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Fleece Density and the Histology of the Merino Skin
in this breed are the first to differentiate from the basal layer of the embryonic
epidermis and appear during a comparatively restricted phase of fleece deve-
lopment in foetus, In the Merino lamb primary follicles appear to produce
almost the entire population of birth-coat kemp fibres, which are gradually
shed at varying intervals after birth. These are replaced ultimately by true
wool fibres, though in some individuals and in some regions of the skin more
than others they may be followed by a second generation of large persistent
hairy fibres often marked by the presence of a fragmentary medulla. Even
when not distinguished by some degree of medullation, mature primary fibres
may often be recognized not only by their position in the groups, but also
as the largest fibres of the population.
Secondary follicles, on the other hand, are initiated at a later stage in
foetal development. They increase relatively more slowly in number, the
period during which they are differentiated extending roughly over the last
third of the gestation period, terminating usually at or perhaps shortly after
birth, secondary follicles never appear to produce anything other than the
true non-medullated wool fibre in the Merino. Broadly speaking, therefore,
the primary follicles tend to contribute the coarsest fibres to the population
and secondary fibres the finest.
DISCUSSION
From what has been briefly stated here, the general significance of the
descriptive terms ' primary ' and' secondary ' is apparent. It becomes clearer,
also, that a simple variation in the relative numbers of primary and secondary
follicles may have a profound influence on the gross characteristics of the
Merino fleece. The significance of the P : (P+S) ratio may therefore be con-
siderable. The observation of type differences in the follicle population adds
much to the importance and value of using the skin itself in the study of fleece
characteristics, but especially of fleece density. A ready differentiation of
fibre types into primary and secondary populations would be quite impossible
in the Merino if fleece samples alone provided the material.
One of the most useful consequences of follicle classification is the element
of precision which it introduces into the comparison of individuals. Where,
by previous methods of fleece analysis, it was only possible to make density
comparisons on the basis of the total population per unit area, now by using
skin sections it is possible to apply three principal criteria :
(i) The absolute density of the total population of follicles.
(ii) The absolute densities of primary and secondary follicles separately.
(iii) The relative densities of primary and secondary follicles expressed
by the ratio P : (P+S).
For example, two animals were shown to have equal absolute densities,
at comparable points on the body, of the order of 78 fibres per square milli-
meter. By ordinary methods of fleece analysis these two animals would be
declared identical in the fibre population density. Examination of the skin,
however, revealed that in one case the mean number of primary fibres was
5.25 per square millimeter and in the other 2.75 per square millimeter. Thus
the two populations were differently constituted, the former having a P : (P+S)
ratio of 1:15.0, the latter a ratio of 1:28.4. By these additional criteria,