Islet of Langerhans

UBERON:0000006 · Atlas One / Scale 05 — Tissue

Compact, ovoid endocrine clusters (100–200 µm) distributed throughout exocrine pancreatic tissue. ~1 million islets per adult pancreas (1–2% of mass; 5–10× higher blood flow per unit weight). β-cells form the central core; α/δ/PP cells form a peripheral mantle. KATP channel-coupled glucose sensing → Ca²⁺ influx → pulsatile insulin exocytosis. Loss or dysfunction underlies T1DM (autoimmune) and T2DM (progressive failure + insulin resistance).

Overview

Islets of Langerhans are compact, ovoid endocrine microorgans dispersed throughout the exocrine pancreatic tissue. Approximately one million islets exist per adult pancreas, representing only 1–2% of pancreatic mass yet receiving 5–10 times higher blood flow per unit weight than the surrounding acinar tissue. Each islet is a highly vascularised, autonomously regulated endocrine unit that monitors blood glucose, amino acid, and incretin levels in real time and secretes hormones to maintain glycaemia within the narrow physiological range of 4–7 mmol/L.

The five cell types of the islet act as a paracrine community, integrating signals from the portal blood, autonomic nervous system, and neighbouring islet cells through connexin-36 gap junctions, paracrine diffusion (aided by the centrifugal portal blood flow: β-cell core → α/δ-cell periphery), and direct neural input. Loss or dysfunction of β-cells underlies both type 1 diabetes mellitus (T1DM, absolute loss via autoimmune destruction) and type 2 diabetes mellitus (T2DM, progressive functional failure combined with insulin resistance), the two most prevalent metabolic diseases globally.

Structure — Cell Types and Architecture

Cell Type	Proportion	Hormone(s)	Location in Human Islet
β-cell	65–80%	Insulin, C-peptide, amylin (IAPP)	Central core; Cx36 gap junctions synchronise Ca²⁺ oscillations
α-cell	15–20%	Glucagon	Peripheral mantle; first bathed by β-cell secretions (paracrine inhibition)
δ-cell	5–10%	Somatostatin (SST-14/SST-28)	Peripheral mantle; SSTR2 on β-cells, SSTR1/2 on α-cells
PP/γ-cell	1–5%	Pancreatic polypeptide	Peripheral; predominantly in head of pancreas; inhibits pancreatic secretion
ε-cell	<1%	Ghrelin	Scattered; mainly fetal pancreas; ↓insulin secretion via growth hormone secretagogue receptor

KATP channel structure: An octameric complex of 4 Kir6.2 pore subunits (KCNJ11) + 4 SUR1 regulatory subunits (ABCC8). SUR1 contains two ABC domains: NBD1 binds ATP (inhibitory, closes channel); NBD2 binds MgADP (stimulatory, opens channel — provides negative feedback after secretion). Gain-of-function KCNJ11 or ABCC8 mutations → permanent channel opening → neonatal diabetes. Sulphonylureas (glibenclamide, gliclazide) bind SUR1 → channel closure → depolarisation → ↑insulin. Diazoxide opens KATP → ↓insulin (used in nesidioblastosis). Meglitinides (repaglinide) bind a distinct SUR1 site for faster meal-coupled secretion.

Function — GSIS Circuit and Paracrine Interactions

  GLUCOSE-STIMULATED INSULIN SECRETION (GSIS) — beta-cell
  ═══════════════════════════════════════════════════════
  Blood glucose rises (postprandial or IV glucose)
       |
       v  GLUT2 (low-affinity, high-capacity; Km ~15 mM)
  Glucose enters beta-cell
       |
       v  Glucokinase (GCK / HK IV; Km ~10 mM, Hill ~1.7)
          ** GLUCOSE SENSOR / THERMOSTAT **
  Glucose-6-phosphate
       |
       v  Glycolysis --> pyruvate --> TCA --> OXPHOS
  ATP/ADP ratio RISES  (from ~1:1 to ~10:1)
       |
       v  KATP channel CLOSES  (Kir6.2/SUR1)
  Plasma membrane DEPOLARISES  (~-70 mV --> ~-40 mV)
       |
       v  L-type Ca2+ channel opens  (Cav1.2, Cav1.3)
  [Ca2+]i RISES  (~100 nM --> ~500 nM)
       |
       v  SNARE-mediated exocytosis
          (VAMP2 / syntaxin-1A / SNAP25 complex)
  INSULIN GRANULE FUSION --> INSULIN + C-PEPTIDE secreted

