The Sertoli cell junctional specialization during spermiogenesis and at spermiation

The relationship between developing spermatids and Sertoli cell junctional specializations was studied with the electron microscope during spermiogenesis and at spermiation. At stage I of the seminiferous cycle, the newly formed spermatids are found in apposition to junctional specializations at the lateral surfaces of the Sertoli cell. Visualization of the junctional site of this early stage appears to be dependent on orientation and plane of section. As differentiation proceeds, the spermatids elongate and come to lie within deep recesses of the Sertoli cell. At this time the junctional specialization is limited to the acrosomal portion of the spermatid. During the maturation phase, the spermatids, while maintaining the same relationship to the junctional specialization, approach the lumen. When stage VIII of the cycle is reached, the stage in which spermiation occurs, the spermatids are at the luminal surface. The relationship of the spermatid head to the junctional specializations is quite variable during this stage. Some spermatids are observed still attached to the Sertoli cell at the junctional site, while others are found completely or partially surrounded by Sertoli cytoplasm, but with no evidence of the normally interposed junctional specialization. Yet, in other instances, the spermatids are observed in a position slightly removed from the junctional site. Also evident are profiles of junctional specializations at a free surface of the Sertoli cell, there being no attached spermatid. In some instances the junctional specializations appeared in apposition to a residual body. In the case of the free surface profiles, the junctional specialization at times lined an empty cleft or crypt-like recess, giving the impression that the spermatid head had just been dislodged from the junctional contact site. The findings indicate that the spermatid is in contact with a junctional specialization from its initial appearance and remains so until spermiation is initiated. It is postulated that spermiation is initiated through a physiological change in the junctional specialization resulting in loss of adhesion and consequent release of the sperm head from its attachment site. A similar mechanism is proposed in relation to the inter-Sertoli junctional complex to account for the means by which the spermatocytes cross this barrier to reach the adluminal compartment of the seminiferous epithelium.