Organogenesis and Vertebrate Axis Formation

Core Concepts

Neurulation: Building the Nervous System

Organogenesis begins with neurulation, the formation of the neural tube. A strip of dorsal ectoderm thickens into the neural plate in response to signals from the underlying mesoderm. The plate then folds upward at its edges, and the folds fuse along the midline, rolling the plate into a tube that sinks beneath the surface ectoderm. The neural tube will become the brain and spinal cord. Some cells at the crest of the folds — the neural crest cells — do not join the tube; instead they migrate throughout the embryo and give rise to the peripheral nervous system, pigment cells, and parts of the face and heart (a commonly taught fate map).

The Notochord and Somites: Scaffolding and Segmentation

The notochord is a rod of mesodermal cells lying beneath the neural tube. It has two jobs: it is a signaling center that induces the overlying ectoderm to form the neural plate, and it provides a temporary stiffening scaffold along the embryo's midline, later replaced by the vertebral column in most vertebrates. On either side of the notochord, the paraxial mesoderm segments into blocks called somites, which appear in sequence from head to tail. Each somite differentiates into the vertebrae and ribs, the skeletal muscle of the body wall and limbs, and the dermis of the back — a repeating segmental plan visible in the ribs and spinal nerves.

Establishing the Body Axes

A vertebrate is organized along three axes: anterior–posterior (head to tail), dorsal–ventral (back to belly), and left–right. Anterior–posterior identity is largely specified by Hox genes, a family of transcription-factor genes expressed in overlapping patterns along the body; the combination of Hox genes active in a segment tells it what to become. In the classic teaching example, a fruit fly with a mutated Hox gene grows legs where antennae should be. Dorsal–ventral patterning of the neural tube depends on signals such as Sonic hedgehog (Shh) from the notochord and floor plate, opposed by signals from the dorsal region; graded concentrations assign different neuron types to different levels. Left–right asymmetry is set up by a signaling cascade that ultimately positions the heart and other organs on their correct sides.

How Scientists Learned This: Organizer Experiments

Much of what we know about axis formation comes from classic experiments with amphibian embryos. In the 1920s, Hans Spemann and Hilde Mangold transplanted the dorsal lip of the blastopore — the site where gastrulation begins — from one newt embryo into the belly region of another. The transplant not only survived but induced the host tissue around it to form a second, complete body axis, producing conjoined twins. This showed that the dorsal lip acts as an organizer, emitting signals that instruct neighboring cells to build an embryo. The experiment is a landmark example of how a small group of cells can organize an entire body plan and earned Spemann a Nobel Prize.

Common Confusions