Several people ask me interesting Qs on regeneration. They read about lost or amputated  limbs. They wonder whether science can help these unfortunate human beings. Several Qs arise from these tormented minds. Let me answer one such Q.

Q: How can some reptiles like Lizards, gecko and salamander regrow their limbs? Why are humans not able to fully understand this process and replicate?
Krishna: A particular field of biology studies animals that are actually capable of regenerating severed parts of their bodies: regeneration biology. By studying salamanders, lizards, and flatworms, (1,2,3)the

scientists look at the mechanisms these animals use to regenerate lost body parts, in the hope that one day humans could too. 
The process these animals use are actually well-documented. When there is a wound, the outermost layer of skin covers up this wound. A bump then appears, called the wound epidermis. It sends instructions to cells near and beneath it and mature cells become immature ones called blastema. And then Nerve cells start to regenerate.

The blastema is key to regeneration. Those immature cells start dividing and differentiating into the cells of the lost limb. The cells that form the blastema are stem cells, cells that have not differentiated into a final cell type. There is also some form of memory that tells the process when to stop, with the blastema only regenerating the lost hand or leg, no more.

This process can be slow. A 4 mm (.16 inches) limb can take as much as 400 days to grow back. What of something as large as an adult arm? Years? Decades?  Human cells don't regenerate as well as some animal cells because of the fact that humans have more complex cells than some animals.

There are several theories regarding why humans can't regenerate like some animals do.

One theory suggests that scar tissue is an adaptation — but one that prevents regeneration. Scar tissue forms quickly and helps seal over a wound, but it’s made of different materials than unmarred skin, and it’s fast and quick, but not performing at the same standard.

Another theory is that the cellular machinery that triggered regeneration was lost, possibly because the growth of cells can look a lot like cancer. Better to prevent growth (that could possibly get out of control) than to risk the development of a cancerous tumour.

We may also not have enough stem cells, or our cells have lost the capacity to naturally regress back to an undifferentiated state. Again, stem cells tend to pop up in cancerous tumours, so this loss isn’t necessarily a bad thing for us.

Finally, some of the challenge may be increased complexity — a human limb is more complex than that of a salamander. That may pose a challenge for regrowth.

Certain human body parts can 'regenerate naturally'.  For instance, Cardiomyocyte necrosis activates an inflammatory response that serves to clear the injured myocardium from dead cells, and stimulates repair, but may also extend injury. Research suggests that the cell types involved in the process play an important role. Namely monocyte-derived macrophages tend to induce inflammation while inhibiting cardiac regeneration, while tissue resident macrophages may help restoration of tissue structure and function (5).

The endometrium after the process of breakdown via the menstruation cycle, re-epithelializes swiftly and regenerates.
There are reports of adult digital tip regeneration (6,7).Studies in the 1970s showed that children up to the age of 10 or so who lose fingertips in accidents can regrow the tip of the digit within a month provided their wounds are not sealed up with flaps of skin – the de facto treatment in such emergencies. They normally won't have a fingerprint, and if there is any piece of the finger nail left it will grow back as well, usually in a square shape rather than round (8,9).

Similarly in the mammalian kidney, the regeneration of the tubular component following an acute injury is well known. Recently regeneration of the glomerulus has also been documented (10).

In 1999 the bladder was the first regenerated organ to be given to seven patients (4). The penis has been successfully regenerated in the lab (12).

It has been shown that bone marrow-derived cells could be the source of progenitor cells of multiple cell lineages, and a 2004 study suggested that one of these cell types was involved in lung regeneration (11). 

Spinal chord nerves have also been regenerated, reconnecting damaged neural circuits by injecting  stem cells, in vivo, into the site of the  injury. A Patient eventually gained feeling, movement and sensation in his limbs, especially on the side where the stem cells were injected; he also reported gaining sexual function. He can now drive and can now walk some distance aided by a frame (13, 14).

 The thymus gland is one of the first organs to degenerate in normal healthy individuals (15).

As always, I would say funding is the main issue for this field not developing as it should really do.

Science is a very slow process and takes years and years of research for making progress. 

And as soon as these problems are sorted out, human organ regeneration too becomes a reality.

Footnotes:

6. McKim, L.H. (May 1932). "Regeneration Of The Distal Phalanx". The Canadian Medical Association Journal. 26 (5): 549–550. PMC 402335. PMID 20318716.
7. Wicker, Jordan; Kenneth Kamler (August 2009). "Current concepts in limb regeneration: A hand surgeon's perspective". Annals of the New York Academy of Sciences. 1172 (1): 95–109. Bibcode:2009NYASA1172...95W. doi:10.1111/j.1749-6632.2009.04413.x. PMID 19735243.
8. Weintraub, Arlene (May 24, 2004). "The Geniuses Of Regeneration". BusinessWeek.
9. Illingworth Cynthia M (1974). "Trapped fingers and amputated fingertips in children". Journal of Pediatric Surgery. 9 (6): 853–858. doi:10.1016/s0022-3468(74)80220-4. PMID 4473530.

10. Song Jeremy J (2013). "Regeneration and experimental orthotopic transplantation of a bioe.... Nature Medicine. 19 (5): 646–651. doi:10.1038/nm.3154. PMC 3650107. PMID 23584091.

11.  Ishizawa K, et al. (2004). "Bone marrow-derived cells contribute to lung regeneration after elastase-induced pulmonary emphysemal". FEBS. 556 (1–3): 249–252. doi:10.1016/s0014-5793(03)01399-1. PMID 14706858. S2CID 1334711.

12. Mohammadi, Dara (4 October 2014). "Bioengineered organs: The story so far…". theguardian.com. Retrieved 9 March 2015.

13. Quinn, Ben (21 October 2014). "Paralysed man Darek Fidyka walks again after pioneering surgery". The Guardian. Retrieved 26 October 2014. The 38-year-old, who is believed to be the first person in the world to recover from complete severing of the spinal nerves, can now walk with a frame and has been able to resume an independent life, even to the extent of driving a car, while sensation has returned to his lower limbs.

14.Walsh, Fergus (21 October 2014). "Paralysed man walks again after cell transplant". BBC. Retrieved 26 October 2014.

15. Blackburn, CC (April 2014). "Regeneration of the aged thymus by a single transcription factor". Development. 141 (8): 1627–1637. doi:10.1242/dev.103614. PMC 3978836. PMID 24715454.