Lizard Tail Structures

McLean and Vickaryous (2011) studied epimorphic regeneration in Eublepharis macularius (Leopard Gecko) and the results showed that the transverse section of the tail is circular to ovoid and the skeletal system is located in the center. The histological observation also showed that the tail is separated into quadrants by the presence of connective tissue known as septa emerging from (1) the neural spine in the dorsal region, (2) chevron bone in the ventral region, and (3) transverse processes on both lateral sides. Both dorsal (epaxial) and ventral (hypaxial) quadrants consist of perivertebral adipose tissue, connective tissue and musculature.

The outer part of the tail is well-protected by integument system modified into scales. Various tissues lie under the skin, from the outside in: segmented musculature, perivertebral adipose tissue surrounding the caudal vertebrae and the spinal cord located inside the vertebral canal. Peripheral nerve fibers can be found in the adipose and muscle layers while the vasculatures are situated ventrally to the vertebrae (McLean & Vickaryous, 2011). 

Generally, the integument system of reptiles consists of two layers, the outer (epidermis) and the inner (dermis) layers separated by a thin layer of basal membrane. The epidermis consists of four distinct layers from the outside in: stratum corneum, stratum granulosum, stratum spinosum and stratum germinativum (Zug et al., 2001) while dermis is a thicker layer compared to epidermis. The dermis is composed of various cells and structures including pigment cells, fibroblasts, nerve endings, glands and vasculatures embedded within the matrix. The skin of reptiles is usually modified into scales and it covers almost all of its body surface (Alibardi & Toni, 2006; Polazzi & Alibardi, 2011). 

The musculature in lizard tail can be categorized into epaxial and hypaxial muscles by the presence of horizontal connective tissue (septum horizontale) (Ritzman et al., 2012). Septum horizontale can be found on both lateral sides of the caudal vertebrae. The epaxial muscles are the groups of muscles located dorsally to the septum horizontale while the epaxial muscles are the ones located ventrally to the septa. The septum horizontale emanates from both transverse processes of caudal vertebrae and expands to the dermis (Cox, 1969). 

Each caudal vertebra consists of a centrum, neural arch and spine, haemal arch and spine (chevron) and bilateral transverse processes (McLean & Vickaryous, 2011). Haemal arch is formed by two pieces of bone in the ventral base of centrum vertebra. Both bones unite in the medio-ventral region, forming a well-protected opening (foramen) allowing the caudal arteries and veins to pass through it (Pratt, 1946). The neural arch is composed of two bony plates in the dorsal part of the centrum. The bones meet in the medio-dorsal centrum forming a continuous canal known as canalis vertebralis where the spinal cord lies in. The spinal cord is sheathed by meninges layer and adipose tissue (Lorenzo Alibardi, 2014b). In most fish, the spinal cord is directly covered by smooth connective tissue membrane namely meninges primitiva while in amphibians, reptiles and the majority of birds, meninges primitiva differentiates into an inner piamater layer, an in between subdural layer and an outer duramater layer (Kent, 1987). In species capable of performing caudal autotomy, the caudal vertebrae are classified into pygal (post-sacral) and postpygal vertebrae (Holder, 1960). Pygal vertebrae are lack of fracture planes, hence autotomy cannot be performed in this section (non-autotomous part). The number of pygal vertebrae in each species ranges from 5 to 9, and the cloaca lies on the first pygal vertebra. The importance of this non-autotomous part in males is to prevent the loss of hemipenis as it is located posterior to the cloaca (Bellairs & Bryant, 2001). Contrary to the pygal vertebrae, postpygal vertebrae are equipped with fracture planes, therefore autotomy may occur in this region. Postpygal vertebrae are situated right after the last pygal vertebra, in G. gecko, the first postpygal vertebra usually starts from the fifth, sixth or seventh caudal vertebrae, and the intravertebral fracture planes can be found in each one of them except in the ones close to the tip of the tail where autotomy infrequently takes place. The aforementioned fracture planes of G. gecko are located posteriorly to the transverse processes, and this structures divide each postpygal vertebra into anterior and posterior parts. The anterior part is slightly smaller compared to the posterior (Rumping & Jayne, 1996).