Embryology - Biology 104, Spring 2006 - Albert Harris and Corey Johnson

 

OUTLINE OF FOURTEENTH LECTURE: Feb 17, 2006, by Corey Johnson

Neurulation and axial structures II

Dorsal Ventral specialization of the brain

The dorsal region is known as the alar plate. It gives rise to the sensory component of the CNS. Mostly interneurons. The actual sensory neurons have their cell bodies in the (neural crest-derived) dorsal root ganglia. The ventral region is the basal plate. Motor neurons are located here. The very bottom is the floor plate and contains some cells of the notochord.

How do the neural cells acquire their identity?

There are a number of distinct cell types in the spinal chord. Roof plate, floor plate, and 11 distinct zones in between! Each is characterized by a distinct transcription factor.

If the notochord is removed, the neural tube expresses only dorsal transcription factors. No motor neurons develop. The notochord signals to the base of the neural tube, a region called the floorplate. The floorplate continues the signal that makes the neural tube have a ventral characteristic. This signal is Sonic Hedgehog (Shh). If an extra notochord is grafted to the side of the neural tube, a second region of Shh is expressed on the side of the tube and a second floorplate develops.

The overlying ectoderm expresses 'bone morphogenetic protein' (BMP) that induces further expression of BMP in the roofplate. So, you have a dorsal signal from the ectoderm and a ventral signal from the notochord which produces distinct dorsal, ventral, and intermediate regions. Cultured intermediate regions can be induced to form dorsal neurons if ectoderm (or BMP) is added. If cultured with notochord (or Shh) it will form ventral neurons.

It was thought that opposing gradients of Shh and BMP might account for the array of cell types. This is partially true, but there's an intermediate signal (Wnt) that emanates from the roofplate and coordinates the functions of proliferation and differentiation.

Anterior-posterior (cranial-caudal) regional specialization of the neural tube

Three main divisions that become five divisions. From anterior to posterior:

Forebrain -

    Prosencephalon
    Telencephalon
    Dielencephalon
Midbrain -
    Mesencephalon
Hindbrain -
    Rhombencephalon
    Metencephalon
    Myelencephalon
These are formed primarily by hydrostatic pressure and differential tensions like balloons of wacky shapes. The tissues don't seem to grow so much as the lumen expands. The Rhombencephalon develops periodic bulges called rhombomeres that are given names from anterior to posterior: r1, r2, r3, etc. Each rhombomere has a distinct developmental fate. Cells within r2 form the ganglia for cranial verve V. Those from r4 form VII and VIII. IX originates from r6.

The neural crest that originates from regions along the AP axis also exhibit a particular fate that is imparted before overt migration to lateral and ventral regions.

It is apparent that there are differences in the neural and neural crest cells along the AP axis.

 

How is the AP axis regionalized?

Signals emanate from several locations to "inform" the neural tube cells of their identity. Signals from the organizer and the endoderm (in amniotes).

There are a series of genes that are common among animals from flies to humans known as hox genes. We will talk about this later in a separate lecture on patterning. For now, we'll learn the basics. Hox genes are a series of up to 13 genes that are divided into for paralogous (similar) groups. 


3'   a1  a2  a3  a4  a5  a6  a7       a9  a10  a11      a13     5'
 
3'   b1  b2  b3  b4  b5  b6  b7  b8   b9                        5'

3'               c4  c5          c8   c9  c10  c11  c12  c13    5'

3'   d1      d3  d4              d8   d9  d10  d11  d12  d13    5'
They are found, from the 3' end of DNA a1, a2, a3, etc. The genes a1, b1, d1 are very similar in sequence so they are considered paralogs. It's thought that they have undergone duplication over the course of evolution to result in 4 groups. The fascinating thing is that these genes are expressed spatially in a manner that correlates to their position on the chromosome! So a1 is expressed anteriorly, followed by a2, a3, etc. This is bizarre!!! The phenomenon is called co-linearity.

The consensus is that somehow these genes impart identity. How they do this is unknown. How they are patterned is also unknown. The current hypothesis (the dominant view) is that anterior and posterior gradients of diffusible or working in a relay system impart identity. More on this later.

As we've seen Saxen and Toivonen demonstrated that there are substances that impart anterior and posterior identity. Many substances are known with such activity. The addition of anterior and posterior substances result in intermediate neural structures. This implies that a gradient model is feasible, but whether it is responsible for the differences we see or is one of several reinforcing systems to set up AP regionalization can't be said. .

Somites The somites are paraxial structures. They are formed from mesoderm that forms segmental units as the node regresses. Some evidence indicates that the cells posterior to the node repress somite formation, but when cells are far from its influence, the paraxial mesenchyme epithelializes and form somites. Some suggest a internal clock regulates the production of a pair of somites as thee paraxial mesoderm is left in the wake of the regressing node.

There are three regions of a somite:

    Dermatome: forms dermis
    Myotome: forms axial and limb muscle
    Sclerotome: forms axial skeleton

For each segment (anterior-posterior level) of the body, the somite forms bone, dermis, and muscle. There are spinal nerves that form from the NT and NC that are associated with a somite and its derivatives so that in the adult, there is a spatial coordination between the nervous system and it's sensory and motor innervations.

 

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