Neurulation – advanced germ layers

Structure

The neural tube consists of a thick, pseudostratified neuroepithelium with crowded nuclei around a slit-like lumen (the future central canal or ventricle). Ventral to it lies the notochord, a round rod of large, vacuolated cells. Lateral to the neural tube is the paraxial mesoderm, which segments into somites (sclerotome, myotome, dermatome). Further laterally is the narrow intermediate mesoderm, which gives rise to the mesonephros and Wolffian duct. Outermost lies the lateral plate mesoderm, which splits into a somatic and a splanchnic layer; the space between is the intraembryonic coelom. Ectoderm covers the embryo on the outside, and endoderm lines the gut tube.

Function

The notochord is an important signalling centre: among other factors it secretes Sonic hedgehog and thereby induces the neural plate, the ventral floor plate of the neural tube and the sclerotome. The neural tube gives rise to the CNS and retina, the neural folds to the neural crest (spinal ganglia, autonomic ganglia, melanocytes, adrenal medulla). The sclerotome forms the vertebrae, the myotome skeletal muscle and the dermatome the dermis of the back. The somatic layer of the lateral plate forms the body wall and limbs, the splanchnic layer the gut wall musculature and heart. The coelom gives rise to the body cavities.

Microscopy tips

At low magnification first look for the neural tube: the most conspicuous, darkest structure with a thick epithelium and narrow lumen. Directly ventral to it you will find the small, round notochord with pale vacuoles – it defines the midline and the orientation of the section. The specimen is often embedded obliquely or rotated, so always orient yourself by the neural tube and notochord, not by the edge of the image. Laterally follow loose mesenchyme and the mesodermal plates, and on the outside thin extraembryonic membranes (amnion, chorion). Blood vessels can be recognised by nucleated erythrocytes, which in birds remain nucleated even in adults.

Clinical relevance

If the neural tube does not close completely, neural tube defects result: caudally spina bifida (up to myelomeningocele), cranially anencephaly. In humans closure occurs as early as the 4th embryonic week, often before the pregnancy is noticed. Folic acid supplementation before conception markedly reduces the risk. Raised alpha-fetoprotein in maternal serum or amniotic fluid may indicate an open defect; notochordal remnants can rarely give rise to a chordoma.

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