Glands: endocrine and exocrine

Exocytosis (exo- = outside, cyt/o = cell) is the fusion of a vesicle with the plasma membrane, which empties the vesicle's contents outside the cell. Endocytosis brings material in; facilitated diffusion moves single molecules through carrier or channel proteins.

• Endocytosis:
• Correct: Exocytosis:
• Facilitated diffusion:

The apical surface faces the lumen or the outside. The basal surface faces the basement membrane and the connective tissue beneath it.

• The basal surface:
• Correct: The apical surface:
• The basement membrane:

A hormone changes only its target cells: the cells with a receptor protein that binds it. Other cells are exposed to it but do not respond.

• Every cell it touches:
• Correct: Target cells that carry a matching receptor protein:
• Only cells next to the gland that made it:

No duct plus a rich capillary supply is the signature of an endocrine gland. Its cells release hormones into the interstitial fluid, and the hormones diffuse into the nearby capillaries.

• A simple tubular exocrine gland: A simple tubular gland is exocrine, so it has a duct running to a surface. The absence of any duct rules it out.
• A mucous gland: A mucous gland is a kind of exocrine gland. It releases mucus through a duct onto a surface, and no duct is present here.
• Correct: An endocrine gland: Correct. Ductless cells surrounded by capillaries release their product toward the blood: that is an endocrine gland.
• A unicellular exocrine gland: A unicellular exocrine gland, such as a goblet cell, is a single cell sitting in a surface lining. This is a cluster of cells with no surface to empty onto.

The pancreas holds two kinds of gland. Its enzyme-making cells are exocrine and drain through the duct, so a blocked duct stops enzymes reaching the gut and fat goes undigested. Its hormone clusters are endocrine: they release into interstitial fluid and capillaries, so a blocked duct does not affect them.

• The tumor destroyed the hormone-making cells first: If the hormone-making cells were destroyed, hormone levels would fall. They stayed normal.
• Enzymes and hormones share the same duct: If both products shared the duct, blocking it would lower both. Normal hormone levels show the hormones take a different route.
• The tumor made hormones that replaced lost ones: Nothing in the case suggests the tumor makes hormones, and it would not explain the undigested fat. The simpler explanation is that the two products take different routes.
• Correct: Enzymes need the duct; hormones go straight into the blood: Correct. Blocking the duct stops the exocrine product (enzymes) but not the endocrine product (hormones), which never used the duct.

Name the duct first, then the secretory part. A branching duct makes the gland compound, and rounded sacs make it alveolar (also called acinar).

• Simple tubular: Simple means one unbranched duct, and tubular means tube-shaped secretory parts. This gland has neither feature.
• Simple alveolar: The sacs fit alveolar, but simple means an unbranched duct. This duct branches repeatedly.
• Correct: Compound alveolar: Correct. A branching duct (compound) that ends in rounded sacs (alveolar) is a compound alveolar gland.
• Compound tubuloalveolar: The branching duct fits compound, but a tubuloalveolar gland has both tube-shaped and sac-shaped secretory parts. Every branch here ends in a rounded sac.

In holocrine secretion the whole cell becomes the product. Cells fill with product, break apart, and are replaced by new cells from dividing stem cells at the base.

• Merocrine: In merocrine secretion the cell releases vesicle contents by exocytosis and stays whole. Here the cells break apart.
• Correct: Holocrine: Correct. Whole cells breaking apart and joining the secretion, with dividing cells at the base replacing them, is holocrine secretion.
• Apocrine: In apocrine secretion only the apical part of the cell pinches off, and the rest of the cell survives. Here whole cells break apart.
• Endocrine: Endocrine describes where a product goes (into the blood), not how a cell releases it. It is not a mode of secretion.

• Number of new secretory cells produced in the gland: down. New holocrine cells come from stem cells dividing by mitosis. Stopping mitosis stops that supply.
• Amount of secretion the gland releases: down. Each holocrine cell is used up when it breaks apart. With no new cells replacing them, fewer cells are left to fill and break apart, so output falls.
• Output of a nearby merocrine gland whose cells are not dividing: no change. A merocrine cell releases its product by exocytosis and survives, so it does not depend on a steady supply of new cells. Its output is unaffected.

Exocrine and endocrine describe the route, not the chemical. A duct to a surface means exocrine; release into interstitial fluid and then blood means endocrine. The student's rule fails whenever a product breaks the pattern, and it cannot classify mucus at all.

• Whether the product is a protein: Enzymes are proteins, and so are many hormones. Knowing the product is a protein does not tell you which way it leaves.
• Whether the cells store their product in secretory vesicles: Both exocrine and endocrine cells can store their product in secretory vesicles and release it by exocytosis.
• Correct: Whether the product leaves through a duct or passes into the blood: Correct. Tracing where the product goes is the test that defines the two kinds of gland.
• Whether the gland is larger than a single cell: Size separates a goblet cell from larger exocrine glands, but it does not separate exocrine from endocrine: both kinds include many-celled glands.

In the milk-making gland of the breast, fat droplets leave from the apical end of the cell wrapped in a piece of its plasma membrane: true apocrine release. Some exams instead name the "apocrine" sweat-producing glands of the armpit, but electron microscope studies show those release mainly by exocytosis, so their name is historical.

• A goblet cell releasing mucus: A goblet cell releases mucus by exocytosis and stays whole. That is merocrine secretion.
• The oil glands of the skin releasing their oily product: The oil glands of the skin are holocrine: whole cells fill with product and break apart.
• The thyroid gland releasing its hormone: The thyroid gland is endocrine. The three mode names describe exocrine glands, and its hormone release is not apocrine.
• Correct: Milk-making cells of the breast releasing droplets of milk fat: Correct. Fat droplets pinching off in a wrap of plasma membrane is the clearest example of apocrine release.