Anatomy 9-3

Anatomy and Physiology, Grades 11 and 12

Module #2: Histology, The Study of Tissues

Lecture 4  —  Glandular Epithelium

Text pages 42 to 44, Figures 2.7 and 2.8  |  40 Minutes  |  Mr. Gibbs

At a Glance

• 0:00 to 0:05  Bell work, silent

• 0:05 to 0:08  Bell work reviewed

• 0:08 to 0:15  Two kinds of gland

• 0:15 to 0:20  The structure of an exocrine gland

• 0:20 to 0:33  Merocrine, apocrine, and holocrine

• 0:33 to 0:38  Endocrine glands

• 0:38 to 0:40  Close, and On Your Own 2.5 and 2.6

Materials: whiteboard, textbook open to Figures 2.7 and 2.8. A tube of toothpaste and a paper towel for the demonstrations.

This lecture changes the question. Every tissue so far was classified by what it LOOKS like. Glands are classified by what they MAKE and HOW they release it. Say that transition out loud at the start, because students will keep trying to sort glands by cell shape.

Bell Work

5 minutes silent.

1.  Why are the cells at the surface of your skin dead?

2.  Stratified transitional epithelium has fewer layers when it is stretched. Why does that make sense?

3.  What makes pseudostratified epithelium only FALSELY layered?

4.  Name the two structures every gland must have in order to deliver something to a surface. Guess if you do not know.

1. Bell Work Reviewed

3 minutes. Question 4 is a guess and it sets up the lecture.

• 1: the basement membrane is avascular, so oxygen and nutrients have to diffuse up from below. The upper cells are too far away to get enough, so they die.

• 2: when things stretch, they get thinner. The cells slide with respect to one another.

• 3: every cell touches the basement membrane, so it is really one layer, but the cells are different heights so not all of them reach the free surface.

• 4, take guesses and do not correct them: somebody will say something that makes something, and something that carries it out. They are right. The book calls them the secretory portion and the duct, and that is where we start.

2. Two Kinds of Gland

7 minutes. Put both definitions on the board word for word.

• Open where the book opens: we want to spend some time on the epithelial tissue that you find in GLANDS. In this section you will learn what a gland is and how it works. First of all, there are TWO types of glands: EXOCRINE GLANDS and ENDOCRINE GLANDS.

• EXOCRINE GLANDS: glands that secrete substances OUTWARD through a DUCT.

• ENDOCRINE GLANDS: DUCTLESS glands that secrete HORMONES into the BLOODSTREAM.

BOOK LANGUAGE:  These two definitions are set apart and underlined in the book on page 42. Write them exactly. The single distinguishing feature is the DUCT. Exocrine glands have one. Endocrine glands do not.

• Use the book’s prefix, since it makes it memorable: the prefix "exo" means "out of," so exocrine glands have ducts that allow them to secrete a product OUT to a surface. Endocrine glands, on the other hand, have NO ducts. They usually secrete chemicals called HORMONES directly into the bloodstream.

• Then follow the book’s order: let’s start with exocrine glands.

Where the product goes

• Make the point the book makes on page 43, because students assume outward means outside the body: exocrine glands such as MAMMARY GLANDS secrete to a surface on the OUTSIDE of the body. In the case of mammary glands, milk is secreted onto the surface of the skin so that it can be ingested by an infant.

• But not always: other exocrine glands, like the ones in the STOMACH, secrete to a surface INSIDE the body. For example, stomach glands secrete onto the surface inside of the stomach, so that their secreted chemicals can be used in digesting what is in the stomach.

• Clarify what outward really means: outward means onto a SURFACE, through a duct. That surface can be the outside of you or the inside of a hollow organ. It does not mean into the blood, and that is the whole contrast with endocrine.

3. The Structure of an Exocrine Gland

5 minutes. Figure 2.7 on page 42.

