Understanding the cellular basis of skin growth - Biomaterials for Treating Skin Loss - page 83

6.3 The structure of a hair follicle.

of developing skin and give rise to concentric rings of differentiated cell types,
including the medulla (the cortex and the cuticle of the cortex). The hair is
surrounded by inner and outer root sheaths. The inner root sheath cells extend to
the dermal papillae, where they are mitotically active and produce cells that
migrate upward. The outer root sheath cells are continuous with the basal epider-
mal cells and do not come into direct contact with the dermal papillae. They are
contiguous with the basal epidermal layer but are distinct and self-propagating.

6.3 Skin development and growth

Skin consists of the epidermis and the dermis. During embryonic development, the
epidermis derives from a single layer of cells called the embryonic ectoderm
( Fig. 6.4 ). This early embryonic ectoderm has a morphology and biochemistry
similar to those of simple epithelial tissues (Dale et al., 1985) and also is a basis for
the neuroectoderm. Midway through mammalian development, an embryonic
epidermis forms consisting of an inner and outer layer. The inner layer is the
embryonic basal layer, which, in reponse to different stimuli, is the precursor of the
epidermis and its appendages. The outer layer, termed periderm and unique to
primates, still possesses properties of simple epithelial cells. The periderm is later
shed from the skin's surface as the true epidermis keratinizes beneath it (Rook et
al., 1986). In all mammals, the first signs of epidermal differentiation appear when
the epidermis stratifies to form a third layer. This happens at around 13 days of
mouse embryogenesis and around 8-11 weeks during human embryonic develop-
ment (Rook et al., 1986). From here on, the development of the epidermis,
sebaceous glands and hair follicles begin to take separate paths. The primary hair