Which epidermal layers are keratinized

These cells are especially abundant on the surfaces of the hands and feet. The second is a melanocyte , a cell that produces the pigment melanin. Melanin gives hair and skin its color, and also helps protect the DNA in the nuclei of living cells of the epidermis from ultraviolet UV radiation damage.

As the name suggests, the stratum spinosum is spiny in appearance due to the protruding cell processes that join the cells via a structure called a desmosome. The desmosomes interlock with each other and strengthen the bond between the cells.

Unstained epidermis samples do not exhibit this characteristic appearance. The stratum spinosum is composed of eight to 10 layers of keratinocytes, formed as a result of cell division in the stratum basale Figure 5.

Interspersed among the keratinocytes of this layer is a type of dendritic cell called the Langerhans cell , which functions as a macrophage by engulfing bacteria, foreign particles, and damaged cells that occur in this layer. If you zoom on the cells at the outermost layer of this section of skin, what do you notice about the cells?

The keratinocytes in the stratum spinosum begin the synthesis of keratin and release a water-repelling glycolipid that helps prevent water loss from the body, making the skin relatively waterproof. As new keratinocytes are produced atop the stratum basale, the keratinocytes of the stratum spinosum are pushed into the stratum granulosum. The stratum granulosum has a grainy appearance due to further changes to the keratinocytes as they are pushed from the stratum spinosum.

The cells three to five layers deep become flatter, their cell membranes thicken, and they generate large amounts of the proteins keratin , which is fibrous, and keratohyalin , which accumulates as lamellar granules within the cells see Figure 5.

These two proteins make up the bulk of the keratinocyte mass in the stratum granulosum and give the layer its grainy appearance. The nuclei and other cell organelles disintegrate as the cells die, leaving behind the keratin, keratohyalin, and cell membranes that will form the stratum lucidum, and the stratum corneum. A similar process of producing cells packed with keratin occurs in the accessory structures of hair and nails. The stratum lucidum is a smooth, seemingly translucent layer of the epidermis located just above the stratum granulosum and below the stratum corneum.

This thin layer of cells is found only in the thick skin of the palms, soles, and digits. The keratinocytes that compose the stratum lucidum are dead and flattened see Figure 5. These cells are densely packed with eleiden , a clear protein rich in lipids, derived from keratohyalin, which gives these cells their transparent i. The stratum corneum is the most superficial layer of the epidermis and is the layer exposed to the outside environment see Figure 5. The increased keratinization also called cornification of the cells in this layer gives it its name.

There are usually 15 to 30 layers of cells in the stratum corneum. This dry, dead layer helps prevent the penetration of microbes and the dehydration of underlying tissues, and provides a mechanical protection against abrasion for the more delicate, underlying layers. Cells in this layer are shed periodically and are replaced by cells pushed up from the stratum granulosum or stratum lucidum in the case of the palms and soles of feet. The cells in this layer can still be anchored to each other by desmosomes which is why the peeling that occurs with a sunburn peels the damaged epidermal layers in one sheet.

The entire layer is replaced during a period of about 4 weeks. It contains blood and lymph vessels, nerves, and other structures, such as hair follicles and sweat glands.

The epidermis is avascular and cells of this layer must get their oxygen and nutrients from capillaries in the dermis. The dermis is made of two layers of connective tissue that compose an interconnected mesh of elastin and collagenous fibers, produced by fibroblasts Figure 5.

The more superficial papillary layer serves as an anchor point for the epidermis above and is intimately connected to the deeper reticular layer. The papillary layer is made of loose, areolar connective tissue, which means the collagen and elastin fibers of this layer form a loose mesh with abundant ground substance supporting the hydration of the skin.

This superficial layer of the dermis projects into the stratum basale of the epidermis to form finger-like dermal papillae see Figure 5. Within the papillary layer are fibroblasts, a small number of fat cells adipocytes , and an abundance of small blood vessels. In addition, the papillary layer contains phagocytes, defensive cells that help fight bacteria or other infections that have breached the skin.

This layer also contains lymphatic capillaries, nerve fibers, and touch receptors called the Meissner corpuscles. In a growing fetus, fingerprints form where the cells of the stratum basale of the epidermis meets the papillae of the underlying dermal layer papillary layer , resulting in the formation of the ridges on your fingers that you recognize as fingerprints.

