At 26 days of postnatal age. At this later time-point, meiotic divisions were observed, and step 3-4 spermatids were the most differentiated type of germ cell present (Fig. 1F). In comparison to the 18-day-old testis (Fig. 1C), the frequency of midpachytene spermatocytes at 26 days of age was increased (Fig. lF), and the appearance of late pachytene spermatocytes and spermatids allowed accurate identification of the different stages of spermatogenesis. These samples were divided into three groups containing cells in stages II-VI, VII-VIII, or IX-I (Table 1). The characteristic cells observed were early pachytene and step 2-4 spermatids for stages II-VI (Fig. 1E), preleptotene and midpachytene spermatocytes for stages VII-VIII (Fig. 1F), and zygotene (Fig. 1G) and late pachytene spermatocytes and step 1, round spermatids for stages IX-I.
Morphological Characterization and TUNEL Staining of Apoptotic Germ Cells
At 8 days of postnatal age. Type A spermatogonia and gonocytes were only rarely seen to undergo apoptosis. Ap-optotic gonocytes were bigger than apoptotic spermatogonia, but accumulation of heterochromatin at the periphery of the nucleus was similar in both cases. No variation was observed in the number of apoptotic cells per 1 ^m3 observed in different segments, nor was this value, as obtained by TUNEL staining, significantly different from that of the morphologic determination (Table 1).
These values are the median (25% and 75% confidence limits). b P < 0.05 compared to the corresponding value for the 8-day-old testis. c P < 0.05 compared to the corresponding value for the 18-day-old testis. d P < 0.05 compared to the corresponding value for stags II-VI in the 18-day-old testis. e P < 0.05 compared to the corresponding value for stages II-VI in the 26-day-old testis.