This is a new one for me. Earlier today I was summoned on Twitter to address an assertion by a creationist, @jarrydtrokis. I was slightly boggled.
He was baffled by eyelid development. It seems he thinks it requires…intelligent design!.
… Here’s one for you to ponder Eye lids in the womb… How are they formed? #IntelligentDesign?
Wait, what? What’s mystifying about eyelid formation?
The section of skin in the middle dies… How does it know to do that? And in a perfectly straight line???
Oh. It forms a straight line. Whoa. And he claims to have done research to get the answer.
The research I’ve done shows the scientists are at a loss for an explanation….
Gosh. I can do research, too. It’s easy to explain, with pictures even.
The eyelids separate in a straight seam because of how they got that way. The eyelids form by expansion of two epithelial sheets from above and below that meet in the middle. When you see how the eyelids develop, it’s easy to see how they separate in a straight line later. This is a series of images over the course of about a day in mouse development. In the first, you can see the eye sans eyelid, but ringed by epithelia. In the second, you can see that epithelium growing, expanding in a sheet over the eye. In the third, the sheet is beginning to close in a line over the middle, and in the fourth it has completely closed, but leaving a seam or scar in a straight line across it.
Wait, you say inquisitively, I’d like a closer look at that seam. Can you show me what is going on postnatally, as the eyelids separate? Sure can.
The first panel is 5 days postnatal in the mouse; the eyelids are still fused. But you can see a difference in the histology of the junctional region (J), and a depression at the arrowhead (you can also see the layers of keratin there). There’s something different in this area.
In the second panel, 10 days postnatally, the depression at the junctional region is deepening and you can see a stratum granulosum (SG) at the seam, while you can also see hair follicles (HF) forming in the adjacent portions of the lid.
The third and fourth panels are at 12 days, and now the keratin layers have extended into the depression from both the inside and outside, completing the separation of the two lids.
Now @jarrydtrokis might be tempted to say that Jesus did the separating, but that’s only true if Jesus is a polypeptide called epidermal growth factor, or EGF. EGF is a molecule that triggers growth and differentiation of keratinocytes, and it turns out that if you treat baby mice with EGF it accelerates the rate of eyelid separation.
I’m sorry, @jarrydtrokis, but your argument from ignorance wasn’t very persuasive, and your talents at ‘research’ are rather pathetic, since the paper describing all that was trivial to find. But then, isn’t this always the case with creationists? There are none so blind as those who will not see.
Findlater GS, McDougall RD, Kaufman MH (1993) Eyelid development, fusion and subsequent reopening in the mouse. J Anat. 183(1):121-9.