Pre-natal development of the wool follicles in Rambouillet sheep

Abstract

A study of the development of wool follicles during the pre-natal stage of the sheep was conducted on a group of Rambouillet fetuses. Factors studied were time of initiation and sequence of follicle development, density per square millimeter and secondary/primary follicle ratios, and the possibility of branching as derived follicles. Sixteen Rambouillet fetuses were obtained at 39, 42, 49, 56, 63, 72, 78, 84, 91, 98, 105, 112, 119, 133, 140 and 146 days of age. Skin samples were obtained from thirteen locations as described by Stephenson (1958) using a 0.65 cm. biopsy punch. Samples were fixed in 10% buffered formalin, dehydrated, embedded and sectioned. After staining with an hematoxylin and eosin stain, sections were studied and three fields counted per section. Follicles were classed as Pf (complete primary with a keratinized fiber), Pi (incomplete primary without fiber), Sf, a complete secondary (with fiber) or Si (an incomplete secondary without fiber). Secondary/primary ratios were derived from (Sf+Si)/ (Pf+Pi). Follicles were first initiated at 56 days of fetal age and appeared earliest on the nose, face and poll. The follicles of the extremities were next to develop, with the dorsal trunk area and hindquarter last. Follicle density ranged from a mean of 7.3 per sq. mm. at 56 days to 129.5 per sq. mm. at 146 days. Secondary/primary ratios at term ranged from 1.01 on the nose to 10.52 at mid-side. High correlations were obtained between primary and secondary density ranging from a low of 0.61 at 84 days to 0.97 at 98 and 119 days and 0. 96 at 140 days. Branching was observed from 105 days of pre-natal life to term, at which time it was obviously not complete. It was observed that during late pre-natal life, as many as six derived follicles may be formed from an original. The formation of branched follicles appears to have an important role in determining the follicle density of Rambouillet sheep.