Study of Sternum and Keel Development via Chick Embryology

The sternum is a critical bone in all terrestrial vertebrates that provides attachment for ribs via cartilage, helps protect vital organs, and provides an area for pectoral muscle attachment. In quadruped organisms, the pectoral muscles play a vital role in lifting the body from the ground, while in bipeds these muscles play a vital role in the movements of the forelimb such as flapping. The more pectoral muscle attachment there is to the sternum translates to a more effective forelimb. For this reason some organisms have adapted sternums to allow for more muscle attachment.  An example of this is a structure that protrudes out of the sternum known as the keel. This keel is an evolutionary adaptation which certain organisms possess (most evident in birds) that provides more surface area for the pectoral muscles (which are critical in a birds ability to fly) to attachment. The size of the keel plays a vital role in the type of locomotion the bird is capable of. Birds such as hummingbirds that are capable of rapid wing movements and possess the ability to fly forward and backwards have rather massive keels while flightless birds such as emus have small keels leaving them unable to fly. Since the sternum is genetically conserved in vertebrates it becomes important to understand the mechanisms of its development in order to help combat sternal deficiencies that negatively affect humans. For this analysis chick embryos are used as a model organism and the development of the sternum and keel is closely monitored. The eggs are held in a 40°C incubator and using Hamilton’s (1951) classic series of normal stages in development of the chick embryo we determine the stage of the chick embryo. They are harvested at various stages leading up to stage 40 (day 14), the thorax is removed, and is stained by Alician blue. These stained thoraxes are then placed in a glycerol series for suspension and a picture is taken at every stage. With this technique the critical stages of sternum development and keel formation will be determined. With preliminary results, it seems that the sternum is full developed at stage 35 day (8-9) and the keel protrudes out at stage 36 (day 10). Simultaneously, an experimental batch of embryos have their limb buds removed at stage 20 (day 3) and the development of the sternum is also examined using the same staining technique. This procedure is necessary in analyzing the development of the sternum since the sternum itself is a derivative of the limbs. This part of the experiment leads to the answer of the question, how does the lack of the precursor of the limb musculature affect sternum development?