A. Primarily ectodermal derivatives.
B. Most important, functionally, are eyes and ears, which are also structurally most complex.
C. Ectodermal placodes
1. Similar to neural crest cells:
a. Can migrate.
b. Neural fate common.
c. May have same phylogenetic precursor - nerve plexus of protochordates that controls many sensory, integrative, and motor functions.
2. Induced by nearby portion of CNS.
3. Confined to head region.
4. Dorsolateral series --> special sense organs.
5. Ventrolateral series (epibranchial) --> sensory ganglia of pharyngeal pouches.
II. EYE DEVELOPMENT
A. Excellent example of fundamental mechanisms of development:
1. Induction
2. Differentiation
3. RNA and protein synthesis
4. Role of extracellular matrix
5. Cell migration
6. Integration between organs
7. Mechanical influences
B. Optic vesicle = outpocketing of wall of diencephalon.
1. 2 layers differentiate rapidly
a. Outer layer - thinner, melanin granules --> pigmented retina.
b. Inner layer - thicker --> sensory retina.
1) Processes of nerve cells invade optic stalk.
2) Converts visual images into signals transmitted to brain via optic nerve.
2. Invagination of vesicle results in continuous gap called choroid fissure, which partly envelopes central artery of retina.
3. Once process from retinal nerve cells invade optic stalk on way to synapses in brain, it becomes known as the optic nerve.
C. Lens
1. Induced in head ectoderm by optic vesicle --> lens placode.
2. Lens placode invaginates into opening of optic cup to form lens vesicle.
3. Cells on deep pole of vesicle begin to elongate as they differentiate into transparent lens fibers.
4. Low lens epithelium on superficial surface is germinative (mitotic) --> daughter cells move toward equatorial zone & begin to elongate in preparation for synthesis of crystallins.
5. Lens sutures = where fibers from opposite sides meet at poles.
6. Polarity of lens depends on signals from retina.
D. Outer covering of eyeball (derived from neural crest mesenchyme cells)
1. Choroid coat = highly vascular inner layer.
2. Sclera = tough, fibrous connective tissue.
E. Cornea
1. Continuous with sclera; transparent
2. Outer epithelial layer sitting on basement membrane.
3. Inner endothelium of fibroblasts & parallel collagen fibers on basement membrane.
F. Retina
1. Multilayered.
2. Cells in center of sensory retina mature.
3. Zone of proliferating cells around margins contributes daughter cells during growth.
G. Position of eyes --> as facial structures develop, eyes move from sides of head inward toward front, until optical axes converge (in cases of binocular vision.).