Question

Describe in detail the development of the neural tube, early brain and spinal cord development in vertebrates complete with relevant signaling systems

Answer 1

Neurulation is the formation of the neural tube from the ectoderm of the embryo. It follows gastrulation in all vertebrates. ... After gastrulation, the notochord—a flexible, rod-shaped body that runs along the back of the embryo—has been formed from the mesoderm.

Embryonic stage

Neurulation

Neurulation is the formation of the neural tube from the ectoderm of the embryo. It follows gastrulation in all vertebrates.

During gastrulation cells migrate to the interior of the embryo, forming three germ layers— the endoderm (the deepest layer), mesoderm and ectoderm (the surface layer)—from which all tissues and organs will arise. In a simplified way, it can be said that the ectoderm gives rise to skin and nervous system, the endoderm to the guts and the mesoderm to the rest of the organs.

After gastrulation, the notochord—a flexible, rod-shaped body that runs along the back of the embryo—has been formed from the mesoderm. During the third week of gestation, the notochord sends chemical signals to the overlying ectoderm, inducing it to become neuroectoderm. This results in a strip of neuronal stem cells that runs along the back of the embryo. This strip is called the neural plate and is the origin of the entire nervous system. The neural plate folds outwards to form the neural groove. Beginning in the future neck region, the neural folds of this groove close to creating the neural tube (this form of neurulation is called primary neurulation). The ventral (front) part of the neural tube is called the basal plate; the dorsal (rear) part is called the alar plate. The hollow interior is called the neural canal. By the end of the fourth week of gestation, the open ends of the neural tube (the neuropores), close off.

Formation of the spinal cord

The spinal cord forms from the lower part of the neural tube. The wall of the neural tube consists of neuroepithelial cells, which differentiate into neuroblasts, forming the mantle layer (the grey matter). Nerve fibres emerge from these neuroblasts to form the marginal layer (the white matter).

The ventral part of the mantle layer (the basal plates) forms the motor areas of the spinal cord, whilst the dorsal part (the alar plates) forms the sensory areas. Between the basal and alar plates is an intermediate layer that contains neurons of the autonomic nervous system.^[2]

Formation of the brain

Late in the fourth week, the superior part of the neural tube flexes at the level of the future midbrain—the mesencephalon. Above the mesencephalon is the prosencephalon (future forebrain) and beneath it is the rhombencephalon (future hindbrain). The optical vesicle (which will eventually become the optic nerve, retina and iris) forms at the basal plate of the prosencephalon.

The embryo's nervous system at six weeks.

In the fifth week, the alar plate of the prosencephalon expands to form the cerebral hemispheres (the telencephalon). The basal plate becomes the diencephalon.

The diencephalon, mesencephalon and rhombencephalon constitute the brain stemof the embryo. It continues to flex at the mesencephalon. The rhombencephalon folds posteriorly, which causes its alar plate to flare and form the fourth ventricle of the brain. The pons and the cerebellum form in the upper part of the rhombencephalon, whilst the medulla oblongata forms in the lower part.

The embryo's brain at four weeks.

The part of the skull where the brain sits is called the cranium. The base, or lower part, of the brain is connected to the spinal cord. Together, the brain and spinal cord are known as the central nervous system (CNS). Many nerves send electrical signals to and from the brain and spinal cord.

Synapses of the mammalian CNS are asymmetric sites of cell-cell adhesion between nerve cells. They are designed to mediate the rapid and efficient transmission of signals from the presynaptic bouton of one neuron to the postsynaptic plasma membrane of a second neuron. Significant progress has been made in the characterization of the structural, functional and developmental assembly of CNS synapses. Recent progress has been made in understanding the molecular and cellular mechanisms that underlie synaptogenesis, in particular, that of glutamatergic synapses of the CNS. The multimeric protein complexes at CNS synapses. In recent years, numerous molecules have been identified that are likely to play fundamental roles in the assembly and maintenance of CNS synapses. A common theme emerging from these studies is that both the presynaptic active zone and PSD are built from several multi-molecular protein complexes.