A965
Wool - Biology

Introduction: The true wool fiber is basically modified hair that is not shed like the hair of most farm animals. Nutrition can cause differences in fiber diameters and fiber strength. A sheep on a high plane of nutrition grows a coarser fiber than one on a poor ration. A break or tender spot in the fiber can be caused by an extreme drop in nutrition or from stress caused by an infection, illness, or pregnancy. Shorter daylight hours also result in a smaller fiber diameter, regardless of nutritional levels. The tip of a fiber on a lamb’s fleece is pointed, while the tip from a mature fleece is flat because of previous shearing. The wool fiber, for descriptive purposes, is divided into three (3) sections: the root, shaft, and tip.

1. Wool Fiber Production: Typical wool is produced by hair follicles that are present on the surface of the skin. Sheep have both primary and secondary follicles. There are potentially three different types of fibers grown from a follicle: true wool, heterotype hair, and kemp. All sheep have the two types of follicles, but do not grow all types of fibers.
   1. Primary follicles:
      1. The primary follicles are typically arranged in rows of threes, called "trio groups."
      2. The primary follicle is unique in that it has a sweat (sudoriferous) gland and an erector (arrector pili) muscle. In addition, it has a sebaceous (wax or grease) gland.
      3. The primary follicle produces a true wool fiber, as well as being the normal source of kemp and heterotype (hair) fibers. The fibers they produce are usually the largest.
         
         
   2. Secondary follicles:
      1. The secondary follicles and their fibers are more numerous and lie to one side of the primary follicles.
      2. The primary trio with its associated secondary follicles constitutes the follicle group, which is the basic unit of wool production.
      3. The secondary follicles are the smallest follicles and tend to grow finer wool fibers than the primary follicles.
      4. The fundamental difference between the primary and secondary follicles is that the secondary follicles do not have a sweat (sudoriferous) gland or an erector (arrector pili) muscle.
         
         
   3. Secondary-Primary follicle ratio (S:P ratio): This ratio denotes the number of secondary follicles for each primary follicle. This ratio varies by breed from 5:1 in the English Down sheep, to a 25:1 ratio in the Superfine Saxon Merino.
      
      

      Breed	No. of Follicles/mm2 (Primary and secondary)	Number of Primary  Follicles/mm2	S:PRatio
      Fine Merino	71.7	3.56	19.1
      Medium Merino	64.4	2.93	21.0
      Strong Merino	57.1	3.27	16.5
      Corriedale	28.7	2.43	10.8
      Southdown	27.8	3.9	6.3
      Dorset	18.5	2.9	5.4
      Suffolk	20.4	3.5	4.8
      Romney	22.0	3.4	5.5
      Border Leicester	15.8	2.9	4.4
      Lincoln	14.6	2.3	5.4
      Cheviot	14.6	2.7	4.5
      Wiltshire	11.4	2.6	3.3

   
   
   
   Section of Sheep Skin Illustrating the Primary and Secondary Follicles:
   
   

   
   

2. Glands and Secretions Associated With Fiber Production:
   1. Sebaceous glands:
      1. Before each fiber reaches the skin surface, it is coated with a greasy secretion from the sebaceous gland.
      2. The function of the greasy secretion is to keep the skin soft and prevent it from drying. It also provides some protection to the fibers from the elements.
         
         
   2. Sudoriferous (sweat) glands:
      1. In the primary follicle, there is a secretion known as suint from the sweat gland.
      2. The suint is soapy in nature and acts like a natural detergent.
         
         
   3. Yolk: Suint and grease combine to form the "yolk" in wool that is responsible for the level of yellow color in the fleece. White, high yielding fleeces have seen a priority in selection for the past 70 years. These fleeces tend to have a low yolk content. Environmental conditions and nutrition can influence the amount of yolk on the fleece.
      
      
3. Other Types of Fibers:
   1. Kemp: Kemp is an opaque, non-conforming, shedding fiber which will not dye uniformly. It appears prominent in the finished fabric unless it is used in a tweed or yarn in which the obvious kemp fibers become part of the pattern. Kemp and heterotype fibers contribute to the coarse-edge of wool and present problems in the worsted wool processing system.
      
      
   2. Heterotype (hair) fibers: These fibers are an intermediate between true wool and kemp fibers and are sometimes called hair. They are known as heterotypes because they have an intermittent medulla (core) in the center of the fiber. Having an intermittent medulla means that the core does not run consistently through the fiber. If 60% or more is medullated, then the fiber can be considered kemp.
      
      
      
   Differentiation Between True Wool, Kemp, and Heterotype (hair) Fibers:
   

   Characteristic	True Wool	Kemp	Heterotype (hair)
   Structure	Epidermis Cortex	Epidermis Cortex Medulla	Epidermis Cortex Medulla
   Length	Long	Short to 1 ½ inch	Long
   Diameter	Same for entire length	Tapers at root and tip	Same for entire length
   Transparency	Semi-transparent	Opaque	Semi-opaque
   Crimp	Yes	No, has wave	Long, open crimp
   Surface	Prominent scales	Scales	Few scales set close to fiber
   Elasticity	Good	Poor	Fair
   Root	None	Present	None