Epigenetics of Reproduction in Animals - Building the Most Complex Structure on Earth - page 110

strict arrangement of the various morphogens in the egg; diffusion of the morphogen
molecules within the egg cytoplasm tends to eliminate any concentration gradient,
let alone allow the accumulation of morphogens to certain sites.
In insects, most cytoplasmic determinants are transported from nurse cells to
oocytes via ring canals in a biphasic process of “slow transport” followed by “dump-
ing” of the nurse cells' content after their apoptosis, which is regulated by ecdysone
( Soller et al., 1999 ), whose synthesis is cerebrally regulated by a neurohormone
called prothoracicotropic hormone (PTTH). In lower invertebrates, such as Hydra
(class Hydrozoa ), oocytes get their cytoplasmic determinants by actively phagocyt-
izing apoptotized nurse cells ( Miller et al., 2000 ).
Another way to deposit cytoplasmic determinants in insect oocytes is receptor-
mediated endocytosis, which is responsible for vitellogenin uptake by oocytes.
Handler and Postlethwait (1977) observed a “cephalic event” that, via ecdysone,
controls the uptake by the egg cell of vitellogenin ( Richard et al., 2000 ), as well as
other maternal cytoplasmic factors ( Chapman, 1998 ) of the hemolymph.
The deposition of cytoplasmic determinants, such as bicoid mRNA ( Cha et al.,
2001 ), oskar mRNA, vasa mRNA ( Tomancak et al., 1998 ), and others, is based on
their transport by motor molecules along microtubules and actin filaments of the
oocyte cytoskeleton. Motors such as dynein, kinesin, and myosins carry out the
transport of determinants in various regions of the oocyte cytoplasm.
If the transport of these determinants takes place along the microtubule rails, this
indicates that the cytoskeleton might play a role in the final localization of determi-
nants in the egg. This implies, in turn, that the egg or the organism can adaptively
modify the structure of the cytoskeleton.
Regulation of the Length of Microtubules—Key to
Transport of Maternal Determinants in the Oocyte
A number of hormones, neurohormones, and neurotransmitters are involved in the
process of lengthening (polymerization) and shortening (depolymerization) cytoskel-
eton microtubules. Many of them perform their microtubule-modifying functions
by acting on a special type of proteins known as microtubule-associated proteins
( MAPs ). By binding microtubules, MAPs induce their polymerization (cadherin) or
depolymerization ( Tournebize et al., 2000; Walczak et al., 1996 ).
Among neuroactive substances that modify the microtubule network by induc-
ing changes in the structure of MAPs are the hypothalamic neuropeptide GnRH
( Drouva et al., 1998 ), VIP, as well as the monoamine neurotransmitters dopamine
and noradrenaline ( Chneiweiss et al., 1992 ). The neuropeptide PTTH stimulates
β-tubulin synthesis ( Rybczynski and Gilbert, 1995 ), and it is believed that tubulin
isotype composition may modulate microtubule polymerization dynamics in cells
( Panda et al., 1994 ). The neurotransmitter acetylcholine (Ach) increases the Ca 2+
in cells ( Mäthger et al., 2003 ), which in turn induces changes in the structure and
length of microtubules.