Biology Reference
In-Depth Information
The sensory neuron is a product of asymmetrical cell division and it inherits a
centrosome with two centrioles (Gomes et al.
2009
; Keil
1997
; Seidl
1991
). After
the sensory neuron generates a long dendrite, the two centrioles are reorganized
and are found in tandem, one of which is attached to a vesicle. This reorganized
structure appears to migrate along the dendrite until it reaches the distal end, where
the associated vesicle fuses with the plasma membrane to form the sensory cilium
(Seidl
1991
). The sensory cilium is composed of a transition zone, also called the
connecting cilium, and the sensory cilia proper, also called the outer segment
(Fig.
1.7
b).
1.3.2 Spermatogenesis
Spermatogenesis is the process that takes place in the testes to form mature male
gametes and begins when a sperm stem cell divides asymmetrically to form
another stem cell and a progenitor spermatogonium. The spermatogonium divides
4 times to form 16 spermatocytes. These spermatocytes grow to *30 times their
original size and ultimately undergo two cycles of meiosis to generate 64 sper-
matids. The spermatids, which are first round, undergo a dramatic differentiation
program, called spermiogenesis. The completion of this differentiation program
results in the formation of a sperm cell that is *2 mm long, a length comparable
to that of the fly itself.
Centrosomes in the Drosophila testes have several interesting properties:
First, unlike the syncytial blastoderm, the centrosome and centrioles of the adult
testes are similar to their vertebrate counterparts (Tates
1971
; Tokuyasu
1975
).
These centrioles have nine triplet microtubules (Fig.
1.9
k). This normal centriolar
structure correlates with the ability of the spermatogenic centrioles to form cilia
and suggests that in ciliated cells, the centriole needs to develop into its mature
state. In this regard, sensory neurons that have cilia also contain centrioles with
triplet microtubules (Keil
1997
).
Second, in spermatogenesis, the centrosomes form two types of cilia. During
spermatocyte growth, each of the spermatocyte's four centrioles forms a primary
cilium-like structure of unknown function (Fig.
1.9
b) (Tates
1971
). Later in
spermiogenesis, each of these primary cilia is modified to form a motile cilium—
the sperm flagellum.
Third, male meiosis is absolutely dependent on centrosomes. Flies that do not
have functional centrosomes fail to accurately separate genetic material and
mitochondria (Fig.
1.10
), one of the reasons why centrosomal mutants are male
sterile. It is currently unclear why centrosomal defects cause abnormalities in male
meiosis but do not disrupt mitosis. However, it is possible that male-specific
meiotic defects are due to the fact that the centrosomes are associated with a
ciliary-like structure that requires centrosomal components for their formation.
Fourth, during spermatocyte growth and spermiogenesis, the centriole elongates
to *2.5 um, much larger than centrioles found in any other Drosophila tissue or
