Biology Reference
In-Depth Information
Upon fusion of the sperm and oocyte, the sperm giant centriole recruits PCM
proteins from the surrounding cytoplasm and forms a centrosome. The zygote
centrosome acts as a microtubule organization center and assembles an aster—a
star-like structure consisting of microtubules. These asters are thought to play a
role in bringing together the female and male pronuclei and in orchestrating the
first cell division (Callaini and Riparbelli
1996
). In support for the critical role of
centrosomes in zygote biology, it has been reported that interfering with centro-
some biogenesis after fertilization inhibits zygote development (Dix and Raff
2007
; Stevens et al.
2007
; Varmark et al.
2007
).
The role of the PCL after fertilization is currently not known; however, one
attractive hypothesis is that the PCL later becomes the second centrosome of the
zygote. The observation that two centrosomes can be observed after fertilization in
mutant oocytes under conditions that block centrosome duplication supports this
hypothesis (Stevens et al.
2007
).
1.2.2 Syncytial Development
Drosophila early embryonic development takes place in a syncytial blastoderm, a
large cell containing many nuclei. In this developmental stage, the embryo
undergoes 13 rounds of nuclear duplication and division without forming indi-
vidual cells surrounded by a plasma membrane; each of these rounds is called a
''nuclear cycle''. For simplicity, we will refer to each dividing unit that includes a
nucleus and its associated centrosomes as a ''cell''. At the end of syncytial blas-
toderm development, the nuclei are partitioned into separate cells where they are
surrounded by a plasma membrane (cellular blastoderm stage). This partitioning,
termed cellularization, is mediated by actin, which forms a cleavage furrow (the
structure that mediates the separation of daughter cells).
The syncytial blastoderm is an excellent system where one can do live imaging
of centriole duplication in real time (Fig.
1.4
). In the embryonic syncytium, the
nuclear cycle is very rapid (*10 min) and many centrioles duplicate synchro-
nously. The nuclear cycle is comprised of two phases: synthesis and mitosis. Early
in synthesis phase, each cell has two centrosomes, each containing one centriole
surrounded by PCM (Callaini and Marchini
1989
; Callaini and Riparbelli
1990
,
1996
; Riparbelli et al.
1997
). During synthesis phase, the centriole within each of
the two centrosomes duplicates, generating a new centriole at the vicinity of the
older centriole. At the onset of mitosis, the centrosomes move to opposite poles
such that each future daughter ''cell'' will inherit one of the of the mother ''cell''
centrosomes. During late mitosis, each of the centrosomes that are associated with
one of the nuclei splits into two centrosomes. As a result, each of the daughter
''cells'' inherently contains two centrosomes, each with one of the centrioles from
the original centrosome (Fig.
1.5
).
The centriole in the Drosophila syncytial blastoderm has a very intriguing
structure.
Unlike
classic
centrioles,
which
are
made
of
nine
triplets
of
