Scalp Hair Characteristics in the Newborn Infant

Susan A. Furdon, RNC, MS, NNP; David A. Clark, MD

Disclosures

Adv Neonatal Care. 2003;3(6) 

In This Article

Hair Formation in Utero

The embryo is formed from 3 distinct germ layers: the endoderm, mesoderm, and ectoderm. The brain, spinal cord, and peripheral nervous system as well as the skin, hair, and nails arise primarily from the ectoderm. Formation of these tissues and structures overlaps in the embryonic and fetal periods.[5] Hair follicles are formed by an interplay between the dermis and epidermis. Hair development begins early in the fetal period; initial hair bud formation occurs during the eighth week of gestation (Fig 1).[5,6] The hair bud develops into a club-shaped hair bulb. The epithelial cells of the hair bulb have the unique function of differentiating cells within the follicle, producing both the outer and inner root sheaths and the hair itself. Hair follicles appear on the scalp by the tenth week of gestation.[7] Follicle induction occurs in a precise pattern so that follicles are spaced equidistant from one another.[6] No new hair follicles are formed after birth.[1]

Note the progressive development of hair during early gestation. The hair follicle passes through an angled canal relative to the skin. Reprinted with permission.[5]

As the epithelial cells within the hair tract proliferate, they are pushed up the dermal root sheath to the surface and are keratinized, forming the hair shaft.[8] Keratinization occurs from the base of the hair shaft and is evident at 15 weeks gestation.[6] The elongating hair follicle is extruded through a straight but slightly angled canal. Consequently, hair growth occurs at an oblique angle relative to the skin. As hair grows, it pierces the epidermis to rise above the surface of the skin. The sloping angle of the scalp hair results from the direction of stretch and tension on the skin of the scalp during the concurrent development and maturation of the underlying fetal brain that is occuring during weeks 10 to 16 of gestation.[3] The parietal hair whorl is a result of the posterior scalp stretching that occurs with the outgrowth of the brain during this time.[3] By 18 weeks gestation the scalp hair pattern is set.[3]

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