Maturation Phase

Spermatids in the maturation phase are termed Sd. This phase is the final stage of spermiogenesis and also spermatogenesis, resulting in spermatid development. During this phase the manchette continues to migrate caudally, supporting the flagellar canal. The annulus migrates to its permanent site at the junction of the mid-piece and the principal piece, drawing down caudally with it the plasma membrane of the flagellar canal. As a result the flagellar canal shortens and subsequently disappears. At this time, and apparently very quickly, the mitochondria migrate and arrange themselves around the flagellum as the annulus migrates down (Johnson et al., 1990) (Fig. 4.21).

Evidence of the speed of this migration is indicated by the fact that it is very rare to identify a spermatid in which the annulus has migrated but the mitochondria are not yet arranged around the mid-piece. Also, in the cross-section of a Sertoli cell (the centre of which is the characteristic site of maturation phase spermatids), some spermatids may show completion of the maturation phase whereas others, of the same cohort, show indications that the phase has only just been initiated.

During these phases, development and changes within the flagellum are evident. The means by which the characteristic nine dense fibres and fibrous sheath of the spermatozoa are formed is unclear. However, it is likely that they develop in a similar fashion to the development of cell cilia and flagella in general - that is, from the microtubules of the distal centriole which develop on to the columns of the capitellum situated at the implantation fossa and hence extend down the flagellum (Kimball, 1983). The fibrous sheath may develop from a spindle-shaped body, which has been identified in developing human flagella (Holstein and Roosen-Runge, 1981), or longitudinal columns found at the distal end of the principal piece in rats (Irons and Clermont, 1982; Johnson, 1991b).

At the end of the maturation phase a significant amount of cytoplasm is left attached to the spermatid. This excess cytoplasm is held within residual bodies and attached to the spermatid by a cytoplasmic stalk. It is subsequently phagocytosed by the supporting Sertoli cells. These spermatids are released from their residual bodies and Sertoli cells into the lumen of the seminiferous tubule. Remnants of this excess cytoplasm still remain on the spermatid as the cytoplasmic droplet, which is evident in all spermatozoa that have not yet matured in the epididymis.

Spermiation

During the process of spermiogenesis, a spermatid undergoes morphological changes from a spherical cell with a spherical nucleus, to an elongated cell with an elongated streamlined head containing a condensed nucleus and a specialist area of penetrative enzymes, plus a tail required for the movement of the cell. In addition, during this process, nucleotide synthesis takes place, probably for DNA repair, along with a change in chromatin proteins, and RNA synthesis to allow the production of specific proteins for unique spermatid structures such as the manchette (Loir, 1972; Johnson, 1991b).

Spermiation is the final release of a mature spermatid, now termed a spermatozoon, into the lumen of the seminiferous tubule. From here it passes to the rete testes and on to the epididymis for final maturation.

Seminiferous epithelium cycle

If a cross-section of seminiferous tubules is taken, a series of layers or changes in spermatozoa development is evident (Le Bland and Clermont, 1952; Johnson et al., 1990). Working from the lamina propria of the seminiferous tubules towards the lumen, these layers are delineated by stages of development of the spermatozoa within the layers (Fig. 4.22). Four to five layers are evident, each having spermatozoa at a set stage of development. The time interval between each layer or stage at a set point is 12.2 days of the spermatogenic cycle.

For example, in a set cross-section of seminiferous tubule epithelium (Fig. 4.22), there may be A and A12 spermatogonia in the layer nearest the lamina propria. Above them and nearer the lumen there will be spermatozoa

Cycle Seminiferous Epithelium

Fig. 4.22. The cycle of the seminiferous epithelium at a given point with the seminiferous tubule of the testis of the stallion. Five cycles of

12.2 days are illustrated, subdivided into eight stages or cellular associations. The lengths of these associations (in days) are as follows: I -

2.1; II - 1.8; III - 0.4; IV - 1.9; V - 0.9; VI - 1.6; VII - 1.5; VIII - 1.9. The diagram shows the progressive development from spermatogonium

(A) near the lamina propria to spermatozoon (Sd) released into the lumen. Spermatogonia A1, A2, A3, B1 and B2; first meiotic division primary spermatocytes Pl1 (preleptotene), L (leptotene), Z (zygotene), P (pachytene) and D (diplotene); second meiotic division primary spermatocytes

Pl2 (preleptotene) the remaining stages of the second meiotic division are not shown; Sa, Sb, Sc and Sd progressively more mature spermatids.The relative time taken for each of the stages is indicated by the distance between them on the diagram. The development of one c spermatogonium (A) to one spermatozoon (Sd) takes 4.7 cycles, which at 12.2 days per cycle results in 57.3 days in total. (Adapted from pp

