Chapter 72. Development of The Integumentary System: Ectodermal Derivatives - Review of Medical Embryology Book

Integumentary system develops from surface ectoderm and the underlying mesenchyme

THE SKIN has a twofold origin: a superficial layer, the epidermis, derived from surface ectoderm, and a deep, thick layer, the dermis, derived from mesenchyme

Epidermis: initially the embryo's surface is covered by a single layer of ectodermal cells which, in month 2, divides to form a superficial protective layer of simple, flattened squamous epithelial cells, the periderm or epitrichium

The cells of the periderm layer continually undergo keratinization and desquamation to be replaced by cells arising from the basal layer

The basal layer of epidermis later becomes the stratum germinativum which produces new cells that are displaced into layers above

The exfoliated cells form part of the vernix caseosa, a white, cheesy, protective substance that covers the fetal skin

The vernix caseosa also includes sebaceous gland sebum, fetal hair, and desquamated amniotic cells

By week 11, the basal layer (stratum germinativum) forms an intermediate skin layer, and by the end of month 4, all the epithelial layers of the adult epidermis of skin have acquired their definitive arrangement. Four successive layers are seen (bottom to top)

Basal (stratum germinativum) layer: responsible for continuous development of new cells. It later forms genetically determined ridges and hollows which are filled by the underlying mesoderm. The patterns so formed are reflected on the surface of the skin (palms, fingers, and soles, including toes) in the form of fingerprints (dermatoglyphics)
Thick spinous (stratum spinosum) layer: large polyhedral cells, on top of the basal layer, connected by fine tonofibrils
Granular (stratum granulosum) layer: cells contain small keratohyaline granules, the first signs of keratinization
Horny (stratum corneum) layer: outermost layer which forms the scalelike hard surface of the epidermis and is loaded with keratin

Replacement of the peridermal cells continues until about week 21 (the cells are lost into the amniotic fluid), thereafter the periderm normally disappears
During the first 3 months of development, neural crest migrates and invades the epidermis, to form melanoblasts and then melanocytes, which synthesize melanin pigment. After birth, these cells cause skin pigmentation and are found in the epidermal-dermal junction

In dark-skinned races, melanin granules are produced by fetal melanocytes; in white-skinned races, the fetal melanocytes contain very little to no melanin pigment

The dermis is derived from mesenchyme of the somatic lateral mesodermal layer which underlies the surface ectoderm

During months 3 and 4, the dermis forms many collagenous and elastic fibers; simultaneously, the superficial dermal layer or corium forms irregular papillary structures, the dermal papillae, which project into the epidermis

Some papillae contain small capillary loops, and others have sensory nerve endings

At birth, the skin is covered by the vernix caseosa, a whitish paste formed by sebaceous gland secretion, degenerated epidermal cells, and hairs. It protects the skin against the maceration action of the amniotic fluid
If the superficial layers of the skin show excessive cornification, the skin has a scaly appearance, a condition spoken of as icthyosis