Already have an account? Get multiple benefits of using own account!
Login in your account..!
Remember me
Don't have an account? Create your account in less than a minutes,
Forgot password? how can I recover my password now!
Enter right registered email to receive password!
Zone of Polarizing Activity (ZPA)
The A-P axis is the 1st axis to be fixed in the presumptive limb mesoderm at a very early stage. On the basis of results of a series of current experiments on chick embryos it has been recommended that determination of the anterior-posterior pattern of wing parts is apparently within the control of a small block of mesodermal cells located at the posterior junction of the wing bud along with the body wall. This mesoderm has been termed as the Zone of Polarizing Activity (ZPA). The usual chick wing has three digits numbered II, III and IV in this anterio- posterior order. In stage 18 or little older embryos an extra ZPA was transplanted within the ectoderm on the anterior side of the bud near or in contact with the AER. This resulted in the creation of a duplicate set of digits in reverse order on the anterior side so the digital formula anterio-posteriorly turns IV, II, II- II, III, IV.
The second set was a mirror image of the original. The normal polarity was kept even in this set as the usually the most posterior digit IV was still nearest to the transplanted ZPA. It was recommended that ZPA secretes some type of substance, a morphogen, whose diffusion creates a gradient of decreasing concentrations along with the A-P axis. The digit IV may be formed in response to the highest concentration and digits III and II in response to in succession decreasing levels of this morphogen. A second such gradient would be established in reverse direction by the morphogen produced by the ZPA transplanted on the anterior side and hence the second set of digits in mirror image of the original set the above suggested role of ZPA is not generally accepted by most biologists. Identical additional set of digits in chick wing is induced by transplantation of mesoderm from another body regions, several different tissues, transplantation of an additional AER and as well by applying retinoic acid (a derivative of Vitamin A) to the anterior side of the bud. Furthermore, removal of ZPA from the posterior side of the bud does not prevent the creation of the normal set of digits while in an experimental situation ZPA does produce a polarizing effect, this role of ZPA in the normal development whereas it is in its usual posterior position in the wing bud is seriously doubled.
What are the indications of implant removal? Failed implants need to be removed and the specific indications include: - Severe Perimplant bone loss (> 50 per cent implant le
What happens to a denatured enzyme regarding its functionality? How can that result be explained with the help of the lock and key model? According to the lock and key model th
Define Carbohydrates needs in Postoperative Nutritional Care? Carbohydrates ensure the use of protein for tissue synthesis and energy required for increased metabolic demands.
What is the digestive enzyme that acts within the stomach? Which type of food does it digest? What are the cells that produce that enzyme? The digestive enzyme that acts in the
Q How does mitosis participate in the growth of pluricellular organisms? All pluricellular beings grow with the raise in quantity of their cells. This raise is produced by mito
Define the Classifications of carbohydrates based on number of carbon? A chain-form monosaccharide that has a carbonyl group (C= O) on an end carbon forming an aldehy
Which of the following is a false statement regarding gel electrophoresis? A. An electric current is used in order move DNA fragments by a semi-porous gel. B. Fragments of n
What is the contraceptive mechanism of the IUD? The IUD (intrauterine device) is a piece of plastic coated with copper that is inserted within the uterus by a doctor. Copper is
Transparency - Aquatic Habitats Transparency affects the extent of light penetration. It is indirectly related to turbidity. Suspended particulate matters such as clay, silt a
1. Discuss how nursing has changed over the last two (2) decades. 2. Review and summarise the philosophy of your enrolled nurse course and its relevance to your clinical practic
Get guaranteed satisfaction & time on delivery in every assignment order you paid with us! We ensure premium quality solution document along with free turntin report!
whatsapp: +1-415-670-9521
Phone: +1-415-670-9521
Email: [email protected]
All rights reserved! Copyrights ©2019-2020 ExpertsMind IT Educational Pvt Ltd