Fig. 4.22. The cycle of the seminiferous epithelium at a given point with the seminiferous tubule of the testis of the stallion. Five cycles of

12.2 days are illustrated, subdivided into eight stages or cellular associations. The lengths of these associations (in days) are as follows: I -

2.1; II - 1.8; III - 0.4; IV - 1.9; V - 0.9; VI - 1.6; VII - 1.5; VIII - 1.9. The diagram shows the progressive development from spermatogonium

(A) near the lamina propria to spermatozoon (Sd) released into the lumen. Spermatogonia A1, A2, A3, B1 and B2; first meiotic division primary spermatocytes Pl1 (preleptotene), L (leptotene), Z (zygotene), P (pachytene) and D (diplotene); second meiotic division primary spermatocytes

Pl2 (preleptotene) the remaining stages of the second meiotic division are not shown; Sa, Sb, Sc and Sd progressively more mature spermatids.The relative time taken for each of the stages is indicated by the distance between them on the diagram. The development of one c spermatogonium (A) to one spermatozoon (Sd) takes 4.7 cycles, which at 12.2 days per cycle results in 57.3 days in total. (Adapted from pp

12.2 days further advanced in development, that is B1 spermatogonia. In the third layer, progressing further again towards the lumen of the seminiferous tubule, there will be pachytene primary spermatocytes (zygotene Z); in the fourth layer, secondary spermatocytes (Pl2); and finally in the fifth layer, nearest the lumen, acrosome spermatids (Sc). No stages other than those given in the example will be seen. However, a section taken from slightly further along the seminiferous tubule may yield in the first layer A2 spermato-gonia against the lamina propria. If so, the second layer would be B2 spermatogonia/preleptotene primary spermatocytes (Pl^, the third layer would be pachytene spermatocytes (P), the fourth layer Golgi spermatids (Sa) and finally the fifth layer acrosome spermatids Sc nearing maturity. Again, the order and stage of spermatozoa development in the five cycles is fixed, no other stages being seen.

As spermatozoa mature and develop through these cycles they are found initially imbedded within the Sertoli cells. As they develop they become increasingly separated from their Sertoli cells until they are released as mature spermatids. The cycle of seminiferous epithelium is 12.2 days in length and is the cycle of spermatozoa development seen within the epithelium of the seminiferous tubules. Hence, at a set point in the seminiferous tubules, gametes can enter as committed A12 spermatogonia every 12.2 days. Similarly, they leave as mature spermatids every 12.2 days at that set point in the tubule (Swierstra et al., 1974, 1975; Johnson et al., 1978a; Johnson, 1991a).

As a further complication each 12.2-day cycle of seminiferous epithelium is further divided into a series of cellular associations (Fig. 4.22). The exact number of cellular associations varies with the criteria used to identify each group of developing spermatozoa cells; for example, the criteria might be morphological changes of the germ cells (Roosen-Runge and Giesel, 1950) or the development of the spermatid acrosome (Le Bland and Clemont, 1952). Using these rather arbitrary criteria, the following number of cellular associations (or stages) per cycle of seminiferous epithelium have been suggested: 14 or eight stages in the bull (Hochereau-de Riviers, 1963; Berndtson and Desjardins, 1974); eight in the ram (Ortavant et al., 1977); and eight in the horse (Swierstra et al., 1974; Johnson, 1991a). The suggestion of eight stages for the horse was arrived at by Swierstra et al. (1974) using morphological changes in germinal cells as the criteria. These eight stages are illustrated in Fig. 4.22, and so in the two previous examples given to illustrate the cycle of the seminiferous epithelium the first example was of cellular association stage IV and the second example was of stage VII.

The relative length of each stage may be arrived at by assessing the relative frequency in appearance of these various stages of spermatid development in a random cross-section sample taken from a normal testis. For example, 16.9% of cross-sections show characteristics of stage I: that is, no mature spermatozoa lining the lumen, an outer (fourth) layer of Golgi spermatids (Sa), a third layer of pachytene primary spermatocytes (P), a second layer of leptotene primary spermatocytes (L) and a first layer, nearest the lamina propria, of A3 primary spermatocytes. The duration of stage I is calculated as 16.9% of 12.2 days, which is 2.1 days. The durations of the other stages are similarly calculated and are also given in Fig. 4.22.

Factors affecting spermatogenesis

There are numerous factors that might affect daily spermatozoan production (DSP) from an individual stallion. These include season, age, testicular size, frequency of use, breed, environmental factors (including temperature and radiation, also nutrition and toxic chemicals or drugs), physical and hormonal abnormalities, disease and infection. These will be discussed in turn, the initial few in some detail. Significant discussion of the latter factors is really beyond the scope of this book and more suited to a text on stallion infertility.

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