Morphometric Description of Spermatogonial Stem Cells and Expansion of Their Clonal Derivatives - Male Germline Stem Cells Developmental and Regenerative Potential - page 92

All spermatogonia in mammals, including the SSCs, are situated on the basal
membrane of the seminiferous tubules. After division all daughter cells of sper-
matogonia will remain in the same two-dimensional space with the basal membrane
on one side and the junctions between Sertoli cells, forming the blood-testis bar-
rier, on the other side (Russell et al. 1990 ). At their last division, spermatogonia
produce spermatocytes that go through a G1 and S phase and then start the prophase
of the first meiotic division. After the start of the meiotic prophase the spermato-
cytes move through the blood-testis barrier and leave the basal membrane (Russell
et al. 1990 ).

4.4 Morphological Characteristics of A s, p r, al Spermatogonia

The A s,pr,al spermatogonia cannot be discerned from each other in testis sections
because the morphological differences between these cell types, if any, are too
small. However, as first described by Clermont and Bustos-Obregon ( 1968 ), it is
also possible to study spermatogonia in whole mounts of seminiferous tubules.
This gives the advantage that one always sees all morphological details of the
nucleus of each spermatogonium, enabling one to make use of the nuclear changes
that take place during the cell cycle. Importantly, in whole mounts one can also
observe the topographical arrangement of the cells in singles, pairs and chains
consisting of up to 16 A spermatogonia (Fig. 4.2 ). By definition, cells with a simi-
lar morphology, the nuclei of which are not farther apart than 25 mm, are supposed
to belong to the same clone of interconnected cells (Huckins 1971c ). The validity
of this criterion has been confirmed in a study in the Chinese hamster in which
internuclear distances were measured between cells of the same clone. The great
majority of the cells had an internuclear distance between 10 and 20 mm (Lok et al.
1982 ). Tokuda et al. immunohistochemically stained A s,pr,al spermatogonia for the
membrane marker CDH-1 (Tokuda et al. 2007 ). It was found that these cells often
had very fine cell processes, longer than 20 mm. However, the authors concluded
that these processes likely did not represent intercellular bridges as they did not
always connect cells.
On the tubule basal membrane, besides A s,pr,al spermatogonia, there also are A1
through A4 and Intermediate (In) and B spermatogonia. In and B spermatogonia
show heterochromatin in their nuclei while A spermatogonia do not, making it easy
to distinguish these cells. In sections, A s,pr,al spermatogonia can only be discerned
from A1-4 spermatogonia morphologically when plastic embedding and a particu-
lar fixation and staining procedure are employed (Chiarini-Garcia et al. 2001,
2003 ). In whole mounts these two categories of spermatogonia can be distinguished
by the fact that A1-4 spermatogonia have a larger internuclear distance, as these
cells are spread out over the basal membrane, starting from A1 spermatogonia in
epithelial stages VII/VIII (Fig. 4.2e ). In contrast, A pr and A al spermatogonia form
pairs and chains the composing cells of which remain close together.
Spermatogenesis is arranged in such a way that, along the seminiferous
tubules, the stages of the epithelial cycle follow each other (Russell et al. 1990 ),