| Table 1-1 Developm ent of Cutaneous Structures |
| | Gestational Age (Estimated) | | Epidermal Development | | Hair, Nail, and Gland Development | | Dermal/Subcutaneous
Development |
First trimester |
| | ~3–4 weeks | | Single layer of ectoderm | | | | |
| | ~6 weeks | | Outer flattened periderm and inner, cuboidal germinal (basal) layer
(Germinal layer produces entire epidermis) | | | | Germinal layer in contact w/ underlying mesenchyme |
| | ~7 weeks | | Fetal basement membrane | | Tooth primordia | | |
| | ~8–12 weeks | | Epidermal stratification begins ~8 week
(Completed by second trimester)
Appearance of
→Melanocytes
→Langherhans cells
→Merkel cells
| | | | Dermal-subcutaneous boundary distinct |
| | ~9–12 weeks | | Appearance of anchoring filaments/hemidesmosomes | | Hair follicle and nail primordia seen | | |
| | Second trimester |
| | ~12 weeks | | Formation of dermo-pidermal junction (DEJ) | | Nail bed starts to keratinize, proximal nail fold forms | | Type III collagen appears |
| | ~12–14 weeks | | Parallel ectodermal ridges (fingerprints) | | Eccrine and sebaceous gland primordia seen | | Fibroblasts actively synthesizing collagen and elastin in dermis |
| | ~12–24 weeks | | Melanin production (12–16 weeks), melanosome transfer (20 weeks) | | Hair follicles differentiate during second trimester (seven concentric layers present) | | |
| | ~15–20 weeks | | | | Follicular keratinization, nail plate completely covers nail bed | | Papillary/reticular boundary distinct, dermal ridges appear |
| | ~22 weeks | | | | Trunk eccrine gland primordia | | Elastic fiber seen |
| | ~22–24 weeks | | Mature epidermis complete (w/ interfollicular keratinization) | | | | Adipocytes appear under dermis |
| | | | | | | | |
