Embryology of the integument

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Describe the embronic skin at 3 weeks. What are the layers derived from?: The embryonic skin at 3 weeks consists of a single layer of ectodermal (simple cuboidal) resting on a layer of mesenchyme. Epidermis is derived from ectoderm and dermis is derived from mesoderm.
Describe what happens at 4-5 wks. What is the periderm? What is the basal germinative layer?: At 4-5 wks the surface ectoderm proliferates and formes a 2 layered structure. Superficial layer of squamous epithelium called the periderm. The second layer of basal cells is called the basal germinative layer. Periderm cells arise from the basal layer and undergo keratinization and are desquamed.
What occurs at 11 wks?: The stratum germinativum produces an intermediate layer.
How long does the replacement of the periderm cells contiune?: Untill week 21.
What do periderm cells do? What happens when they die?: They secrete amnionic fluid. Sloughed periderm cells form a greasy layer around the fetus. This is called the vernix caseosa. IT also contains periderm cells and sebum from forming sebaceous glands. THe function of this is to protect the fetus from the amnionic fluid and provide a slippery coating on the baby to ease delivery.
What happens after 21 weeks to the periderm?: The periderm stops sloughing and the normal stratum corneum is formed. (based on the prodcution of keratin by the forming keratinocytes)
Describe two types of genetic disorders taht are the result of defects in keratinization.: Hyperkeratinization of skin is characterized by dry, fishscale-like skin or alligator skin. Lamellar ichthyosis with scale-like appearance of skin. Usually lacks hair and sweat glands.
Harlequin fetus- extreme hyperkeratinization, characterized by "alligator skin" rigid, deeply fissured epidermis, fatal, usually still born.
When do the fingerprints form? What composes them?: 10-17 weeks the epidermal ridges form. Both dermis and epidermis interact to form the ridges (genetically determined).
What are melanocytes derived from? Describe the journey they make towards become mature melanocytes.: Melanocytes are derived from neural crest cells that migrate from the neuroectoderm at 6wks gestation and migrate through the forming dermis. In the dermis they differentiate into melanoblasts. They continue migration into the basal layer of the epidermis by 10-11 wks of gestation where they finish differentiating into melanocytes. They also migrate into the forming hair follicles by 10 wks.
From where/when do langerhans cells arise?: Langerhans (immune) cells arise from bone marrow and enter the epidermis by the 4th week and are fully differentiated by 12-14 wks.
When/where do merkel cells appear?: Merkel cells (light touch) appear in the stratum basale during the 8-12 wks.
What is the hair follicle formed by? When does development begin?: It is formed by both the epidermis and dermis. During the 3rd month.
Describe each of the 3 stages of hair development.: Stage I - slight invangination of epidermis into dermis. THis is sometimes called the hair germ.
Stage II - is characterized by elongation of the bud into the hair bud (13-15 wks)
Stage III- characterized as club-shaped and is called the hair bulb.
The hair shaft passes through the epidermis by wk 18. Dermal sheath and arrector pili muscles are derived from the mesenchyme.
Where/when are apocrine and sebaceous glands formed?: They empty into the hair follicle and are formed during follicle formatin. They both begin forming during the hair bud stage around wk 15-16.
What are the 3 types of hair and when/where are they associated with?: Lanugo hair is the first hair to form in the fetus (3rd month of gestation). Very fine hair that covers the entire body by 5-6 months.
The lanugo hair is replaced by very downy hair on the body at birth. 65% of the adult female body is covered with this.
Terminal hair is course hair of the scalp and eyebrows. IT makes up 95% of the body hair of male.
What is hirsutism?: Hirsutism is excessive hairiness due to the conversion of vellus hair into terminal hair by androgens.
What is hypertrichosis?: Hypertrichosis is excessive hairiness due to the formation of excessive follicles or the persistence of hair that normally disappear during development.
When do eccrine sweat glands form? How do they form? Do eccrine sweat glands form after birth?: Eccrine sweat glands starts forming at 18-20 wks as epidermal bud that elongates down into the dermis and forms coil. Central cells degenerate to form lumen. Outer cells differentiate into secretory cells and myoepithelial cells around 22 wks. No eccrine sweat glands are formed after birth.
When do fingernails/toenails first start to form? Where does this first occur? How does this occur, describe it. What patholgy can you determine by the status of the nails in a newborn?: Fingernails first start to form at 10 wks and toenails start about 4 wks later. The nail first starts on the palmar or plantar surface of the forming digit. The thickened epidermis then migrates to the dorsal surface almost immediately. This forms the primary nail field that forms the nail bed. The nail field then flattens on the surface and drives the wedge of cells deeper into the dermis. Wedge forms the nail matrix that produces the nail plate. By 11-12 weeks the nial is delinated by lateral and proximal nail folds. By 14 wks the nail plate can be seen still covered by periderm/epidermis that degenerates by birth. The nail plate reaches the end of the digit by about 32 weeks. Prematurity is indicated if the nail plates do not reach the tips of fingers.
