Advanced Lizard Embryo

27 December

I received an unexpected surprise right around the holidays. It was an egg filled with an advanced embryo. As an embryo geek, this made me quite excited! Normally egg production ends between September and October, but a student in our lab found the egg at the end of December. It appeared that the embryo died of natural causes, as evidenced by deformations in the skull. Deflation of the skull and eyes are common in eggs, which have become dehydrated.

I began preparing the Stage 18+ lizard embryo for a skeletal prep. Once I remove the scales and some of the soft tissue, I will stain it with Alcian Blue (for cartilage) and Alizarian Red (for bone). We should be able to see parts of the bone that were beginning to calcify.

Very meticulously, I began to remove the hardened yolk (and some mold) around the embryo.

 

I was unable to remove a spot of yolk attached to the center of the eye, although you can still see the iris. 

You can also see the teeth. They are triangular and as clear as glass.

Here is the arm and hand before I began removing the tissue.

After carefully removing the scales of the arm, I was able to see the humerus emerge. The bright white cyclinder-shaped areas are presumably being ossified, while the the clear part (cartilage) has not yet been remodeled into bone.

Here is a close-up of the arm. You can see ossification was occurring in this embryo. The uppermost region of ossification is presumably occuring from the epiphyseal plate.

Here is how the development of bone normally occurs (for endochondrial ossification). A cartilage matrix is laid down first. Then a cluster of mesenchymal cells differentiate into osteoblasts. A group of osteoblasts build bone matrix. The osteoblasts secret something calle osteoid, which mineralizes into a bone matrix. For long bones, like the humerus and tibia, the bone lengthens at the epiphysis. In this growth, or profilieration zone, cartilage cells are mitotically dividing and pushing adjecent cells outward. The cartilage cells will later be replaced by bone.

(Midlands Tech 2012)

Next, I began working on the forearm. Below you can see the radius and ulna. They are still clear and appear to be unossified cartilage.

I proceeded to carefully remove the skin around the wrist. Below, you can see the carpals (the bones of the hand) and the metacarpals (the finger bones). The metacarpals show white strips located closest to the carpal. On the other hand, these metacarpal proximal "ossified" strips aren't yet present. It seems that ossification is asymmetrical between the left and right hand.

A few days later I returned to the skeletal prep. Here is the leg. You can see spots of white forming in the tibia. The calcification appears to occur in non-homogenous patches at first. Once I stain it with the Alizarian Red, I will be able to make more sense of the patterning of calcification. I can then assess if calcification occurs asymmetrically in the respective limb bones.

Here is a picture of the the lizards lamellae, on the pads of their toes The lamallae are subdigital pads (covered with millions of microscopic setae) and are thought to increase their adhesiveness to leaves and branches (Losos 1997).

Lastly, here is a picture of the lizard ear. It has no external pinna, like humans have. Unfortunately the quality of the picture isn't good enough to see the tympanic membrane. However, under the microscope I could see it shimmering through the auditory canal.

I just want to add that this lizard embryo was ethically treated according to our IUCAC protocol, as it was already dead before it hatched. You can see in the next picture that the skull is caved in. The tissue in the indent is soft. Once I do a skeletal prep, I will be able to find out more about the pathology of the skull development.

All embryos were treated ethically according to IUCAC protocol.


 

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