1. By Jittipan Chavadej, Ph.D. Anatomy Dept.,Fac. of Science yr., 2000

2. Structural development begin as a single layer of ectodermal cells formation of a thin outer layer periderm at the end of the first month third month- three layers - basal layer, intermediate layer&sup l layer of peridermal cells

3. Diagram showing the formation of the skin at various stages of development

4. 6 th month -epidermis beneath periderm differentiation into the definitive layers Sloughs of peridermal cells into the amniotic fluid Immigrant cells in the epidermis -2 nd month -melanoblast -late in 1 st trimester -Langerhans cells -Merkel cells

5. Melanoblast (-cyte) : -derived from neural crest -midpregnancy-begin producing pigment granules = melanosomes DDark skin- more pigment granules/cell Albinism -genetic trait, lack of enzyme-tyrosinase: tyrosine melanin

6. Epidermal differentiation Movement of epidermal cells - loss of adhesiveness e.g. fibronectin, laminin&collagen types I&IV Keratohyalin granules (histidine&sulphur- rich), (closely associated with keratin filament) - stratum granulosum (keratinocyte) Keratin filaments-stratum corneum

7. Unspecialized cells - mitotic activity Produce keratin filament (desmosome) Flattened cell-breaking up nuclear membrane Lost nuclei -bag of densely packed with keratin filaments 15-20 layers of dead cells

8. One of the prominent features of thick skin=presence of epidermal ridges and creases ->loops & whorls Dermatoglyphics-basic patterns (epi. ridge) for genetic analysis or criminal investigation (unique to individual)

9. Dermis Mesodermal cells from dermatome / beneath ectoderm 3 rd month -transition from highly cellular embryonic form fibroblast +fibrous intercellular matrix Becomes highly vascularized + sensory nerve

10. Future dermis-loosely aggregated mesenchymal cells

11. Epidermal appendages Hair -induction from dermis -12 th week :down growth of hair bud -hair papillae hair shaft epithelial hair sheath -arrector pili muscle : dermal root sheath & dermal papilla -sebaceous gl. fat-like substance hair follicle

12. Diagram showing development of a hair and a sebaceous gland

13. *5 th mo. hair shaft-keratinization forming granules of trichohyalin(hardness of the hair) *Late stage of hair formation-hair bulb--> melanocytes -->color

14. Mammary Gland Mammary line or ridge (6 wk.) :base of forearm region of hind limb Small solid outbudding lactiferous duct formation+alveoli of the gland epithelial pit (nipple-after birth)

15. Stages in the embryonic development of the human mammary gland Diagram showing position of mammary line

16. Abnormalities of Skin Development Ichthyosis-genetically transmitted disorders of keratinization /Harlequin -autosomal recessive disorder Hypertrichosis- formation of hair follicle Atrichia -congenital absence of hair

17. A: milk line-mammary glands form along this line B: common sites of formation of supernumerary nipples/mammary gland

18. Polythelia - accessory nipples (axillary region) Polymastia -supernumerary brests