The unincubated avian blastoderm : its characterization and an investigation of developmental quiescence

Foulkes, Adrian George (1990) The unincubated avian blastoderm : its characterization and an investigation of developmental quiescence. University of Southampton, Doctoral Thesis.

Abstract

The avian embryo particularly that of the domestic hen (Gallus domesticus) has long been used as a model system for studying embryonic development. However, due to the earlier inaccessibility of the in utero embryonic stages, the early events of avian embryonic development have been less extensively studied than the older post-laying stages of development. Partly as a result of this the unincubated embryo at the time of laying was generally regarded as an amorphous mass of pluripotential cells and only relatively recently has recognition been made that it is at this stage in development that the first morphogenic event in the development of the embryo occurs. Nevertheless although some information concerning the fundamental characterization of the unincubated chick embryo is available much of it is ill defined or is ambiguous in reference to the actual developmental stage being described. Consequently many inconsistencies concerning the description of the unincubated avian embryo at the time of laying exist. Therefore the work included in this thesis has been conducted in an attempt to rectify some of these inconsistencies using unincubated blastoderms at accurately determined developmental stages. In addition, an investigation into the phenomenon of developmental quiescence, whereby continued development of the embryo is retarded upon laying, has been conducted. This has been performed in an attempt to determine if the failure to resume development following a prolonged arrested period correlates with any changes in the parameters investigated during the characterization of the unincubated embryo. Using a number of different strains the mean linear dimensions of the unincubated chick blastoderm and the number and size of its constituent cells has been determined and the effects of preincubation storage at different temperatures investigated. Unincubated blastoderms from each of the strains consist of between 34,000-62,000 cells, the majority of which fall within a size range of between 20-30μm. No change in the blastoderm's mean cell size occured upon storage at any of the selected temperatures, while the mean cell number per blastoderm consistently decreased within the first week of storage suggesting that the phenomenon occurs as a normal consequence of arrested development. In addition, the need for a re-evaluation of the temperature currently regarded as that below which development cannot occur is discussed. No obvious heterogeneity in unincubated blastoderm morphology was detected using scanning electron microscopy while the most obvious effect of preincubation storage in this respect is a general decrease in cell surface projections and an increase in cell surface blebbing particularly on the ventral surface. Morphological and immunocytochemical evidence which suggests a dual origin for lower layer (hypoblast) formation is also presented. A quantitative investigation of the behavioural characteristics of the cells of the two embryonic layers (the epiblast and hypoblast) comprising the unincubated and early incubated embryo shows that distinct differences exist between the cells of these two issues. Furthermore an immunochemical investigation of the unincubated chick blastoderm revealed the presence of antigenic heterogeneity both between and within the epiblast and hypoblast layers. At least one of these antibodies appears to specifically identify the cells of the forming lower layer (hypoblast) and in this respect has been used to investigate the development of this tissue during the early stages of avian embryonic development.

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