521The yolk amount determines the cleavage type, organization of the cleaving
embryo, and distribution of the yolk in the developing embryo. Xenopus oocytes are
mesolecithal with yolk concentrated mainly in the vegetal half and undergo radial
holoblastic (complete) cleavage, which results in sequestering yolk platelets into
blastomeres of the cleaving embryo. Yolk consumption in Xenopus embryos does
not appear to be triggered by embryonic cells declining to a critically small size, and
it is among the earliest aspects of differentiation (Jorgensen et al. 2009 ). The yolk
contribution to development in the zebrafish goes even farther. Zebrafish oocytes are
telolecithal and undergo partial discoidal cleavage (cleavage furrows do not reach
into the yolk), and the early embryo essentially develops around the yolk cell, which
not only has a nutritional role but also, together with the associated extraembryonic
syncytial layer, participates in germ layer patterning (Chen and Kimelman 2000 ;
Ober and Schulte-Merker 1999 ).
Taken together, there is a great variability in yolk deposition and consump-
tion in vertebrate embryos, ranging from negligible amounts of yolk in mam-
malian oocytes through different amounts and distributions of yolk deposits in
fish, amphibian, reptile, and bird eggs. Importantly, in addition to its nutritional
role, yolk may contribute to developmental control, as exemplified by embry-
onic patterning in the zebrafish model. Finally, with respect to the clearance of
parental products, yolk proteins deposited in the oocyte are probably among the
longest-lasting maternal factors, which are being used up by the developing
embryo.
10.5.2 Elimination of Paternal Mitochondria
Mitochondria are semiautonomous energy-generating organelles, which are in
most animals inherited maternally, although some species exhibit strong paternal
inheritance (Zouros et al. 1992 ). The uniparental inheritance may be an adaptation
to avoid an intragenomic conflict between parental populations of mitochondria
(Hurst 1992 ) and/or maintenance of mitochondrial genome integrity through segre-
gation of wild-type and mutant mitochondrial DNA during oocyte development
(Cree et al. 2008 ). Mechanisms controlling uniparental mitochondrial inheritance
are diverse (reviewed in Luo et al. 2013a; Song et al. 2014 ; Sato and Sato 2013 );
here we will mainly focus on mammals.
In mammals, mitochondria are almost exclusively transmitted through the
female, although exceptions were observed in interspecific crosses of mice
(Gyllensten et al. 1991 ) and livestock (Zhao et al. 2001b), and there is one well-
documented human case report (Schwartz and Vissing 2002 ). Although many
reports implied and repeated that sperm mitochondria do not enter the embryo, the
evidence shows the opposite (reviewed in Ankel-Simons and Cummins 1996 ).
Although the midpiece of the giant Chinese hamster sperm is excluded from the
zygote (Yanagimachi et al. 1983 ), in other mammals the midpiece is detectable in
the cleaving zygote through several cleavages (Szollosi 1965 ; Fleming et al. 1986 ;
10 Clearance of Parental Products