Wool denotes soft, curly fleece fibers from domesticated sheep. It differs from hair in that its scales are more numerous, smaller, and pointy. Curliness makes wool very resilient. This high tensile strength and elasticity help wool fabrics to retain their shape. These properties, wool's lightness, and its fine insulating ability make wool fabrics desirable.
Wild sheep have sparse, woolly undercoats and coarse hair, useless in fabrics. The hair has been bred out in domestic breeds. Sheep fleeces are usually shorn annually, from late spring to early summer. The wool is cut off very close to the skin and removed in one piece, weighing nine to eleven pounds. Wool from different fleece parts varies in its fiber length, fineness, and structure. The shoulder and sides yield the best fibers. Merino sheep yield the best overall wool. It makes up 40 percent of all wool produced commercially. Crossbreeds of merino sheep and strains that produce longer, coarser wool yield most of the rest. Some apparel wool comes from alpacas, goats, and llamas.
Woolen cloth manufacture begins by pulling fleeces apart and choosing the best fibers for given uses. Next, fibers are cleaned to remove lanolin and dried-on sweat. The clean fibers are disentangled and drawn straight by carding, which entails passing them between rotating cylinders to yield a thin film or web. Web processing varies, depending on whether it will be used in tweed or worsted yarn. Tweeds, woven from bulky yarn made of short, randomly arranged fibers, are thick and fuzzy. Worsted fabrics such as gabardines are woven from web made of longer, thinner fibers, tightly twisted for smoothness.
At every hair's base is a saclike hair follicle. The hair grows from the bottom of the follicle, nourished by blood vessels in a structure called a papilla. The papilla extends into the follicle and into the hair's root. A tiny muscle is attached to each hair follicle. Action of the nervous system can cause the muscle to contract to make hair "stand on end."
A hair forms from cells that grow from the surface of the papilla, which means that it grows from the root, not the free end. As new cells develop, they push forward old ones, which become part of the shaft. Hair growth continues as long as follicle and papilla are functional. The lifetime of a hair from start of growth until it is shed depends upon the organism which produces it. When an old hair falls out a new one takes its place.
Hair follicles produce hairs in cycles of hair growth, in which the hair follicle and the shaft pass through a complex series of morphological changes. During hair growth, the follicle penetrates into the dermis, and cells of the shaft are joined together. In addition, the follicle's mela nocyte cells deposit pigment into shaft cells. Once a hair shaft attains its characteristic length, the follicle contracts and a "dead" hair protrudes from it. The growth period of a single hair ranges from three years in humans to around two weeks in rodents.
Hairs are continually replaced, or shed, throughout the life of a mammal. However, their development and loss occur asynchronously, so mammals are never completely naked of pelage. In rodents, replacement is in waves, across the body. In primates, each follicle passes through the growth cycle, independent of those around it. Hormones control hair growth; however, there are other, as yet unknown components that must also affect the process, because hormones are carried in the blood, and if they acted alone they would simultaneously affect all follicles in the body. This would be disastrous, because if all follicles grew in together, at the same rate, all hairs would be shed at the same time. Then the mammal involved would have naked periods where it was deprived of hair's protection and insulation. Continuous growth of hairs can also be hazardous. For example, in merino sheep, long growth phases pro duce long-stranded wool. However, if these sheep strayed, were not minded well, or were sheared irregularly, they might starve to death from becoming entangled in underbrush.
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