ABSTRACT Previous autoradiographic and chemical studies gave incompatible results as regards the incorporation of cystine into and cystine content of the 2 cortical segments of the wool fibre. In this study the incorporation of [35S]cystine into the wool fibre cortex was therefore re-examined by electron-microscope autoradiography. Skin samples were taken from a Merino sheep 1 h and 5 h after intradermal injections of L-[35 S]cystine. At both times there was very little incorporation of 35S in the follicle bulbs. By 5 h incorporation occurred distally from the suprabulbar region throughout the zone of macrofibril (filament bundle) formation. More 35S was incorporated per unit area in the paracortex than in the orthocortex, and at the level of maximal uptake near the middle of this zone there was about a 2-fold difference per unit area. However, when the relative cross-sectional areas of the cortical segments were also considered, the actual amount of 35S incorporated at this level was slightly greater in the orthocortex than in the paracortex. These differences in the incorporation of [35S]cystine by the cortical segments agreed with previous results from chemical studies on cortical fractions separated from wool.

Electron microscopy of wool follicles has revealed that in each layer of the inner root sheath formation of filaments and trichohyalin continues until Henle's layer, the cuticle and Huxley's layer harden, respectively, one-eighth, one-quarter, and one-third the distance up the follicle. Above a level about two-fifths the distance up the follicle progressive extraction of the inner root sheath occurs, commencing with Henle's layer from which protrusions and lamellated structures extend into the outer root sheath. At a level slightly less than threequarters the distance up the follicle the extracted inner root sheath cells compact and slough into the pilary canal. Adjacent outer root sheath cells cornify and also slough into the pilary canal. Distal to the zone of sloughing the outer root sheath progressively develops a granular layer containing keratohyalin, and becomes indistinguishable from epidermis.

The intensity of nucleoside triphosphatase activity in the fiber cortex is bilaterally asymmetric in the bulb and suprabulbar region of wool follicles of Merino sheep. In most follicles the region of greater activity is on the concave side of the deflected bulb. The activity is associated with cell membranes and in the follicle bulb is more intense in that portion of the fiber cortex adjacent to the side of the dermal papilla on which the basal lumina is thickened. The region of greater activity coincides with the orthocortex of the fiber at the suprabulbar level. Cortical segmentation therefore originates in the proliferative zone of the follicle bulb and is revealed by the bilateral asymmetry in the intensity of the enzyme reaction.

Differentiation of the cortical segments of the wool fiber commences in the suprabulbar region of the follicle with the formation of small macrofibrils adjacent to desmosomes. In orthocortical cells, the macrofibrils are without distinct microfibril formation below a level about one-quarter the distance up the follicle, at which level a microfibril-matrix complex appears to resolve throughout the macrofibrils formed so far. In paracortical cells, the macrofibrils consist initially of distinct microfibrils between which matrix subsequently forms at a faster rate until matrix production catches up with microfibril formation at a level about one-quarter the distance up the follicle. Synthesis of both microfibrils and matrix continues in each cortical segment until the cells are replete with microfibril-matrix complex at about two-fifths the distance up the follicle. Beyond this is a zone of low sulfhydryl content extending to a level three-fifths the distance up the follicle where ultrastructural changes indicative of final keratinization occur.

Cystic enlargements of the outer root sheath in the suprabulbar region of wool follicles have been studied by electron microscopy. The first indication of cyst formation is a perinuclear condensation of filaments in cells that otherwise resemble normal cells of the outer root sheath. Keratohyalin and membrane-coating granules are rarely seen. A mass of keratinized cells progressively differentiates in the cyst. In cells close to the center of the cyst, organelles are centripetally located in the cytoplasm. A gradation of glycogen exists radially in the cyst wall, the higher concentrations being found in peripheral cells. The cyst may eventually reach a diameter of about 150 μ.

SUMMARY Previous autoradiographic and chemical studies gave incompatible results as regards the incorporation of cystine into and cystine content of the 2 cortical segments of the wool fibre. In this study the incorporation of ("S)cystine into the wool fibre cortex was therefore re-examined by electron-microscope autoradiography. Skin samples were taken from a Merino sheep 1 h and 5 h after intradermal injections of L-P'Sjcystine. At both times there was very little incorpora- tion of 36S in the follicle bulbs. By 5 h incorporation occurred distally from the suprabulbar region throughout the zone of macrofibril (filament bundle) formation. More 35 S was incorpo- rated per unit area in the paracortex than in the orthocortex, and at the level of maximal uptake near the middle of this zone there was about a 2-fold difference per unit area. However, when the relative cross-sectional areas of the cortical segments were also considered, the actual amount of "S incorporated at this level was slightly greater in the orthocortex than in the paracortex. These differences in the incorporation of (35S)cystine by the cortical segments agreed with pre- vious results from chemical studies on cortical fractions separated from wool.