by Jim Watts. M.V.Sc., Ph.D.,
The SRS® breeding system was developed for fleece-coated animals by Dr. Jim Watts between 1986-88. It is based on the pre-papilla cell research of Moore (1984) and Moore et al (1989, 1991, 1995, 1996,1998). The working hypothesis is that the extent of wool follicle formation and fibre output of an animal is genetically predetermined by the number and activity of pre-papilla cells (which induce follicle formation and fibre growth ) in the foetal skin.
The objective of the SRS® breeding system is to maximise the density and length of fibres grown by animals such as alpacas. The logic for this is if there are many fibres on the animal’s body, and if these fibres are long, then the alpaca will produce a high fleece weight. Also, since high fibre (follicle) density is genetically correlated with low fibre diameter, the fleece will have a high value because it is fine and there is a lot of it.
Examples of the distinctive fleece structure of Huacaya alpacas with measurably high levels of fibre density and length has a fleece like shown in Figure 1.
High fibre density is indicated by high crimp amplitude (deep crimp) and thin staples. High fibre growth rate (long fibres), is indicated by both high crimp amplitude and low crimp frequency (‘bold crimp).
Measurements made at this laboratory reveal that adult alpacas have, on average, a fibre density of about 35 fibres per square millimetre (range, 9 to 89) and fibre length growth rate of about 0.25 millimetres per day (range, 0.20 to 0.57). The scope for genetic improvement of both traits is at least two-fold, or a four fold increase in the fibre production of the animal. This would change the fleece production of an alpaca, for example, from producing 3 kgs of 25 micron wool to one producing about 7.5 kgs of 20 micron wool.
Figure 1. The fleece structure of a Huacaya with measurably high levels of fibre density and length.
Follicle and Fibre Formation
Three types of wool follicles (primary follicles, original secondary follicles and derived secondary follicles) are formed in the skin of fleece-coated animals (Figure 2).
Figure 2. Diagrammatic representation of the structure of the sheep skin. The dotted line show the level of microtome sectioning to produce a horizontal section to measure follicle density.
The skin has five layers. Of these layers, the outer epidermis is supported by the underlying dermis.
The timetable for the development of follicles is outlined here for the unborn lamb. The time intervals could be approximately doubled for the unborn alpaca.
From about 60 after conception of the lamb, particular cells in the dermis begin to aggregate into clusters of pre-papilla cells of, apparently, genetically pre-determined sizes.
Each cluster stimulates above it the multiplication of epidermal cells which grow down into the dermis to form a primary follicle. Primary follicles develop in groups of three, called trio groups. As well as its earlier development, the primary follicle is distinguished by having a sweat gland and a muscle attached to it.
The process is repeated for the original secondary follicles from about 80 days. A third wave of follicle development occurs about 100 days, when derived secondary follicles develop as branches of the original secondary follicles. Figure 3 illustrates the successive stages in the formation of secondary follicles. Derived secondary follicles are shown as branching from the neck of an original secondary follicle.
Figure 3. Formation of derived (branched) secondary follicles
Follicle groups, each comprising its three primary follicles and the associated cluster of secondary follicles, are formed. Figure 4 shows a cross section through follicle groups of an adult Huacaya alpaca with a high follicle (fibre) density of 89 follicles per square millimetre.
Figure 4. Horizontal skin section (magnified view) of an adult alpaca with a high density of 89 follicles per square millimetre. .
The numbers of follicles per group is characteristic for a particular alpaca but can vary widely between animals (in our studies, from 17 to 50). Because of the difficulty of distinguishing between the two sorts of secondary follicles in skin sections, secondary follicle development is generally assessed as the ratio of the total secondary to primary follicles, the S/P ratio.
In the cup at the base of the follicle, the original cluster of pre-papilla cells, which stimulated formation of the follicle, becomes the dermal papilla. The dermal papilla controls the multiplication of cells in the overlying follicle bulb. As the cells in the follicle bulb divide, they move up the follicle and differentiate to form the wool fibre and the concentric inner and outer root
sheaths which surround it (Figure 5).
Figure 5. Diagrammatic representation of the lower portion of a wool follicle.
Moore and colleagues suggest that, for a given number of committed pre-papilla cells, the number of derived secondary follicles would be greater if the diameter and density of the primary and original secondary fibres was less. They propose that a decrease in primary fibre diameter or density would leave more pre-papilla cells available to multiply and form derived secondary follicles.
Dr. Ken Ferguson and I examined the effect of primary fibre diameter on the number of secondary fibres in the follicle group using the extensive data of Dr. Harold Carter (Carter, 1968), covering most breeds of sheep and many crosses between them. We also included our data for Merino rams of different levels of fibre density. The results are shown in Figure 6.
