1. Development of skin

2. Skin develops at 4—5 wk of gestation & consists of single layer of ectoderm overlying mesoderm

3. Epidermis *Single layer of ectoderm is called primordium *proliferation of primordium will form 1- Squamous epithelium----------- periderm 2- Basal layer

4. *Periderm cells undergo keratinization & desquamation *Desquamated periderm cells + sebum from sebaceous G. Vernix caseosa (greasy material covers skin of fetus)

5. *Basal layer will form intermediate layer &later on differentiates into stratum germinativum *Proliferation of cells in stratum germinativum produce Epidermal ridges

6. *Neural crest cells migrate into mesenchyme of dermis &differentiate into melanoblasts &later migrate into dermoepidermal junction &differentiate into melanocytes

7. *Melanocytes produce melanin before birth &amount of Melanin production will increase after birth in response to Ultraviolet

8. Dermis *Develops from mesoderm—from 2 regions 1-Somatic layer of lateral mesoderm 2-Dermatomes of somite

10. *When epidermal ridges form,the dermis projects into epidermis *Capillary loops develop in some ridges *Sensory nerve endings form in other ridges *By the end of 1 st trimester, blood supply of dermis is well established

11. Glands * Sebaceous G.:As a buds from side of epidermal root sheath of follicles *These buds grow into surrounding C.T.& branches to form alveoli & duct

12. *Central cells of alveoli break down forming oily secretion (sebum)

13. Sweat G 1-Eccrine G. * Develops as epidermal downgrowth(bud)into underlying mesenchyme * Bud elongates &later coils to form secretory part of G.

14. *Central cells of bud degenerate to form lumen *Peripheral cells of bud differentiate into myoepithelial cells & secretory cells *They begin to function shortly after birth

15. 2- Apocrine G. *Develops as downgrowth of stratum germinativum *They begin to function during puberty

16. Hair *Begin to develop during 9—12 wk First sites of development are— eyebrow—upper lip—chin *It begins as a proliferation of stratum germinativum &extends into dermis

17. Lower part of hair bud expands to form hair bulb *Cells of hair bulb form hair matrix *Hair bulb is invaginated by mesenchymal T. to form hair Papilla *Peripheral cells of hair follicle differentiate to form epidermal root sheath *Cells of matrix proliferate to form hair shaft

18. *First hairs that appear is lanugo(downy hair)-soft-fine-mild pigmented *Lanugo is replaced during perinatal period by coarse hair all over the body skin except in axilla &pubic region where lanugo is replaced at puberty *Melanoblasts migrate into hair bulb &differentiate into melanocyte - melanin pigment transferred into hair matrix

19. *Arrector pili M.develops from mesenchyme -it is poorly developed in axilla & face -it is abscent in eyebrow & eyelashes

21. Nail *Toe nails& fingernails begin to develop at tips of distal Phalanx at 10 wk *Development of fingernail precedes that of toenails by 4 wk

22. *Primordia of nails appear as thickened area of epidermis called (field) *Nail fields migrate into dorsal surface of distal phalanx *nail fields are surrounded by proximal &lateral nail folds

24. *Cells from proximal nail fold grow over nail field &keratinized to form nail plate *At the beginning-nail plate is covered by superficial layer of epidermis which is called eponychium *Later,eponychium degenerates to expose nail plate except at its base where it persists as cuticle *fingernails reach fingertips at 32 wk *Toenails reach toetips at 36 wk