Embryology
Embryology (from Greek ἔμβρυον, embryon, "unborn, embryo"; and -λογία, -logia) 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.
Morula, 8 cell stage
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.
1 - morula, 2 - blastula
In bilateral animals, the blastula develops in one of two ways that divides the whole animal kingdom into two halves (see: Embryological origins of the mouth and anus). 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.
1 - blastula, 2 - gastrula with blastopore; orange - ectoderm, red - endoderm.
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. Embryos in many species often appear similar to one another in early developmental stages. The reason for this similarity is because species have a shared evolutionary history. These similarities among species are called homologous structures, which are structures that have the same or similar function and mechanism having evolved from a common ancestor.
Human embryo at six weeks gestational age
History
As recently as the 18th century, the prevailing notion in human embryology was preformation: the idea that the egg or sperm contains an embryo--a preformed, miniature infant, or "homunculus"--that simply becomes larger during development. The competing explanation of embryonic development was epigenesis, originally proposed 2,000 years earlier by Aristotle. According to epigenesis, the form of an animal emerges gradually from a relatively formless egg. As microscopy improved during the 19th century, biologists could see that embryos took shape in a series of progressive steps, and epigenesis displaced preformation as the favored explanation among embryologists.[1]
![Ernst Haeckel embryo drawings](/2009-wikipedia_en_wp1-0.7_2009-05/I/180px_Haeckel_drawings.jpg)
Modern embryological pioneers 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.[2] Other important contributors include William Harvey, Kaspar Friedrich Wolff, Pander, Karl Ernst von Baer, and August Weismann.
Histological film 10 day mouse embryo
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.[3] 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 its parent or 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.
Beetle larvae
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
- French flag model
- 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.
External links
Developmental biology > Human embryogenesis (development of embryo) and development of fetus (some dates are approximate - see Carnegie stages and a timeline) |
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Week 1 |
Fertilization - Egg activation - Zygote - Cleavage - Morula - Blastula (Blastomere) - Blastocyst - Inner cell mass
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Week 2 |
Bilaminar disc (Hypoblast, Epiblast)
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Week 3 (Trilaminar embryo,
germ layers) |
Archenteron/Primitive streak (Primitive pit, Primitive knot/Blastopore, Primitive groove) - Gastrula/Gastrulation - Regional specification
Ectoderm: Surface ectoderm - Neuroectoderm - Somatopleure - Neurulation - Neural crest
Endoderm: Splanchnopleure
Mesoderm: Chorda- - Paraxial (Somite/Somitomere/Sclerotome/Myotome/Dermatome) -
Intermediate - Lateral plate (Intraembryonic coelom, Splanchnopleure/Somatopleure)
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Extraembryonic/uterus |
Trophoblast (Cytotrophoblast, Syncytiotrophoblast)
Blastocoele - Yolk sack/exocoelomic cavity - Heuser's membrane - Extraembryonic coelom - Vitelline duct
Umbilical cord (Umbilical artery, Umbilical vein, Wharton's jelly) - Allantois
Placenta - Decidua (Decidual cells) - Chorionic villi/Intervillous space - Gestational sac (Amnion/Amniotic sac/Amniotic cavity, Chorion)
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Histogenesis |
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Organogenesis |
Limb development: Limb bud - Apical ectodermal ridge/AER
other structures: Eye development - Cutaneous structure development - Heart development - Development of the urinary and reproductive organs
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Prenatal development/Mammalian development of digestive system |
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Gut |
Stomodeum
Foregut (Buccopharyngeal membrane, Rathke's pouch, Tracheoesophageal septum, Pancreatic bud, Hepatic diverticulum)
Midgut
Hindgut (Urorectal septum )
Proctodeum
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Mesentery |
Dorsal mesentery - Ventral mesentery
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Other |
Septum transversum
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Prenatal development/mammalian embryogenesis - Development of the urinary and reproductive organs |
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Common Urinary/Reproductive system |
Cloacal membrane - Cloaca - Urethral groove - Urogenital sinus - Urachus - Urogenital folds
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Kidney development |
Nephrogenic cord - Nephrotome - Pronephros - Mesonephros (Mesonephric tubules)
Ureteric bud - Metanephric blastema
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Fetal genital development |
Primarily internal
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Gonadal ridge - Sex cord (Cortical cords, Testis cords)
Pronephric duct/Wolffian duct/mesonephric duct - Müllerian duct/paramesonephric ducts (Vaginal plate)
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Primarily external
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Genital tubercle - Phallus - Labioscrotal folds
Gubernaculum - Processus vaginalis
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see also list of homologues of the human reproductive system
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Prenatal development/Mammalian development of nervous system |
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General neural development/
neurulation/neurula |
Notochord - Neuroectoderm - Neural plate - Neural folds - Neural groove
Neural crest - Neural tube (Neuromere/Rhombomere, Cephalic flexure)
Alar plate (sensory) - Basal plate (motor)
Glioblast - Neuroblast
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Eye development |
Optic vesicles - Optic stalk - Optic cup - Lens placode
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Auditory development |
Auditory vesicle - Auditory pit
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Note: mostly ecoderm, but mesoderm is precursor for epineurium, perineurium, and endoneurium
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Prenatal development/Mammalian development of circulatory system |
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Vascular |
Blood island
arteries: Dorsal aorta - Aortic arches - Vitelline arteries - Ductus arteriosus - Umbilical artery
veins: Cardinal veins - Ducts of Cuvier - Vitelline veins - Ductus venosus - Umbilical vein
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Heart development |
Primitive heart tube: Truncus arteriosus - Bulbus cordis - Primitive ventricle - Primitive atrium - Sinus venosus
Septum primum (Ostium primum, Ostium secundum) - Septum secundum (Foramen ovale) - other septa (Endocardial cushions/Septum intermedium, Aorticopulmonary septum) - Atrial canal
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Prenatal development/Mammalian development of respiratory system (overview) |
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Upper |
Nasal placode
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Lower |
Laryngotracheal groove - Lung buds
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Human development of head and neck |
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Branchial region |
Pharyngeal arch (1st, 2nd) - Pharyngeal pouch - Pharyngeal groove - Cervical sinus
Frontonasal prominence - Maxillary prominence - Mandibular prominence (Meckel's cartilage)
tongue: Lateral lingual swelling - Tuberculum impar
nose: Nasal placode - Olfactory pit - nasal prominences (Lateral, Medial) - Intermaxillary segment
palate: Primitive palate - Secondary palate
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Tooth development |
Dental papilla - Odontoblast - Ameloblast - Hertwig's epithelial root sheath - Epithelial cell rests of Malassez - Cementoblast
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Thyroid |
Thyroid diverticulum - Thyroglossal duct - Ultimobranchial body
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