Alfred AmmouryAlfred Ammoury

Division of Dermatology, St George Hospital Division of Dermatology, St George Hospital UMCUMC

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Skin is a complex organ

Horizontally stratified into 3 compartments-epidermis-dermis-subcutis (hypodermis)

Vertically penetrated by appendages-hair follicles-sebaceous glands-eccrine and apocrine sweat glands

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Bilaminar embryonic disc :

Epiblast and the

hypoblast

Primitive streak – raised groove on the dorsal surface of the epiblast

Gastrulation – a process of invagination of epiblast cells

Endoderm – formed from migrating cells that replace the hypoblast

Mesoderm – formed between epiblast and endoderm

Ectoderm – formed from epiblast cells that stay on dorsal surface

Skin layers are derived from 2 different germ layers:

Ectoderm (lateral to the neural plate): epidermis

Mesoderm: dermis

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Earliest skin study (4 week): single layered epidermis and a thin mesenchymal dermis.

Progressive development over the first 6 months: by the end of the second trimester, the skin is a stratified squamous epithelium.

Dermis lags behind epidermis in developemnt.

Dermal bulk increases post nataly and its maturation continues

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Day 20-30: skin of embryo starts to develop (organogenesis).

Day 60: most organ systems have formed (including the skin). Embryo enters fetal period of growth and differentiation.

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Gastrulation occurs during the third week after fertilization

Complex process of involution and cell redistribution resulting in the formation of the three primary embryonal germ layers:

Ectoderm, Mesoderm, endoderm

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Shortly after gastrulation, the ectoderm further subdivides into neuroectoderm and the presumptive epidermis that covers the embryo

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At this stage, the ectoderm (presumptive epidermis) that covers the embryo is made of basal cells and superficial periderm cells

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Form part of the vernix caseosa

Form a pavement epithelium that blanket the developing epidermis.

Exfoliated peridermal cells and sebum form the Vernix caseosa , a white greasy substance that covers the fetal body, protects it from amniotic fluid contents and facilitates delivery.

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Hematopoiesis has switched from the extraembryonic yolk sac to the bone marrow. The classical division between embryonal and fetal development.

Epidermal stratification occurs: formation of an intermediate layer, highly proliferative cells that eventually evolve into a multi layer structure that will replace the periderm.

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Basal layer begins to elaborate proteins that will anchor them to the developing basal lamina.

Late fetal development reveals the further differenciation of keratinocytes in the epidermis: granular layer, stratum corneum that replaces the periderm at W21.

Stratum germninativum starts to extend downward growth (ridges) into the developing dermis.

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Melanocytes, Merkel cells and Langerhans cells can be detected by the end of the embryonic period.

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Neural crest derived

W8 neural crest cells mgrates into mesenchymal tissue and differentiate into melanoblasts.

Melanoblasts localize to DEJ and hair bulbs, and differenciates into melanocytes

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Bone marrow derived

Detectable by 40 days

By the third trmester most of the adult number of these cells will have been produced

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Slowly adapting touch receptors.

Controversial origin: neural crest Vs in situ differentiation of ectodermal cells.

Present at 11-12 weeks.

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Develops from mesenchyme

W11: mesenchymal cells produce collagenous and elastic tissue.

Part of the dermis project into the epidermis forming pappillae.

Capillary loops and nerves develop in these papillae

Major vasculature : by the end of the 1st trimester.

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Hair follicles, sebaceous glands and sweat glands develop from the epidermis and they grow into the dermis.

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W12: Hair first appears on eyebrow & upper lip

1st hair is lanugo hair (soft, fine, lightly pigmented and non mediullated).

Replaced by coarse terminal hair

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Stratum germinativum proliferates into the dermis.

Hair bud becomes club shaped=Hair bulb containing germinal matrix cells that willlatinize to form the hair shaft.

Hair bulbs are soon invaginated by small mesenchymal hair papilla.

Arrector pilorum develop from the surrounding mesenchyme.

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Buds from the sides of developing epithelial root sheath of hair follicles

Inactive until puberty.

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They develop as epidermal downgrowthInto the underlying dermis

Starts at about W20

Functional at birth

Develop from downgrowths of the stratum germinativum that gives rise to hair follicle

Begin secretion around 7-10 years.

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Fingernails reach the edge at W32

Toenails reach the edge at W36

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Highly modified and highly specialized type of sweat glands (development similar to that of SG)

W6: downgrowths of epidermal ridges in the pectoral area

Primary duct forms many 2ndary ducts that in turn develop to form lactiferous duct (15-20 at term).

Epidermis at the site forms a shallow pit mammary pit where the nipple will develop.

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