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 is the concept of minimum and maximum way of tolerance
What is Cellular Respiration? Cellular Respiration : The most important of the metabolic processes, cellular respiration, provides energy for most chemical reactions in an or
what is fluid mosaic model?
Feline panleukopenia It is arguably the most important of all feline viral diseases and all members of the family Felidae are susceptible. This disease is commonly found in kitten
what is holozoic, holophytic, saprozoic nutrition
Define Etiology - Anorexia Nervosa? The exact cause of eating disorders is not known. It is multi factorial in origin in which the personality of the patient, family relationsh
#question.name any two plants having heterotrophic mode of nutrition .
L A W S OF CLEAVAGE - 1. Sac's Law - According this law, during cleavage each blastomere to form similar blastomere. New cleavae always take place at the right angle of th
Define some Usual Doubts of Children related to Food? Before embarking yourself to go through this unit, here is an activity for you to perform, Talk to 2-3 school' boys and gi
Calcium The calcium content in forages is affected by species, plant part (leaf versus stem), plant maturity, soil content of calcium and climate. Forages are usually a good so
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: +91-977-207-8620
Phone: +91-977-207-8620
Email: [email protected]
All rights reserved! Copyrights ©2019-2020 ExpertsMind IT Educational Pvt Ltd