Floor plate
Encyclopedia
The floor plate is a structure integral to the developing nervous system
of vertebrate organisms. Located on the ventral midline of the embryonic neural tube
, the floor plate is a specialized glial structure that spans the anteroposterior axis from the midbrain to the tail regions. It has been shown that the floor plate is conserved among vertebrates with homologous structures in invertebrates such as the fruit fly Drosophila and the nematode C. elegans. Functionally, the structure serves as an organizer to ventralize tissues in the embryo as well as to guide neuronal positioning and differentiation along the dorsoventral axis of the neural tube.
, an axial mesoderm derived signaling structure. This is supported experimentally in chick, in which floor plate induction, as well as associative ventral nervous tissue differentiation, is mediated by the secreted signaling molecule Sonic hedgehog
(Shh). Shh is expressed in a gradient with highest concentration localized in the notochord and floor plate. In vitro tissue grafting experiments show that removal of this molecule prevents differentiation of the floor plate, whereas its ectopic expression induces differentiation of floor plate cells. An alternative view proposes that neural tube floor plate cells stem from precursor cells which migrate directly from axial mesoderm. Through chick – quail hybrid experiments as well as genetic interaction experiments in zebrafish, it appears that notochord and floor plate cells originate from a common precursor. Furthermore, in zebrafish, Nodal
signaling is required for differentiation of medial floor plate cells whereas Shh is expendable. These data may indicate that the floor plate induction mechanism in amniotes and anamniotes differs. To reconcile these differences, a dual-mode induction model has been proposed in chick. In this model, exclusively ectodermal cells are induced to become medial floor plate during gastrulation
by prechordal mesoderm, possibly through Nodal signaling. Later in development during neurulation
, extended contact and interaction between notochord and fated floor plate cells causes differentiation, suggesting a cooperative effect between Nodal and Shh signaling.
The signaling molecules guiding the growth and projections of commissural neurons have well studied homologs in invertebrates. In the Netrin/DCC chemoattraction pathway the C. elegans homologs are Unc-6/Unc-40 and Unc-5 while the Drosophila homologs are Netrin-A and Netrin-B/Frazzled and Dunc5. In the Slit/Robo chemoreppelant pathway the C. elegans homologs are Slt-1/Sax-3 whereas the Drosophila homologs are also known as Slit/Robo(1-3).
Nervous system
The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...
of vertebrate organisms. Located on the ventral midline of the embryonic neural tube
Neural tube
In the developing vertebrate, the neural tube is the embryo's precursor to the central nervous system, which comprises the brain and spinal cord...
, the floor plate is a specialized glial structure that spans the anteroposterior axis from the midbrain to the tail regions. It has been shown that the floor plate is conserved among vertebrates with homologous structures in invertebrates such as the fruit fly Drosophila and the nematode C. elegans. Functionally, the structure serves as an organizer to ventralize tissues in the embryo as well as to guide neuronal positioning and differentiation along the dorsoventral axis of the neural tube.
Induction
Induction of the floor plate during embryogenesis of vertebrate embryos has been studied extensively in chick and zebrafish and occurs as a result of a complex signaling network among tissues, the details of which have yet to be fully refined. Currently there are several competing lines of thought. First, floor plate differentiation may be mediated by inductive signaling from the underlying notochordNotochord
The notochord is a flexible, rod-shaped body found in embryos of all chordates. It is composed of cells derived from the mesoderm and defines the primitive axis of the embryo. In some chordates, it persists throughout life as the main axial support of the body, while in most vertebrates it becomes...
, an axial mesoderm derived signaling structure. This is supported experimentally in chick, in which floor plate induction, as well as associative ventral nervous tissue differentiation, is mediated by the secreted signaling molecule Sonic hedgehog
Sonic hedgehog
Sonic hedgehog homolog is one of three proteins in the mammalian signaling pathway family called hedgehog, the others being desert hedgehog and Indian hedgehog . SHH is the best studied ligand of the hedgehog signaling pathway. It plays a key role in regulating vertebrate organogenesis, such as...
