Embryology of the Integumentary system “The making of “
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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
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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
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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
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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
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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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