Embryology

   
 
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