Thursday, January 23, 2014

Lab 1: General anatomy of reptiles

In this lab we learned about characteristics of reptile eggs and development, as well as the basic anatomy of their integument and skeletal system.

Eggs, embryos and development:

Fig. 1.  Amniote egg showing extra-embryonic membranes and shell

One evolutionary change of the clade Amniota is the cleidoic egg.  Some key features of this egg are the presence of extra-embryonic membranes and a hard shell. The shell is made of units of calcium carbonate which can vary in thickness and hardness. These "units" are interspersed with pores which allows for gas exchange. Within the amniote egg are the chorion (just inside hard shell), amnion (surrounds embryo), allantois (collects nitrogenous wastes) and yolk (provides nutrition to the developing embryo) (Fig. 1).  Other important features of the reptile egg are the chalaza, albumin and air cell (Fig. 2).

Fig. 2.  Amniote egg showing more detailed internal anatomy.

Reptiles are oviparous (in which young hatch from eggs) and less commonly ovoviviparous (in which eggs are retained inside the mother until young are ready to hatch).  Additionally, they exhibit internal fertilization and direct development in which young are born as miniature versions of the adults.  Breaking out of the hard shelled egg is a challenge which hatchlings overcome with an egg tooth, or protuberance at the tip of the snout which is lost shortly after they emerge from their shell (Fig. 3).
Fig. 3.  Example of an egg tooth on a tortoise hatchling.

Integument (skin and scales):

Fig. 4.  Histological section of reptile skin.  Photo by Meghan Kelley.
Reptile skin is thick, rough and impermeable to gases and water.  In general, reptile skin can be divided into layers: Stratum corneum (outermost layer composed of dead cells), Stratum germinativum (basal layer of epidermis; living cells) and the dermis (with nerves, blood vessels and adipose tissue) (Fig. 4).  Most reptiles shed their skin regularly as they grow.  During this process, the superficial layers of the stratum corneum (beta keratin and alpha keratin) are lost.

Fig. 5.  Examples of scale types.
Most reptiles have a tough layer of scales covering the body surface which can have a variety of characteristics. Some examples include: small rectangular scales which are either longer than wide or vice versa (often found on the ventral surface) (Fig. 5A), granular scales which appear small and circular (Fig. 5B), imbricate keeled mucronate scales (overlapping, with prominent ridges down the center and tapering off to a V-shape or point posteriorly) (Fig. 5C), and imbricate cycloid scales (overlapping and rounded) (Fig. 5D).  Femoral pores and pre-cloacal glands are most often found on male lizards and secrete a waxy substance used for scent marking and chemical communication with the opposite sex (Fig. 6A).  Additionally some snakes have scales with small pores called apical pits (Fig. 6B).
Fig. 6.  Femoral pores (A) and apical pits (B).

Fig. 7. Three methods of counting dorsal scales on snakes.
In addition to the dorsal and ventral scales shown above, reptiles have unique scale types covering the head.  Species can be differentiated by the placement, size and number of dorsal, ventral or head scales. There are several techniques used for counting dorsal scales in snakes, all of which give the same result (See Fig. 7). Some commonly counted head scales are shown in the image below (Fig. 8).  For more detailed descriptions of these scales:

Fig. 8.  Head scales of a typical non-venomous snake.



 As tetrapods, reptiles have a more robust skeleton for structural support and locomotion in a terrestrial habitat. The post-cranial skeleton is composed of the vertebral column, ribs, pectoral & pelvic girdles and their associated limbs (See Fig. 9). The vertebrae of reptiles (and other amniotes) are unique because the centra and neural arches are fused, which provides structural support for locomotion on land. In addition, they have an atlas-axis complex which allows for greater mobility of the head and neck.

Fig. 9.  Appendicular skeleton of Crocodylian.

One way to study the skeletal elements of small vertebrates is by a method called clearing and staining. In this method, you skin and eviscerate the organism and add chemicals which cause the soft tissues to become clear and jelly-like. Then the cartilage is stained blue and the bones are stained red, resulting in a fully visible, articulated skeleton (Fig. 10).

Fig. 10.  Cleared and stained specimens.

Fig. 11.  Patterns of temporal fenestration.   

An important trend in the cranial anatomy of Amniotes is the modification of the skull structure to lighten and strengthen the skull and allow for modification of muscle mass. These changes were made possible by the presence and position of the temporal fenestrae. The anapsid condition (seen in turtles, Fig. 11A) is lacking fenestrae, synapsids (mammals, Fig. 11B) have only one subtemporal fenestra, and the diapsid condition (seen in most other reptiles, Fig. 11C) exhibits two openings in the temporal region of the skull. Common bones found in the diapsid skull are indicated in Fig. 12.  An additional condition, called euryapsid, had one supratemporal opening, however these clades are extinct. 

Fig. 12.  Bones of the Crocodylian skull.

KJ and JT

1 comment:

  1. There was some confusion in class about the layers of the skin that are visible. This may clarify some parts:
    "β-keratin or beta-keratin (not to be confused with β-carotene) is rich in stacked β pleated sheets, in contrast to alpha-keratin, a fibrous protein rich in alpha helices.
    β-keratin is found in reptiles.[1][2] It adds much more rigidity to reptilian skin than alpha-keratin does to mammalian skin. β-keratin is impregnated into the stratum corneum of the reptilian skin, providing waterproofing and the prevention of desiccation." --Wikipedia