H. B.
CARTER
321
FOLLICLE TYPES IN THE MERINO SKIN
No special attention has been drawn to the existence of more than one
type of follicle in the skin of the Merino, though the fact is well known as a
general feature in all mammals. The classical description of a wool follicle in
available works of reference depicts this structure as being characteristically
associated with two accessory glands, the sebaceous and the sudoriferous and
with a small involuntary muscle, the m. arrector pili. However, in the Merino
such follicles, provided with the full complement of accessory structures, con-
stitute in fact only a small fraction of the total number even in animals whose
skin carries an extremely sparse population. The majority of follicles con-
tributing wool fibres to the Merino fleece, though seldom lacking a sebaceous
gland, are not provided with either a sudoriferous gland or a m. arrector pili.
These different anatomical relationships make possible a fundamental classifica-
tion of follicle types into : (i) primary follicles, which are distinguished by their
association with (a) a simple coiled tubular sudoriferous gland, (b) a well-
defined bilobar acinous sebaceous gland, and (c) a double band of involuntary
muscle, the m. arrector pili ; and (ii) secondary follicles, with which are
typically, but not invariably, associated only the smaller sebaceous glands,
irregular in size and shape, though generally of the simple acinous type.
Differential counts of these follicles are readily made in tangential sections
of skin at the appropriate level that is, in a region which usually lies within
100 to 300 microns of the epithelial surface, the variation occurring according
to the average depth of follicle implantation.
In the adult Merino skin the secondary follicles in the more superficial
layers of the dermis are gathered into compact clusters of irregular size sur-
rounded by clearly defined trabeculae of dermal connective tissue. Toward
the epithelial surface in the region of the sebaceous glands the primary follicles
are invariably found slightly removed into the connective tissue stream and
consistently to one side of the aggregation of secondaries. This typical
relationship is lost in the deepest levels of the dermis surrounding the papilae
of the follicles which are dispersed more widely in the available space, with
only the faintest suggestion of the clustering so apparent toward the surface.
In order to observe the characteristic distribution of the follicle population
and to make a satisfactory differentiation, it is necessary to use sections no
deeper than the level of the sebaceous glands. Here the primaries may be
readily detected as much by their relative position as by their structural
associations and cytology. The method of charting the follicular arrangement
has been noted elsewhere [Carter, 1939], but it is important to refer to the sys-
tem of recording.
It was observed in the course of collateral studies on the development of
the follicle population that the relative numbers of primary and secondary
follicles were of the greatest importance. The relationship may be conveniently
summarized as a ratio in the equation :
P: (P+S) = l:x,
where P = number of primary follicles per unit area and S = number of
secondary follicles per unit area.
To appreciate the importance of this simple relation some elementary
facts in the ontogeny of the Merino require to be noted. Primary follicles
F 2