Development of the sense organs: Eye and ear Flashcards
Where are the 3 places where sensory neurons can originate from? What are examples of these?
1) Neural crest (eg. sensory neurons of the DRG)
2) Neural tube (eg. retina)
3) Placodes (eg. otic (ear) placode)
Where is the ‘eye field’ set up and how?
In the anterior NEURAL PLATE
Set up by anterior transcription factors, which upregulate other transcription factors in cells which are characteristic of the eye field
What are the stages of eye formation in vertebrates?
1) Proliferation in the neural tube, causing a change in shape of the neural tube - forming the optic grooves
2) Once the neural tube has closed, the optic grooves from optic vesicles and contact the ectoderm
3) At the ectoderm contact, there is a thickening of the ectoderm - forming a LENS PLACODE
4) The lens placode invaginates and eventually pinches off to form the lens
5) The lens comes to sit in a double layered cup, formed by the optic vesicles
When do the optic grooves form?
As the anterior neural tube proliferates
Where does the lens come from?
The invagination of the lens placode, which pinches off to forma a circle of cells
What happens to the optic vesicle as the lens placode invaginates?
Causes the optic vesicle to buckle in on its self - forming a double layered cup, where the inside layer is the retina
Where does the retina come from and how is it formed?
Outgrowth of the ventral forebrain (diencenphalon)
Formed by the optic vesicle buckling in on its self to form a double layered cup
The inner layer of the cup is the retina
Where do retinal neurons differentiate from?
The inner layer of the retinal cup
Where do retinal neurons extend their axons?
Through the optic nerve, from the inner layer of the retinal cup
Which part of the double layered optic cup is the retina?
The inner layer
What do the outer layer cells of the optic cup become?
- Retinal pigment epithelium
What does the retinal pigment epithelium produce?
Melanin
What are the properties of the inner layer cells of the optic cup and what do they become?
Stem-cell population:
1) Can self-renew
2) Differentiate into ganglion cells, interneurons or sensitive cells in the neural retina
What is PAX6?
A master eye regulator
A transcription factor which is conserved through evolution causes the outgrowth of the eye to form the retina
What is PAX6 normally restricted to?
The optic vesicles
What happens if ectopically express PAX6?
Induces a second retina
How is the sensory retina built up?
In the same way as the neurons in the spinal chord:
- Build up may layers of differentiated neurons, while retaining a stem cell progenitor in the neuroepithelial layer (at the VZ)
- Made of different neuronal types, each born at a different time period
What determines what neurons are born in the retina?
The time at which the wave of neurons is born
how long the cells has been sitting as a progenitor (stem) cell
When do neurons begins to be born in the retina?
6-7 weeks, before this (5 weeks) there is no differentiation of cells
What layer is formed in the 1st wave of neurons and what cells are formed?
- Innermost neuroblastic layer (closest to the eyeball)
- Ganglion cells
What layer is formed in the 2nd wave of neurons and what cells are formed?
- Inner neuroblastic layer
- Bipolar cells
What layer is formed in the 3rd wave of neurons and what cells are formed?
- Outermost neuroblastic layer (furthest away from the eyeball)
- Rods and cones
Why is it important to know how long each neuron type remains as a progenitor?
- Can begin to manipulate pluripotent cells and direct them down particular routes in order to restore function
What is the structure of the vertebrate eye?
Retina is a laminar structure made of many layers
Where are sensory neurons derived from in the vertebrate eye?
From the neural plate
Which TF does eye formation in the vertebrate require?
Pax-6
What are placodes?
Embryonic structures that give rise to structure such as the eye, ear and olfactory epithelium
Where do placodes originate from?
From the anterior neural plate border cells (NOT the neural crest)
What are cranial placodes?
Thickened areas formed in the neural plate border, where there is contact between the neural plate and the ectoderm - at early development, before the neural plate elongates and the neural tube forms
What is the neural plate border important in forming anteriorly?
Formation of cranial placodes
How are the cranial placodes arranged?
With bilateral symmetry, at the neural plate border, until the neural tube forms, where they spread out
What happens to the placodes as the neural plate forms the neural tube?
They spread out and form additional placodes
What is the neural plate border important in forming posteriorly?
The neural crest (DRG, peripheral nervous system etc)
What are the cranial placodes?
Which ones are formed when the original cranial placodes spread out?
- Olfactory
- Lens
- Trigeminal
- Otic
Formed when the cranial placodes spread out:
- Nodose
- Petrosal
- Geniculate
How does the ear develop?
- From the otic placode, which folds to give a pit
- Pinches off from the surface ectoderm to form a vesicle
What do the cells in the otic vesicle give rise to?
- Different structures of the ear (eg. hair cells, auditory sensory neurons)
- Dependant on where the cell lies in the otic placode
What are neurogenic regions of the otic placode and what comes from this?
At the base of the vesicle
Sensory neurons come from this
What splits the eye field into 2 bilateral regions?
Expression of shh which down regulates the genes in the middle
