General Structure and Development
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 course through the stroma, and encapsulated nerve endings—lamellar corpuscles (Vater-Pacini corpuscles acting as baro- and vibroreceptors)—can occasionally be found there.
Embryonic development of the pancreas begins from two outgrowths (dorsal and ventral) in the wall of the endodermal gut tube. These buds 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 tubule walls: they pinch off, lose their connection with the ducts, and transform into islets of Langerhans.
Exocrine Part and Acinar Structure
The exocrine portion constitutes 97% of the gland's mass. Its main structural unit is the pancreatic acinus, which has a saccular shape. It consists of a secretory unit (a group of 8–12 large acinar cells) and an intercalated duct. Acinar cells synthesize pancreatic juice rich in enzymes for digesting proteins (trypsinogen), carbohydrates (amylase), lipids (lipase), and nucleic acids.
Exocrine pancreatocytes are conical in shape and exhibit pronounced cytoplasmic polarity:
- Basal region (homogeneous zone): stains strongly basophilic (appears dark on histological preparations). Rough endoplasmic reticulum (RER) is highly developed here for intensive protein synthesis.
- Apical region (zymogen zone): acidophilic (light). Contains the Golgi apparatus and large granules containing proenzymes (zymogens).
This polarity creates the characteristic microscopic appearance of the acinus: a "dark periphery and light center."
Duct System
The duct system begins with intercalated ducts. They are lined by simple squamous epithelium and may originate sequentially from the acinus or invaginate directly 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 first appears around intralobular ducts.
- Interlobular ducts: simple columnar epithelium. Their distinguishing marker is a thick layer of loose connective tissue surrounding them.
- Main pancreatic duct: opens into the duodenum, equipped with a muscular sphincter at its termination.
Large ducts contain mucous goblet cells and endocrine I cells, which secrete the hormone cholecystokinin to stimulate juice secretion.
Endocrine Part (Islets of Langerhans)
Endocrine islets account for about 3% of the organ's mass (1–2 million in total). On histological sections, these are pale, rounded, or oval areas permeated by a dense network of sinusoidal capillaries. Islet cells (insulocytes) are smaller than acinar cells and stain less intensely. Blood drains from them into the hepatic portal vein, delivering hormones directly to the liver.
Main types of insulocytes:
- B cells (~70%): located predominantly in the center, pyramidal in shape. Synthesize insulin (lowers blood glucose, promotes nutrient uptake).
- A cells (~20%): located at the periphery of the islet. Secrete glucagon (mobilizes stored carbohydrates, raises blood glucose).
- D cells (5–8%): secrete somatostatin, which inhibits the activity of other pancreatic cells.
- PP cells (2–5%): produce pancreatic polypeptide, which stimulates gastrointestinal secretions.
- 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 Diagnosis on Histological Sections
Due to its prominent basophilia, the pancreas can easily be confused with other organs. Key distinguishing criteria:
- From the parathyroid gland: parathyroid glands completely lack duct systems.
- 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 by simple epithelium.
- Specific markers: the presence of islets of Langerhans (pale patches of endocrine tissue) and the biphasic heterogeneous staining of acini (dark basophilic periphery and light acidophilic center).