One final, but often misunderstood consequence of the organization of the seminiferous epithelium is the cycle wave (13). The stages of the cycle occupy segments of the seminiferous tubule that are aligned more or less in sequence along the length of the tubule. This alignment of all stages of the cycle is the wave. Moreover, the length of a segment in a particular stage was assumed to be constant from one wave to another. Indeed, the wave was believed by many early investigators of spermatogenesis to represent in space what the cycle was in time, i.e., an orderly sequence that could be used to know precisely that the stage of the segments immediately adjacent were numerically one stage earlier or one later in the cycle.
A detailed study of the cycle wave in the rat indicated that the wave was not a spatial arrangement of stages that approached the orderly temporal sequence of the cycle (29). The wave instead varied from an orderly spatial sequence in two ways. First, the segments of tubule in a particular stage were not consistently of similar length, and, in turn, the length of tubule containing the entire wave was also variable. Secondly, two types of modulations of the wave were observed. In the first type, the consecutive arrangement of the cycle stages along the tubule was interrupted, and the stages would be repeated. For example, if segments of tubule were in Stages 1, 2, and 3, then the next segment would not be Stage 4 as expected, but instead this segment would contain Stage 1 again, and subsequent segments would contain the remaining cycle stages. In the second variant, the sequence of stages would be reversed. For example, if the sequence was Stages 3, 4, and 5, then the next segments would be 4 and 3. The segments would then continue in sequence; that is, 4, 5, and 6.
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