Figure 18.3 Development of a hair and a sebaceous gland. A. 4 months. B. 6 months. C. Newborn.
peripheral cells become cuboidal, giving rise to the epithelial hair sheath (Fig. 18.3, B and C).
The dermal root sheath is formed by the surrounding mesenchyme. A small smooth muscle, also derived from mesenchyme, is usually attached to the dermal root sheath. The muscle is the arrector pili muscle. Continuous proliferation of epithelial cells at the base of the shaft pushes the hair upward, and by the end of the third month the first hairs appear on the surface in the region of the eyebrow and upper lip. The first hair that appears, lanugo hair, is shed at about the time of birth and is later replaced by coarser hairs arising from new hair follicles.
The epithelial wall of the hair follicle usually shows a small bud penetrating the surrounding mesoderm (Fig. 18.3C). Cells from these buds form the sebaceous glands. Cells from the gland degenerate, forming a fatlike substance secreted into the hair follicle, and from there it reaches the skin.
CLINICAL CORRELATES Abnormalities of Hair Distribution
Hypertrichosis (excessive hairiness) is caused by an unusual abundance of hair follicles. It may be localized to certain areas of the body, especially the lower lumbar region covering a spina bifida occulta defect or may cover the entire body.
Atrichia, the congenital absence of hair, is usually associated with abnormalities of other ectodermal derivatives, such as teeth and nails.
Was this article helpful?
The best start to preventing hair loss is understanding the basics of hair what it is, how it grows, what system malfunctions can cause it to stop growing. And this ebook will cover the bases for you. Note that the contents here are not presented from a medical practitioner, and that any and all dietary and medical planning should be made under the guidance of your own medical and health practitioners. This content only presents overviews of hair loss prevention research for educational purposes and does not replace medical advice from a professional physician.