Final answer:
A morphogen is a signaling molecule that guides embryonic cells to form specific tissues and organ patterns through its concentration gradient. Bicoid in fruit flies and Sonic hedgehog (Shh) in vertebrates are two examples demonstrating how morphogens direct pattern formation. Hox genes also play a critical role in embryonic development by determining the positioning of body segments.
Step-by-step explanation:
A morphogen is a signaling molecule that emanates from a localized source and forms a concentration gradient across a developing tissue. The concentration of the morphogen provides positional information to cells, effectively telling them what type of cell to become during the process of embryonic development. This process, termed pattern formation, is critical for the correct development of tissues and organs.
One classic example of a morphogen at work is the protein Bicoid in fruit flies (Drosophila melanogaster). Bicoid is crucial in establishing the anterior-posterior axis of the embryo. In the early embryo, Bicoid is more concentrated at what will become the head end and less concentrated towards the tail end. Cells in the high-concentration region will develop into head structures, whereas those at lower concentration levels will form other parts of the body.
Another well-known morphogen is Sonic hedgehog (Shh), which is important in vertebrates. Shh gradient influences the patterning of the neural tube, the precursor to the central nervous system. Cells exposed to higher levels of Shh typically become different types of neurons, while those in lower concentrations can become structures such as the vertebrae and muscles.
Hox genes are another important aspect of animal development. These regulatory genes are critical in positioning body segments in an embryo. For example, a mutation in a Hox gene in a fruit fly can cause a leg to grow where an antenna should be, indicating a disruption in the normal pattern formation directed by Hox genes.