Building an Embryo
On Wednesday 21 July 2021 Associate-Professor Peter Pfeffer (Victoria University of Wellington) gave a Royal Society of New Zealand Wellington Branch public lecture at Victoria University, entitled Building an Embryo.
An early-stage human embryo
Peter’s present research in developmental biology at Victoria University involves study of lineage determination (differentiation of cells from the cells that gave rise to them), gastrulation – the process in which an embryo transforms from a one-dimensional layer of epithelial cells (blastula) – to a multidimensional structure (the gastrula) and the development of the placental lineage, using mice and cattle as model systems. He combines genetic approaches and classical physiological methodology to investigate fundamental embryological and evolutionary questions on a gene expression and mechanistic level.
Peter gave a brief overview of the key ideas of embryology and spoke about his own research in embryology on the development of embryos. He gave a brief account of historical thinking about embryos (for example, the homunculus – a sixteenth century representation of the embryo as a small human being). He referred to embryology as the self-guided developmental process that allows a single cell (the fertilised egg) to develop into over two hundred cell types that later become the tissues, organs and body systems that make up a complex animal. Using examples of his research, he outlined the main principles leading to the lineage decisions and patterning events that are important in the building of an embryo.
He explained the cell-division process leading from a single cell to a four-cell embryo to a Blastocyst embryo, as seen in the following photographs:
Cell-division in action
He summarised the critical stages in gastrulation and discussed diagrams of the developmental stages of embryos of different species, including fish, amphibians, birds and humans (Carnegie Stages). He noted that, in humans, all organs are formed by the end of the eighth week.
The Carnegie Stages
He spoke of the generation of lineage differences through inherent processes and chance, producing polarised cells through radial cleavage and tangential cleavage. He discussed the role of master regulators (such as Oct4) in producing non-polarised and polarised cells. He emphasised significant differences in the developmental processes between different species, noting, for example, a different process order between a mouse embryo (Blastocyst at day 3.5, leading to implantation at day 4.5 and then to Gastrulation at day 6.5) and a cattle embryo (Blastocyst at day 7, leading to Gastrulation at day 14 and then to implantation at day 20).
Peter discussed experiments involving grafting of Hensen’s node (a group of cells that constitutes the organizer of a bird embryo) from one species to another and explained how a new axis is induced in the host. He outlined the separation of the midbrain and hindbrain by an organiser and mentioned patterning in the brain.
Peter concluded his lecture by summarising embryo development as a series of successive autonomous steps in which:
- inherent properties of cells lead to differences
- communication between different cell types leads to new cell types and
- cell migration that creates new possibilities.
The lecture was held from 6.00 pm – 7.30 pm at Victoria University, Kelburn Campus. Approximately thirty-five people attended.