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Type VI collagen is a subendothelial constituent that binds von Willebrand factor (vWF) and platelets. The interaction of platelets with type VI collagen and the roles of platelet glycoprotein (GP) receptors and vWF were studied under flow conditions using epi-fluorescent videomicroscopy coupled with digital image processing. We found that surface coverage was less than 6% on collagen VI at a relatively high-wall shear rate (1,000 s-1) and was approximately 60% at a low-wall shear rate (100 s-1). The molecular mechanisms involved in low-shear platelet binding were studied using monoclonal antibodies to platelet GPIb and GPIIb-IIIa, and polymeric aurin tricarboxylic acid. Anti-GPIIb-IIIa was the most effective in eliminating adhesion (surface coverage, 0.8%), followed by anti-GPIb (4.3%), and ATA (12.6%). Experiments with von Willebrand disease blood indicate that vWF is involved in platelet adhesion to collagen VI at 100 s-1. In the absence of vWF, there may be direct binding of platelet GPIIb-IIIa complexes to collagen VI. Adhesion and aggregation on collagen VI are different in shear rate dependence from collagen I. Our results suggest a possible role for collagen VI and vWF in platelet adhesion and aggregation in vascular regions with low shear rates.

Frequencies of G1, G2, and G3 kemps in the fleeces of N-type strains have been analysed. The G1 frequencies have been related to fibre and follicle type differences. It has been shown that shedding is restricted to fibres formed by primary follicles, and that primary central follicles shed most of their fibres, while primary laterals shed few. Differences between genotypes in total kemp frequency were highly significant. In N/N and nr/nr there was more G1 kemp than in N+ in succeeding generations the difference between N/N and N+ was maintained, but the frequency in nr/nr only equalled that in N/+ . The N gene has a greater effect than the nr gene on shedding. The proportion shedding in the different pre-curly-tip fibres (those formed by primary central follicles) was related to their length and degree of medullation, and not to N-genotype. Genotype differences in G1 kemp frequency can be explained in terms of fibre type frequency differences.

Several types of follicles occur in the skin of sheep. These have been shown to differ in the types of fibres they form, when these are classified on shape of tip curl, occurrence of medulla, size of crimps, rate of growth, and frequency of shedding as kemps. Fibre type frequency distributions have been analysed. In non-N-type, halo-hairs and fibres with a sickle-shaped tip, forming the pre-curly-tip group, can only be formed in primary central follicles, which ordinarily all grow pre-curly-tip fibres, although occasionally these follicles produce curly-tip fibres. The primary lateral follicles all grow curly-tip fibres which grow also in the early secondary follicles. The late secondary follicles grow fibres with a tip of indefinite shape, histerotrichs. In N-type sheep primary centrals always form pre-curly-tip fibres, with the reservation that in Plateau arrays with few or no fibres classed as supersickle- fibres it is believed that some fibres called hairy-tip–curly-tip fibres grow in primary central follicles; primary laterals form hairy-tip–curly-tip fibres save in some N/+ lambs; and the secondaries form curly-tip and histerotrich fibres. The primary follicles are strongly affected by the N and nr genes, which have much less effect, if any, on the secondaries. Fibre growth rates and consequently size of crimps, and frequency of occurrence of brown fibres and of shedding as kemps, are all higher in primary than in secondary fibres in N-type sheep, and in N-type than in non-N-type, showing that the N and nr genes affect the primary more than the secondary follicles in these characteristics.

The development of the follicle population in N-type sheep is described for the period from 40 days of gestation to birth. No major differences from the description given for the non-N-type Romney or the Merino were found. The development and growth of follicles after their first initiation appear to be slightly faster in N-type. The tips of fibres which had pierced the skin at 113 and 119 days' gestation were examined and found to belong to all birthcoat fibre types except hiserotrichs. Since it has been shown that secondary follicles form, in addition to histerotrichs, only curly-tips, it follows that primary follicles form some types of fibres which do not grow in secondary follicles.