Introduction
Hyperglycaemia in type 2 diabetes results from insufficient and inappropriately sluggish insulin secretion in a context of diminished insulin action in target tissues [
1,
2]. These secretory defects often coexist with a relatively low beta cell number, commonly referred to as a decrease in beta cell mass. On average, this decrease amounted to 30–50% of normal in postmortem morphological comparisons of pancreatic samples from non-diabetic and type 2 diabetic subjects [
3‐
7]. Converging observations also indicate that the disease is bi-hormonal and entails abnormalities of glucagon secretion by alpha cells. Type 2 diabetic patients show excessive plasma glucagon levels for their glycaemia. This relative hyperglucagonaemia aggravates the consequences of their relative hypoinsulinaemia by activating glucose production in the liver [
8‐
10].
Inhibition of glucagon secretion by glucose encompasses direct effects on alpha cells and indirect actions via signals produced by neighbouring islet cells [
11]. Although intrinsic abnormalities in alpha cells are possible in type 2 diabetes, dysregulation of glucagon secretion is often attributed to perturbations in paracrine interactions between beta and alpha cells [
10]. Changes in alpha cell mass could also underlie such perturbations. Old postmortem studies using histochemical methods to distinguish beta and alpha cells have reported that alpha cell mass was unchanged [
12] or decreased by about 35% [
13,
14] in maturity onset-diabetes (type 2 diabetes) when compared with non-diabetic controls. More recent studies using immunohistochemical identification of the two cell types have reported that the relative volume density of alpha cells vs beta cells tended to be higher [
15,
16] or was significantly higher [
3,
4,
6,
17‐
19] in type 2 diabetic patients than in non-diabetic subjects. In some of these studies, alpha cell mass could be calculated and was reported to be decreased by about 45% [
16], unchanged [
15] or increased by 40–80% [
4,
17]. In all of these series, the small number of subjects (2–15 per group) and the high inter-subject variability unfortunately undermine the reliability of certain conclusions. We have therefore measured alpha cell mass in a larger series of non-diabetic and type 2 diabetic subjects whose beta cell mass was reported recently [
7].
Discussion
We report that the average pancreatic alpha cell mass is similar in a series of 52 non-diabetic subjects and 50 type 2 diabetic subjects. However, owing to the ∼36% decrease in beta cell mass in these type 2 diabetic subjects, their alpha cell/beta cell mass ratio is increased ∼1.7× compared with non-diabetic subjects.
The proportion of alpha and beta cells in human islets has previously been determined, usually in small numbers of islets from limited numbers of non-diabetic subjects. When identified nucleated cells were counted in isolated islets or in islets in situ, the ratio of alpha/beta cell numbers ranged from 0.37 to 0.55 [
18,
21‐
23]. When alpha/beta cell areas were measured by semi-automatic imaging programmes, the ratio ranged from 0.24 in isolated islets [
24] to 0.56 in islets in situ [
19]. Using a point counting method to examine islets in situ, we measured an average ratio of alpha/beta cell areas of 0.42 in 52 non-diabetic subjects. This value is in the upper range of previous measurements by point counting (0.28–0.44) in smaller series [
4,
6,
16,
17]. It is important to underline that measurements of relative cell area do not adequately reflect relative cell numbers because alpha cells are smaller than beta cells [
25]. Our measurements therefore underestimate the ratio of alpha/beta cell numbers but, unlike the latter, permit the correct calculation of alpha cell mass.
Recent studies focusing on the topography of the different cell types in islets have emphasised that, in contrast to rodent islets, the majority of alpha cells do not form a peripheral rim in normal human islets, but are concentrated in the centre [
21‐
24] in close proximity to vessels [
19]. Our observations agree with these descriptions. Moreover, our analysis of 100–300 islet profiles in each of 30 pancreas samples shows that the ratio of alpha/beta cell areas increases with apparent size of the islet, providing further quantitative support to previous conclusions [
19] that alpha cells are preferentially localised in the centre of large islets and in the mantle region of small islets. The reason why the proportion of alpha cells increases with the size of the islet [
6,
19] is not known, but this variation may complicate comparisons of data obtained in different studies if not all sections of islets are counted. Most importantly, we have also established that this relative distribution of alpha and beta cells is not perturbed in islets from type 2 diabetic subjects: the proportion of alpha cells also increases with the size of the islet profile, so that the difference between diabetic samples and controls is not restricted to one particular islet category.
Beta cell mass slightly increases with BMI in non-diabetic subjects [
5‐
7]. This increase is usually viewed as an adaptation to greater insulin needs to overcome the resistance of peripheral tissues. It was therefore unexpected that the ratio of alpha/beta cell areas did not decrease with the increase in BMI. We have no explanation, but wish to point out that beta cell mass was only 20% higher (972 vs 804 mg) in our group of overweight non-diabetic subjects (mean BMI 30 kg/m
2, no individual >40 kg/m
2) than in our group of lean non-diabetic subjects (mean BMI 22 kg/m
2) [
7]. It is also notable that ageing was accompanied by a small decrease of both alpha and beta cell mass in non-diabetic subjects.
From our measurements, we can conclude that the relative hyperglucagonaemia of type 2 diabetic subjects is not the consequence of an increased alpha cell mass. The elevated ratio of alpha/beta cell areas measured in the pancreas of these subjects is attributable to the decrease in beta cell number [
7]. If alpha cell function is controlled by direct contact between the two cell types or by products released by beta cells (insulin or others), and reaching alpha cells via either a paracrine route or the islet microvasculature, perturbations of glucagon secretion may obviously result from the lower relative number of beta vs alpha cells. This may be particularly true in the generally large islets quasi-exclusively composed of alpha cells. However, qualitative and quantitative relationships between both cell types are not consistently perturbed in all islets from all type 2 diabetic subjects. Some type 2 diabetic individuals have beta and alpha cell masses similar to those of non-diabetic individuals. One insurmountable limitation in the interpretation of our results, obtained from autopsy specimens, is the impossibility of correlating the extent of dysregulation of glucagon secretion with changes in the cellular composition of islets. In the absence of changes in islet cell numbers, relative hyperglucagonaemia might be secondary to functional perturbations of beta cells [
10] or intrinsic defects of alpha cells.
Acknowledgements
We thank S. Godecharles for technical assistance, S. Lagasse and M. Nenquin for help in the preparation of figures, and Y. Guiot for advice on technical issues. This work was supported by grant ARC 05/10-328 from the Direction de la Recherche Scientifique de la Communauté Française de Belgique and grants 3.4615.05 and 3.4530.08 from the Fonds National de la Recherche Scientifique et Médicale, Bruxelles.