Figure 6. Relation of primary fibre diameter to number of follicles per group.
The number of follicles per group increases rapidly as primary fibre diameter decreases below 30 microns. Above this value, few derived secondary follicles are formed.
Fleece markers for density and length
As we can deduce from Figure 6, it is likely that as we reduce primary fibre diameter (reduce or eliminate ‘guard hair’) in alpacas, the first obstacle to increasing fibre density is removed. This may result in more follicles per group, that is, follicles that are closely packed to one another. Then, it would be reasonable to expect the fibres will grow as a highly aligned and distinctive cluster from each follicle group in the skin into the fleece.
Also note from Figure 3 that derived secondary follicles form as branches from the root sheath wall of an original secondary follicle, and therefore share a common piliary canal as the fibres to emerge into the fleece. Usually about 3 or 4 derived secondary follicles branch from an original secondary follicle, but there can be a lot more. Clearly, increasing secondary follicle branching as the means of increasing fibre density is an appealing means of perfecting fibre alignment.
The cluster of fibres emerging from a follicle group we call ‘a fibre bundle’. Since the follicle group is no more than 1.5 millimetres in diameter, so the fibre bundle is no more than 1.5 millimetres in diameter, that is, much thinner than a staple (Figure 6).
Figure 6. This animal’s fleece consists almost entirely of long fibre bundles. Staples have been replaced.
A fibre bundle is likely to form in the fleece whenever follicles are packed closely together in a follicle group. To ensure that the alpaca produces a high fleece weight of fine diameter and high quality fibre, the follicle group needs contain a high number of secondary follicles (equivalent to a high S/P ratio), and the adjacent follicle groups need to be packed closely together (ensuring high follicle density) as well as the follicles having the functional capacity to sustain high fibre output (fast length growth rate) of fibres of uniform size and shape throughout life and on low nutritive regimes.
To emphasise this point by way of an example, we are not selecting for : alpacas with fibre bundles with very small follicle groups containing closely packed but small numbers of follicles (low S/P ratio); groups that are widely spaced apart (and therefore have low density); and which have low fibre output – these are ‘pretty wools’ or animals producing low fleece weights of fine diameter and short fleece length.
High density imparts high crimp amplitude (‘deep’ crimp) to the fibre. It also makes the fibre cylindrical in shape and smooth surfaced (low height of the cuticular scales), presumably as a consequence of even distribution of orthocortical and paracortical proteins within the substance of the fibre).
When high fibre length is also present, these low scales are longer, accentuating the ‘silkiness’ or skin comfort of the fibre product. These fast growing fibres, whilst retaining the high crimp amplitude, have fewer crimps per unit length, or low crimp frequency.
The visual sensations of softness of handle and lustre are important fleece markers and expressions of changes in fibre properties wherein the fibres are re-aligned, re-sized and reshaped. Importantly, these are genetic outcomes resulting from selection for high levels of fibre density and length.
We also select for fast early body growth, good muscularity and high fecundity in alpacas.
We encourage alpaca breeders to participate in genetic evaluation systems that are independently conducted and in which results are placed in the public domain. Well-conducted systems can provides a ‘common language’ for alpaca producers across the world to assess genetic animal performance and buy with added confidence.
Moore, G.P.M. (1984). Growth and development of follicle populations and critical stages of growth. In: Proceedings of Wool Production in Western Australia (Baker, S.K., Masters, D.G. and Williams, I.H., eds) pp. 69-76, Perth W.A. Australian Society of Animal Production.
Moore, G.P.M., Jackson, N. and Lax, J. (1989). Evidence of a unique developmental mechanism specifying both wool follicledensity and fibre size in sheep selected for single skin and fleece characters. Genet. Res. Cambridge 53, 57-62.
Moore, G.P.M. Du Cros, D. L., Pisansarakit, P. and Wynn, P.C. (1991). Hair growth by growth factors. In: The Molecular and Structural Biology of Hair (Stenn, K.S., Messenger, A.G. and Baden, H.P., eds) pp. 308-325. New York: Annals of the New York Academy of Sciences.
Moore, G.P.M., Jackson, N., Isaacs, K. and Brown, G. (1995). Estimating densities of original secondary and derived secondary wool follicles in the sheep. Wool Tech. Sheep Breed. 43, 263-267.
Moore, G.P.M., Jackson, N., Isaacs, K. and Brown, G. (1996). Development and density of wool follicles in Merino sheep selected for single fibre characteristics. Aust. J. Agric. Res. 47, 1195-1201.
Moore, G.P.M., Jackson, N., and Brown, G. (1998). Pattern and morphogenesis in skin. J. theor. Biol. 191, 87-94.