EMBRYONIC DEVELOPMENT & STEM CELL COMPENDIUM
EMBRYONIC DEVELOPMENT & STEM CELL COMPENDIUM
74. Development of The Hair and Associated Structures
Review of MEDICAL EMBRYOLOGY Book by BEN PANSKY, Ph.D, M.D.
- The hair appears as a solid epidermal downgrowth of the stratum germinativum into the underlying dermis and is called the hair bud
- THE DEEPEST PORTION OF THE HAIR BUD becomes club-shaped and forms the hair bulb
- The epithelial cells of the hair bulb constitute the germinal matrix, which later will give rise to the hair
- The hair bulb is then invaginated by a small mesenchymal hair papillae
- As the cells of the germinal matrix, in the center of the hair follicles, proliferate, they are pushed upward and become keratinized forming the hair shaft. The peripheral cells of the developing hair follicle form the epithelial root (hair) sheath
- The surrounding mesenchymal cells differentiate into the dermal (connective tissue) root sheath
- The hair grows, penetrates the epidermis, and appears above the skin surface
- Melanoblasts invade the hair bulb and form melanocytes. They produce melanin which is transferred to the hair-forming cells in the germinal matrix before birth
- HAIRS BEGIN TO DEVELOP during early fetal life, but become visible at about week 20 on the eyebrows, upper chin, and lips and are called the lanugo hairs
- The lanugo are shed at about the time of birth and are later replaced by coarser hairs called vellus hairs which arise from new hair follicles
- The vellus persists over most of the body except in the axillary and pubic regions where, at puberty, they are replaced by coarse terminal hairs (seen also on the chest and face in males)
- THE ARRECTOR PILI MUSCLES are smooth muscle fibers which form from the surrounding mesenchyme and become attached to the connective tissue sheath of the hair follicle and dermal papillary layer
- The sebaceous glands develop as buds from the side of the developing epithelial root sheath of the hair follicle
- THE BUDS GROW into the surrounding connective tissue and branch to form the primordia of the glandular alveoli and ducts
- The central cells of the alveoli break down and form an oily secretion, the sebum, which is extruded into the hair follicle and onto the skin surface to mix with the desquamated peridermal cells to help form the vernix caseosa
- INDEPENDENT GLANDS (not with hair follicles) also develop from the epidermis in the areas of the glans penis and the labia minora
- Sweat (eccrine or merocrine) glands develop as a solid epidermal bud which grows down into the underlying dermis
- AS THE BUD ELONGATES, its end coils to form the primordium of the glands secretory portion, while the epithelial attachment to the epidermis forms the duct primordium
- The central cells of the primordia degenerate to form a lumen
- The peripheral cells of the secretory portion of the gland differentiate into secretory and myoepithelial cells, the latter being specialized ectodermal smooth muscle cells which aid in expelling the glandular secretion
- Sweat (apocrine) glands in humans, are confined to the axilla, pubic areas, and areola of the mammary glands
- THEY DEVELOP as downgrowths of the stratum germinativum of the epidermis
- Their ducts open into the hair follicles and not on the skin surfac They open just above the sebaceous glands
- Their chief function seems to be the production of small amounts of secretions which, on the surface, give rise to distinctive odors that enable animals to recognize each other. Human apocrine sweat glands have no odor in their secretion but contain substances readily degraded by bacteria into odiferous breakdown products
