embryology

Embryology is the branch of biology that studies the development of embryos, from fertilization to birth. It covers the formation, growth, and differentiation of tissues and organs, and the cellular and molecular processes that guide development. Classical embryology examined morphological changes across species and fostered ideas such as von Baer’s laws, while later theories like Haeckel’s recapitulation have been tempered by modern evidence that emphasizes conserved developmental pathways rather than simple replay of evolutionary history.

Major stages of embryonic development include gametogenesis (formation of sperm and eggs), fertilization (fusion of gametes), cleavage (rapid cell division without growth), formation of the blastula, gastrulation (establishment of germ layers), organogenesis (formation of organs), and fetal development leading to birth. Across many animals, axis formation and tissue patterning are orchestrated by signaling pathways (such as BMP, Wnt, and FGF) and gene networks that regulate cell fate.

Embryology employs observational and experimental methods, including lineage tracing, advanced imaging, and genetic manipulation. Model organisms such as mice, chicks, zebrafish, Xenopus, and Drosophila have been central in uncovering conserved mechanisms of development. In humans, embryology informs reproductive medicine, prenatal diagnosis, and the understanding of congenital anomalies.

Subfields include descriptive and comparative embryology, developmental biology, and teratology. The discipline intersects with genomics, cell biology, and anatomy, underpinning advances in regenerative medicine, stem cell research (embryonic and induced pluripotent stem cells), and assisted reproductive technologies.