The Integumentary System

2. The Integumentary System

3. The Integumentary System • Consists of skin & • Its derivatives: • Sweat glands • Sebaceous glands • Arrector pili muscles • Nails • Hair • Mammary glands

4. Development of Skin • Skin consists of two layers that are derived from two different germ layers • Epidermis, superficial epithelial tissue derived from surface ectoderm • Dermis, deeper layer of connective tissue derived from the mesoderm

5. Epidermis • In the 4-5th week, the skin of the embryo consists of simple cuboidal epithelium….the surface ectoderm • By the 7th week, the surface ectodermal cells proliferate and form a layer of squamous epithelium, the periderm (epitichium) and a basal germinative layer. • During 1st & 2nd trimesters, epidermal growth occurs in stages, which result in an increase in epidermal thickness

6. Periderm • The peridermal cells continually undergo keratinization and desquamation and are replaced by cells arising from the basal layer • The exfoliated cells form part of the white greasy substance, the vernix caseosa, that covers the body of the fetus • Replacement of peridermal cells continue untill about the 21st week, thereafter the periderm disappears and the stratum corneum forms

7. Basal Germinative Layer • This layer becomes the stratum germinativum of the epidermis • It proliferates and the new cells are displaced into the layers superficial to it. • By 11th week, an intermediate layer, containing several cell layers, is interposed between the basal cells and the periderm.

8. Basal Germinative Layer cont’d • Proliferation of stratum germinativum also forms epidermal ridges which extend into the developing dermis. • These ridges begin to appear in embryo of 10 weeks and are permenantly established by the 17th week. • These ridges produce grooves on the surface of palms of the hand and soles of the feet including digits

9. Melanoblasts & Melanocytes • During the early fetal period the epidermis is invaded by melanoblasts, cells of the neural crest origin. • Melanoblasts move to dermoepidermal junction and differentiate into melanocytes • The melanocytes have several long processes.

10. Melanoblasts & Melanocytes cont’d • The cell bodies of melanocytes are confined to the basal layers of the epidermis, and their processes extend between the epidermal cells • The melanocytes begin producing melaninbefore birth and distribute it to the epidermal cells

11. At birth all layers of the adult epidermis are present

12. Dermis • The dermis is derived from the mesenchyme underlying the surface ectoderm • This mesenchyme is derived from the: • Somatic layer of the lateral mesoderm (most of it) • Dermatomes of the somites (some). • By 11th week, the mesenchymal cells begin to produce collagenous and elastic connective tissue fibers

13. Dermal Papillae • As the epidermal ridges are formed, the dermis projects upward into the epidermis and forms the dermal papillae • Capillary loops and sensory nerve endings develop in these papillae DP DP

14. Hair • Begin to develop during the 3rd month, but they do not become visible until the 20th week • Begins as an epidermal proliferation, the hair bud, into the underlying dermis. • The deepest part of the hair bud becomes cup-shaped, forming a hair bulb • The hair bulb gets invaginated by mesenchymal hair papilla

15. Hair cont’d • The central epithelial cells of the hair bulb give rise to the shaft of the hair, that grows through the epidermis and protrudes above the surface of the skin • The peripheral cells of the hair bulb form the epithelial root sheath. • The cells of the epithelial root sheath proliferate to form a sebaceous gland bud.

16. Hair cont’d • Surrounding mesenchymal cells differentiate into dermal root sheath. • The arrector pili muscle differentiates from the surrounding mesenchyme • Melanoblasts migrate into the hair bulb and differentiate into melanocytes

17. Hair cont’d • Hairs are first recognizable in the region of eyebrows, upper lip and chin • The first set of hairs that appear are fine and colorless and are called ‘lanugo’ hair • Lanugo hair are replaced during the perinatal period by coarser hair

18. Sweat Glands • Develop at about 20 weeks as solid growth of epidermal cells into the underlying dermis • Its terminal part coils and forms the body of the gland • The central cells degenerate to form the lumen of the gland • The peripheral cells differentiate into secretory cells and contractile myoepithelial cells

19. Vernix Caseosa • Vernix caseosa, is the waxy or cheesy white substance found coating the skin of the newborn. • The vernix is secreted by the sebaceous glands around the 20th week of gestation • It is composed of: • Sebum (the secretion of the sebaceous glands) • Desquamated epithelial cells • Fetal hair (lanugo hair) • It protects the baby's skin from dehydation and from constant exposure to the amniotic fluid.

20. Nails • Begin to develop at about 10th week of gestation, as thickened areas of the epidermis at the tips of the digits. • Later, these nail fields extend to the dorsal surface and become surrounded by the nail folds. • Cells from the proximal nail fold grow over the nail field and form keratinized nail plate, the primordium of the nail.

21. The MammaryGlands • Begin to develop during the 6th week as thickened strips of the ectoderm (mammary ridges) that extend from the axillary to the inguinal regions. • They regress in most locations except in the area of the pectoral muscle, where they proliferate.

22. The MammaryGlands cont’d • The downgrowth of epithelial tissue continues to proliferate into 16 to 24 solid outbuddings which give rise to the lactiferous ducts. • Fibrous connective tissue and fat of the mammary gland develop from the surrounding mesenchyme. • The lactiferous ducts at first open into a small mammary pit.

23. Postnatal Development • Newborn (nipple is inverted) • Child (nipple elevates to form the usual nipple) • Puberty (breast enlarges due to development of the mammary glands) & deposition of fat • Late puberty • Young adult • Pregnant female

24. Anomalies • Gynecomastia • Polythelia • Inverted nipples

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