Dermal papillae push up on the epidermis creating unique epidermal ridge patterns. Fingerprints are unique to each individual and are used for forensic analyses because the patterns do not change with the growth and aging processes. Underlying the papillary layer is the much thicker reticular layer , composed of dense irregular connective tissue which resists forces in many directions attributing to the flexibility of the skin.

The reticular layer appears reticulated net-like due to a tight meshwork of fibers. Elastin fibers provide some elasticity to the skin, enabling movement. Collagen fibers provide structure and tensile strength, with strands of collagen extending into both the papillary layer and the hypodermis.

In addition, collagen binds water to keep the skin hydrated. Collagen injections and Retin-A creams help restore skin turgor by either introducing collagen externally or stimulating blood flow and repair of the dermis, respectively. The hypodermis also called the subcutaneous layer or superficial fascia is a layer directly below the dermis and serves to connect the skin to the underlying fascia fibrous tissue surrounding the muscles.

It is not strictly a part of the skin, although the border between the hypodermis and dermis can be difficult to distinguish. The hypodermis consists of well-vascularized, loose, areolar connective tissue and abundant adipose tissue, which functions as a mode of fat storage and provides insulation and cushioning for the integument.

Fascia is a thick connective tissue wrapping that surrounds skeletal muscles anchoring them to surrounding tissues and investing groups of muscles.

The hypodermis is home to most of the fat that concerns people when they are trying to keep their weight under control. Adipose tissue present in the hypodermis consists of fat-storing cells called adipocytes.

This stored fat can serve as an energy reserve, insulate the body to prevent heat loss, and act as a cushion to protect underlying structures from trauma. Where the fat is deposited and accumulates within the hypodermis depends on hormones testosterone, estrogen, insulin, glucagon, leptin, and others , as well as genetic factors.

Fat distribution changes as our bodies mature and age. Men tend to accumulate fat in different areas neck, arms, lower back, and abdomen than do women breasts, hips, thighs, and buttocks. These keratinized cells are called horny cells. The stratum corneum is very thick in areas subject to frequent abrasion such as the palms and soles e. In thin skin, this layer is relatively thin and delicate. Since the upper layer of skin is heavily keratinized and the lower layer stratum germinativum is not, it is clear that as cells of the epidermis progress toward the external surface, one of the things that is happening is that higher and higher concentrations of keratin are being synthesized and deposited in the cytoplasm.

Also, since cells of the stratum corneum lack organelles, something is occurring to rid the cells of these structures. So, what we find is that as keratinization proceeds, more and more autophagosomes and lysosomes are present in the cytoplasm of epidermal cells. The autophagosomes are responsible for engulfing and degrading cytoplasmic organelles. Second skin cell type - melanocytes - pigment producing cells. Melanocytes are derived from neural crest cells which you'll recall are also involved in the formation of the adrenal medulla.

Cell bodies of melanocytes are found between or beneath those of the stratum basale and stratum spinosum. They extend cytoplasmic processes between cells of these two layers. These processes end in invaginations in the plasmalemma of the cells of the stratum spinosum and stratum basale. Melanin is synthesized by melanocytes via the action of an enzyme called tyrosinase on the amino acid tyrosine. Melanin synthesis occurs in vesicles formed by the Golgi body and consists of 4 stages.

Tyrosinase precursors are synthesized on ribosomes of endoplasmic reticulum and accumulate in vesicles formed by the Golgi body. These vesicles are transported into the processes of the melanocytes and are transferred to epidermal cells in the stratum basale and stratum spinosum. Pigmentation of the skin is due to accumulations of melanin in epidermal cells rather than in the melanocytes.

Darker skin is due to greater accumulation of melanin in these cells and not to a change in the number of melanocytes. Thus, differences in skin color in people of the same race and of different races are due to differences in the concentration of melanin granules in epidermal cells and not to differences in the number of melanocytes present.

As skin cells mature and become keratinized, melanin is enzymatically broken down such that dead cells of the stratum corneum have lost their pigmentation by time they are shed. Third skin cell type - Langerhans cells. These cells are found in the stratum spinosum. They may have an immune function and are a specialized type of macrophage that "entraps" foreign antigens that enter the skin. Fourth type of skin cell - Merkel cells. Present in hair follicles, thick skin of palms and soles, and oral mucous membrane.

