Biology Reference
In-Depth Information
Fig. 2.2 Mitosis of a guinea
pig spermatogonium. At one
pole of the mitotic spindle a
centriole (C) is surrounded by
dense PCM (inset detail).
Note that the proximal ends
of spindle microtubules
(m) converge toward
nucleation sites on the area
surrounding the centriole (*).
The distal ends of spindle
microtubules (m) are
anchored to metaphase
chromosomes (Chr). Bar
represents 3 lm (This figure
belongs to the author and is
originally reproduced in the
present text)
modifications that result in the elaborated structure of mature spermatozoa. The
Golgi complex develops into the sperm acrosome and mitochondria organize
around the sperm axoneme giving rise to the midpiece (mitochondrial sheath).
Spermatids derive from meiosis II of secondary spermatocytes, the last cells to
divide in spermatogenesis. Since spermatids will not enter a new mitotic cycle,
their centrosomes undergo a functional shift to basal bodies that serve as templates
for the assembly of axoneme doublet microtubules by direct tubulin nucleation on
subunits a and b of distal centriolar triplets. As flagellar axonemes grow, basal
bodies migrate to the cell periphery where distal centrioles dock perpendicular to
the plasma membrane as the axoneme sprouts toward extracellular spaces
(Fig.
2.4
a). In successive steps the basal body-flagellar complex invaginates and
attaches to the nuclear envelope at the concave implantation fossa (Fawcett
1981
;
Holstein and Roosen Runge
1981
). ODF2 (a protein component of sperm outer
dense fibers) is involved in the initial docking of centrioles to membranes
(reviewed by Hoyer-Fender
2010
). As spermatid nuclei elongate, acrosomes
occupy their cranial pole while basal bodies take up the caudal pole. This topo-
graphical arrangement is critical for normal sperm development since it defines the
bipolarity of spermatid nuclei and the alignment of heads, midpieces, and tails
along the sperm longitudinal axis. As we will discuss later, alterations in this
