The exocrine pancreas makes up 90% of the pancreatic tissue, the acinar cells being the most numerous.  Acinar cells are recognized easily as they make up most of cells in histological analysis of the pancreas.  They are easily recognized as being the most numerous and by their cuboidal features, usually surrounding a pancreatic duct. Because acinar cells make a lot of digestive enzymes, a large nucleus is necessary and thus easily identifiable.

Conversely, the endocrine pancreas makes up <10% of the pancreatic tissue. However, the endocrine cells cluster together in ‘little islands in the sea of acinar cells’. Hence, the term Islet cells.  Islet cells do not stain as intensely as acinar cells, mostly because their nuclear size and activity is not as great as acinar cells.  As mentioned, the islet cells are a combination of alpha, beta, gamma, and delta cells.  Histologically, one can not differentiate between the cells based on shape nor nuclear size.  Immunological histochemistry (i.e. using dyes attached to antibodies that are specific for each islet cell type) would be needed to identify the cells within the islets.  Because islet cells must sense blood glucose levels for the purpose of releasing their hormones,  islet cells are often interspersed with capillaries and are often near a blood vessel within the pancreas.

^{DHPLC Specimen PATH 304-016 – Normal pancreas with H&E staining. Created by Jennifer Kong licensed under All rights reserved
Video Summary –  The bulk of pancreatic tissues are made of acinar cells.  Acinar cells make digestive enzymes (lots of protein synthesis) which are secreted into a central ductule. Thus giving acinar clusters staining dark pink/purple in a circular pattern: acinar nuclei on the outside border and the ductule in the centre.  In contrast, islets are clusters of cells that don’t stain as dark (i.e. not as much protein present) with a central nuclei. These islet cells are interspersed with capillaries, identifiable by the spindly endothelial cells of the capillary wall and the red blood cells.  Islets make up <10% of the pancreatic tissues and are often found near blood vessels.}

Brief Review of Insulin Release and Glucose Entering Tissue

Revisit this link , from the “Anatomy and histology of the endocrine pancreas and glucose homeostasis” chapter to view an animation describing the role of insulin.

^{Video was created by Sarah Perkins, according to CC-Attributions – NC  license.
Video Summary – After a meal, there is a steep rise in nutrients in the blood.  These blood nutrient levels – especially glucose (gold hexagons) – will be sensed by the endocrine pancreatic islet cells. When beta cells sense high blood glucose, it releases insulin into the bloodstream.}

^{Video was created by Sarah Perkins, according to CC-Attributions – NC  license.
Video Summary – When insulin (purple triangle) reaches the tissue, it will bind to the insulin receptor (blue).  Insulin receptors are on almost every cell type in the body.  Upon binding with insulin, the insulin receptors activates  glucose transporter (green) to allow for glucose (gold hexagons) to enter the cell via facilitated diffusion.  Once inside, glucose can be immediately metabolized to produce ATP or stored as glycogen (e.g. in liver in the process known as glycogenesis)}

^{Video was created by Sarah Perkins, according to CC-Attributions – NC  license.
Video Summary – For metabolically active tissues such as contracting skeletal muscle, insulin is needed for immediate glucose influx and metabolism within muscle cells.  The resulting ATP will fuel the muscle contraction process.}

Although not quite fully understood, contracting muscles can also allow for glucose entry into cells in an insulin-independent manner.^[1]

Section Summary

The pancreas is mostly made out of acinar cells which perform the exocrine function.  Acinar cells produce digestive enzymes which drain into pancreatic ducts.   Pancreatic islet cells are less numerous and are distributed infrequently within the pancreas, but are close to blood vessels.  Islet cells are responsible for the performing the endocrine function, constantly monitoring blood glucose levels and releasing hormones such as insulin and glucagon as necessary in order to maintain blood glucose homeostasis.

Review questions