Spermatogenesis in birds.

It is concluded that in the birds which have been studied there is a well-defined, regular cycle of the seminiferous epithelium, and that spermatogenesis involves synchronization of the activities of germ cells within and between successive generations to produce cellular associations of the seminiferous epithelium which are essentially similar to those described in mammals. However, the area of a cellular association is smaller in birds than in most (and probably all) mammals so that numerous cellular associations are present in a cross-section of a seminiferous tubule. As in primates, in which a cellular association also covers a small area of seminiferous tubule, the wave of spermatogenesis in the Japanese quail follows a spiral path along and around a seminiferous tubule with consecutive stages of the cycle always occurring as adjacent stages along the tubule. The literature indicates that there are fewer mitotic divisions during spermatogonial proliferation in birds than has been reported for mammals and it is suggested that this difference explains the difference between birds and mammals in the area occupied by a cellular association. Work on the Japanese quail indicates that stem cell renewal and spermatogonial proliferation is simpler in the quail than has been described in some mammals. In the quail, both processes occur in the same compartment of the seminiferous epithelium and are synchronized with subsequent phases of spermatogenesis, and there does not seem to be a reserve pool of non-proliferating stem spermatogonia in the quail as has been reported for some mammals. Findings to date also indicate that spermatogenesis in birds is four times faster and produces four times the number of spermatozoa/g testis than in mammals, and that this difference is associated with a faster transit and poorer survival of spermatozoa in the male extragonadal ducts of birds than mammals. These differences between birds and mammals are interpreted as a response to selection pressure due to competition between males to inseminate partners frequently in order to provide the most viable spermatozoa at the site of fertilization when ova are ready for fertilization. In this respect, it is suggested that sperm production in birds and mammals have progressed along different lines of evolutionary development due to differences in ovulatory pattern and mating system. In particular, the high sperm production of birds is associated with the need to mate a (usually monogamous) partner numerous times a day over a period of days or weeks when a clutch of eggs is laid.(ABSTRACT TRUNCATED AT 400 WORDS)