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Encyclopedia > Limb development

The vertebrate limb arises out of a general morphogenetic area called a limb field. The position of the limb field correlates with the anterior boundary of the Hoxc6 gene expression. Retinoic acid also plays a role in limb positioning. Blocking retinoic acid eliminates limb formation. Retinoic acid, or Retin-A or vitamin A acid, is a carotenoid organic compound that is a component of visual pigments. ...

Limb formation occurs out of a reciprocal interaction between the mesenchymal cells and the overlying ectodermal cells. Cells from the lateral plate mesoderm and the myotome migrate to the limb field and proliferate to create the limb bud. The lateral plate cells produce the skeletal portions of the limb while the myotome cells produce the muscle components. The lateral plate mesodermal cells express FGF10 and induce the apical ectodermal ridge (AER) cells to express FGF8 and FGF4 which maintains the FGF10 signal and proliferation in the mesoderm. The position of FGF10 expression is regulated by Wnt8c in the hindlimb and Wnt2b in the forelimb. The forelimb and the hindlimb are specified by their position along the anterior/posterior axis and possibly by two T-box containing transcription factors: Tbx5 and Tbx4 respectively. In embryology, the limb bud is a structure formed by the developing limb, derived from lateral plate mesoderm[citation needed]. It is intimately related with the apical ectodermal ridge, which secretes factors inducing the initial differentiation of the limb bud. ... There are very few or no other articles that link to this one. ...

Programmed cell death removes the spaces between the digits and joints. BMP signaling induces cell death and Noggin block cell death in the digits.


Proximal/Distal Patterning

The limb is made up of three sections: stylopod, zeugopod, and autopod (in order from proximal to distal). Hox genes contribute to the specification of these segments. Mutations in Hox genes lead to proximal/distal losses or abnormalities[1]. There are two competing models for explaining the patterning of these sections.

Progress Zone Model

The AER creates a zone of cell proliferation and lays down the limb from proximal to distal. The time cells leave the AER determines their positional value. Proximal structures are formed earlier than distal structures.

The Progress Zone Model was proposed 30 years ago but recent evidence has conflicted with this model.

Experimental evidence:

  • Removing the AER at a later period of development results in less disruption of distal structures than if the AER was removed early in development.
  • Grafting a new bud tip on top of an old bud tip results in a deletion and duplication of structures.

Early Allocation and Progenitor Expansion Model or Prespecification Model

Cells are specified for each segment in the early limb bud and this population of cells expand out as the limb bud grows. This model is consistent with the following observations. Cell division is seen throughout the limb bud. Cell death occurs within a 200mm zone adjacent to the AER when removed--cell death removes some patterning. FGF beads are able to rescue limb development by preventing cell death.

Experimental evidence:

  • Labeled cells in different position of an early limb bud were restricted to single segments of the limb[2].
  • Limbs lacking expression of required FGF4 & FGF8 showed all structures of the limb and not just the proximal parts[3].

Anterior/Posterior Patterning

The Zone of Polarizing Activity (ZPA) in the limb bud has pattern-organizing activity by action of a morphogen gradient of Sonic hedgehog (Shh). Shh is both sufficient and necessary to create the ZPA and specify the anterior/posterior pattern in the distal limb (Shh is not necessary for the polarity of the stylopod). Shh is turned on in the posterior through the early expression of Hoxd genes, the expression of Hoxb8, and the expression dHAND. Shh is maintained in the posterior though a feedback loop between the ZPA and the AER. Shh induces the AER to produce FGF4 and FGF8 which maintains the expression of Shh. Sonic hedgehog homolog (SHH) is one of five proteins in the vertebrate hedgehog (HH) family thus far described, the others being desert hedgehog (DHH), Indian hedgehog (IHH), echidna hedgehog (EHH) and tiggywinkle hedgehog (TwHH). ...

Digits 3,4 and 5 are specified by a temporal gradient of Shh. Digit 2 is specified by a long-range diffusible form of Shh and Digit 1 does not require Shh. Shh cleaves the Ci/Gli3 transcriptional repressor complex to convert the transcription factor Gli3 to an activator which activates the transcription of HoxD genes along the anterior/posterior axis. Loss of the Gli3 repressor leads to the formation of generic (unpatterned) digits in extra quantities[4].

Dorsal/Ventral Patterning

Dorsal/Ventral Patterning arises from Wnt7a signals in the overlying ectoderm not the mesoderm. Wnt7a is both necessary and sufficient to dorsalize the limb. Wnt7a also influences the anterior/posterior axis and loss of Wnt7a causes the dorsal side of limbs to become ventral sides and causes missing posterior digits. Replacing Wnt7a signals rescues this defect. Wnt7a is also required to maintain expression of Shh.


