Regeneration

Regeneration is the sequence of morphogenetic events that restores the normal structure of an organ after its partial or total amputation.

Regeneration in invertebrates:• Hydra: Cross amputation of hydra led every part to regenerate

to whole hydra species. • Planaria: The planaria contains neoblast cells which migrate

toward the amputated region and form the lossing parts.• Annelids : The blastema of the amputated region formed of

ectoderm , mesoderm and neoblast cells which co-ordinated in regeneration of the lossing parts.

Regeneration of vertebratesThere are two types of regeneration:1. Epimorphosis or epimorphic regeneration : This type of regeneration involve the reconstruction of

the missing parts by local proliferation from the blastema, or addition of parts to remaining piece . For example: regeneration of tail, limbs and lens in anurans and urodels and other vertebrates.

2. Morpholaxis or morphollactic regeneration: This type of regeneration involving reorganization of the

remaining part of the body of an animal.For example: Hydra, planaria and other invertebrates e.g. regeneration of the new individual from body pieces.

Epimorphosis or epimorphic regeneration

Regeneration of Tail in amphibians & reptilia :

Amphibia : The tail lacks vertebrae and has an unsegmented cartilaginous tube, which contains the regenerated spinal cord which form mainly of the ependymal lining of the central canal . At first very few cells accumulate under the wound epithelium . The ependyma and the various connective tissues dermis, muscle septa, adipose tissues and osteocytes of vertebrae are the sources of cells for the generate. The non-nervous elements proliferate behind the apex, forming both the muscle and cartilage tube ,then, the ependyma proliferate and gradually extend dorsally.

Reptilia : For example lizard, the regenerated tail is a quite imperfect tail. It lacks vertebrae and in their place, has an unsegmented cartilaginous tube. This tube contains the regenerated spinal cord, including the extension of the ependymal lining of the central canal of the spinal cord.

Regeneration of lens: 1. The dorsal region of the iris thickens and a cleft arises between

inner and outer lamellae of the iris.2. Amoeboid cells move from the stroma into the cleft followed by

marked increase of RNA and DNA synthesis as well as of mitotic cell division.

3. The pigmented cells of the dorsal region is engulfed by invading amoeboid cells.

4. The formed non- pigmented cubical cells form hollow epithelial vesicle and extends with inner and outer lamellae.

5. The vesicle inner wall cells elongated into the lumen and form primary lens fibers.

6. The lens-specific crystalline proteins is formed.7. The primary lens fibers push to the front of vesicle to form a

nucleus behind the lens epithelium which form the secondary lens fibers.

8. The nucleus of primary lens fibers is enclosed by secondary lens fibers.

9. In the central lens fibers the nucleus degenerate ,primary and secondary lens fibers are the components of the lens.

Regeneration of Limb

Regeneration begins in 3 phases :1. Phase of wound healing or pre -blastema stage : Blood clotting and migration of epidermal cells from the basal

layer of epidermis toward the centre of the wound. The wound is covered with epithelium which is thicker than the epidermis of the limb .

2. Phase of blastema formation : Cells accumulate beneath the epithelial covering and formed the

blastema. Mesenchymal cells accumulate beneath the cap . Mesenchymal – blastemal cells differentiate into myoblasts and muscle cells, early cartilage cells and cartilage. During the dedifferentiate phase Hyaluronate (HA) increases in the distal stump to form blastema . As the blastema forms, the HA will be decrease. The production of HA and break down of collagen represent the establishment of migration from stump tissues .

3. Phase of dedifferentiate and morphogenesis : The blastema begins to restore the part of which the limb was

deprived. Specifically, if the fore arm is removed, the blastema differentiated directly into the muscle, bone, cartilage and skin of the fore arm.

Factors Controlling Regeneration

1. nervous system.2. animal size .3. pituitary gland .4. Vitamin A and its derivatives.5. Insulin .

Steps of regenerated limb