Ultrastructural observations were made on fowl spermatids and free spermatozoa, fixed in situ, in the lumina of the seminiferous tubules, the rete testis, the ductuli efferentes, the connecting ductules, the ductus epididymidis and different levels of the ductus deferens.
Changes occurred in the acrosome during the differentiation of the spermatid. The early ellipsoidal form of the acrosome attains a mature, elongated shape at the late spermatid stage. During this stage, remodelling and condensation of chromatin of the nuclear region also occurs. The acrosomal cap changes from a loosely fitting structure before spermiation to become relatively closely apposed to the sperm nucleus in the excurrent ducts. The plasma membrane overlying the acrosomal cap of the free spermatozoon is closely applied whilst that over the head is slightly loose and swollen, especially in the ductus epididymidis. The inner mitochondrial membrane becomes thicker when the spermatozoon reaches the rete testis.
A cytoplasmic droplet is not seen in the middle piece of the fowl spermatozoon.
It is concluded that the structural differentiation of the fowl spermatozoon is almost complete directly after spermiation, and is earlier than the achievement of the fertilizing capacity which is reported to occur in the ductus epididymidis. There is no obvious correlation at an ultrastructural level between the morphological and the functional maturation of the fowl spermatozoon.