Spermatogenesis is the process by which a complex, interdependent population of germ cells produces spermatozoa [7]. Spermatogenesis begins at puberty after a long preparatory period of "prespermatogenesis" in the fetus and the infant. Three major stages can be distinguished: spermatogoniogenesis, maturation of spermatocytes and spermiogenesis, which is the cytodifferentiation of spermatids.
Spermatogoniogenesis
Several types of spermatogonia are distinguished by their position in the basal part of the germinal epithelium, their morphology and stainability of nuclei: A pale type-, A dark type- and B type-spermatogonia [
8]. A type spermatogonia belong to the stem cell pool of spermatogenesis. B type-spermatogonia represent the onset of germ cell development up to spermatids.
Spermatogonia multiplicate continuously in successive mitoses. Spermatogonial cell divisions are usually incomplete. The daughter cells remain interconnected by cytoplasmic bridges so that a clone derived from one stem cell forms a syncytium of cells. Syncytial connections are maintained through spermatogonial and spermatocytic stages and are dissolved only in advanced phases of spermatid development. It is thought that the formation of these clones are the basis for the synchronuous development of germ cells (Fig. 3B).
Both A type spermatogonia are necessary for intact spermatogenesis. In reduced spermatogenesis A dark-type spermatogonia are often absent. Of course, in the absence of both types of spermatogonia, no spermatogenesis takes place and the germinal epithelium consists of Sertoli cells only. Spermatogonia may be absent from birth (congenital Sertoli cell-only Syndrome) or may be destroyed by different noxes, e.g. x-radiation, (acquired Sertoli cell-only Syndrome). In cases of disturbed ability of spermatogonia to develop B-type spermatogonia the number of A pale type spermatogonia increases and bi- or multilayered groups (Fig. 4A) of spermatogonia in the basal compartment are formed without further developed germ cell stages. This aspect represents an arrest of spermatogenesis at the stage of spermatogonia (Fig. 4B)[9]. The barrier of Sertoli cells can not normally be passed by A type-spermatogonia. Under special conditions, e.g. intratubular tumor cells, the barrier is interrupted and spermatogonia are dislocated into the adluminal compartment where they disintegrate [10].
| Figure 4(A) Section of the germinal epithelium with multilayered spermatogonia. 52 years old infertile patient with arrest of spermatogenesis at the stage of spermatogonia. Drawing on the basis of a semithin section. × 600 (B) Arrest of spermatogenesis (more ...) |
In the basal compartment of the seminiferous tubules tumour cells may be found. In semithin sections they differ noticeable from spermatogonia because of the their larger size, the prominent nucleolus, increase glycogen content and a clear peripheral border (Fig. 4C)[11]. In paraffin sections the detection of single tumour cells may be difficult and the PLAP (Placental alkaline phosphatase)-reaction is required to demonstrate a characteristic dark border [12]. Occasionally hypospermatogenesis is caused by the presence of neoplastic cells in the basal compartment of the germinal epithelium along the basal lamina. These basally situated neoplastic cells in the seminiferous tubules are characteristic of carcinoma-in-situ. They appear to be the stem cell population for most germ cell tumours including both seminomatous and teratomatous tumour types. Sporadic tumour cells may be found within tubules in association with active spermatogenesis, but as the neoplastic cells increase in number, spermatogenesis ceases and the remaining spermatogonia become detached and are released into the tubular lumen. After further proliferation of the neoplastic cells, these also appear in the lumen of the tubule or penetrate the peritubular tissue giving rise to the development of intertubular tumour cell clusters.
Meiosis of spermatocytes
The stage of meiosis is manifested through changes in chromatin configuration in the nucleus after the last spermatogonial division. Cells in meiosis are called spermatocytes. As the process of meiosis comprises two divisions, cells before the first division are called primary spermatocytes and before the second division secondary spermatocytes.
