Lizard Tail Regeneration

Lizard ability to regrow its tail following tail amputation is known as regeneration (Hutchins et al., 2016). Regeneration process is initiated by the formation of blastema from the dedifferentiated cells (Daniels et al., 2003; Alibardi & Lovicu, 2010). Later on, Alibardi (2015) stated that progenitor cells also play an important role in this process. These blastemal cells will proliferate and then differentiate to form new tissues to replace the lost/damaged tissue (Carlson, 2007; Narayanan, 2015).

Regeneration process is divided into four stages: (1) wound healing stage, lasting up to 10 days post-autotomy (dpa); (2) blastema formation ranging from 10-15 dpa; (3) tail growth and differentiation ranging from 15-25 dpa; and (4) tail maturation when the tail becomes fully scaled (Alibardi, 2009) which is usually studied from 25-60 dpa (Fisher et al., 2012) with no currently known endpoint. Ependymal cells lining the central canal of the original spinal cord appear to be crucial for initiating regeneration (Gilbert et al., 2015). This process yields a regenerated tail with an axial skeleton in the form of a hyaline cartilage tube instead of segmented caudal vertebrae. Groups of myoblasts are formed around the cartilage tube and they will differentiate into myofibers (Simpson & Cox, 1967). 

The wound healing phase is initiated by the formation of a highly proliferative wound epithelium started hours after amputation (Echeverri et al., 2001) and dominated by the degradation of the stump bone by osteoclasts (Fernando et al., 2011). The wound site will be covered by wound epithelium, a stratified squamous epithelium with a prominent apical thickening (epithelial cap), within days following autotomy. Without the wound epithelium, the regeneration process will not progress suggesting how important this structure is to the continuity of regeneration (Gilbert et al., 2015). In the wound healing phase of Anolis carolinensis, osteoclasts appear at day two and a small area of dissolved bone is observed indicating their activity. By day four, a large number of osteoclasts are concentrated in the area between the broken caudal vertebra and the newly-formed wound epithelium and this last vertebra is extensively degraded so that the initial flat chondrocytes are in contact with the bone (Cox, 1969; Lozito & Tuan, 2015; Alibardi, 2015a). The osteoclasts then disappear two or three days after the completion of wound healing process (Cox, 1969). 

Numerous mesenchymal cells are present in the autotomy planes from the dermis, muscle myoseptum, periosteum of vertebra, adipose tissue up to the bone marrow, that are believed to be the main source of cells participating in the formation of regenerative blastema (Bellairs & Bryant, 2001; Alibardi, 2010). Those mesenchymal cells are believed to redifferentiate into the same tissue of origin. All injured tissues of the tail stump contribute to the formation of blastema but the specific contribution of each tissue is still unclear (Alibardi, 2009). 

When these mesenchymal cells are concentrated underneath the wound epithelium, the ependymal tube keeps growing in the center of blastemal and acts as the guidance for the regeneration process (Alibardi & Miolo, 1990; Alibardi, 2009; Shieh & Cheng, 2015). A population of mesenchymal cells around the ependymal tube forms the precartilaginous aggregation, rapidly followed by the differentiation of these cells into hyaline chondrocytes with little intercellular matrix which then quickly grow to surround the newly-formed ependymal tube. This cartilage tube isolates the ependymal tube from other tissues except blood vessels which are still in contact with the ependymal tube through few small holes present on the cartilage tube. Pro-muscular aggregates are formed outside the cartilage tube, this tissue is originated from clusters of myoblasts (Alibardi, 2009). 

The autotomized tail does not elongate at the wound healing and the initiation of blastema formation stages but it will grow rapidly several weeks afterwards (Bellairs & Bryant, 2001). In the tail growth phase, there are a heterogeneous population of cells consisting of connective cells, pigment cells, nerves and small adipose cells in the connective tissue between the forming muscle bundles and the cartilaginous tube. The adipocytes gain more amount of lipids a few days later. Tissue proliferation, mostly the axial tissues, determines the rate of tail growth (Alibardi, 2009). 

In the tail scalation and maturation stage, the differentiation process continues in the more distal part of the regenerating tail. A tiny cluster of mesenchymal cells still exists beneath the wound epithelium forming small amount of tissue at the tip of the new tail. At this stage, the new tail usually has been covered by smaller cornified scales with simpler pattern compared to the scales of the original tail (Alibardi, 2009). Overall, this regeneration process yields a new regenerated tail which is slightly different than the original tail, and those differences are (1) the axial skeleton of regenerated tail is replaced by cartilage tube, (2) the spinal cord in regenerated tail is only formed by meninges and the ependymal cells lining the central canal, and (3) the pattern of scales in regenerated tail slightly differs (Higham et al., 2013).