Abstract:
Spermatogenesis is a process in which spermatogonial stem cells
undergo a series of biochemical and morphological changes resulting in the
production of highly differentiated haploid cells called spermatozoa. The
entire process of spermatogenesis can be divided into three phases: 1) stem
cell renewal and differentiation, 2) meiosis and 3) spermiogenesis. In
mammals, spermatogenesis is characterised by a unique chromatin
remodeling process, in which somatic histones are sequentially replaced by
testis specific variants, followed by the replacement of both somatic and
testis specific histones with a class of basic proteins, transition proteins
(TP1, TP2 and TP4).
There are three types of spermatogonia or stem cells, the A type, Intype (intermediate) and the B-type, which are located in the periphery of
seminiferous tubules. A-type cells that are undifferentiated (Courot et al.,
1970; Clermont, 1977) undergo four successive divisions giving rise to cell
types A1 to A4 in rat and mouse. The A4 cells undergo several rounds of
division producing A0 or A1 and In-type cells. The In-type cells then undergo
further division and differentiation giving rise to B-type spermatogonia. There
is a progressive decrease in the size of the nucleus, followed by the
appearance of dark, spherical structures with coarse granules attached to
the nuclear membrane as the cells undergo differentiation. After successful
mitotic division, the B-type spermatogonia produce preleptotene primary
spermatocytes (Courot et al., 1970). Most of the meiotic DNA and nuclear
protein synthesis takes place at this stage of preleptotene primary
spermatocyte. After this synthesis phase the cells enter the prophase of the
first meiotic division.
