Evolution under the microscope

Embryonic development of human eye

This is an outline of how human eyes form embryonically, for reference from other pages.

Morphological description

In humans, eyes begin to develop at about day 22 after fertilisation when optic grooves form either side of the developing forebrain (see Figure 1). As the neural tube closes, these grooves become outpocketings of the forebrain and are now called optic vesicles, which extend towards the surface ectoderm. The latter thickens to form the lens placode.

formation of optic vesicle

Figure 1. Transverse section through developing forebrain, showing formation of optic vesicles.

When an optic vesicle approaches the ectoderm the adjacent area of ectoderm responds by (see Figure 2):

Meanwhile the optic vesicle also invaginates and forms the double-layered optic cup, most of which subsequently develops into the retina.

formation of lens vesicle

Figure 2. Section showing development of lens vesicle.

Development of the lens

The cells of the posterior surface of the lens vesicle elongate until they completely fill the cavity of the vesicle. Towards the end of this process these cell lose their internal organelles, including their nucleus, to form the primary lens fibres. This foetal lens becomes the central part (sometimes called the ‘lens nucleus’) of the mature lens; it has the highest protein concentration and optical density.

formation of lens

Figure 3. Section showing development of lens (note that the cells are much smaller/thinner than indicated).

Cells around the periphery of the lens continue to divide; the daughter cells elongate and wrap around the lens nucleus, lose their organelles, and become secondary lens fibres (where fibres from opposite sides of the lens meet is called the lens suture). This process is largely complete by childhood, but continues slowly throughout life. In cross-section these fibres are hexagonal and packed together somewhat like a honeycomb.

Genetic aspects

Because of the relative ease of access to the eye it has been one of the first organs to elucidate some of the genetic mechanisms involved in effecting embryonic development.

Induction

It has become evident that there is a sophisticated interplay between the various developing tissues - an exchange of chemical signals, called induction - which serves to coordinate the development of the various components of the eye. Interestingly, the stimulus for the formation of the lens placode was the first embryonic inductive mechanism to be discovered.

It was found that if the very early optic vesicle is removed before it contacts the overlying ectoderm (Figure 1) then the lens placode does not form. Conversely, if the optic vesicle is transplanted to another part of the ectoderm, such as the abdomen, then a lens placode forms there.

It was evident that the optic vesicle releases some chemical signal which is recognised by the ectoderm and which responds by forming the lens placode. Similar procedures, and the use of variants having missing or corrupted genes, have revealed that throughout the development of the eye there are many inductive signals between the various developing tissues. Some key ones identified so far are illustrated in Figure 4.

induction

Figure 4. Schematic showing inductions (green dashed arrows) involved in development of vertebrate eye.

Of particular note is the reciprocal nature of some of the inductions.

For example, on one hand the optic vesicle induces the lens ectoderm to become a lens placode; but on the other, further development of the optic vesicle into the optic cup is dependent on induction from the lens placode. (If a barrier is placed between the lens vesicle and ectoderm, not only does the lens placode not form, but the optic vesicle does not develop into an optic cup, and development of the eye ceases.)

Subsequently, induction from the neural retina is required to prompt transformation of the cells on the posterior side of the lens vesicle into elongated lens fibres. And there is a longer-term effect to ensure correct polarity of the lens (long fibres posteriorly, thin epithelium anteriorly).

Once the lens vesicle has formed, not only is it necessary for further development of the optic cup to form the retina, but it also induces development of surface ectoderm to form the cornea.

It is evident that there is a constant interplay between the various parts of the embryonic eye to ensure that all develop in step with each other, and that the many complex components within the eye are properly related to neighbouring structures.

 


Notes

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Page created May 2017.