  1. ^ Wellik D, Capecchi M (2003). "Hox10 and Hox11 genes are required to globally pattern the mammalian skeleton.". Science 301 (5631): 363-7. PMID 12869760.
  2. ^ Dudley A, Ros M, Tabin C (2002). "A re-examination of proximodistal patterning during vertebrate limb development.". Nature 418 (6897): 539-44. PMID 12152081.
  3. ^ Sun X, Mariani F, Martin G (2002). "Functions of FGF signalling from the apical ectodermal ridge in limb development.". Nature 418 (6897): 501-8. PMID 12152071.
  4. ^ Chiang C, Litingtung Y, Harris M, Simandl B, Li Y, Beachy P, Fallon J (2001). "Manifestation of the limb prepattern: limb development in the absence of sonic hedgehog function.". Dev Biol 236 (2): 421-35. PMID 11476582.
Mammalian development of embryo and development and fetus (some dates are approximate - see Carnegie stages) - edit - discuss

Week 1: Zygote | Morula | Blastula/Blastomere/Blastosphere | Archenteron/Primitive streak | Blastopore | Allantois | Trophoblast (Cytotrophoblast | Syncytiotrophoblast | Gestational sac) Mammalian embryogenesis is the process of cell division and cellular differentiation which leads to the development of a mammalian embryo. ... Bold text--203. ... This article or section is in need of attention from an expert on the subject. ... Fetal (U.S. English; Foetal UK English) development is the process in which a fetus (U.S. English; Foetus UK English) develops during gestation, from the times of conception until birth. ... A human fetus A fetus (or foetus, or fœtus – see below) is a developing mammal after the embryonic stage and before birth. ... In embryology, Carnegie stages are a standardized system of 23 stages used to provide a unified developmental chronology of the embryo. ... This article or section does not cite its references or sources. ... Morula is a stage of embryonic development in animals, including the 16-cell phase, the 32-cell phase, and the 64-cell phase. ... Blastulation. ... A blastomere is the structure which results from the divisions of a fertilised egg during embryonic development . ... Blastulation. ... The archenteron is an indentation that forms early on in a developing blastula. ... The primitive streak is a structure that forms during the early stages of avian, reptilian and mammalian embryonic development. ... A blastopore is an opening into the archenteron during the embryonic stages of an organism. ... Allantois is a part of a developing animal embryo. ... The trophoblast (from Greek threphein: to feed) is considered to be the first of all embryonic annexes. ... The introduction to this article provides insufficient context for those unfamiliar with the subject matter. ... Syncytiotrophoblasts are cells found in the placenta of human embryos. ... The gestational sac is the only available intrauterine structure that can be used to determine if an intrauterine pregnancy (IUP) exists, until the embryo is identified. ...

Week 2: Yolk sac | Vitelline duct | Bilaminar disc The yolk sac is the first element seen in the gestational sac during pregnancy, usually at 5 weeks gestation. ... At the end of the fourth week the yolk-sac presents the appearance of a small pear-shaped vesicle (umbilical vesicle) opening into the digestive tube by a long narrow tube, the vitelline duct. ... This article, image, template or category should belong in one or more categories. ...

Week 3: Hensen's node | Gastrula/Gastrulation | Trilaminar embryo Branchial arch (1st) | Branchial pouch | Meckel's cartilage | Somite/Somitomere | Germ layer (Ectoderm, Endoderm, Mesoderm, Chordamesoderm, Paraxial mesoderm, Intermediate mesoderm, Lateral plate mesoderm) Hensens Node is the organizer for gastrulation in birds. ... 1 - blastula, 2 - gastrula; orange - ectoderm, red - endoderm. ... 1 - blastula, 2 - gastrula; orange - ectoderm, red - endoderm. ... It has been suggested that this article or section be merged with Embryogenesis. ... In the development of vertebrate animals, the branchial arches (or pharyngeal arches) develop during the fourth and fifth week in utero as a series of mesodermal outpouchings on the left and right sides of the developing pharynx. ... It has been suggested that this article or section be merged into branchial arch. ... In the development of vertebrate animals, the branchial arches (or pharyngeal arches) develop during the fourth and fifth week in utero as a series of mesodermal outpouchings on the left and right sides of the developing pharynx. ... The cartilaginous bar of the mandibular arch is formed by what are known as Meckel’s cartilages (right and left) ; above this the incus is developed. ... In the developing vertebrate embryo, somites are masses of mesoderm distributed along the two sides of the neural tube and that will eventually become dermis (dermatome), skeletal muscle (myotome), and vertebrae (sclerotome). ... In the developing vertebrate embryo, the somitomeres are loose masses of paraxial mesoderm derived cells that form along each side of the neural tube towards the end of the third gestational week. ... It has been suggested that organogenesis be merged into this article or section. ... The ectoderm is outermost of the three germ layers of the developing embryo, the other two being the mesoderm and the endoderm. ... The endoderm, sometimes refered to as entoderm, is one of the three germ layers of the developing embryo, the other two being the ectoderm and the mesoderm. ... The mesoderm is one of the three germ layers in the early developing embryo, the other two layers being the ectoderm and the endoderm. ... Chordamesoderm is a type of mesoderm that lies along the central axis, under the neural tube. ... Paraxial mesoderm is the area of mesoderm that forms just lateral to the neural tube on both sides. ... Intermediate mesoderm is a type of mesoderm that is located between the paraxial mesoderm and the lateral plate. ... Lateral plate mesoderm (or hypomere) is a type of mesoderm that is found at the periphery of the embryo. ...