The primary spermatocytes are the largest germ cells of the germinal epithelium (Fig. 1C). The aspect of their nuclear chromatin represents the meiotic stages. Meiosis of spermatocytes starts with the leptotene stage of prophase already in the basal compartment of the germinal epithelium. After passing the Sertoli cell barrier, spermatocytes reach the adluminal compartment and continue with the further prophase stages, namely the zygotene stage, the pachytene and the diplotene stage. During the prophase the reduplication of DNA, the condensation of chromosomes, the pairing of homologuous chromosomes and the "crossing over" take place. After division the germ cells become secondary spermatocytes. They undergo no DNA-replication and divide quickly to the spermatids. The two maturation divisions of each spermatocyte result in four haploid cells, namely the spermatids. These differentiate into mature spermatids, a process called spermiogenesis which ends when the cells are released from the germinal epithelium. At this point, the free cells are called spermatozoa.
Many defects of meiosis are known indicating the vulnerability of this complicate process. Apoptotic spermatocytes are frequent. In some cases very large spermatocytes, so called megalospermatocytes (Fig. 4D) [13] appear. In these cells asynapsis of homologuous chromosomes occurs and the cells become abortive. Genetic disorders may cause these defects. Often, arrest of spermatogenesis at the stage of primary spermatocytes appears without any special aspect of changed morphology of the cells. The primary spermatocytes border the lumen of the seminiferous tubule and do not develop further (Fig. 4E). They disintegrate and spermatids are missing.
Spermiogenesis
During the cytodifferentiation of spermatids the following three processes take place: (fig
5A)
| Figure 5(A) Steps of spermatid differentiation: (1) Immature spermatid with round shaped nucleus. The acrosome vesicle is attached to the nucleus, the tail anlage fails contact to the nucleus. (2) The acrosome vesicle is increazed and flattened over the nucleus. (more ...) |
• Condensation of the nuclear chromatin to about one tenth of the volume of an immature spermatid
• Formation of the enzyme filled acrosome cap by the Golgi apparatus and its attachment to the nucleus
• Development of flagellum structures and their implantation to the nucleus.
The spermatids develop thus the configuration which enables them to leave the germinal epithelium during a complex process, called spermiation.
In summary, the differentiation of spermatids may be divided into 8 steps, demonstrated in figure 5A[14].
Normally, a large number of spermatids is malformed. Malformations may affect only the acrosome, the nucleus or flagellum or may be combined thus sometimes producing bizarrely abnormal spermatozoa. They are abortive germ cells.
A large variety of malformed spermatids may develop:
Malformations of the acrosome, absence of acrosome in cases of round-headed spermatids, disturbances of nuclear condensation, malformations of the flagellum, absence of parts of the flagellum, e.g. the middle piece, appearance of multinucleated spermatid giant cells and more (Figs. 5B, 5C, 6A, 6B, 6C, 6D) [9].
| Figure 6(A) Development of headless spermatozoa. Only the proximal centriole contacts the basal plate of the nucleus of the spermatid. The distal centriole is separated and develops the headless flagellum. Drawing on the basis of electron micrographs. From Ref. (more ...) |
Spermiation
The delivery of mature spermatids from the germinal epithelium (spermiation) is managed by the Sertoli cells. As a result of the complex cooperation of intermediate filaments and cytoplasmic tubules of the Sertoli cells spermatids are advanced to the border of the lumen of the seminiferous tubule [
5]. There the mature spermatids close their intercellular bridges, disconnect their contact to the germinal epithelium and become free cells, now called spermatozoa. Smaller parts of the spermatids with cumulated RNA granules, a few mitochondria, lipid droplets and membranes are released, forming the so-called residual bodies. Most of them are incorporated and digested by the Sertoli cells [
15].
Characteristics of normal spermatogenesis on the basis of histological sections
- Diameter of the seminiferous tubule 180 μm at the minimum
- Presence of A pale type-, A dark type-, B type-spermatogonia
- Presence of primary and secondary spermatocytes
- Differentiation of spermatids
- Zones of spermiation
- Score count of 8 at the minimum (see section - "Score count for the evaluation of spermatogenesis").
- Lumen of the seminiferous tubule
- Normal lipid distribution in the Sertoli cell cytoplasm
- Presence of stages of spermatogenesis
- Formation of clones of germ cells
- Thickness of the lamina propria of the seminiferous tubule of 8 μm at the maximum
-Normal structure and distribution of Leydig cells