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The medulla refers to the hollow, cylindrical cell that is found along the long center axis of some animal fibers. The degree of medullation can vary from broken or interrupted to fully medullated fibers, in which there is a thick hollow core or medulla, with a thin cortex. This hollow core may run continuously along the length of the fiber or it may be intermittent. Alpacas tend to have more medullated fiber than sheep, although when guard hair is present in sheep, it is medullated and referred to as kemp. Medullation can occur in both primary and secondary fibers. Medullated fibers cannot be discerned by the naked eye. While characteristics that can be indicative of medullation may be easily discernable, such as with thick and/or stiff fibers, medullation itself cannot be verified except by means of microscopic examination. Guard hair and thick primary fibers are relatively easy to detect in Huacaya alpacas, sometimes simply because they lack crimp; but to determine the degree to which those fibers may be medullated, they must be examined by microscope.
The impact of medullation in a fleece depends on the desired end product. An end product that is intended to provide superior insulation may benefit from the presence of medullated fibers, as those hollow and/or semi-hollow cores can help trap heat, effectively helping a person stay warm in cold weather or cool in hot weather. Medullation can help provide an effective thermal barrier. On the other hand, if a dyed end product is desired, medullation is problematic. The hollow areas within a fiber do not absorb dyes the way the substantive parts of the fiber do. As a result, highly medullated fibers tend not to produce colors that are either as vivid or as consistent as non-medullated fibers. In general, coarser alpaca fiber tends to be more medullated than finer fibers. However, with respect to any particular fiber, a microscope is needed to accurately determine the degree of medullation. The degree of medullation within a fleece is not only a function of genetics; it can also be affected by climate, stress, nutrition, and parasite load.
Fully medullated fiber is true "hair" fiber – straight, smooth, hollow, and ending in a point.
Any fibers over 30 microns will create a prickly, itchy, or scratchy effect, whether or not they are medullated. Both the shear-cut end of the fiber and the outside, pointed end of the fiber contribute to the prickle factor. Frequently, those prized, thick Scottish and Irish wool sweaters are warm, as a result of medullation, but also prickly because the fibers are relatively coarse.
Most Huacayas carry guard hair on the chest and in the britch area, and many carry it also up the neck. In some, guard hair creeps further into the fleece and through the blanket. Coarse primary fibers are undesirable in alpacas, particularly when carried within the blanket. With careful selection techniques, breeders should be able to make good progress at producing alpacas with less variation in micron size between primary and secondary fibers within a fleece. In other words, thoughtful breeding choices can result in increased fiber consistency and decreased overall AFD. However, both degree of medullation and AFD can be influenced by outside factors, including environmental stressors, general health, and nutrition. In addition, it appears that as density increases, often the number of guard hairs decrease, and primary fibers instead evolve into finer fibers, less distinguishable from the secondaries within the blanket.
Primary fibers can be fully medullated, partially medullated, or not medullated at all.
Secondary fibers, likewise, can be fully medullated, partially medullated, or not medullated at all, even when the micron is extremely low. Medullation has been noted to as low as 12 microns.