Histogenesis and Organogenesis

Circulatory system: Primitive atrium | Primitive ventricle | Bulbus cordis | Truncus arteriosus | Ostium primum | Foramen ovale | Ductus venosus | Ductus arteriosus | Aortic arches | Septum primum | Septum secundum | Cardinal veins An Introduction to Histogenesis Histogenesis is defined as the formation of tissues and organs from undifferentiated cells (Encarta Dictionary). ... Organogenesis is a stage of animal development where the ectoderm, endoderm, and mesoderm are formed. ... The Circulatory System is a Psychedelic Rock musical ensemble formed by musician/painter Will Cullen Hart, and featuring Hannah Jones , Derek Almstead , Peter Erchick , John Fernandes , and Heather McIntosh. ... The primitive atrium grows rapidly and partially encircles the bulbus cordis; the groove against which the bulbus cordis lies is the first indication of a division into right and left atria. ... The primitive ventricle becomes divided by a septum, the septum inferius or ventricular septum, which grows upward from the lower part of the ventricle, its position being indicated on the surface of the heart by a furrow. ... When the heart assumes its S-shaped form the bulbus cordis lies ventral to and in front of the primitive ventricle. ... For the medical condition with the same name, see Truncus arteriosus. ... In the developing heart, for a time the atria communicate with each other by an opening, the ostium primum of Born (interatrial foramen primum), below the free margin of the septum. ... In the fetal heart, the foramen ovale allows blood to enter the left atrium from the right atrium. ... In the fetus, the ductus venosus connects the left umbilical vein with the upper inferior vena cava. ... In the developing fetus, the ductus arteriosus (DA) is a shunt connecting the pulmonary artery to the aortic arch that allows much of the blood from the right ventricle to bypass the fetus fluid-filled lungs. ... This article focuses upon the multiple aortic arches present in the embryo. ... The cavity of the primitive atrium becomes subdivided into right and left chambers by a septum, the septum primum, which grows downward into the cavity. ... The septum secundum, semilunar in shape, grows downward from the upper wall of the atrium immediately to the right of the primary septum and foramen ovale. ... During development of the veins, the first indication of a parietal system consists in the appearance of two short transverse veins, the ducts of Cuvier, which open, one on either side, into the sinus venosus. ...

Nervous system: Neural development/Neurulation | Neurula | Neural folds | Neural groove | Neural tube | Neural crest | Neuromere (Rhombomere) | Notochord | Optic vesicles | Optic stalk | Optic cup The nervous system of an animal coordinates the activity of the muscles, monitors the organs, constructs and also stops input from the senses, and initiates actions. ... The study of neural development draws on both neuroscience and developmental biology to describe the cellular and molecular mechanisms by which complex nervous systems emerge during embryonic development and throughout life. ... Neurulation is a part of organogenesis in vertebrate embryos. ... A Neurula is an embryo at the early stage of development in which neurulation occurs. ... In front of the primitive streak two longitudinal ridges, caused by a folding up of the ectoderm, make their appearance, one on either side of the middle line. ... Between the neural folds is a shallow median groove, the neural groove. ... The neural tube is the embryonal structure that gives rise to the brain and spinal cord. ... The neural crest, a component of the ectoderm, is one of several ridgelike clusters of cells found on either side of the neural tube in vertebrate embryos. ... Neuromeres are transient segments during the early development of the human brain. ... In the vertebrate embryo, a rhombomere is a segment of the developing rhombencephalon. ... The notochord is a flexible, rod-shaped body found in embryos of all chordates. ... The eyes begin to develop as a pair of diverticula from the lateral aspects of the forebrain. ... The optic vesicles project toward the sides of the head, and the peripheral part of each expands to form a hollow bulb, while the proximal part remains narrow and constitutes the optic stalk. ... The outer wall of the bulb of the optic vesicles becomes thickened and invaginated, and the bulb is thus converted into a cup, the optic cup (or ophthalmic cup), consisting of two strata of cells). ...