  FIRST PHASE  (0-5 min): Pre-docked release-ready granules
  SECOND PHASE (5-60 min): Reserve pool granule recruitment via cytoskeleton

  ── AMPLIFYING SIGNALS (KATP-independent) ────────────────
  GLP-1 (L-cells) --> GLP-1R --> Gs --> cAMP --> PKA + Epac2
        --> potentiates GSIS; protects beta-cell survival
        ** Basis of semaglutide, liraglutide therapy **
  GIP (K-cells) --> GIPR --> Gs --> cAMP  (blunted in T2DM)
  FFA (NEFA) --> FFA1R/GPR40 --> Gq --> IP3 --> Ca2+
  Acetylcholine (cephalic phase) --> M3R --> Gq --> IP3/DAG

  ── ALPHA-CELL GLUCAGON SECRETION ────────────────────────
  Low glucose --> partial KATP open --> T-type Ca2+ + Na+ channel
        --> action potential --> glucagon granule exocytosis
  Paracrine INHIBITION by:
    - beta-cell insulin --> IR on alpha-cell --> Akt --> inhibition
    - beta-cell Zn2+ (co-secreted with insulin) --> K+ channel opening
    - delta-cell SST --> SSTR1/2 --> Gi --> cAMP reduction

  ── DELTA-CELL SOMATOSTATIN ─────────────────────────────
  Stimulated by: high glucose, amino acids, GLP-1
  Acts via SSTR2 (beta-cells, Gi --> cAMP reduction --> exocytosis)
  Acts via SSTR1/2 (alpha-cells, Gi --> glucagon inhibition)
  Paracrine brake on both insulin AND glucagon secretion

Biphasic Insulin Secretion and Pharmacology

First Phase (0–5 min)

Pre-docked, release-ready insulin granules at the plasma membrane. Immediately responsive to acute glucose rise. Blunted or absent in early T2DM — the earliest functional defect and important clinical biomarker. Captured by intravenous glucose tolerance test (IVGTT) or hyperglycaemic clamp.

Second Phase (5–60+ min)

Reserve pool granules recruited from cytoplasm via actin/tubulin cytoskeletal tracks. Dependent on ongoing Ca²⁺ influx + amplifying signals (GLP-1, GIP, ACh). Reduced in established T2DM; partially restored by weight loss, GLP-1 agonists, and sulphonylureas.

Sulphonylureas (SU)

Glibenclamide, gliclazide, glipizide, glimepiride → bind SUR1/ABCC8 → KATP closure → depolarisation → Ca²⁺ influx → ↑insulin secretion (glucose-independent). Risk: hypoglycaemia, weight gain. First-line T2DM after metformin in many guidelines.

GLP-1 Receptor Agonists

Semaglutide, liraglutide, dulaglutide → GLP-1R/Gs → cAMP → PKA + Epac2 → potentiate GSIS + ↓glucagon + β-cell survival. Weight loss via hypothalamic appetite suppression. Cardiovascular and renal benefits independent of glycaemia (LEADER, SUSTAIN-6, REWIND trials).

