**2. Spermatogenesis in Urodeles**

earliest stages are more cephalic and the latest stages are more caudal, in contrast with the tubular structure with radial disposition of the spermatogenesis in the testis of amniotes.

**Figure 1.** Testis of *Ambystoma dumerilii* in longitudinal sections. Spermatogenesis advancing in successive regions along the cephalocaudal axis of the testis, where the different types of spermatogenic cells are seen. (A) Panoramic morphology of the testis. H-E. Bar = 0.3 mm. (B, C) Testicular lobules containing the spermatogenic cells and surrounded of connective tissue. H-E. Bar = 0.1 mm. Spermatogonia (Sg), primary spermatocytes (S1), spermatids (St), spermatozoa

Spermatogenic cells of Urodeles are quite big, compared to amniotes germ cells, as example, the spermatogonia may attain 55 μm in *A. dumerilii* [1, 3] and the spermatozoa may attain

840 μm long in *Necturus maculosus* [4, 5].

(Z), and interlobular connective tissue (c).

10 Spermatozoa - Facts and Perspectives

Spermatogenesis in Urodeles was studied by several authors who described stages of germ cell maturation in a variety of species as: in *Desmognathus fusca* [7]; in *Ambystoma tigrinum* [8–10]; in *Trituroides hongkongensis* [11]; in *Necturus maculosus* [12]; in *Salamandrina terdigitata* [13]; in *Salamandra salamandra* [14, 15]; in *A. mexicanum* [16, 17]; Ricote et al. in *Triturus marmoratus* [18]; in *A. dumerilii* [1, 3, 17, 19]; and in *Salamandrella keyserlingii* [20].

The spermatogenic cells of *A. dumerilii*, as in all Urodeles, are in synchronous groups called cysts, where all the cells are at the same stage of development. A cyst is formed when a spermatogonium becomes surrounded by a Sertoli cell. Then, the distribution of cysts in the testicular lobules displays a longitudinal sequence of stages of spermatogenesis, in respect to the cephalocaudal gradient: spermatogonia, primary spermatocytes, secondary spermatocytes, spermatids in spermiogenesis, and spermatozoa. The Sertoli cells are involved in essential functions of the spermatogenesis: they maintain a permeability barrier to the germinal cells into the cyst during all the process of differentiation, determine the endocrine activity that controls the spermatogenesis, and phagocytose degenerating spermatogenic cells, residual bodies, and abnormal spermatozoa during the spermiogenesis [1, 17, 19].

Spermatogonia of *A. dumerilii* are spherical cells with 45–55 μm in diameter (**Figure 2A**). These cells are diploid and have mitotic activity. When spermatogonia initiate the meiotic process become a primary spermatocyte (**Figure 2A**–**C**).

The primary spermatocytes are also spherical cells; their size is 40–45 μm in diameter. These cells initiate the meiosis; then, their nuclei contain duplicated chromosomes at different stages of prophase I of meiosis exposed clearly in the chromatin changes: leptotene with fine reticular chromatin, zygotene with fine fibrillar pattern of duplicated chromosomes, pachytene with more thick fibrillar pattern of duplicated chromosomes in crossing-over, and diplotene when occurs the separation of homologous duplicated chromosomes, remaining some chiasms (**Figure 3A** and **B**). The primary spermatocytes enter metaphase I, anaphase I, and telophase I (**Figure 3C**), resulting in two secondary spermatocytes.

Secondary spermatocytes are spherical cells and are smaller than primary spermatocytes; they have in average 18–20 μm in diameter. As the result of the first division of meiosis, the secondary

**Figure 2.** Spermatogenesis in the testis of *Ambystoma dumerilii*. (A) Cephalic region of the testis. Spermatogonia surrounded by connective tissue. Lobules with cysts containing cells in different stages of spermatogenesis. Masson's trichrome. Bar = 30 μm. (B, C) Periphery of the testis in the adjacent region to spermatogonia. Cysts containing primary spermatocytes during the first meiotic prophase, in pachytene, with thick fibrillar chromatin, in diplotene with pairs of chromosomes showing chiasms and during the first meiotic division, secondary spermatocytes and early spermatids. Around the lobules, there is connective tissue. The tunica albuginea with blood vessels surrounds the testis. Spermatogonia (Sg), primary spermatocytes (S1), primary spermatocytes in pachytene (S1p), primary spermatocytes in diplotene (S1d), primary spermatocytes during the first meiotic division (S1md), secondary spermatocytes (S2), spermatids (St), connective tissue (c), tunica albuginea (T), and blood vessels (v). B: H-E, C: PAS. Bar = 20 μm.

**3. Morphology of spermatids in** *A. dumerilii* **during spermiogenesis**

spermatids (St1), and connective tissue (c).

The spermatids of *A. dumerilii* initiate the spermiogenesis, occurring during a sequence of morphological changes transforming the spermatids into spermatozoa. Early spermatids are spherical in shape and attain a diameter of 14–17 μm; their nuclei contain light fibrillar chromosomes.

**Figure 3.** Spermatogenesis in the testis of *Ambystoma dumerilii*. (A) Primary spermatocytes during pachytene and early spermatids. Compare the size of both type of germinal cells. H-E. Bar = 10 μm. (B) Primary spermatocytes during diplotene and early spermatids. Compare the size of both type of germinal cells. PAS. Bar = 10 μm. (C) Primary spermatocytes during the first meiotic division. B: H-E, C: PAS. Bar = 10 μm. Primary spermatocytes in pachytene (S1p), primary spermatocytes in diplotene (S1d), primary spermatocytes during the first meiotic division (S1md), early

Sequence of Germ Cells Differentiation During Spermiogenesis of the Amphibian Urodele *Ambystoma dumerilii*

http://dx.doi.org/10.5772/intechopen.71508

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spermatocytes contain a haploid, but duplicated, number of chromosomes (**Figure 2C**). These cells are seen less frequent, since they divide during the second part of meiosis after a very short interphase, rapidly giving rise to two spermatids.

Sequence of Germ Cells Differentiation During Spermiogenesis of the Amphibian Urodele *Ambystoma dumerilii* http://dx.doi.org/10.5772/intechopen.71508 13

**Figure 3.** Spermatogenesis in the testis of *Ambystoma dumerilii*. (A) Primary spermatocytes during pachytene and early spermatids. Compare the size of both type of germinal cells. H-E. Bar = 10 μm. (B) Primary spermatocytes during diplotene and early spermatids. Compare the size of both type of germinal cells. PAS. Bar = 10 μm. (C) Primary spermatocytes during the first meiotic division. B: H-E, C: PAS. Bar = 10 μm. Primary spermatocytes in pachytene (S1p), primary spermatocytes in diplotene (S1d), primary spermatocytes during the first meiotic division (S1md), early spermatids (St1), and connective tissue (c).
