Embryology
Morula, 8 cell stage
1 - morula, 2 - blastula
1 - blastula, 2 - gastrula with blastopore; orange - ectoderm, red - endoderm.
Embryology (from Greek ἔμβρυον, embryon, "unborn, embryo"; and -λογία, -logia) is a science which is about the development of an embryo from the fertilization of the ovum to the fetus stage. After cleavage, the dividing cells, or morula, becomes a hollow ball, or blastula, which develops a hole or pore at one end.
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 include 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. 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.
History
Human embryo at six weeks gestational age
Histological film 10 day mouse embryo
Beetle larvae
As recently as the 18th century, the prevailing notion in human embryology was preformation: the idea that semen 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]
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, Heinz Christian 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.[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
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
References
Embryology - History of embryology as a science." Science Encyclopedia. Web. 06 Nov. 2009. <http://science.jrank.org/pages/2452/Embryology.html>.
"Germ layer." Encyclopædia Britannica. 2009. Encyclopædia Britannica Online. 06 Nov. 2009 <http://www.britannica.com/EBchecked/topic/230597/germ-layer>.
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 |
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First three
weeks |
Week 1
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Fertilization · Egg activation · Zygote · Cleavage · Morula · Blastula (Blastomere) · Blastocyst · Inner cell mass
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Week 2
(Bilaminar)
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Hypoblast · Epiblast
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Week 3
(Trilaminar)
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Germ layers
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Archenteron/Primitive streak (Primitive pit, Primitive knot/Blastopore, Primitive groove) · Gastrula/Gastrulation · Regional specification
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Ectoderm
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Surface ectoderm · Neuroectoderm · Somatopleure · Neurulation · Neural crest
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Endoderm
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Splanchnopleure
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Mesoderm
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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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(Some dates are approximate—see Carnegie stages and a timeline.)
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Human development of head and neck / branchial apparatus (GA 1.65) |
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Nose |
Nasal placode · Olfactory pit · nasal prominences (Lateral, Medial) · Intermaxillary segment
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Mouth/neck |
Palate
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Primitive palate · Secondary palate
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1st arch
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Frontonasal prominence · Maxillary prominence · Mandibular prominence (Meckel's cartilage)
Anterior tongue: Lateral lingual swelling · Tuberculum impar
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Thyroid (pouches 3-5) |
Thyroid diverticulum · Thyroglossal duct · Ultimobranchial body
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General |
Pharyngeal groove (Cervical sinus) · Pharyngeal arch (1st, 2nd) · Pharyngeal pouch (Ultimobranchial body) · Copula linguae
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anat(n, x, l, c)/phys/devp
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noco(c)/cong/tumr, sysi/, injr
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Ossification (GA 2.80) |
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Upper limb |
Ossification of humerus · Ossification of ulna · Ossification of radius
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Lower limb |
Ossification of tibia
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Head |
cranium: Ossification of occipital bone · Ossification of frontal bone · Ossification of temporal bone · Ossification of the sphenoid · Ossification of ethmoid
facial bones: Ossification of maxilla · Ossification of the mandible
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Other |
Ossification of scapula
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anat(c/f/k/, u, t/p, l)//devp/cell
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noco/cong/tumr, sysi/, injr
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Prenatal development/Mammalian development of circulatory system (GA 5) |
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Vasculogenesis,
angiogenesis,
and lymphangiogenesis |
Development of arteries
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Dorsal aorta · Aortic arches
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Development of veins
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Cardinal veins · Ducts of Cuvier
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Fetal circulation
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umbilical cord: Umbilical vein → Ductus venosus → Inferior vena cava → Heart → Pulmonary artery → Ductus arteriosus → Umbilical artery
yolk sac: Vitelline veins · Vitelline arteries
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Heart development |
Primitive heart tube
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Truncus arteriosus · Bulbus cordis · Primitive ventricle · Primitive atrium · Sinus venosus