Pathology

Type 1 Diabetes Mellitus (T1DM)

CD4⁺ and CD8⁺ T cell autoimmune destruction of β-cells. Autoantigens: GAD65, IA-2/ICA512, ZnT8, proinsulin. HLA-DR3/DR4-DQ2/DQ8 haplotypes confer ~50% of genetic risk; PTPN22, INS VNTR, IL2RA contribute additionally. Complete β-cell loss → absolute insulin deficiency → fasting hyperglycaemia + ketoacidosis (unopposed glucagon → ↑hepatic ketogenesis). Treatment: basal-bolus insulin analogues; closed-loop systems (artificial pancreas); Edmonton protocol islet transplantation; Vertex VX-880 stem cell-derived β-cells (Phase 1/2: insulin independence achieved in trial).

Type 2 Diabetes Mellitus (T2DM)

Progressive β-cell functional failure superimposed on peripheral insulin resistance. Islet amyloid (IAPP/amylin aggregation → β-cell toxicity, membrane disruption), glucolipotoxicity (chronic ↑glucose + ↑FFA → ER stress + oxidative stress + mitochondrial dysfunction), and HIF-1α hypoxia → ~50% β-cell mass loss by T2DM diagnosis. First-phase insulin secretion loss is the earliest functional defect. Treatment escalation: lifestyle → metformin (↓hepatic glucose output) → sulphonylureas → GLP-1 agonists → DPP-4 inhibitors → SGLT2 inhibitors → insulin.

Insulinoma

β-cell adenoma (~90% benign); autonomous insulin secretion independent of blood glucose → fasting hypoglycaemia. Whipple's triad: symptomatic hypoglycaemia + documented low glucose + relief with glucose administration. C-peptide elevated (distinguishes from exogenous insulin use). Localised by endoscopic ultrasound + CT/MRI; curative: enucleation or distal pancreatectomy. Medical bridge: diazoxide (KATP opener → ↓insulin), octreotide (SSTR → ↓insulin).

Glucagonoma

α-cell tumour → chronic glucagon excess → necrolytic migratory erythema (NME, pathognomonic skin rash), new-onset diabetes, normochromic anaemia, weight loss, DVT/PE risk. ~80% malignant at diagnosis. Measure fasting glucagon (>500 pg/mL, often >1000). Octreotide (SSTR agonist) for symptom control; surgical resection; everolimus/streptozocin for metastatic disease.

Nesidioblastosis / PHHI (Persistent Hyperinsulinaemic Hypoglycaemia of Infancy)

Loss-of-function mutations in ABCC8 (SUR1) or KCNJ11 (Kir6.2) → constitutive KATP channel closure → permanent depolarisation → unregulated insulin secretion regardless of glucose → severe neonatal hypoglycaemia. Treatment: diazoxide (if SUR1-responsive); if unresponsive → near-total pancreatectomy. Gain-of-function GCK mutations also cause PHHI by ↑glucose sensitivity threshold.

Cross-Atlas Connections

Part ofPancreas (Scale 06) ModulatesLiver (portal insulin) ModulatesAdipocyte (anti-lipolysis) ModulatesHepatocyte (glycogen synthesis) Targeted byGLP-1 Agonists / SU / DPP-4i Damaged bySARS-CoV-2 (ACE2+ β-cells)

References

Rorsman P, Ashcroft FM. Pancreatic β-cell electrical activity and insulin secretion: of mice and men. Physiol Rev. 2018;98(1):117-214. doi:10.1152/physrev.00008.2017 · PubMed 29212789
Hall JE, Hall ME. Guyton and Hall Textbook of Medical Physiology. 14th ed. Elsevier; 2021. Ch. 78.
Alberts B et al. Molecular Biology of the Cell. 7th ed. W.W. Norton; 2022.
Atkinson MA, Eisenbarth GS, Michels AW. Type 1 diabetes. Lancet. 2014;383(9911):69-82. doi:10.1016/S0140-6736(13)60591-7 · PubMed 23890997
Drucker DJ. The biology of incretin hormones. Cell Metab. 2006;3(3):153-65. doi:10.1016/j.cmet.2006.01.004 · PubMed 16517403