Microtubular mass defect of spermatozoa in the stallion.

A microtubular mass (MM) defect was found in the spermatozoa of 7 Standardbred stallions; 3 stallions were sons of the same sire. Two of these 3 stallions and 2 other stallions (for a total of 4 out of the 7 stallions) were considered subfertile when the defect was first observed. Fertility improved with time, either during the first breeding season or when a given stallion was used less frequently; however, the MM defect persisted, consisting of tortuous arrays of small abnormal microtubules visible only by transmission electron microscopy. The MM probably contained the protein tubulin as indicated by immunofluorescence. Microtubules of the MM were different from normal microtubules in being highly coiled, rather than straight, without interconnecting bridges or arms and had a 20% smaller cross-sectional diameter than did normal microtubules. Synthesis of the MM may occur near areas of the nuclear membranes normally associated with the manchette of the developing spermatid.