The mouse embryo as a whole
The EMAGE ontology of mouse development has been constructed in an on-going project to list all components of the developing embryo, foetus and adult. Annotation of the embryonic period (from fertilisation to the laying down of organ primordia, days 0–10) can be considered complete. Annotation of the foetal period is patchy: the 'whole body' information is there up to about day 18 and some individual organ systems have been annotated thoroughly for other projects such as the GUDMAP urogenital project (www.gudmap.org), but the majority of organs still await this highly detailed treatment. The EMAGE whole embryo can therefore be considered as a reliable source of information on total numbers of cell types only during the embryonic period, and for this reason only the ontologies up to day 11.5 (Theiler stage TS19), a little beyond the embryonic period, were included in the analysis.
The result of plotting the number of ontological entities in the embryo against developmental time is shown in figure 1A. From 4 days (the blastocyst- the beginning of differentiation until 10 days, the number of entities rises, following a line that is very close to exponential (the best-fit exponential line fits with R2= 0.97). After this point, the curve flattens (see below).
The rudiment of the metanephros (permanent kidney) appears in mouse at E10 and is patterned over the next 5 days or so: almost all cell types are present by E15. A detailed ontology of the process has been constructed for the GUDMAP project. A plot of the number of entities listed in this ontology, against developmental time, is shown in figure 1B. From the beginning of metanephros development until E15, the rise in cell types is remarkably close to exponential (R2= 0.99). Thereafter, as patterning gives way to maturation, the trend flattens considerably and even begins to fall as entities that are not present in the mature organ disappear.
The reproductive system
Development of the reproductive system begins at about E10.5 and continues until puberty, in the case of males, and arguably until menopause in the case of females (the menstrual cycle and pregnancy involve developmental events). The accurately timed GUDMAP ontology for the reproductive system extends to TS 28 (about 19.5 days). Plots of the number of entities in the ontologies for the male and female reproductive systems against time are shown in figures 1C, D: both show approximately exponential rises (R2= 0.98, 0.95 respectively) with very similar slopes. The male shows a fall in cell types in the last data point, when the patterning phase has given way to maturation.
Complexity does not simply track growth
One possible explanation for an exponential increase in anatomical entities might be an underlying exponential increase in cell number. Direct counts of cell numbers do not seem to have been published for mouse embryos beyond the earliest stages, but volume is a reasonably proxy for total cell number given that most mammalian cells are of similar size. It can be seen from figure 1E that entity number grows much less steeply than volume, with respect to time, and from figure 1F that the correlation between entity number and volume is very far from linear. Complexity is therefore not a direct, linear effect of cell proliferation in this system.
All 4 ontologies examined indicate an exponential rise in complexity in the early period of their system's development, a period that corresponds well with that of patterning. The metanephros and whole-embryo data show a clear transition to a much flatter rise and eventual small fall in complexity during maturation (the same may be true of the reproductive system but the available ontologies end too soon to test this). The apparent timing of this flattening in the whole-embryo data may be premature, an artefact of the incomplete annotation in the foetal period mentioned above: a fully annotated whole embryo would of course contain information on all of its developing organs at the same level of detail as the three organ systems described here.