Brief Answer:
Conversion of blood vessel cells into bone cells.

Detailed Answer:
Hi Wendee,
Thanks for consulting us.
Cutaneous calcification, or calcinosis cutis (CC), is the name given to the calcium deposits under the skin. It is seen in some patients undergoing chronic dialysis.
Well! the pathogenesis or cause is not fully understood, it may be similar to those for vascular calcification seen in patients with ischaemic heart disease.
This is supposed to be an actively regulated process.
This condition was found to be associated with female sex, vascular disease, secondary hyperparathyroidism, and elevated levels of plasma calcium-phosphorus product.
Some patients had a favorable outcome after lowering of plasma calcium levels during dialysis or after parathyroid gland removal.
The frequency of CC in ESRD ( End stage renal disease has been reported to increase with the age of the patients. It is quite rare in comparison with non-cutaneous vascular calcification. Calcification of non-cutaneous ( other then skin) blood vessels is also a common feature of ESRD. This is often associated with the increasing age, diabetes mellitus, time since dialysis therapy began, dyslipidemia, arterial hypertension, hyperhomocysteinemia, hyperphosphatemia, high calcium-phosphorus product (Ca × P), hypervitaminosis D, and excessive daily intake of calcium salts.
Other risk factors for CC are low levels of serum albumin, high serum phosphate levels, high levels of total alkaline phosphatases and morbid obesity.
It occurs mostly because of the transformation of smooth muscle vascular cells, or myofibroblasts, into osteoblast like cells ( bone forming cells). These in turn may produce calcium deposits like they do in bone!
This may promote the conversion of vascular cells into bone making cells.
I admit that this is a complicated discussion. I tried to mention some simple & common facts in this answer. Please write back if you need any more simplification.
Wish you good health.

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