• Give the two parts: there are lots of exocrine glands in the body, and they come in different shapes and sizes. Despite their different shapes and sizes, however, they have a basic structure. They ALL have a SECRETORY PORTION, which contains the cells that do the secreting, and they ALL have a DUCT, through which the chemicals travel to reach the surface.

SHOW OR DEMONSTRATE:  Figure 2.7 on page 42 shows six different exocrine gland shapes. The book says plainly that you need NOT learn or memorize the figure. Say that to your students out loud, because a page of six diagrams looks like something to memorize and it is not. What they should take from it is that regardless of shape, they all have secretory regions and ducts.

• Have them find the three labels on the figure that appear on the sebaceous gland: surface, duct, and secretory portion. Then have them locate the same two parts on two other shapes.

• The six examples in the figure, for reference: stomach and colon glands, glands in the lower portion of the stomach and intestines, sebaceous glands in the skin, salivary glands, mucus glands of the duodenum, and pancreas and mammary glands.

• Then flag what matters next, as the book does: the NEXT figure is more important for you to learn, because it details HOW these exocrine glands do their jobs.

4. Merocrine, Apocrine, and Holocrine

13 minutes. Figure 2.8 on page 43. This is the heart of the lecture.

• Frame it as a classification: there are THREE ways that an exocrine gland can secrete chemicals, and that provides a way of classifying exocrine glands.

The three definitions, exactly as the book gives them

• MEROCRINE GLANDS: exocrine glands that secrete WITHOUT LOSING CELLULAR MATERIAL.

• APOCRINE GLANDS: exocrine glands that have CYTOPLASM in their secretions.

• HOLOCRINE GLANDS: exocrine glands whose secretions are made up of DISINTEGRATED CELLS.

• Then say what the book says next, because it is reassuring and true: "Don’t worry if you find these definitions hard to understand." Figure 2.8 illustrates these different modes of secretion, and the figure is far clearer than the definitions are.

SHOW OR DEMONSTRATE:  Figure 2.8 on page 43 shows all three side by side with captions. Merocrine: cells secrete via exocytosis, no cellular material in the secretion. Apocrine: cells secrete by pinching off a part of themselves, which puts cellular material like cytoplasm in the secretion. Holocrine: cells secrete by being ejected into the duct, then rupture, releasing their secretion. Walk all three captions aloud and have students trace the labeled parts on each.

What is actually happening, in order

• MEROCRINE, pronounced mer’ oh krin: the cells excrete the chemicals via EXOCYTOSIS, as described in Module 1. Nothing of the cell is lost.

• Call back to Module 1: they already know exocytosis. A vesicle fuses with the membrane and dumps its contents outside. The cell is unharmed. That is merocrine secretion, and they learned the mechanism weeks ago.

• APOCRINE, pronounced ap’ oh krin: these glands actually PINCH OFF a portion of the epithelial cell that is secreting. That cellular material then becomes part of the secretion. That is why the definition says that cytoplasm is a part of an apocrine gland’s secretions. After all, the cell contains cytoplasm. If a part of the cell gets pinched off, there will be cytoplasm in it, and that cytoplasm will become a part of the secretion.

• HOLOCRINE, pronounced hol’ oh krin: these glands actually EJECT ENTIRE CELLS into the duct. Those cells RUPTURE, spilling their contents into the duct. The cells then get replaced by MITOSIS.

The prefixes, and the three examples

• Give the book’s memory device, because it works: if you have trouble remembering the definitions, take the words apart. The suffix "crine" means SECRETION.

• "MERO" means A SMALL PART. Thus merocrine means only a PRODUCT is secreted. Certain SWEAT GLANDS are an example of merocrine glands. They secrete a substance with water and salt and some other products in it.

• "APO" means A PART OF. Thus the cells in apocrine glands release A PART OF THEMSELVES. MAMMARY GLANDS are apocrine glands. That explains why a mother’s milk is such a nutritious food. It does not just contain this or that, it contains CELL PARTS.