What are the two types of wound healing?: Epidermal wound healing and deep wound healing.
When does epidermal wound healing occur?: It occurs with slight scratches, burns, abrasions chemical trauma. THe epidermis is scraped away in center of scratch.
What are the four steps of epidermal wound healing?: 1. Clot formation
2. Activation/Proliferation
3. Migration
4. Maturation
What occurs during the clot formation stage of epidermal wound healing? What do clots consist of? What things are stored in it? What do they do?: Blood clot is formed by leakage of blood from capillaries in underlying dermis. This also provides a matrix through which cells can migrate during the healing process. The clot consists of platelets embedded in a meshwork of cross-linked fibrin fibers together with small amounts of firbronectin. The clot acts as a reservoir of cytokines and growth factors that are released by the platelets. This cocktail of growth factors initiate the wound closure process.
What occurs during the Activation/Proliferation stage of epidermal wound healing? What regulates keratinocyte proliferation?: Chemotactic factors recruit neutrophils and monocytes. This requires activation of p-selectin on endothelial cell surface for extravasation to occur. Neutrophils arrive within minutes and produce cytokines to activate local keratinocytes and fibroblasts to proliferate. Growth factors released at the wound site regulate keratinocyte proliferation. (karatinocytes in stratum basale within 24 hrs)
What occurs during the migration stage of epidermal wound healing? What fibrinolytic enzyme is produced by keratinocytes? What do they do? What other changes occur?: Keratinocytes migrate on the basement membrane toward the center of the wound below the clot. THis requires the production of metalloproteinases to break hemidesmosomes attaching to basal lamina for the cells to be able to move. Plasmin is a fibrinolytic enzyme produced by keratinocytes. Other metallo proteinases cut collagen IV and anchoring fibrils (Type III collagen) of basal lamina. ALso the type of integrin chages as focal contacts are produced that are used during diapedesis.
What occurs during the Maturation stage of epidermal wound healing?: WHen keratinocytes meet in the center of the wound, contact inhibition stops the cells proliferating at the wound edge. The process then shifts over to the resurfacing epidermas and normal keratinization builds up the thickness of the epidermis. The focal attachement integrin and metalloproteinases production is down regulated. Basal lamina synthesis occurs. THe scab is sloughed off as the epidermis fills in the cavity.
What defines deep wound healing?: When trauma extends down into the dermis.
What are the four steps of deep wound healing?: 1. Inflammation phase
2. Proliferative phase
3. Migratory phase
4. Maturation phase
Describe the inflammation phase of deep wound healing.: Blood clot forms and unites the edges of the wound. Also vasodilatation of the calipparies occurs due to release of histamines which causes an influx of neurophils and macrophages.
Describe the proliferative phase of deep wound healing.: Both fibroblasts and keratinocytes proliferate.
They begin to secrete matrix to form granulation tissue.
What occurs during the migratory phase of deep wound healing?: Keratinocytes and fibroblasts migrate across the granulation tissue and through the blood clot. Fibroblasts begin to migrate into the wound 3-4 days after injury. Blood vessels grow into granulation tissue. Within 7 days after injury the fibroblasts have completely infiltratedclot and granulation tissue. Fibroblasts at edge are transformed into myofibroblasts which produce actin and myosin fibers in the cells. This causes contraction of the wound. Wound contraction has been shown to be driven by growth factors and mechanical tension.
What occurs during the maturation phase of deep wound healing?: THe granulation tissue slowly turned into dense irregular CT. The epidermis returns to normal thickness and the scab is sloughed off.
What is effect/the appearance of a first degree burn?: WHen only the epidermis is affected, it is considered partial thickness because the entire layer is not affected. This is characterized by redness and a mild pain. (Mild sunburn without blisters)
What is effect/the appearance of a second degree burn?: In second degree burns all the epidermis and the upper part of the dermis is affected. It is characterized by blisters, pain and edema. (picking up a hot object)
WHat is the effect/ appearance of a third degree burn?: In a third degree burn the full thickness of the skin is affected. Both the epidermis and dermis has been affected and is characterized by charred skin. There is no pain in the center of the burn and only at the peripehry where the pain receptors are intact. There is a loss of multiple skin functions.
Describe some of the systemic effects of burns. (4): Dehydration
-Due to loss of water and plasma protien
-can cause electrolyte imbalance
Bacterial infection- the barrier function of the skin is lost
Shock - blood circulation is reduced due to loss of fluid. There is low blood volume because of reduced urine production.
Renal shut down
-Causes uremic poisoning

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