The structural and biochemical ontogeny of 14 mid-trimester (12.0–19.5 wk) human fetal endocrine pancreata were examined. Each pancreas was halved at the region of the isthmus into duodenal (D) and splenic (S) portions, and islets, isolated from each of the two portions, were aliquoted and grouped according to diameter into 4 categories: (1) > 300 μm (2) 300–200 μm (3) 200–100 μm (4) 100 μm.
At all fetal ages, the number of islets isolated from the splenic half was greater than from the duodenal half, and the number of islets increased as their diameter decreased. The mean total cell number per islet for each of the 4 categories are reported: (1)(D) 30,810 (S) 28,450; (2)(D) 10,970 (S) 10,250; (3)(D) 3,390 (S) 3,460; (4)(D) 1,930 (S) 1,840.
Measurement of islet insulin (IRI), glucagon (IRG), and somatostatin (IRS) by radioimmunoassay showed a progressive increase with time in the content of all three hormones in all islets from both halves of the pancreas (with the exception of the smallest islets of <100 μm diameter). After 18 wk, a dramatic increase in the content of all three hormones was observed particularly in the case of the largest (>300 μm) islets. In contrast, pancreatic polypeptide (IRPP) shows no significant increase. With the exception of the smallest islets (100 μm), all splenic-half islets contain a significantly greater content of IRI, IRG, and IRS compared with their duodenal-half match while the content of IRPP is significantly greater in the duodenal-half islets. In addition, all islets with the exception of the largest (>300 μm), contained equal quantities of IRG and IRS and these results were apparent at 12 wk gestation.
These data provide new insights into the ontogeny of individual islet cells during the mid-trimesteric period and serve as a basis for future studies of both normal and abnormal pancreatic development.