• "HOLO" refers to the WHOLE cell. The SEBACEOUS GLANDS of the skin are holocrine cells. They secrete oil which softens the skin and, at certain points of your life, is responsible for ACNE. That oil contains entire cells that have ruptured. Those cells, of course, must be constantly replaced by mitosis.

The acne line will get their attention, and it is worth using. Sebaceous glands eject whole cells that burst, and that oil is what causes acne. Sixteen year olds have a personal stake in holocrine secretion.

SHOW OR DEMONSTRATE:  A three step demonstration with a tube of toothpaste and a paper towel. MEROCRINE: squeeze a little out of the tube. The tube is undamaged. APOCRINE: tear off a small piece of the tube itself and put it with the paste. Part of the container is in the product. HOLOCRINE: drop the whole tube onto the towel and squash it. The entire container is destroyed and everything spills out. Crude, and students will remember it.

5. Endocrine Glands

5 minutes. Back to the second half of the definition from the start of class.

• Introduce them as the book does: now let’s turn our attention to endocrine glands for just a moment. These are glands which secrete messenger chemicals called HORMONES into the BLOOD. Remember, they have NO DUCTS. They secrete directly into the blood.

• Why that matters, and this is the key advantage: from there, the hormones can be taken ALL OVER THE BODY, since the blood circulates all over the body. So the hormones from the THYROID, for example, go into the blood and can affect cells FAR AWAY from the thyroid.

• Set the scope, as the book does: in a future module we will talk about how hormones can cause profound changes. For right now, we just want to stress how DIFFERENT their MICROSCOPIC ANATOMY is compared to exocrine cells.

What an endocrine gland looks like under the scope

• The structure: in an endocrine gland, you have got CELL PACKED ON CELL. That is typical of epithelial tissue.

• The big difference: there is NO DUCT in endocrine glands. There are tiny blood vessels called CAPILLARIES running through the cells.

• And how the delivery works: the blood takes oxygen and nutrients TO the cells, but it also PICKS UP anything the cells secrete. The endocrine cells secrete their hormones by EXOCYTOSIS, as described in Module 1, and the blood in the capillaries carries the hormones throughout the body.

• Draw both on the board side by side: an exocrine gland with a duct running to a surface, and an endocrine gland with capillaries running through it and no duct at all. That one picture is the whole distinction.

Notice that BOTH merocrine exocrine glands and endocrine glands secrete by exocytosis. The mechanism at the cell is the same. What differs is where the product goes, into a duct or into the blood. If a student catches that, they have understood the section.

6. Close, and On Your Own

2 minutes.

• Put these two on the board and have students work them aloud. They are On Your Own 2.5 and 2.6 from page 44:

• 2.5  The secretions of an exocrine gland are examined, and it is determined that there is cytoplasm in the secretions. This narrows the type down to two. Which two types could it be?

• 2.6  What analysis could be done to determine which of the two types of exocrine gland is being studied?

• For your reference on 2.5: apocrine and holocrine. Both put cellular material into the secretion. Merocrine is ruled out because it loses no cellular material at all.

• For your reference on 2.6: look for WHOLE CELLS, or the remains of whole cells, in the secretion. Apocrine secretions contain pinched off parts of cells, so cytoplasm but not entire cells. Holocrine secretions contain entire ruptured cells. Finding complete cells or cell membranes and nuclei points to holocrine.

• Preview: the book next moves to a microscope experiment on glands in tissue samples. Tell them whether you will be running that with the slide set.

• Assign the reading. Dismiss by procedure.

Objectives

• Students will define exocrine and endocrine glands and identify the duct as the distinguishing feature.

• Students will identify the secretory portion and duct of an exocrine gland.

• Students will distinguish merocrine, apocrine, and holocrine secretion and give an example gland for each.

• Students will explain how endocrine glands deliver hormones using capillaries rather than ducts.

• Students will analyze a described secretion and determine which type of exocrine gland produced it.