(Shh). Shh is expressed in a gradient with highest concentration localized in the notochord and floor plate. In vitro tissue grafting experiments show that removal of this molecule prevents differentiation of the floor plate, whereas its ectopic expression induces differentiation of floor plate cells. An alternative view proposes that neural tube floor plate cells stem from precursor cells which migrate directly from axial mesoderm. Through chick – quail hybrid experiments as well as genetic interaction experiments in zebrafish, it appears that notochord and floor plate cells originate from a common precursor. Furthermore, in zebrafish, Nodal
Nodal
Nodal can refer to* Nodal, a protein encoded by the gene NODAL and responsible for left-right asymmetry* Nodal , a novel music composition program...
signaling is required for differentiation of medial floor plate cells whereas Shh is expendable. These data may indicate that the floor plate induction mechanism in amniotes and anamniotes differs. To reconcile these differences, a dual-mode induction model has been proposed in chick. In this model, exclusively ectodermal cells are induced to become medial floor plate during gastrulation
Gastrulation
Gastrulation is a phase early in the embryonic development of most animals, during which the single-layered blastula is reorganized into a trilaminar structure known as the gastrula. These three germ layers are known as the ectoderm, mesoderm, and endoderm.Gastrulation takes place after cleavage...
by prechordal mesoderm, possibly through Nodal signaling. Later in development during neurulation
Neurulation
Neurulation is the stage of organogenesis in vertebrate embryos, during which the neural tube is transformed into the primitive structures that will later develop into the central nervous system....
, extended contact and interaction between notochord and fated floor plate cells causes differentiation, suggesting a cooperative effect between Nodal and Shh signaling.
Axon Guidance
In the development of the central nervous system, the decision of a neuron to cross or not cross the midline is critical. In vertebrates, this choice is mediated by the floor plate, and enables the embryo to develop successful left and right body halves with respect to nervous tissue. For example, while ipsilateral neurons do not cross the midline, commissural neurons cross the midline forming a single commissure. These particular neurons develop in the dorsal region of the neural tube and travel ventrally toward the floor plate. Upon reaching the floor plate, commissural neurons cross through the structure to emerge on the opposite side of the neural tube, whereupon they project anteriorly or posteriorly within the tube.- Netrins: Netrins are proteins expressed and secreted by cells of the floor plate. Experiments using floor plate extracts and commissural neurons embedded in a collegen matrix show attraction of neurons towards the floor plate in vitro. Moreover, Isolation and transfection of Netrin-1 and Netrin-2, two secreted proteins, into Cos cells has similar effects. Further research confirmed that Netrins act as attractant proteins in addition to Shh to guide commissural axons toward the floor plate. Netrins are secreted by the floor plate cells and function to bind the axon receptor DCC in a chemotactic manner.
- Slit: SlitSlitA slit is a vertical or horizontal space that can open and close. It can also refer to:* Slit , a vertical cut from the hem of the skirt which makes the skirt easier to walk in* Vagina, slang...
is a secreted ligand expressed in the floor plate and functions to inhibit axonal crossing of the neural tube. While netrins attract commissural neurons toward the midline, slit proteins repel and expel neurons from the midline. As axons not destined to cross the midline project through the neural tube they are repelled by the ligand slit which is expressed in the cells of the floor plate. Slit acts through its receptors Roundabout (Robo) 1 and 2. This interaction inhibits the chemotaxis provided by the Netrin/DCC pathway. However, Robo-3 (Rig-1) is upregulated during growth of commissural axons during migration toward the floor plate, which sequesters Robo-1/2 inside the cell within vesicles. Consequently, the Netrin/DCC attraction pathway dominates over the Slit/Robo repulsion pathway and the axon can grow toward the midline and enter the floor plate. Upon entering, through a mechanism not yet fully understood, Robo-3 becomes downregulated and this liberates and upregulates Robo-1/2, effectively repelling the neuron from the floor plate midline. Through this complex cross talk of Slit, Robo-1/2, and Robo-3, commissural axons are guided toward the midline to cross the neural tube and prevented from crossing back.
The signaling molecules guiding the growth and projections of commissural neurons have well studied homologs in invertebrates. In the Netrin/DCC chemoattraction pathway the C. elegans homologs are Unc-6/Unc-40 and Unc-5 while the Drosophila homologs are Netrin-A and Netrin-B/Frazzled and Dunc5. In the Slit/Robo chemoreppelant pathway the C. elegans homologs are Slt-1/Sax-3 whereas the Drosophila homologs are also known as Slit/Robo(1-3).