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Integumentary systemMin-Chuan Huang

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Integumentary system

Skin:

Epidermis

Dermis

Skin appendages

Sebaceous glands

Sweat glands

Mammary glands

Hair

Arrector pili muscles

Nails

Teeth

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• Embryonic skin: begins from embryo of 4-5 weeks

• Epidermis: surface ectoderm

Dermis: mesoderm

Development of skin: overview4 weeks

Mutual inductive mechanism for ectodermal/mesenchymal

interactions (epidermal / dermal interactions)WNT, fibroblast growth factor (FGF), transforming growth factor-β (TGF-β),

sonic hedgehog (SHH)

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Development of epidermis: early stage

• 4 weeks: primordial

skin from surface

ectoderm

• 7 weeks:

●Periderm: single layer of

squamous epithelium- Continue keratinization,

desquamation

- Exfoliated cells form part of

vernix caseosa

●Basal layer: proliferation

4 weeks

7 weeks

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Development of Epidermis at 11 weeks

• Stratum germinativum–Epidermal ridges: begin at 10 weeks, permanently established at 19th

week; genetically determined (fingerprint), Dermatoglyphics皮紋學

• Intermediate layer: from stratum germinativum

• Migration of neural crest cells to developing dermis (melanoblasts), later melanocytes (as early as 40-50 days) in stratum germinativum

11 weeks

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Development of epidermis: 21st week

forward

• Stratum corneum: originally periderm

• Appearance of stratum lucidum, granulosum, spinosum Melanocytes fail to produce melanin:

1. Generalized albinism

2. Localized albinism (piebaldism)

(periderm disappears)

ichthyosis: severe keratinization

Newborn

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Development of Dermis• Mesenchyme from mesoderm: (major) somatic layer of

lateral mesoderm & (minor) dermatomes of somites

• By 11 weeks: mesenchymal cells produce collagenous &

elastic fibers

• Dermal papillae: with capillary loops & sensory nerve

endings

• Capillary-like vessels: derived from mesenchyme

(vasculogenesis), begin at the end of 5th week

11 weeks

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Development of

hair (1/2)

•Hair follicle: stratum germinativum into dermis

•Hair bud•Hair bulb (primordium of hair root) –Germinal matrix, invaginated by mesenchymal hair papilla–Hair shaft: keratinized portion

Week 9-12

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Development

of hair (2/2)

• Epidermal root sheath:

from peripheral cells of

hair follicles

• Dermal root sheath: from

mesenchymal cells

• Lanugo: the first hair, end

of 12th week, replaced

during perinatal period

• Melanocyte in hair bulb

from migrating

melanoblast

• Arrector pili muscles from

mesenchyme, goose

bumps

Week 12 Week 20

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Hypertrichosis

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Nature. 2007Hypertrichosis

Alopecia

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Folliculogenesis Hair cycling

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Stem Cells 2005;23:150–165

Hair follicle stem cells in bulge region

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Hair follicle stem cells

• Hair follicles

• Schwann cells

• Neurons

• Glial cells

• Keratinocytes/sebaceous glands

• Smooth muscle cells

• Blood vessels

• Adipocytes

• Hematopoietic cells

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Development of

sebaceous glands

Week 16 Week 20

• Glandular buds from sides of developing epidermal root sheaths of hair follicles, branch to form primordia of alveoli & ducts

• Central cells of alveoli: break down, release into hair follicle, mix with desquamated peridermalcells (vernix caseosa)

• Sebaceous glands independent of hair follicles: in external genital organs

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Development of eccrine sweat glands• Epidermal downgrowth into dermis by elongation, and coiling

• End: primordium of secretory part; (1) myoepithelial cells, (2)

secretory cells

• Epithelial attachment: primordium of duct, central cells

degenerate

• Begin to function after birth20 weeks

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Development of apocrine sweat glands

• Axilla, pubic, perineal regions, areolae of the nipples

• Downgrowths of stratum germinativumgiving rise to hair follicles

• Open into the upper part of hair folliclessuperficial to the openings of sebaceous glands

• Secrete during puberty

• Pheromone / body odor

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Development of mammary glands

• Mammary ridges: 4th week; thickened strips of ectoderm from axillary to inguinal regions

• Mammary buds: downgrowth of epidermis at 6th week

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Supernumerary nipples

1% of the female

5.6%

Supernumerary breasts

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Development of mammary glands

Flat or inverted nipples (10-20%)

• Mammary buds: primary bud → secondary buds → lactiferous ducts/branches: canalization by placental sex hormones

• Mammary pits: depressed epidermis → nipples rise by proliferation of connective tissues (areola)