Laboratory #1: General Anatomy of Reptiles by Alex and Julie (17 January 2014)

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 Welcome to Reptilia!

Background: Today’s lab was an overall introduction to the basic anatomical features that unite the reptilian clade. The reproductive, developmental, integumentary, and skeletal adaptations required to exploit terrestrial environments were a central theme of this lab.

Eggs, Embryos, and Reproduction

• Cleidoic Eggs 

There is a great diversity of egg sizes and hardness, which is due in part to the organization of the calcium carbonate shell units and the degree of porosity created by that organization. For example, some eggs are soft and flexible like the ping-pong ball-shaped snapping turtle eggs, while others are rigid-shelled like the dark emu egg.

            In order for some early tetrapods to complete the transition to an exclusively terrestrial lifestyle, a method of reproduction that protected eggs from desiccation and gravity was required. These abiotic challenges were met by changing from aquatic, soft-shelled, externally fertilized anamniotic eggs (as seen in most amphibians) to internally fertilized, hard-shelled cleidoic eggs (as seen in Amniota).

  

This figure from the Herpetology 4th ed. text book (p. 121) is a good visualization of the reptilian egg structure and highlights the extraembryonic membranes found in members of Amniota.

           Amniotes are characterized by a cledoic egg with extraembryonic membranes, which grow from the embryo. These extraembryonic membranes include the amnion, allantois, yolk, and chorion. The amnion surrounds and protects the developing embryo. The allantois is considered the “waste basket” of the egg and segregates nitrogenous excretions so the embryo can grow in an enclosed environment without poisoning itself. The yolk provides the embryo with nutrients for growth and development. The chorion functions in the exchange of gases and water with the environment.
 

Internal anatomy of a chicken egg. The internal albumin immediately surrounds the yolk. Most of the albumin present is middle albumin, which is found between the internal and exterior albumin membranes. The exterior albumin is difficult to see in the picture, but is the thinnest and furthest membrane from the yolk.

           A cracked chicken egg displays surprising structural complexity. The nutritive yellow yolk is conspicuous and has two sections of white chalaza attached, which function in centering and providing support to the embryo. The multiple membranes of the albumin supply water and protect the developing embryo from mechanical stress. 

          With the benefit of a protective hard shell came the cost of reduced permeability to gas and waste exchange, which was easily achieved via diffusion for anamniotic eggs in aquatic environments. In reptilian eggs, gas exchange occurs through the air cell, which is present at the “rounded” end of the egg. However, if gas exchange only occurred at one end of the egg, the gases would need to diffuse the entire length of the egg, making the process inefficient. Thus, the porous outer layer of the shell allows gas exchange to occur over the entire surface of the egg.  

• Embryos and Development

On the left is a stained Chelydra serpentina embryo showing cartilage (blue) and bone (red). On the right is a series of  embryonic stages of Trachemys scripta (Greenbaum 2002).

            In contrast to the free-living larvae of many amphibians that metamorphose into adults in the process of indirect development, reptiles exhibit direct development from miniature versions of adults that grow into full-size adults. The reptilian embryo undergoes various developmental stages including chondrification, ossification of bones, elaboration of face and sensory organs, and formation of the intestinal tract. The above picture shows a turtle embryo that has been cleared of soft tissue and double-stained, so that blue stain indicates the presence of cartilage, while bone is stained red. 

• Hatching

            In order for a young reptile to emerge from the egg, it must break through the membranes and hard outer shell. A specialized structure called an egg tooth helps young reptiles penetrate the shell. Some reptiles have an egg tooth that is merely a protuberance extending from the snout. Alternately, in snakes and lizards, the egg tooth is a true midline tooth that is lost after hatching. The conspicuous bulge on the abdomen of the turtle specimen showing an egg tooth is the remaining yolk of the egg. When turtles hatch from their eggs, they do not immediately emerge from the nest. There is a period where hatchlings hibernate in the nest and absorb the last of the yolk.

Integument

• Skin

Some examples of shed snake skins.

When we think reptile skin, we often think of scales. Reptiles have evolved skin that met the challenges that came with terrestrial life such as gravity, friction, abrasion and evaporative water loss.  Reptile skin is actually composed of three distinct layers, the stratum corneum, stratum germinativum, and dermis.

1) Stratum corneum – This is the outer most layer that is comprised of dead cells arranged in alpha and beta keratin layers. A process known as ecdysis allows the stratum corneum to be shed and replaced by an identical new layer of cells beneath it. Snake sheds are a great example of this process.

