Migration and Keratinization of Cells in Wool Follicles

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Abstract
Migration of cells in wool follicles of an adult Merino sheep was studied autoradiographically in skin samples taken at intervals after an i.v. injection of [3H]thymidine. Fiber and inner root sheath cells incorporated [3H]thymidine in a cone-shaped region of the follicle bulb. Labeled inner sheath cells migrated out of the bulb ahead of contemporaneous cells in the fiber and remained in advance, although to a progressively lesser extent, until the inner sheath cells sloughed into the follicle lumen. Outer root sheath cells incorporated [3H]thymidine along the length of the follicle. Cells in the proximal half of the outer sheath migrated inwards and distally and sloughed into the follicle lumen before contemporaneous inner sheath cells. Other cells in the distal half of the outer sheath migrated past the level where cells from the proximal population were shed and also sloughed into the lumen. In the most distal part of the outer sheath, which formed the epidermis-like lining of the follicle canal, little migration of cells was observed during an 8-day period. The specific activity of tritium in fibers plucked from the same sheep at intervals after the i.v. injection of [3H]thymidine was determined by scintillation counting and assessed in terms of cell migration and fiber hardening. The time at which the specific activity of solvent-degreased fibers reached a maximum gave an estimate of the time for cells in the fiber to migrate to the upper limit of the keratogenous zone. When the plucked fibers were extracted with 8 M urea the times of the maximum specific activities of the urea-dispersible and urea-insoluble material provided estimates of the times at which hardening of the fibers began and ended. The effects of different planes of nutrition were examined in 2 other sheep by radioassay of fibers plucked after i.v. injections of [3H]thymidine given after an equilibration period of at least 2 mo. on each level of feeding. A high plane of nutrition increased the rate of cell migration and hastened the onset of hardening of the fibers, but prolonged the hardening process, as confirmed by the specific activiites of fibers plucked after i.v. injections of [35S]cystine.