Developmental Mechanisms
Introduction
These notes are intended to introduce a number of different mechanisms at the all levels that form structures within the embryo. These are general notes about these mechanisms giving only a few examples. For specific tissues and organs read the system notes. What is remarkable, given our biological diversity, is the strong evolutionary conservation of these developmental mechanisms.
Axes
All embryos occupy 3 D space and appear differently externally and internally within this space. This gives us 3 different axes which need to be determined: Head/Tail (rostro/caudal), Front/Back (anterior/posterior), and Left/Right. The left/right axis appears at first mirror symetrical, and looks very similar externally, but clearly many internal organs are not duplicated (heart, liver, brain etc).
Cellular Mechanisms
These notes are intended to introduce a number of mechanisms at the cellular level that form structures within the embryo. These cellular dynamic and physical interactions form basic tissue structures of epithelia, connective tissues and features such as tubes.
Cell Migration
Molecular Mechanisms
This page is a link to many different resources related to Molecular Development. In current years we have turned from wanting to merely describe the events of embryogenesis, to a desire to understand the mechanisms of development. This has been a boon in allowing the use of many (easier) model systems such as the genetist's tool the fruitfly, and the worm, frog, chicken, zebrafish and mouse (see other embryos page).
A continuing theme also seems to be the reuse of signals at different times and places within the embryo, for diiferent jobs. This has given rise to the concept of "switches" which by themselves may contain no "information" but to activate other genes or switches. Finally, you can imagine that of our 20,000-25,000 protein-coding genes, a large number of these may only be expressed during development or if reused, have a completely different role in the mature animal.
Sex Determination
Eukaryotic organisms reproduce by generating haploid gamete cells, oocytes and spermatozoa, that in fertilization recombine to form the first diploid cell of the new organism. In normal human development, each gamete contains a single sex chromosome, the oocyte carries an X chromosome and the spermatozoa carries either an X or a Y chromosome. Initial sex determination is therefore determined by the spermatozoa and the specific sex chromosome it contains.
In development, the term sex determination used to apply more broadly to how the Y chromosome determined male development. It was thought that female sex determination was the "default mechanism", but we now know that there are also female specific mechanisms.
Links: Genital System Development
Epigenetics
Epigenetics, as the name implies, is the inheritance mechanisms that lie outside the DNA sequence of our genome and genes. These molecular mechanisms include: DNA methylation, histone modification, and those related to the microRNA machinery.
- Links: Epigenetics
External Links
External Links Notice - The dynamic nature of the internet may mean that some of these listed links may no longer function. If the link no longer works search the web with the link text or name. Links to any external commercial sites are provided for information purposes only and should never be considered an endorsement. UNSW Embryology is provided as an educational resource with no clinical information or commercial affiliation.
Glossary Links
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Cite this page: Hill, M.A. (2026, September 11) Embryology Developmental Mechanisms. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Developmental_Mechanisms
- © Dr Mark Hill 2026, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G
