General structure and development of the organ
The pancreas is covered by a thin connective tissue capsule. Its anterior surface is additionally covered by the visceral peritoneum. Septa extend inward from the capsule, dividing the organ parenchyma into lobules and creating a characteristic lobular structure. Blood vessels run within the gland stroma, and in rare cases, encapsulated nerve endings—lamellar corpuscles of Vater-Pacini (baro- and vibroreceptors)—can be found there.
Embryonic development of the gland begins from two outgrowths (dorsal and ventral) in the wall of the entodermal gut tube. These rudiments fuse, and a system of branching tubules forms within them. Exocrine tissue (acini) differentiates at the ends of these tubules. Endocrine tissue forms from epithelial "buds" on the walls of the tubules: they pinch off, lose connection with the ducts, and transform into Islets of Langerhans.
Exocrine part and structure of the acinus
The exocrine part makes up 97% of the gland's mass. Its main structural unit is the pancreatic acinus, which has a saccular shape. It consists of a secretory portion (a group of 8–12 large acinar cells) and an intercalated duct. Acinar cells synthesize pancreatic juice rich in enzymes for breaking down proteins (trypsinogen), carbohydrates (amylase), lipids (lipase), and nucleic acids.
Exocrine pancreatocytes are conical in shape and exhibit pronounced cytoplasmic polarity:
- Basal part (homogeneous zone): stains strongly basophilic (appears dark on the slide). Rough ER is highly developed here for intensive protein synthesis.
- Apical part (zymogenic zone): oxyphilic (light). Contains the Golgi apparatus and large granules with proenzymes (zymogens).
This polarity creates a characteristic microscopic picture of the acinus — "dark periphery and light center."
System of excretory ducts
The excretory system begins with intercalated ducts. They are lined with squamous epithelium and can branch off from the acinus sequentially or penetrate deep into it (in which case the duct cells are called centroacinar cells).
As the caliber of the ducts increases, the structure of their walls changes:
- Intercalated ducts: simple squamous epithelium.
- Intralobular and interacinar ducts: simple cuboidal epithelium. A connective tissue sheath appears around intralobular ducts for the first time.
- Interlobular ducts: simple columnar epithelium. Their distinctive marker is a thick layer of loose fibrous connective tissue surrounding them.
- Main duct: opens into the duodenum, equipped with a muscular sphincter at the outlet.
Large ducts feature mucus-secreting goblet cells and endocrine I-cells that secrete the hormone cholecystokinin to stimulate juice secretion.
Endocrine part (Islets of Langerhans)
Endocrine islets make up about 3% of the organ's mass (1–2 million units). On a slide, these are light, round, or oval areas penetrated by a dense network of sinusoidal capillaries. Islet cells (insulocytes) are smaller than acinar cells and stain lighter. Blood drains from them into the portal vein, delivering hormones to the liver.
Main types of insulocytes:
- B-cells (~70%): located predominantly in the center, pyramidal in shape. Synthesize insulin (lowers blood sugar, ensures nutrient uptake).
- A-cells (~20%): lie on the periphery of the islet. Secrete glucagon, which mobilizes carbohydrates from stores (raises blood sugar).
- D-cells (5–8%): secrete somatostatin, which inhibits the activity of other pancreatic cells.
- PP-cells (2–5%): produce pancreatic polypeptide, which stimulates the secretion of gastrointestinal juices.
- D1-cells (1–2%): secrete VIP (vasoactive intestinal peptide), which dilates blood vessels and lowers blood pressure (the existence of this cell type is not universally recognized).
Differential diagnostics on slides
Due to pronounced basophilia, the pancreas is easily confused with other organs. Main distinguishing criteria:
- From the parathyroid gland: excretory ducts are completely absent in the parathyroid gland.
- From the parotid salivary gland: in the salivary gland, interlobular ducts have a stratified epithelium, and intralobular ducts are striated. In the pancreas, interlobular ducts are lined exclusively with simple epithelium.
- Specific markers: presence of Islets of Langerhans (light patches of endocrine tissue) and two-tone heterogeneous staining of acini (dark basophilic periphery and light oxyphilic center).
Briefly: answers to topical questions
Which arteries supply blood to the pancreas?
Blood supply to the pancreas is provided by branches of two arterial systems: the celiac trunk and the superior mesenteric artery.
From the celiac trunk basin (truncus coeliacus):
- Superior pancreaticoduodenal arteries (aa. pancreaticoduodenales superiores) — branch off from the gastroduodenal artery, supplying the head of the organ.
- Splenic artery (a. splenica) gives off pancreatic branches (rami pancreatici) to the body and tail of the gland.
From the superior mesenteric artery basin (a. mesenterica superior):
- Inferior pancreaticoduodenal arteries (aa. pancreaticoduodenales inferiores) — supply the lower part of the head.
An arterial arch forms between the branches of these systems, creating an important anastomosis for collateral circulation.
Which hormones and neural mechanisms stimulate pancreatic juice secretion, in addition to cholecystokinin?
Pancreatic juice secretion is stimulated by the parasympathetic nervous system and a number of humoral factors.
Neural mechanisms:
- Parasympathetic stimulation via the vagus nerve (n. vagus, ACh) — enhances exocrine pancreatic secretion.
Hormones and humoral factors:
- Secretin — stimulates the exocrine part of the gland.
- Vasoactive intestinal polypeptide (VIP) — stimulates the secretion of pancreatic juice and hormones.
- Pancreatic polypeptide (PP) — stimulates the secretion of pancreatic and gastric juice.
- Bombesin, insulin, and bile salts are also listed among pancreatic secretion stimulants.
To memorize the topography of the main cells in the islet: The "Base" (B-cells) is always in the center of the islet and forms the majority (70%). A-cells (Vanguard) occupy the periphery.
Class and exam questions
Why does an exocrine acinus look two-toned?
This is due to the polarity of acinar cells. The basal part is rich in granular ER (stained basophilically dark), while the apical part contains zymogen granules (stained oxyphilically light).
What are centroacinar cells?
These are cells of the intercalated excretory duct in the case of an invaginated arrangement, where the duct does not simply continue the acinus but penetrates deep into it, forming a second layer.
Which cells produce cholecystokinin in the pancreas?
Cholecystokinin (pancreozymin) is produced by endocrinocytes (I-cells) located in the wall of large excretory ducts.
Where do hormones from the islets of Langerhans enter?
The islets are richly vascularized by sinusoid-type capillaries. Blood drains from them directly into the portal vein, allowing hormones to reach the liver.
What else to study on this topic
- Microcirculation features: fenestrated capillaries of acini and the portal system of the islets.
- Morphology and physiology of intermediate acino-insular cells.
- Mechanisms of neural regulation: influence of intramural ganglia and parasympathetic stimulation.
- Detailed ultrastructural composition of B-cell granules (crystalline state of insulin).
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