Development and density of wool follicles in Merino sheep selected for single fibre characteristics

Wool follicles are classified into 3 major types: primary (P), original secondary (SO), and derived secondary (SD). They are formed during fetal life as successive waves of initiation pass through the skin. P follicles are the first to be initiated. SO follicles develop between the primaries and are separated from them at non-randomly distributed sites. SD follicles are the last to be initiated and branch from SO and other SD follicles. We have measured the densities of these follicles in 4 lines of sheep selected for different fleece characters. Primary follicle and total follicle densities (N-P and N-P + N-S) were estimated by conventional procedures. The densities of pilary canals were also obtained to provide a measure of N-P + N-SO. Follicle counts in both adult and fetal animals showed that N-P and N-P + N-SO were relatively constant across the lines. Predominantly, density differences were due to variations in the numbers of follicles initiated during the last wave, forming the derived secondary population. Changes in follicle densities were therefore effected by developmental mechanisms that increase or decrease the extent of branching rather than by altering the numbers of P and SO follicles. The results suggest firstly that the numbers of initiation sites for P or SO follicle formation in the fetus, corresponding to the pilary canals of adult skin, are limited. Secondly, the skin has the capacity to continue to initiate follicles after most or all of the sites have been occupied. It is concluded that the mechanisms controlling follicle initiation site densities and total follicle densities are Independently regulated in the sheep. The observations are discussed in relation to factors that influence the densities of the different follicle types. The results have practical implications for changing fleece weight and fibre diameter through selective breeding.