DEVELOPMENT AND DENSITY OF WOOL FOLLICLES IN MERINO SHEEP SELECTED FOR SINGLE-FIBER CHARACTERISTICS

Wool follicles are classified into 3 major types: primary (P), origina l secondary (SO), and derived secondary (SD). They are formed during f etal life as successive waves of initiation pass through the skin. P f ollicles are the first to be initiated. SO follicles develop between t he primaries and are separated from them at non-randomly distributed s ites. 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 estima ted by conventional procedures. The densities of pilary canals were al so obtained to provide a measure of N-P + N-SO. Follicle counts in bot h adult and fetal animals showed that N-P and N-P + N-SO were relative ly constant across the lines. Predominantly, density differences were due to variations in the numbers of follicles initiated during the las t wave, forming the derived secondary population. Changes in follicle densities were therefore effected by developmental mechanisms that inc rease or decrease the extent of branching rather than by altering the numbers of P and SO follicles. The results suggest firstly that the nu mbers of initiation sites for P or SO follicle formation in the fetus, corresponding to the pilary canals of adult skin, are limited. Second ly, 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 fo llicle densities are Independently regulated in the sheep. The observa tions are discussed in relation to factors that influence the densitie s of the different follicle types. The results have practical implicat ions for changing fleece weight and fibre diameter through selective b reeding.