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Septa/ostia
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Endocardial cushions/Septum intermedium · Ostium primum · Septum primum (Ostium secundum) · Septum secundum (Foramen ovale) · Aorticopulmonary septum
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Other
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Atrial canal · Interventricular foramen
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anat(a:h,u,t,a,l,v:h,u,t,a,l)/phys/devp/cell/
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proc, drug(C2s/,C3,C4,C5,,C8,C9)
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proc, drug (C1A/1B/1C/), blte
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Prenatal development/Mammalian development of nervous system and integument (GA 9.733) |
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Neurogenesis |
General neural development/
neurulation/neurula
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Notochord · Neuroectoderm · Neural plate · Neural folds · Neural groove
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Neural crest
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Cranial neural crest · Trunk neural crest · Cardiac neural crest
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Neural tube
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Neuromere/Rhombomere, Cephalic flexure, Pontine flexure
Alar plate (sensory) · Basal plate (motor)
Glioblast · Neuroblast
Germinal matrix
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Integument |
Cutaneous structure development · Hair growth
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note: NS is mostly ectoderm, but mesoderm is precursor for epineurium, perineurium, and endoneurium
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anat(s,m,p,,e,b,d,c,,f,,g)/phys/devp/cell
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noco(m,d,e,h,v,s)/cong/tumr,sysi/,injr
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proc,drug(N1A/2AB/C/3/4/7A/B//)
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noco(i,,d,q,u,,p,,,v)/cong/tumr(n,e,d), sysi/
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Prenatal development/Mammalian development of eye and ear (GA 10.1002) |
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Eye development |
Neural tube: Optic vesicles · Optic stalk · Optic cup
Surface ectoderm: Lens placode
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Auditory development |
Auditory vesicle · Auditory pit
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anat(g, a, p)/phys/devp/cell/
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Prenatal development/Mammalian development of respiratory system (overview) (GA 11.1071) |
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Upper |
Nasal placode
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Lower |
Laryngotracheal groove · Lung buds
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anat(n, x, l, c)/phys/devp
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noco(c)/cong/tumr, sysi/, injr
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Prenatal development/Mammalian development of digestive system (GA 11.1101) |
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Gut |
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Stomodeum
Foregut: upper GI (Buccopharyngeal membrane, Rathke's pouch, Tracheoesophageal septum) · accessory (Pancreatic bud, Hepatic diverticulum)
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Midgut
Hindgut: Urorectal septum
Proctodeum
Cloacal membrane
Cloaca
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Abdominopelvic |
Mesentery
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Dorsal mesentery · Ventral mesentery
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Septum transversum
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anat(t, g, p)/phys/devp/cell/
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proc, drug(A2A/2B/3/4//6/7/14/16), blte
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Prenatal development/mammalian embryogenesis · Development of the urinary and reproductive organs (GA 11.1204) |
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Common urinary and
reproductive system |
Mesoderm → Intermediate mesoderm → Urogenital folds ("UF")
Mesoderm → Lateral plate mesoderm ("LPM")
Endoderm → Cloaca → Urogenital sinus ("US")
Endoderm+Ectoderm → Cloacal membrane
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Urinary system
development |
UF → Nephrogenic cord → Kidney
UF → Nephrotome → Pronephros · Mesonephros (Mesonephric tubules)
UF → WD → Ureteric bud + Metanephric blastema
US → Urinary bladder + Urethra + Urethral groove + Urachus
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Reproductive system
development |
Primarily internal
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UF → Gonadal ridge → Indifferent gonad → Sex cord (Cortical cords, Testis cords)
UF → Pronephric duct/Wolffian duct/mesonephric duct + Müllerian duct/paramesonephric ducts (Vaginal plate)
US → Prostate or Skene's gland
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Primarily external
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LPM → Genital tubercle → Labioscrotal folds → Scrotum or Labia majora
LPM → Genital tubercle → Phallus → Penis or Clitoris
IM → Gubernaculum
Peritoneum → Processus vaginalis or Canal of Nuck
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Homologues |
List of homologues of the human reproductive system
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noco/acba//tumr, sysi/, urte
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proc/itvp, drug (G4B), blte,
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proc/asst, drug (G1/G2B/G3CD)
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