Zygote, development

In mollusks, sexes are separate. Bivalves release their eggs and sperm into the water, and fertilization occurs there. In other types of mollusks, internal fertilization, a process in which sperm are transferred to the body of the female, is the norm. Following internal fertilization, females deposit strings or cases of eggs on the sand, seaweed, or rocks. In both internal and external fertilization, zygotes develop into swimming larvae that metamorphose into adult forms. 

Grotendorst, C. A., and Carter, R. (1987). Complement effects of the infectivity of Plasmodium gallinaceum to Aedes aegypti mosquitoes. II. Changes in sensitivity to complement-like factors during zygote development. ]. Parasitol. 73, 980-984. 

A fertilized egg develops into a hatched blasto-cyte in vitro when cultured in a protein-free medium at a slower growth rate than that observed in vivo. Growth factors and cytokines appear to accelerate the zygotic development in both autocrine and paracrine manners. CDF/LIF is one of the factors produced by preimplantation embryos (Conquet and Brulet, 1990 Murray et al., 1990). The addition of hLIF to day-5 parthenogenetic bovine morulae produced in vitro stimulates their further development to blastocysts (Fukui et al., 1994). So far, studies of this process remain descriptive (Adamson, 1993) and hence the role of CDF/LIF is not well understood (see reviews by Lee, 1992 and Adamson, 1993). 

These observations are important for understanding the mechanism by which fertilization triggers the onset of development, and relevant to the successful fertilization of human oocytes in cases of assisted conception, particularly since the introduction of intracytoplasmic sperm injection (ICSI). Whereas normal sperm - oocyte fusion in humans is always followed by a train of periodic Ca spikes, ICSI only sometimes elicits similar Ca oscillations (Tesarik, Sousa Testart, 1994). Much attention is focused on the function of periodic Ca spikes in successful fertilization by ICSI (Taylor, 1994 Tesarik, 1994). In some species, such as sea urchin, a single Ca spike at fertilization induces zygote development, while a larger number of spikes may be needed in mammalian species. The pattern of Ca " spiking associated with successful fertilization may thus vary from species to species. 

If haploid cells fuse, nuclear fusion follows and a diploid nucleus (one containing two sets, twice the haploid number, of chromosomes) is formed. The immediate product of such a fusion process is termed a zygote. From the zygote develop diploid cells which reproduce vegetatively by budding the nucleus divides mitotically and the cells are said to be in the diplophase. 

DEKENS M p, PELEGRi F J, MAiscHEiN HM and NussLEiN-voLHARD c (2003) The matemal-effect gene futile cycle is essential for pronuclear congression and mitotic spindle assembly in the zebrafish zygote. Development, 130,3907-3916. 

LUO D, HU w, CHEN s, XIAO Y, SUN Y and ZHU z (2009) Identification of differentially expressed genes between cloned and zygote-developing zebrafish (Danio rerio) embryos at the dome stage using suppression subtractive hybridization. Biol Reprod, 80, 674-684. 

Seeds of some species such as Linum usitatissimum, Citrus spp, Empetrum nigrum, Poa alpina and Opuntia show polyembryony. In different species this can result from one of several causes, including cleavage of the zygote, development of one or more synergidae, existence of more than one embryo sac per nucellus and various forms of apogamy and adventitious embryony. Clearly, in some types of polyembryony haploid embryos are formed, such as in Linum. 

Martinez-Salas, E., Linney, E., Hassell, J., and DePamphilis, M. L. (1989). The need for enhancers in gene expression first appears during mouse development with formation of the zygotic nucleus. Genes Dev. 3 1493-1506. 

Arora K, Niisslein-Volhard C 1992 Altered mitotic domains reveal fate map changes in Drosophila embryos mutant for zygotic dorsoventral patterning genes. Development 114 1003-1024... 

McGrath J, Solter D 1984 Inability of mouse blastomere nuclei transferred to enucleated zygotes to support development invitro. Science 226 1317-1319 Miyazaki S 1988 Inositol 1,4,5-trisphosphate-induced calcium release and guanine nucleotidebinding protein-mediated periodic calcium rises in golden hamster eggs. J Cell Biol 106 345-353... 

Lee, C.-C. (1975) Dirofilaria immitis. ultrastructural aspects of oocyte development and zygote formation. Experimental Parasitology 37, 449-468. 

Schoenwaelder MEA (2002b) Physode distribution and the effects of Thallus Sunburn in Hormosira banksii (Fucales, Phaeophyceae). Bot Mar 45 262-266 Schoenwaelder MAE, Wiencke C, Clayton MN, Glombitza KW (2003) The effect of elevated UV radiation on Fucus spp. (Fucales, Phaeophyta) zygote and embryo development. Plant Biol 5 366-377... 

Provided these two processes are in phase, the conditions are then optimal for fertilisation, proliferation of the zygote to form the blastocyst, implantation of the latter and maintenance of pregnancy. Before ovulation, the development of the ovum and hormone secretion take place in a single structure, the follicle. 

The term zygote refers to the immediate result of fertilisation, i.e. one cell, whereas tiie embryo contains more than one cell. The term foetus refers specifically to mammalian development but has more of a legal meaning fiian a biological one. It is defined as file embryo from the end of the eighfii week after fertilisation. Nonetheless, the term is still used in biology and biochemistry and is used in this text to describe the developing embryo (also known as the conceptus). 

Fig. 1. Dynamics of DNA methylation levels during mouse development. The methylation patterns of the oocyte and the rapidly demethylated after fertilization sperm create the combined methylation patterns in the early mouse zygote. During the first two to three cleavage divisions, the 5mC levels decrease further and stay low through the blastula stage. Post-implantation, the mouse embryo genome is methylated de novo the CpG islands remain mostly unmethylated. The primordial germ cells remain unmethylated. During gametogenesis specific parental (maternal or paternal) patterns of DNA methylation are established at imprinted loci (for further details see Refs. [13, 14]) (re-drawn from Ref [4]).

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