Another factor that may limit the overall growth of the avian embryo is the possibility that the growth of critical individual organs may limit the overall growth of the embryo. The upper limit of growth of any of several tissues could set upper limits to the growth of the embryo as a whole. A good example of such an organ would be the brain and nervous system, which are some of the initial tissues formed early in embryogenesis and, relative to other tissues, grow to immense size during the embryonic period relative to the body and other tissues.[2,4] Lilja and Olsson divided organs by their supply and demand functions. Demand tissue growth (breast, wings, legs, and feathers) compared to supply tissue growth (esophagus, proventriculus, gizzard, intestines, heart, and liver) is accelerated in avian embryos whose parents have been selected for rapid growth. Thus, growth of embryonic organs in larger eggs from larger adults may be disproportional and lead to overall growth-limiting functions.
The maturation of organs into functional entities may also limit overall embryonic growth. Glycogen acquisition by individual organs is a requisite process to survive the plateau stage in oxygen consumption. Organs must have adequate nutrients for anaerobic metabolism to support them through the hypoxia and hypercapnia of hatching. Selection for reproduction has decreased the amount of glycogen found in vital tissues compared to unselected controls. Similarly, selection for rapid growth has also depressed the acquisition of glycogen prior to the plateau stage in oxygen uptake. Thus, the ability of individual organs to continue their maturation and function late in development may be impaired by the lack of nutrients for vital organs.
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