Skin

Skin

Staci Bryson, MD

Key Facts

Embryology

Dermis
- Originates from somatic mesoderm and dermomyotome population of somites
- Elastin and collagen fibers produced at week 11
Epidermis
- Forms from ectoderm
- Simple epithelium present by week 4
- Superficial periderm forms by week 7
- Intermediate layer forms from stratum germinativum by week 11
- Periderm cells are replaced by epidermal cells by week 21
- All adult layers are formed by birth
Melanocytes
- Melanoblasts migrate from neural crest to basal layer by week 8
Adnexa
- Hair follicles form from downgrowths of stratum germinativum during weeks 9-12
- Sebaceous glands form as buds from hair follicle root sheath
- Arrector pili muscles form from surrounding mesoderm
- Eccrine sweat glands form as solid epidermal downgrowths into dermis by week 20

Macroscopic Anatomy

Skin remains thin with visible vessels until week 17
Vernix caseosa forms white protective covering
- Formed from desquamated periderm cells, sebum, lanugo hairs, and amniotic cells
Epidermal ridges form from downgrowths of stratum germinativum into dermis
- Arise during weeks 13-17 of development
- Palmar surfaces of hands and fingers, plantar surfaces of feet and toes
- Give rise to unique pattern of whorls, e.g., fingerprints

Microscopic Anatomy

Definitive layers of skin are not present until week 21 of development
Dermis
- Contains collagen and elastic fibers in ground substance of mucopolysaccharides and mucoproteins
- Contains adnexal structures, nerves, blood vessels
- Cellular composition is a mix of fibroblasts, mast cells, macrophages, lymphocytes, and dermal dendrocytes
Stratum germinativum
- Basal layer
- Cuboidal cells with high nucleus:cytoplasmic ratios
- Produces other keratinocytes
Intermediate layer
- Layers of cells between stratum germinativum and periderm
- Lacks spiny bridge of stratum corneum
- Present from week 11 of development until definitive layers form (week 21)
Periderm
- Earliest layer of squamous cells that keratinize and desquamate
Stratum spinosum
- Several layers of cells
- Cells are larger and have more cytoplasm than stratum germinativum
- Progressive maturation with cells becoming increasingly flat and eosinophilic toward surface
- Cells are attached by spiny bridges
Stratum granulosum
- 1-3 cell layers thick
- Cells contain intensely basophilic keratohyaline granules
Stratum corneum
- Cornified or horny layer
- Definitive keratinizing and desquamating layer
- “Basket weave” pattern of multiple polyhedral cells without nuclei

(Left) Overview of the skin at 20 weeks gestation shows the basic layers: Epidermis

, dermis

, and subcutis

. Note the relatively thin epidermis without rete ridges and the relative paucity of fat in the subcutis. (Right) At 20 weeks gestation, the skin lacks the definitive layers of the epidermis. The superficial layer is the periderm