The Questions of Developmental Biology

epidermal pathway of differentiation. The alternative pathway for basal cells leads to the
generation of hair follicles.
Cutaneous appendages
The epidermis and dermis also interact at specific sites to create the sweat glands and the
cutaneous appendages: hairs, scales, or feathers (depending on the species). In mammals, the
first indication that a hair follicle primordium, or hair germ, will form at a particular place is an
aggregation of cells in the basal layer of the epidermis. This aggregation is directed by the
underlying dermal fibroblast cells and occurs at different times and different places in the
embryo. It is probable that the dermal signals cause the stabilization of β-catenin in the ectoderm
(Gat et al. 1998). The basal cells elongate, divide, and sink into the dermis. The dermal
fibroblasts respond to this ingression of epidermal cells by forming a small node (the dermal
papilla) beneath the hair germ. The dermal papilla then pushes up on the basal stem cells and
stimulates them to divide more rapidly. The basal cells respond by producing postmitotic cells
that will differentiate into the keratinized hair shaft (see Hardy 1992; Miller et al. 1993).
Melanoblasts, which were present among the epidermal cells as they ingressed, differentiate into
melanocytes and transfer their pigment to the shaft (Figure 12.34).
As this is occurring, two epithelial
swellings begin to grow on the side of
thehair germ. The cells of the lower
swelling may retain a population of stem cells that will regenerate the hair shaft periodically
when it is shed (Pinkus and Mehregan 1981; Cotsarelis et al. 1990). The cells of the upper bulge
form the sebaceous glands, which produce an oily secretion, sebum. In many mammals,
including humans, the sebum mixes with the shed peridermal cells to form the whitish vernix
caseosa, which surrounds the fetus at birth.