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Follicle Density Analysis

An increasing number of breeders are including results of follicle density testing in their promotional materials.  Do these refelect an objective assessment of an animal's quality?

In "Development of hair coat and skin glands in fetal porcine integument" (1986), W Meyer and S Görgen write:

"Beginning at 70-73 days GA (about 300g body weight), the first secondary hair follicles appeared. Their density ... increased up to 85 days GA (550g body weight). The density of secondaries increased and decreased later during the late fetal stages, between 90 and 100 days GA. After birth, only a few secondary hair follicles could be found in each body region...

"...The structural development of the hair folicle is similar in domestic mammals, laboratory animals and man until the hair peg stage. However, clear differences occur when the gland anlagen appear, and also in relation to the development of the wool hair coat, especially in rodents and sheep (Danneel, 1931; Galpin, 1935; Davidson & Hardy, 1952" Hardy & Lyne, 1956; Ryder, 1956; Lyne & Heideman, 1959; Pinkus & Tanay, 1968; Lyne & Hollis, 1972; Montagna & Parakkal, 1974)...

"...hair follicle growth follows a definite pattern which probably depends on hair density and/or mammalian group. In densely haired animals like the Angora goat, there is an initial period of rapid growth occurring in the first two thirds of gestation which is followed by a period of quiescence during the last third of gestation, until one month postpartum (Wentzel & Vosloo, 1975)...

"Findings presented for sheep (Ryder, 1956), the Angora goat (Wentzel & Vosloo, 1975), cattle (Lyne & Heideman, 1959), and for the pig in the present study , suggest that the initiation of primary hair follicle stops after the first half or two thirds of gestation, at the time when the fetus has achieved the highest density of primaries. Later, this density decreases in relation to fetal growth or increase in body surface area. Further changes in hair coat density are mainly due to the formation or non-formation of secondary hair follicles."

This is a very important point when it comes to follicle density testing, and on that is widely overlooked.  "Later, this density decreases in relation [to]...increase in body surface area."

Let's try a visual representation of this. Imagine a polka dotted balloon, each dot representing a primary/secondary cluster of follicles. As we inflate the balloon the dots get both larger and further apart. It doesn't magically produce more dots in order to maintain a consistent density!  Instead, the density of decreases in relationship to the expanding surface area.

Similarly, the absoulute number of primary hair follicles is set during gestation, and the relative density decreases in relationship to the body surface area.

I have not found any general agreement on the development of secondary follicles, but there is some body of literature suggesting that they have formed within the first few months after birth. In "Gene expression in sheep skin and wool (hair)"(2003), D.L. Adelson, G.R. Cam, U. DeSilva, and I.R. Franklin write:

"Hair (fiber) diameter is highly correlated with the size of the hair follicle dermal papilla [6], whose origin can be traced to the dermal condensate, one of the earliest features of the developing hair or wool follicle. The dermal condensate is an aggregation of dermal cells at the site of follicle initiation, which is characterized by a thickening of the basal layer of the epidermis. After initiation, the follicle rudiment grows down into the dermis and the dermal condensate remains as a discrete structure, or prepapilla, that maintains its position at the base of the extending follicle plug. Eventually, the developing follicle bulb encloses the pre-papilla, at which time it is termed the dermal papilla. Once the papilla has formed, epithelial cells surrounding it begin to proliferate and migrate up the hair follicle, eventually differentiating and keratinizing, forming the mature fiber. In the Merino sheep, the majority (f80%) of follicles are actually derived from branching, rather than initiation from the epidermis ( and D.L.A., personal observations]. Therefore, it is essential that we understand three stages of follicle development: patterning and initiation of original follicles (f73 days of gestation in sheep, midflank skin), branching of secondary follicles (f105 days of gestation, midflank skin), and regulation of fiber growth (adult mid-flank skin). Detailed descriptions of the sequence and timing of wool follicle initiation and development have been reported previously."

The literature suggests that a process  exists that establishes a set number of follicles long before physical maturity.

The actual mechanism of the development of follicles continues to be debated and researched, and no doubt there are any number of environemntal factors involved - not the least of which is nutrition.

Then there is the fact that just because a follicle exists does not mean it is active. Absolute density of follicles is not, therefore a totally accurate measurement of density - only a measurement of the potential density were all the follicles to be active at any given time.

The hair growth cycle includes a period of growth, called anagen, followed by a transitional stage, called catagen, and then a period of inactivity in the hair follicle, called telogen, lasting until the cycle starts again. The duration of each stage varies with the species, anatomical location, genetic influence, and a variety of environmental and physiological factors.

The current fad of testing for follicular density is fatally flawed on several points.

It lacks protocols to account for age. The original purpose as proposed by Watts (as part of the SRS breeding protocol) was to test mature potential rams, and then only to confirm what had been identified by careful examination of physical markers in skin and fleece traits (this due, in part, to the expense of the analysis).

It lacks protocols to account for size

It fails to measure follicle activity

If this procedure is conducted without regard to age/physical maturity, then the results are open to question, since, as Meyer and Görgen have pointed out, "density decreases in relation to...increase in body surface area."

 
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