The Pilosebaceous Unit: Week 18

Bulbous hair peg

The glands of the pilosebaceous unit differentiate from hair peg keratinocytes.

The hair peg becomes rounded (bulbous) at the dermal end and mostly encloses the cells of the dermal papilla.

Bulbous hair peg

The apocrine and sebaceous primordia, the attachment site for the arrector pili muscle, the hair canal, and the hair itself form simultaneously.

Three protrusions form along the shaft of the bulbous hair peg - one in each of the upper, middle, and lower portions of the hair peg. The uppermost usually involutes, but in certain locations it will differentiate into an apocrine gland; the middle bulge will differentiate into a sebaceous gland; the lowermost bulge will develop into the attachment site for the arrector pili muscle (See dermis cells).

The growing hair advances up the hair canal and hair tract, pushing debris ahead of it until it finally erupts from the scalp.

Until week 18 of gestation, melanocytes are uniformly distributed among the cells of the extending hair peg and now begin to migrate into the hair bulb, where they will be active in melanogenesis.