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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:
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.
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. 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. 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.
Ectoderm: Surface ectoderm - Neuroectoderm - Somatopleure - Neurulation - Neural crest
Endoderm: SplanchnopleureMesoderm: Chorda- - Paraxial (Somite/Somitomere/Sclerotome/Myotome/Dermatome) - Intermediate - Lateral plate (Intraembryonic coelom, Splanchnopleure/Somatopleure)
|Extraembryonic/uterus||Trophoblast (Cytotrophoblast, Syncytiotrophoblast) Decidua (Decidual cells) - Chorionic villi/Intervillous space - Gestational sac (Amnion/Amniotic sac/Amniotic cavity, Chorion)|
|Histogenesis||Programmed cell death - Stem cells - Germ line development|
|Organogenesis||Limb development: Limb bud - Apical ectodermal ridge/AER - Eye development - Cutaneous structure development - Heart development - Development of the urinary and reproductive organs|
Prenatal development/Mammalian development of digestive system
Foregut (Buccopharyngeal membrane, Rathke's pouch, Tracheoesophageal septum, Pancreatic bud, Hepatic diverticulum)
Hindgut (Urorectal septum )Proctodeum
|Mesentery||Dorsal mesentery - Ventral mesentery|
Prenatal development/mammalian embryogenesis - Development of the urinary and reproductive organs
|General Urinary/Reproductive system||Cloacal membrane - Cloaca - Urethral groove - Urogenital sinus - Urachus - Urogenital folds|
|Kidney development||Nephrogenic cord - Nephrotome - Pronephros - Mesonephros (Mesonephric tubules)|
Ureteric bud - Metanephric blastema
|Fetal genital development - primarily internal||Gonadal ridge - Sex cord (Cortical cords, Testis cords)|
Pronephric duct/Wolffian duct/mesonephric duct - Müllerian duct/paramesonephric ducts (Vaginal plate)
|Primarily external||Genital tubercle - Phallus - Labioscrotal folds|
Gubernaculum - Processus vaginalis
|see also list of homologues of the human reproductive system|
Prenatal development/Mammalian development of nervous system
|General neural development/|
|Notochord - Neuroectoderm - Neural plate - Neural folds - Neural groove
Neural crest - Neural tube (Neuromere/Rhombomere, Cephalic flexure)Alar plate - Basal plate
|Eye development||Optic vesicles - Optic stalk - Optic cup - Lens placode|
|Auditory development||Auditory vesicle - Auditory pit|
Prenatal development/Mammalian development of circulatory system
|Vascular||Blood island Umbilical vein|
|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
Prenatal development/Mammalian development of respiratory system (overview)
|Lower||Laryngotracheal groove - Lung buds|
Human development of head and neck
|Branchial region||Branchial arch (1st/Meckel's cartilage, 2nd)/Branchial pouch - Cervical sinus - Lateral lingual swelling - Tuberculum impar - Frontonasal prominence - Olfactory pit - Nasal processes - Globular process - Primitive palate - Maxillary prominence|
|Tooth development||Dental papilla - Odontoblast - Ameloblast|