Chapter 2 43 level downward and is contiguous with the base of the DP through a stalk. The DS consists of three layers of collagen fibers running in different directions, with fibroblasts mostly residing in the middle of thickened collagen layer. DS has been considered as a supply source for DP renewal during each hair follicle cycle.11,12 Hair follicle morphogenesis takes place during fetal and perinatal development, and involves tightly coordinated ectodermal– mesodermal interactions. However, de novo hair follicle formation may also occur after skin wounding in adult mice. In the following sections, we will discuss the embryonic morphogenesis and postnatal cyclic growth of human and murine hairs in detail. Hair Follicle Morphogenesis The mouse has a hair coat comprising a mixture of four different hair types: guard, awl, auchene, and zigzag.13 There are three successive waves of hair follicle induction that employ distinct molecular mechanisms. The first wave of hair follicle morphogenesis occurs around embryonic day 14 (E14), giving birth to primary hair placodes. Primary hair placodes produce guard hairs that are straight, containing two columns of medulla cells, and are relatively long. Each primary hair placode has two sebaceous glands and a unique sensory function. They represent 1–3% of the total number of hairs. Secondary hair follicle development comes in two major waves: at E16.5 for the intermediate awl and auchene hairs, and around postnatal day 0 (P0) for the downy zigzag hairs. Awl hair represents 30% of the coat, which is about one-half to two-thirds the size of guard hair. Awl hair is straight and contains two to four columns of medulla cells. Zigzag hairs, also known as undercoat hair, are the most abundant hairs in the coat (65–70%). The size of zigzag hair is the same size as that of awl hair, and contains only one column of medulla cells. Zigzag hair also has several constrictions along the hair shaft forming three to four bends alternately pointing in opposite directions. Auchene hair is present at very low frequency (around 0.1%), sharing common features with both awl and zigzag hairs except only having one bend.14,15 Both primary and secondary