Digestive system: Foregut | Midgut | Hindgut | Proctodeum | Rathke's pouch | Septum transversum For the Physics term GUT, please refer to Grand unification theory The gastrointestinal or digestive tract, also referred to as the GI tract or the alimentary canal or the gut, is the system of organs within multicellular animals which takes in food, digests it to extract energy and nutrients, and... The foregut is the anterior part of the alimentary canal, from the mouth to the intestine, or to the entrance of the bile duct. ... The midgut is the portion of the embryo from which most of the intestines are derived. ... Hindgut is the posterior (caudal) part of the alimentary canal. ... A proctoduem is the back ectodermal part of an alimentary canal. ... The introduction to this article provides insufficient context for those unfamiliar with the subject matter. ... The liver arises in the form of a diverticulum or hollow outgrowth from the ventral surface of that portion of the gut which afterward becomes the descending part of the duodenum. ...

Urinary/Reproductive system: Urogenital folds | Urethral groove | Urogenital sinus | Kidney development (Pronephros | Mesonephros | Ureteric bud | Metanephric blastema) | Fetal genital development (Wolffian duct | Müllerian duct | Gubernaculum | Labioscrotal folds) The urinary system is the organ system that produces, stores, and carries urine. ... It has been suggested that this article or section be merged into sex organ. ... The urogenital folds are an embryological structure which give rise to a portion of the external genitalia. ... The urethral groove is a temporary linear indentation on the underside (ventral side) of the male penis during embryonic development. ... The urogenital sinus (also known as the persistent cloaca) is a part of the human body while it is an embryo. ... In humans, the metanephros (adult kidney) begins as an aggregate of mesenchymal cells that are detectable by the fifth gestational week as two small areas in the intermediate mesoderm close to the pelvic aorta. ... Pronephros the most primitive of the three excretory organs that develop in vertebrate, corresponding to the first stage of kidney development. ... The mesonephros (Latin for middle kidney) is one of three excretory organs that develop in vertebrates. ... The Ureteric bud is a portion of the mesonephric duct. ... The Metanephric blastema (or metanephric mesenchyme) is one of the two embryological structures that give rise to the kidney (the other is the ureteric bud. ... This article or section is in need of attention from an expert on the subject. ... The Wolffian duct (also known as archinephric duct, Leydigs duct, and the mesonephric duct) is an paired organ found in mammals including humans during embryogenesis. ... The Müllerian ducts are paired ducts of the embryo which empty into the cloaca, and which in the female develop into the upper vagina, cervix, uterus and oviducts; in the male they disappear except for the vestigial vagina masculina and the appendix testis. ... The gubernaculum is a fold of peritoneum which attaches to the caudal end of the testes. ... The labioscrotal folds (or labioscrotal swellings) are paired embryonic structures that represent the final stage of development of the caudal end of the external genitals before sexual differentiation. ...

Glands: Thyroglossal duct A gland is an organ in an animals body that synthesizes a substance for release such as hormones, often into the bloodstream (endocrine gland) or into cavities inside the body or its outer surface (exocrine gland). ... The thyroglossal duct is an embryological anatomical structure which forms the connection between the initial area of development of the thyroid gland and its final position. ...

Uterine support: Placenta | Umbilical cord (Umbilical artery, Umbilical vein, Wharton's jelly) | Amniotic sac (Amnion, Chorion) The uterus or womb is the major female reproductive organ of most mammals, including humans. ... The placenta is an ephemeral (temporary) organ present only in female placental vertebrates during gestation (pregnancy). ... A newborn at 45 seconds. ... Umbilical arteries carry deoxygenated blood from the fetus to the placenta in the umbilical cord. ... Fetal circulation; the umbilical vein is the large, red vessel at the far left The umbilical vein is a blood vessel present during fetal development that carries oxygenated blood from the placenta to the growing fetus. ... Whartons jelly is a gelatinous substance within the umbilical cord. ... Amnion. ... This article or section may be confusing or unclear for some readers, and should be edited to rectify this. ... Chorion can refer to the following things: Chorion is the outer membrane of the amniotic sac. ...

Limb development: Limb bud | Apical Ectodermal Ridge/AER In embryology, the limb bud is a structure formed by the developing limb, derived from lateral plate mesoderm[citation needed]. It is intimately related with the apical ectodermal ridge, which secretes factors inducing the initial differentiation of the limb bud. ... There are very few or no other articles that link to this one. ...



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