Localized mRNAs are critical early in embryogenesis to establish axes, and have been identified via molecular screening in frogs and genetic studies in Drosophila. Region Genes/Proteins Involved Effect Anterior/Posterior (A/P) Axis Bcd & Nos Hb, others Termini Torso Receptor Signal transduction cascade → Tailless, Huckebein (zygotic) Dorsal/Ventral (D/V) Axis Toll receptor Signal transduction cascade → [...]

The brain develops from the neural tube. Before even fully developing at the posterior, the neural tube (initially straight) begins to form bulges at the anterior that will give rise to the brain. First, three primary vesicles are formed; starting at the most anterior, these are the forebrain (prosencephalon), midbrain (mesencephalon) and hindbrain (rhombencephalon). Many [...]

Similar to the tracheal and hepatic buds and their branches, the peancreatic bud is an evagination of the endoderm which branches profusely to form the liver. This branched endodermal endothelium differentiates into two types of glandular tissue: exocrine glands, which produce essential digestive enzymes that are secreted into the small intestine; and endocrine glands (aka [...]

Similar to the tracheal and pancreatic buds and their branches, the hepatic (liver) bud is an evagination of the endoderm which branches profusely to form the liver.

The lung bud (aka tracheal bud) evaginates from the ventral foregut endoderm. The lung bud bifurcates (branches out) to form the two (left and right) bronchial buds. The bronchial buds bifurcate repeatedly to form a huge surface area. Each bronchiolus (small, individual branch) ends in an a small sac-like alveolus. Mature alveoli are surrounded by [...]

1st Induction Specific positions along the anterior-posterior axis of the archenteron (cells arising from the organizer to form dorsal mesoderm and endoderm) induce specific regions of the brain to form. For the developing eye, the diencephalon is induced along with its outpatching, the optic vesicle. 2nd Induction The optic vesicle induces the lens placode in [...]

An axon often has to navigate great distance to reach its target cell (ie, another neuron or muscle fiber) and establish the synaptic connection. This process is axonal pathfinding. The axon does not navigate its natural pathway due to any single cue, but via combinations of signals. This redundancy makes axonal pathfinding extremely complex. Axonal [...]