Hypoplasia describes the situation where individual nephrons are structurally normal but their number is significantly reduced. It is a congenital problem and should be distinguished from acquired causes of reduced renal mass. When the renal tissue is abnormal, the term dysplasia is more accurate, but often hypoplasia and dysplasia coexist in the same kidney.
A number of variations are described:
Simple hypoplasia
Congenital small kidneys which are normal in shape. Intrauterine growth retardation is a major cause.
Oligomeganephronia
In this condition, as well as having small kidneys with reduced numbers of nephrons, the nephrons themselves are markedly hypertrophied. The glomeruli are at least twice normal size and tubules are enlarged.