A current perspective on the biology of fibre production in animals

South American camelids produce commercially important fibres from hair follicles. These are embedded in the skin and have close similarities to those of other fibre-producing animals. Two major types of follicle have been characterised. These are primary follicles, which are usually larger and produce fibres that are longer, have a greater diameter and are less commercially valuable than fibres from the more numerous secondary follicles with which they are associated anatomically. Animals with high ratios of secondary to primary follicles are generally favoured in breeding programmes. Follicles develop in the embryo from interactions of mesodermal and overlying ectodermal tissue, which extends into the dermis to form the hair follicle unit. Postnatal fibre is produced from division and subsequent differentiation of basal epidermal cell "keratinocytes" in the follicular bulb matrix under the influence of signals from the contiguous dermal papilla (DP). The volume of the DP affects the diameter of the hair fibre. The presence of pigmentation depends on the presence of melanocytes in the bulb matrix. The anatomical structure and activity of post-natal follicles vary according to species and location on the body. All follicles are characterised by cycles of fibre growth which differ in length of active growth (anagen) followed by no growth (telogen) subsequent to apoptosis (programmed cell death) of epidermal cells in the lower bulb matrix. Regeneration involves the production of a new bulb matrix with existing DP and frequently shedding of the previously produced fibre. Control mechanisms involve hormones including those mediating response to photoperiod and physiological state (e.g. pregnancy and lactation), and local growth factors and signalling pathways directing communication within and between dermis and epidermis. Fibre production also depends on the provision of nutrients including minerals, vitamins and sulphur-containing amino acids. This review considers the above topics with reference to information derived from a range of animal species and relates these to more limited information currently available from studies on South American camelids.