2) Stratum germinativum – This is the layer of living cells of the deep epidermis.

3) Dermis – This is the layer of skin which contains nerves, blood vessels and in some cases osteoderms. Osteoderms present in the dermis can form a protective layer that is seen in some lizard, turtle and crocodile species.

          It is through the folding of the dermis and epidermis that scales form on the outer layer of the skin. 

• Scales

The specimen on the left has rectangular, juxtaposed scales on its ventrum, while the beaded lizard has bead-like scales covering its body.

Often, reptiles have multiple types of scales covering their body. There are various terms used to describe the specific type of scale: cycloid scales – scales with a rounded edge, granular – appear like tiny beads, keeled – prominent ridge running down the center, mucronate - terminate in a pointed "V" shape, rectangular - scales that appear longer than wide. There are also terms used to describe the position of the scales: dorsal scales - covering the upper body surface, ventral scales - covering the lower body surface, subcaudal scales - on the underside of the tail, juxtaposed – placed adjacent to one another, and imbricate – those scales overlapping like shingles. Snake-specific scale terms include anal scales, which appear as a flap-like cover on or near the cloaca, and apical pits, which are small pits found near the edge or center of the scale. Additionally, there are modified lamellae scales, which are associated with the fingers or toes.

          It is important to understand that when describing scales multiple terms are often applied, for example, a specimen can have “imbricate keeled mucronate scales” (as exhibited by the specimen pictured above). 

          Further, reptiles can have different scale types of different parts of the body. Particular attention is given to the scales of the head of snakes and lizards. Scales and scale pits (snakes) are often used as identifying characteristics, with specific methods of counting employed to differentiate taxa. 

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          The carapace (upper bony shell) and the plastron (ventral bony plate) of turtles are covered by scutes that are made of keratin.

• Glands

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Compared to our amphibian friends, reptiles have fewer glands in the skin that are visible to the naked eye. Femoral or precloacal glands are typically seen in males, and if these glands are present on females they are noticeably smaller. Femoral glands are used to emit a waxy secretion used for communication between sexes.

Skeleton 

• General Regions

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Reptiles can demonstrate morphological variation within the skeleton. However, in terms of general skeletal regions, most reptiles exhibit elements of a typical tetrapod skeleton. Some of the notable features include:

1) Atlas – Axis complex: the first two vertebrae of the cervical region allowing for the movement of the head in all directions.

2) Dorsal/trunk region: some of these vertebrae attach to ribs.

3) Sacral region: vertebrae that interact with the ilium of the pelvic girdle.

3) Caudal region: the vertebrae of the tail.

4) Pelvic and pectoral girdles along with their respective limbs. The development of a robust set of girdles was critical for supporting weight and allowing for locomotion on land. 

• Skull

          Overall trends in cranial anatomy as reptiles evolved from fish and amphibian ancestors included compaction of the skull and a reduction in the number of bones. 

          Large alligator skulls provide excellent models for identifying specific elements of anatomy. 

•  Temporal Fenestration

            A fenestra is a “window” or large opening between bones. Modification of the temporal fenestra within Amniota shows adaptation to the pressures of moving from an aquatic to a more terrestrial lifestyle. For example, it is thought that the anapsid skull, which contains no temporal fenestra, is the plesiomorphic condition. Testudines (an order including turtles) are the only extant reptiles with anapsid skulls.

          The dense, anapsid skull of a sea turtle would be supported by water and thus not have to endure the pressure of gravity.  

          In addition to reducing the density of the skull, fenestration allowed for a change in the origin and insertion of musculature associated with the jaw. Synapsids are represented by Mammalia and have skulls with one subtemporal fenestra. In herbivorous ungulates, this singular cranial opening accommodates muscles that function in grinding the jaw from side to side to chew vegetation. 

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          Euryapsids also only have one fenestra, but in contrast to synapsids, these extinct reptiles only possessed a supratemporal fenestra. 

          The diapsid condition, in which the skull has both supratemporal and subtemporal fenestrae, is characteristic of most reptiles.

Sources
1. Greenbaum E (2002) A standard series of embryonic stages for the emydid turtle Trachemys scripta. Can. J. Zool. 80: 1350-1370
2. Vitt LJ, Caldwell JP (2014) Herpetology: An introductory biology of amphibians and reptiles, 4th ed. Elsevier inc., Massachusetts pp 121