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Let's Talk about Fleece: The Basics

Originally published in the March 2000 issue of Marker, quarterly magazine of the National Colored Wool Growers Association. Reproduced here with permission. See also Part 2, Genetics and Nutrition and Part 3, Harvesting and Handling.

The History
Ancestors of modern sheep had a double coat consisting of long hairy fibers, which served to protect from the weather and insulate from the elements, and a soft downy undercoat, which gave additional insulation and was shed annually.

The primary follicles of these primitive sheep produced the outer coat of hair. The secondary follicles produced the seasonal soft fine undercoat. The undercoat grew during the warm months, was idle during the winter months, and was shed in the spring. Wool, as we know it, developed from this undercoat, and sheep of today have a variety of fiber diameters from superfine to coarse.

While remnants of these ancient breeds still exist, sheep have evolved into four distinct breed types: the fine wool type of African and Spanish origin; the medium wool type and the long wool type, both of European and English origin; and the carpet wool type, mostly of Asiatic origin. In modern sheep the primary fibers have been reduced in size and numbers, the density of secondary fibers have increased, and the seasonal effects of growth reduced, eliminating shedding. The result is a relatively uniform coat of wool fibers.

The Follicles

Wool fibers are outgrowths of the skin; the follicle is an indentation of the skin's outer layer. Basically, the follicle consists of a bulb down deep and a shaft leading from the bulb up to the skin surface. The fiber originates in the bulb and is pushed up the shaft, where it undergoes elongation and hardening or keratinization. From two types of follicles, the primary and secondary, three types of fibers are produced: true wool, heterotype/medullated, and kemp.
The larger primary follicles produce wool fibers, as well as being the normal source of heterotype, often medullated, fibers and kemp. The primary follicle is unique as it has a sweat gland, a sebaceous or grease gland, and an erector muscle. The sebaceous gland coats each fiber with a greasy secretion before it reaches the skin surface. This greasy secretion also keeps the skin soft and protects both the skin and the fiber from drying out. The sweat gland secretes a soapy perspiration called suint, which combines with the grease to produce the yellow colored yolk. When the erector muscle contracts, the outer coat hairs stand up to trap the air and cause pressure on the sebaceous gland releasing the suint. One primary follicle is capable of producing only one fiber. Primary Follicle
Secondary Follicle In contrast, the smaller but more numerous secondary follicles grow only wool fibers and lack the sweat gland and erector muscle. One secondary follicle can produce up to eight wool fibers. The secondary-primary ratio is the number of secondary follicles for each primary follicle, and varies by breed, i.e. 20:1 in a merino to a 6:1 ratio in a Lincoln. This ratio can be a useful indicator of the quality, density and fineness of wool production in sheep.

The Fibers

  • True wool fibers are solid, strong, elastic and take a dye evenly. They can originate from primary or secondary follicles. Crimp or the degree of waviness is a distinguishing feature. The fiber is made up of overlapping scales or cuticles that protect the central cortex. The para-cortex can accept a dye, whereas the ortho-cortex will not accept dyes. The edges of the scales are glued down; scratchy wool results when the scales are not glued down completely.
  • Medullated fibers are coarse, hollow, often straight and brittle, with a thick core or medulla and a thin cortex. They can be difficult to see with the naked eye. These intermediate fibers tend to take up less dye than wool fibers. Degrees of medullation, due to the medulla in the center of the fiber, are possible.
  • Kemp fibers have sharp, pointed tips, are short, chalky white, often kinky in appearance, but kemp is not medullated. Kemp is shed in the fleece and originates from a primary follicle. It will not dye uniformly. Wool-containing kemp is often used in tweeds where the fibers become part of the pattern.
  • Colored or pigmented fibers contain melanin granules, which are produced by a cell called a melanocyte. These granules are present in the skin, medulla and cortex.
The Development

The central primary follicles develop in the fetus about 60 days into gestation. At between 70 to 90 days the two lateral primaries complete the trio group. After 95 days the secondary follicles begin to develop, the first ones are more apt to be branched and can produce up to eight wool fibers from one follicle. After about 120 days the secondaries are more apt to be an unbranched-type, thus producing only one fiber. All of the primary follicles are formed and growing fibers by the time the lamb is born. While most of the secondaries are developed before birth, they may not mature or produce fiber until after birth. Normally all follicles are producing fibers by a month after birth.

The Benefits

By increasing or assuring adequate energy or protein levels in the ewe at 95 days into gestation, the branching secondaries can be encouraged, resulting in finer, denser fleeces. By increasing the secondary/primary ratio, we can possibly effect the quality, fineness and density of the wool production in our sheep. The amount of wool produced by a sheep depends on the numbers of follicles actually growing a fiber and how fast that fiber grows; which in turn is related to nutrition, health, genetics, and other factors...but thatís next time.

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