Large, oval, poorly stained. Unmyelinated axons penetrate the stratum basale and terminate as expanded disks on the base of Merkel cells - suggests sensory function. Granules resembling neurosecretory vesicles sugget a possible hormonal function for these cells.

This is the connective tissue that supports epidermis and binds it to subjacent tissue. The dermis is composed of two layers:. Papillary layer. Reticular layer. Composed of loose connective tissue , mostly fibroblasts, but also mast cells, macrophages, and extravasated leukocytes. This portion of the dermis interdigitates with the epidermis forming folds of the dermis that are called dermal papillae.

Most papillae contain a capillary bed that provides nutrients to overlying and surrounding cells. Some palillae are "tactile papillae" and house sensory structures for touch called Meissner's corpuscles. From this layer, collagen fibrils penetrate the dermal papillae and insert into the basal lamina of the epidermis, just beneath the stratum germinativum. This binds the two layers together. The collagen fibrils are called anchoring fibrils.

This layer is thicker than the papillary layer and is composed of dense irregular connective tissue that contains collagen and elastic fibers. Fibers are the major component. There are fewer cells in the reticular layer than in the papillary layer.

Thick elastic fibers emerge from this layer and penetrate the papillary layer. These thick fibers end in the fine fibrotubular component of the elastic fibers. As is the case with the anchoring fibers mentioned above, the ends of these thick fibers penetrate the papillary layer and insert on the basal lamina below the stratum basale.

This elastic network is responsible for the elasticity of the skin. The functional cells of hair follicles, sweat glands, and oil glands are found in the reticular layer of the dermis. Hair follicles may be considered specializations of the epidermis that penetrate into the reticular layer of the dermis. While the epidermis contains no blood or lymph vessels, the dermis contains many. Underlying the dermis is a fatty, loose connective tissue layer called the hypodermis.

There are a number of structures that can be defined as appendages of the skin:. The epidermis is epithelium composed of multiple layers of cells. The basal layer consists of cuboidal cells, whereas the outer layers are squamous, keratinized cells, so the whole epithelium is often described as being keratinized stratified squamous epithelium. If you zoom on the cells at the outermost layer of this section of skin Figure 2 , what do you notice about the cells?

The stratum basale also called the stratum germinativum is the deepest epidermal layer and attaches the epidermis to the basal lamina, below which lie the layers of the dermis.

The cells in the stratum basale bond to the dermis via intertwining collagen fibers, referred to as the basement membrane. Dermal papillae increase the strength of the connection between the epidermis and dermis; the greater the folding, the stronger the connections made Figure 3.

Figure 3. The epidermis of thick skin has five layers: stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum. The stratum basale is a single layer of cells primarily made of basal cells.

A basal cell is a cuboidal-shaped stem cell that is a precursor of the keratinocytes of the epidermis. All of the keratinocytes are produced from this single layer of cells, which are constantly going through mitosis to produce new cells. As new cells are formed, the existing cells are pushed superficially away from the stratum basale.

Two other cell types are found dispersed among the basal cells in the stratum basale. The first is a Merkel cell , which functions as a receptor and is responsible for stimulating sensory nerves that the brain perceives as touch.

These cells are especially abundant on the surfaces of the hands and feet. The second is a melanocyte , a cell that produces the pigment melanin. Melanin gives hair and skin its color, and also helps protect the living cells of the epidermis from ultraviolet UV radiation damage.

In a growing fetus, fingerprints form where the cells of the stratum basale meet the papillae of the underlying dermal layer papillary layer , resulting in the formation of the ridges on your fingers that you recognize as fingerprints. Fingerprints are unique to each individual and are used for forensic analyses because the patterns do not change with the growth and aging processes. As the name suggests, the stratum spinosum is spiny in appearance due to the protruding cell processes that join the cells via a structure called a desmosome.

The desmosomes interlock with each other and strengthen the bond between the cells. Unstained epidermis samples do not exhibit this characteristic appearance. The stratum spinosum is composed of eight to 10 layers of keratinocytes, formed as a result of cell division in the stratum basale Figure 4. Interspersed among the keratinocytes of this layer is a type of dendritic cell called the Langerhans cell , which functions as a macrophage by engulfing bacteria, foreign particles, and damaged cells that occur in this layer.

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.