Some events such as the early attachment of zygotes of Fucus vesiculosus L. to a substratum, their polarity establishment, the histological localization of extracellular polysaccharides (regarding their origin, transport and secretion) and the general histology of the developing zygotes have been studied by five methods. (1) Vital staining or staining the whole zygotes after brief fixation (2) Shake culturing (3) Histochemistry on 1 to 2 micrometers thick sections of fixed material embedded in plastics (4) Transmission electron microscopy (5) Scanning electron microscopy.
Results show that the zygotes immediately following fertilization secrete a polysaccharide believed to attach them to the substratum, so that by 4 hours after fertilization, the majority of zygotes are firmly attached. Young zygotes secrete a fibrillar cell wall (probably mainly alginic acid at this stage). At about 12 hours post-fertilization, another extracellular polysaccharide (fucoidan) is polarly secreted from the presumptive site of rhizoid formation and later, mainly from the rhizoid cell. At about this time, the walls are now shown to contain both alginic acid and fucoidan.
At about 12 hours post-fertilization, the zygotes cultured under stationary conditions, begin to bulge at one end so that by 16 hours after fertilization a polar zygote is formed. Corresponding to the time of polarity determination at about 10 to 13 hours after fertilization, histological and histochemical studies have revealed that an asymmetrical distribution of a material (presumably a sulphated polysaccharide) is present. This is the only sign of an intracellular morphological asymmetry which is indicative of the polarity expression of the 11 hour old zygotes. This asymmetrical accumulation of the material intracellularly at the presumptive site of rhizoid formation has been suggested to relate to the polar secretion of the fucoidan to theexternal medium from this site.
Significantly, when all the external gradients are eliminated for a sufficient length of time under shaking conditions, all the individually suspended zygotes become apolar. Furthermore, all of these apolar zygotes are unable to attach to a substratum and subsequently, most of these will develop into apolar multicellular embryos. These results have been discussed in relation to polarity establishment of the zygotes in conjunction with the secretion of the extracellular polysaccharides to the external medium and with their early attachment to the substratum.