Morphometric Description of Spermatogonial Stem Cells and Expansion of Their Clonal Derivatives - Male Germline Stem Cells Developmental and Regenerative Potential - page 100

the A s spermatogonia decreases at about stage VI and that of the A pr,al spermatogonia
at about stage III.
The higher growth fraction of the A al spermatogonia during the active period
may be related to the fact that the cells composing the chains are connected by
intercellular bridges. Factors stimulating cells to cross the G1/S barrier induced in
one cell of a chain may well diffuse to other cells and initiate cell cycle progress in
the whole chain. Then the more cells present in a chain, the larger the chance of cell
cycle initiation.
While in the Chinese hamster A s,pr,al spermatogonia start to actively proliferate at
about the same epithelial stage, they do not decrease their proliferative activity
simultaneously. The A s spermatogonia keep proliferating considerably longer than
A pr and A al spermatogonia (Lok and de Rooij 1983b ). A similar phenomenon was
seen in the rat (Huckins 1971d ).

4.9 Regulation of the Proliferative Activity of A s , A pr , and A al

Spermatogonia at the Cellular Level

Which mechanisms determine the stages of the epithelial cycle at which inhibition
and stimulation of A s,pr,al spermatogonial proliferation take place? Several pieces of
evidence indicate the nature of the cellular cause for the inhibition of the prolifera-
tion of A pr and A al spermatogonia from stage II onwards.
First, in the 1970s experiments were carried out to detect a so-called chalone for
the regulation of spermatogonial proliferation. Chalones are supposed to be factors
secreted by the differentiated cells in a tissue that inhibit the proliferation of the
preceding stem cells and early amplifying cell types. The general idea is that, in this
way, an equilibrium can be formed between the numbers of cells needed by a tissue
and cell production (Iversen 1973 ). When the number of differentiated cells is low,
little chalone is produced and proliferation of progenitor cells will increase because
these cells are no longer inhibited and vice versa. With one exception (Cunningham
and Huckins 1979 ), testicular extracts have been found to inhibit the proliferation
of early spermatogonial cell types (Clermont and Mauger 1974 ; Irons and Clermont
1979 ; de Rooij 1980 ; Thumann and Bustos-Obregon 1978, 1982 ). Of course, this
may also have been caused by an effect of the testicular extracts via the somatic
component of the testis. However, in the mouse, injection of extracts from mouse
testes from which the spermatogonia have been removed by way of administration
of busulfan, fails to produce a diminution of the formation of differentiating
spermatogonia making this possibility unlikely (de Rooij 1980 ). Hence, these
results suggest an inhibitory action of differentiating type spermatogonia on A s,pr,al
spermatogonial proliferation. No reports have been published as yet describing a
successful purification of the inhibiting factor.
Second, more direct evidence for a role of differentiating type spermatogonia in
the proliferation of A s,pr,al spermatogonia came from an experiment in which
Chinese hamsters were given cytosine 1-b-D-arabino-furanoside (Ara-C), which