficial granular layer (Stratum granulosum) of the epidermis. ^[19]

The cells in the stratum granulosum do not divide, but instead form skin cells called keratinocytes from the granules of keratin. These skin cells finally become the cornified layer(stratum corneum), the outermost epidermal layer, where the cells become flattened sacks with their nuclei located at one end of the cell. After birth these outermost cells are replaced by new cells from the stratum granulosum and throughout life they are shed at a rate of 0.001 - 0.003 ounces of skin flakes every hour, or 0.024-0.072 ounces per day.^[20]

Epidermal development is a product of several growth factors, two of which are:^[19]

• Transforming growth factor Alpha (TGF?) is an autocrine growth factor by which basal cells stimulate their own division.
• Keratinocyte growth factor (KGF or FGF7) is a paracrine growth factor produced by the underlying dermal fibroblasts in which the proliferation of basal cells is regulated.

Function[edit]

Barrier[edit]

The epidermis serves as a barrier to protect the body against microbial pathogens, oxidant stress (UV light) and chemical compounds and provides mechanical resistance. Most of that function is played by the stratum corneum.^[11]

Characteristics of the barrier[edit]

• Physical barrier through keratinocytes attached together via cell

Answer 3

How thick is the epidermis, in number of cells? Its thickness is not uniform so give a range from minimum to maximum

outermost layer; a broad zone 20-30 cell layers thick

12. What kind of epithelial cells does the epidermis contain?

keratinized stratified squamous

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4. Compare the structure of the dermis to the epidermis. What structures are found in the dermis?

Epidermis

Structure: Superficial part of the skin; stratified spamous epithelium; composed of four of five strata.
Function: Prevents water loss and the entry of chemicals and microorganisms
Protects against abrasion and ultraviolet light
produces vitamin D
Gives rise to hair, nails, and glands

Stratum Corneum

Structure: Most superficial stratum of the epidermis; 25 or more layers of dead spamous cells.
Function: Provides structural strength due to the keratin within cells; prevents water loss due to the lipids surrounding cells; sloughing off of most superfisial cells resist abrasion.

Stratum lucidum

Structure: Three to five layers of dead cells; appears transparent; present in thick skin, absent in most thin skin.
Function: Disperses Keratohyalin around keratin fibers

Stratum granulosum

Structure: Two to five layers of flattened, diamond-shaped cells.
Function: produces keratohyalin granules; lamellar bodies release lipids from cells; cells die

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Stratum Spinosum

Structure: A total of 8-10 layers of many-sided cells.
Function: Produces keratin fibers; lamellar bodies from inside keratinocytes

???

Stratum Basale

Structure: Deepest stratum of the epidermis; single layer of cuboidal or columnar cells; basement membrane of the epidermis attaches to dermis.
Function: Produes cells of the most superficial strata; melanocytes produce and contribute melanin, which protects against ultraviolet light.

???

Dermis

Structure: Deep part of skin; connective tissue composed of two layers.
Function: Is responsible for the structural strength and flexibility of the skin; the epidermis exchanges gases, nutrients, and waste products with blood vessels in the dermis.

???

Papillary layer

Structure:Papillae project toward th epidermis; loose connective tissue.
Function: Brings blood vessels close to the epidermis; dermal papillae form fingerprints and footprints

???

Reticular layer

Structure: Mat of collagen and elastic fibers; dense irregular connective tissue.
Function: Is the main fibrous layer of the dermis; strong in many directions; forms cleavage lines

???

Subcutaneous tissue

Structure: NOT PART OF THE SKIN; loose connctive tissue with abundant deposits of adipose tissue.
Function: Attaches the dermis to underlying structures; adipose tissue provides energy storage, insulation, and padding;

blood vessels and nerves from the subcutaneous tissue supply the dermis.