Embryology

Embryology is the study of the development of an embryo. An embryo is defined as any organism in a stage before birth or hatching, or in plants, before germination occurs.

Embryology refers to the development of the fertilized egg cell (zygote) and its differentiation into tissues and organs. After cleavage, the dividing cells, or morula, becomes a hollow ball, or blastula, which develops a hole or pore at one end.

In animals, the blastula develops in one of two ways that divides the whole animal kingdom into two halves. If in the blastula the first pore (blastopore) becomes the mouth of the animal, it is a protostome; if the first pore becomes the anus then it is a deuterostome. The protostomes include most invertebrate animals, such as insects, worms and molluscs, while the deuterostomes includes more advanced animals including the vertebrates. In due course, the blastula changes into a more differentiated structure called the gastrula.

The gastrula with its blastopore soon develops three distinct layers of cells (the germ layers) from which all the bodily organs and tissues then develop:

• The innermost layer, or endoderm, gives rise to the digestive organs, lungs and bladder.
• The middle layer, or mesoderm, gives rise to the muscles, skeleton and blood system.
• The outer layer of cells, or ectoderm, gives rise to the nervous system and skin.

In humans, the term embryo refers to the ball of dividing cells from the moment the zygote implants itself in the uterus wall until the end of the eighth week after conception. Beyond the eighth week, the developing human is then called a fetus.

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History

With a very long history, embryology is a subject that has engaged the minds of some of the most brilliant and original biological thinkers and experimentalists.

In modern times, these include Gavin de Beer, Charles Darwin, Ernst Haeckel, J.B.S. Haldane, and Joseph Needham, while much early embryology came from the work of Aristotle and the great Italian anatomists: Aldrovandi, Aranzio, Leonardo da Vinci, Marcello Malpighi, Gabriele Falloppia, Girolamo Cardano, Emilio Parisano, Fortunio Liceti, Stefano Lorenzini, Spallanzani, Enrico Sertoli, Mauro Rusconi, etc.^[1] Other important contributors include William Harvey, Kaspar Friedrich Wolff, Pander, Karl Ernst von Baer, and August Weismann.

After the 1950s, with the DNA helical structure being unravelled and the increasing knowledge in the field of molecular biology, developmental biology emerged as a field of study which attempts to correlate the genes with morphological change; and so tries to determine which genes are responsible for each morphological change that takes place in an embryo, and how these genes are regulated.

Vertebrate and invertebrate embryology

Many principles of embryology apply to both invertebrate animals as well as to vertebrates.^[2] Therefore, the study of invertebrate embryology has advanced the study of vertebrate embryology. However, there are many differences as well. For example, numerous invertebrate species release a larva before development is complete; at the end of the larval period, an animal for the first time comes to resemble an adult similar to it parents. Although invertebrate embryology is similar in some ways for different invertebrate animals, there are also countless variations. For instance, while spiders proceed directly from egg to adult form many insects develop through at least one larval stage.

Modern embryology research

Currently, embryology has become an important research area for studying the genetic control of the development process (e.g. morphogens), its link to cell signalling, its importance for the study of certain diseases and mutations and in links to stem cell research.

See also

• Ontogeny
• Embryogenesis
• Recapitulation theory
• Prenatal development
• Protostomes
• Deuterostomes
• Germ layers
• Epigenesis (biology)
• Developmental biology
• Cell signalling
• Hedgehog signaling pathway
• Morphogens
• Embryo drawing

References

• UNSW Embryology Large resource of information and media
• [1] Definition of embryo according to Webster

Further reading

• Scott F. Gilbert. Developmental Biology. Sinauer, 2003. ISBN 0-87893-258-5.
• Lewis Wolpert. Principles of Development. Oxford University Press, 2006. ISBN 0-19-927536-X.

Ectoderm: Surface ectoderm - Neuroectoderm - Somatopleure - Neurulation - Neural crest

Endoderm: Splanchnopleure

Blastocoele - Yolk sack/exocoelomic cavity - Heuser's membrane - Extraembryonic coelom - Vitelline duct

Umbilical cord (Umbilical artery, Umbilical vein, Wharton's jelly) - Allantois