Skin

Front 1

Describe the components of the skin - epidermis and dermis.

Back 1

Epidermis- avascular squamous epithelium,with melanocytes,wandering antigen-presenting cells (Langerhans cells); divided into 5 layers, protective function

Dermis-mesenchymal origin, vascular network feeds epidermis and for thermoregulation, sensory nerve endings, immune cells, fibroblasts, mesenchymal cells instruct epithelial differentiation in development

Front 2

Understand the concept of stem cells, and the location and functions of the stem cells for the epidermis.

Back 2

True stem cells are specialized in that they have the capacity to proliferate indefinitely. The cells of the epidermis are derived from basal keratinocytes, which are themselves descendents of stem cells located in the stratum basale.

Pluripotent stem cells also reside in the bulge, or the center of hair follicles. These divide for self-renewal and produce committed stem cells of different cell types (keratinocyte, sweat glands, hair follicles, sebaceous glands)
Transit amplifying cells are committed stem cells that can divide 2-3 times before undergoing differentiation.

Front 3

Describe the structural characteristics of the differentiated cells in various layers of the epidermis

Back 3

Stratum basale: most undifferentiated keratinocytes. regular/cuboidal in shape. They are single layer and attached to a basement membrane and can be seen undergoing mitosis.

Stratum Spinosum: have visible "spines" that contain desmosomes.

Stratum granulosum: cells that contain visible keratohyaline granules and keratin filaments.

Stratum lucidum: (only in thick sin) contains cells that have lost their nuclei

Stratum Corneum: no nuclei, become flattened sheets called squames.

Front 4

Provide a general outline of the processes of formation of squames and pigmentation.

Back 4

Squames are formed as the keratinocytes of the epidermis lose their nuclei and their intracellular keratin filaments become highly crosslinked and impervious. Also lost their desmosomal connections.

Pigmentation of the skin is due to melanocytes. Melanocytes use tyrosinase to convert L-DOPA into eumelanin and pheomelanin pigments, which are localized in the cell's melanin granules. Exposure to sun/UV light causes melanin to darken and stimulates melanin production by melanocytes.

Front 5

Describe the variation in structure of the epidermis over different regions of the body.

Back 5

Though the overall organization of the skin remains the same all over the body, different regions may have a larger layer of stratum lucidum and stratum corneum in order to compensate for the added abrasive forces of that region.

Additionally, different regions of the skin can have a different number and density of sebaceous glands, sweat glands, and hair follicles.

Front 6

Describe the pattern of sensory innervation of the skin.

Back 6

Specialized sensory endings sense touch and pressure in the dermis, including Meissner's (under epidermis and perceive light touch) and Pacinian corpuscles (deeper in dermis and perceive harder pressure). These are surrounded by onion-like layers of fibroblasts, Ruffini's endings, spindle-shaped receptors, and unmyelinated fibers that surround hair follicles.

Merkel Cells are found in the basal layer of the epidermis and are though to play a role in cutaneous sensation and contain neurosecretory granules.

Front 7

Describe the general structure of hair and its mechanism of growth.

Back 7

Hair is an epidermal derivative composed mainly of keratin. The bulb is located at the base of each hair, which is responsible for its growth. Stem cells for hair reside in the bulge region of the hair follicle. After division, epithelial cells of the hair bulb differentiate into concentric rings of specific cell types. Within the lower part of the hair follicle, these are surrounded by the internal root sheath, which degenerates around the level of the sebaceous glands. This is surrounded by the external root sheath, which is continuous with the epidermal cells of the skin.

Dermal papilla instructs epidermal cells to form hair follicles as these cells divide they grow up the follicle to protrude out of the surface of the skin --> anagen phase of growth

During catagen and telogen rest phases, the entire hair follicle is pushed up to the top fo the epidermis and shed while the dermal papilla pulls out of the follicle and instructs stem cells to form a new hair bulb to replace the shed hair follicle.
The existing hair is shed at the beginning of the new anagen cycle.

Front 8

Describe the structure and function of sebaceous glands, sweat glands, and mammary glands.

Back 8

Sebaceous glands: usually attached to hair follicle. They are composed of acini attached to a duct that empties into the hair follicle. These glands secrete sebum, which lubricates the surface of the skin and hair. Sebum is composed of the entire contents of dead cells filled with fat droplets. Holocrine secretion: dead cells are secreted into the hair follicle.

Sweat glands are coiled and tubular, and have ducts that open at the surface of the skin. The secretory epithelium contains dark mucoid cells and cells; it is surrounded by myoepithelial cells that contract to expel sweat. Sweat glands are used for thermoregulation via evaporative heat loss.
Eccrine: secrete clear, watery/salty sweat for thermoregulatory function.
Merocrine sweat glands: create viscous section that smells with bacterial decomposition. innervated by adrenergic neurons.

Mammary glands are modified sweat glands. The basic components of the mammary gland are the alveoli (hollow cavities) lined with milk-secreting cuboidal cells and surrounded by myoepithelial cells. These alveoli join up to form groups known as lobules, and each lobule has a lactiferous duct that drains into openings in the nipple. Mammary glands secrete milk that nourishes the young. --> merocrine secretion

Front 9

Compare the changes in the epidermis and dermis that take place during the healing of a cutaneous wound, with that of self-renewal during skin maintenance.

Back 9

In normal skin maintenance, basal keratinocytes differentiate into cells of the ascending four other layers of the epidermis. Squames of the stratum corneum slough off, and are replaced.

In the healing of a cutaneous wound, epithelial migration and proliferation occurs in the dermis, increasing vasculature and forming granulation tissue. This weak tissue is mainly stabilized by collagen III, which is then replaced by collagen I. This collagen becomes increasing insoluble as healing complete, evolving into a scar.

Keratinocytes of the stratum basale migrate and divide as transit amplifying cells until they meet each other beneath the developing clot and form the initial healed epidermis. All the cells of the basal layer are proliferative in this stage and once they meet they begin to differentiate into the layers of epidermis. Below the migrating epithelium is the forming granulation tissue made up of vasculature and inflammatory response --> provisional structure under the initial healed dermis.

Front 10

Describe the major cellular components of the epidermis and dermis

Back 10

no

Front 11

Describe the location and function of stem cells in the epidermis

Back 11

Pluripotent stem cells are located in the bulge area of hair follicle, divide for self-renewal and produce committed stem cells of different cell types. (keratinocyte, sweat glands, hair follicles, sebaceous glands, etc)
- transit amplifying cells are committed stem cells that can divide 2-3 times before differentiating

Epidermis renews itself every 2-4 weeks, important to have these stem cells in the basal layer of the epidermis to give rise to keratinocytes which then undergo differentiation.