Thursday, January 26, 2012

Laboratory #1: General Anatomy of Reptiles - Chris K & Cait F

by Cait Falasco and Chris Koch
Laboratory #1: General Anatomy of Reptiles

Who came first? The chicken or the egg?

In this case, we’re starting with the egg! (If you guessed chicken, unlucky!)
The beginning of the life of reptiles…these transitioning land pioneers needed to start their life in a warm, protective environment so nature’s selective pressure gave them the cleidoic egg.

The Cleidoic Eggs comes from the Greek kleistos meaning “closed” and these eggs, sometimes called “amniotic eggs”, are characteristic of amphibians and REPTILES! Assuming life begins at conception – controversial, we know - these developing little embryos live a very sheltered life in their beginning stages, simply because the egg has everything they need!

Protective force field? Check. That’s the outer layer “egg shell” made up of reinforced with calcium carbonate shell units. Some eggs are hard, like ostrich or bird eggs, and others are soft and leathery like the turtle eggs shown. The crystalline columns of shell units help the embryo conserve food and water and resist desiccation and other threats of the terrestrial environment, while dimple-like pores on the shell surface to help the babies exchange gases.

All-you-can-eat buffet? Check. The yolk and three layers of albumen are full of proteins for the embryos to chow down on throughout their stay. The yolk is the orange-yellow ball in the center, the external albumen is the runny liquid clear portion of the egg, the middle albumen is the runny clear substance within, and the inner albumen immediately surrounds the yolk.

Comfort/Stability? Check. If membranes were gold chains, this egg would be in a rap music video. Not only does the shell have stabilizing outer and inner shell membranes, but the yolk is surrounded by the vitelline membrane as well. There is the air cell “air bag” at one end of the egg and two white bungee cord-like yolk suspenders called chalazae for support within the center of the albumen.

Skin. The skin of reptiles is covered by scales OR in those trying to show off, highly derived scales: feathers!

The 3 layers of reptile skin are:
  1. The Stratum corneum (the outer layer of skin)
  2. Stratum germinativum (under the outer layer)
  3. Dermis (inner layer of skin)
The outer dead layer of skin (Stratum corneum) is what is shed through a process called ecdysis. The 2 remaining layers (Stratum germinativum and Dermis) contain living cells. The Stratum germinativum is composed of living cells of the deep epidermis and the Dermis forms a armor-like protective layer that can be found in some lizards, turtles and crocodilians.
We observed snake sheds to look at the outer layer of Stratum corneum.
The snake skins allowed us to see the outer layer of skin as well as look at the different scale types...speaking of which...
Scales: Reptiles have a wide range of different types of scales. Scale type can be used to identify specimens by classifying them with granular, rectangular, cycloid, juxtaposed or keeled scales. These different scale types can be small or large, smooth or keeled with a ridge in the middle of the scale, overlap or not overlap and some can have pits. It is important to note that an individual specimen can have several different types of scales on different parts of the body. In lab we looked at many different specimens and looked at the different types of scales.
Large rectangular scales can be seen in the ventral surface of this caiman specimen above.
Here to the left, this 5-lined skink has smooth cycloid scales.

Glands. Reptiles have a diversity of pores but only a few can be seen by the naked eye. In lab we observed femoral or precloacal pores. These glands are used for communication between sexes and marking territories.
In this lizard specimen femoral pores can be seen along the femur.

Skeletal structure varies morphologically among reptiles but all demonstrate the general regions of the skeleton of a Tetrapod. Notice the skull and cervical region of the neck, esp. bones 1 and 2 (Axis-Atlas). After the neck lies the trunk (neck to the end of the hip bones) which is broken into 2 parts: the upper thoracic region (with ribs directly connected via gastralia) and the lower lumbar region without ribs.
Also much like the anatomy of humans, the reptile skeleton has a lot of bones with fancy names. The sacral region is composed of the hip structure withthe pelvic, ilium and ischium bones. The tail beyond the sacral region is the caudal region.

Many groups of reptiles are defined by their cranial structures thought explain certain evolutionary history. In particular, fenestrae and vacuities are very important structures for these ends. The number and position of fenestra, Latin for “window” or natural openings in the skull are helpful for categorizing amniotes as anapsid (0), synapsid (1 subtemporal), euryapsid (1 supratemporal), or diapsid (2).

That’s it for this week! We’ll leave you with an extremely mature and thought-provoking picture of one of us examining a specimen very thoroughly.

1 comment:

  1. I love this post! It is very easy to read, and I feel like anyone outside of the field would be able to read this and really understand what we learned. Awesome job, steelhead brethren!