🥚 Developmental Biology

Fertilisation

The fusion of sperm and egg nuclei to form a diploid zygote.

Zygote

The single cell formed at fertilisation that divides to become an embryo.

Embryo

The early developmental stage of an organism produced by division of the zygote.

Cleavage

Rapid cell divisions of the early embryo without growth, producing a ball of smaller cells.

Blastula

A hollow ball of cells formed by cleavage, containing a fluid-filled cavity.

Gastrulation

The folding of the blastula that creates the three germ layers.

Germ Layers and Their Fates

Ectoderm forms skin and nerves, mesoderm forms muscle and bone, and endoderm forms the gut lining.

Organogenesis

The formation of organs from the germ layers after gastrulation.

Fetal Development

In humans the embryo becomes a fetus after eight weeks, growing until birth around nine months.

Placenta

An organ connecting mother and fetus that exchanges nutrients, gases and wastes.

Growth

An increase in cell number and size that produces a larger organism.

Metamorphosis in Amphibians

Thyroid hormones trigger the tadpole to frog change, including tail loss and limb growth.

Twins

Identical twins arise from one zygote splitting, while fraternal twins come from two separate eggs.

Birth

Hormonal changes and contractions expel the baby, with oxytocin driving labour.

Ageing and Development

Growth, maturity and decline form the human life cycle from infancy to old age.

Intermediate topics (17)

Totipotency

The ability of a cell to give rise to all cell types, including embryonic and extraembryonic tissue.

Pluripotency

Capacity to form all body cell types, as in embryonic stem cells, but not a whole organism.

Cell Fate Determination

Cells commit to specific lineages as signals activate particular genes.

Morphogens

Signalling molecules whose concentration gradient instructs cells to adopt different fates.

Induction

One group of cells signals a neighbouring group to change its developmental path.

Cell Migration in Embryo

Cells move along tracks and chemical cues to reach their destination, as neural crest cells do.

Neurulation

Formation of the neural tube from ectoderm, the precursor of the brain and spinal cord.

Somites

Blocks of mesoderm beside the neural tube that give rise to vertebrae and muscle.

Hox Genes

Master regulators that specify body segment identity along the head-to-tail axis.

Body Axes Formation

Anterior-posterior and dorsal-ventral axes are established by maternal factors and early signalling.

Maternal Effect Genes

Mother's mRNAs deposited in the egg, such as bicoid in Drosophila, set up the embryo's polarity.

Segmentation Genes

Gap, pair-rule and segment polarity genes progressively divide the fly embryo into segments.

Apoptosis in Development

Programmed cell death sculpts tissues, such as separating fingers and toes.

Limb Development

Signalling centres such as the apical ectodermal ridge and ZPA pattern limb growth and digits.

Sex Determination

The SRY gene on the Y chromosome triggers male development in mammals.

Embryonic Stem Cells

Pluripotent cells from the blastocyst inner cell mass that can be grown and directed in culture.

Cloning

Producing a genetically identical organism or cell, as with Dolly the sheep by nuclear transfer.

Advanced topics (22)

Induced Pluripotent Stem Cells

Adult cells reprogrammed to a pluripotent state with factors Oct4, Sox2, Klf4 and c-Myc.

Reprogramming and Transdifferentiation

Converting one cell type to another directly or via an intermediate pluripotent state.

Notch Signalling

Cell-contact signalling that controls cell fate choices through lateral inhibition.

Wnt Signalling

Pathways involving beta-catenin that regulate axis formation, proliferation and stem cell maintenance.

Hedgehog Signalling

A morphogen pathway patterning the neural tube and limbs through Gli transcription factors.

BMP and TGF-Beta Signalling

Signals that pattern dorsal-ventral axes and regulate differentiation via SMAD proteins.

Spemann Organiser

A signalling region in the amphibian gastrula that induces a second body axis when transplanted.

Planar Cell Polarity

Coordinated orientation of cells within the tissue plane, important for convergent extension.

Epithelial-Mesenchymal Transition

Cells lose adhesion and gain motility, important in gastrulation, healing and cancer spread.

Left-Right Asymmetry

Cilia-driven flow in the embryonic node breaks symmetry, leading to asymmetric organ placement.

Developmental Gene Regulatory Networks

Logic circuits of transcription factors and enhancers that specify cell states in the embryo.

Regenerative Development

Some animals reactivate developmental programs to regrow organs after injury.

Epigenetic Reprogramming in Embryos

Waves of DNA demethylation and remethylation reset the genome in germ cells and early embryos.

Organoids

Three-dimensional miniature organs grown from stem cells that model development and disease.

Teratogens

Agents such as thalidomide, alcohol and rubella virus that cause birth defects by disrupting development.

Neural Crest Cells

Multipotent migratory cells that form facial bones, pigment cells and peripheral neurons.

Developmental Timing

Heterochrony and microRNA pathways such as let-7 control when developmental events occur.

Model Organisms in Development

Fruit flies, zebrafish, mice and C. elegans reveal conserved developmental mechanisms.

Stem Cell Niches

Specialised microenvironments that regulate whether stem cells divide, stay quiet or differentiate.

Evo-Devo of Body Plans

Changes in the regulation of conserved toolkit genes explain how new body forms emerge.

Preimplantation Genetic Testing

Screening embryos created by IVF for chromosomal or genetic disorders before implantation.

Gastruloids

Aggregates of stem cells that self-organise to mimic early embryo axis formation.