Lining Epithelium Histology Laboratory -‐ Year 1, Fall Term Dr

Epithelium 1: Lining Epithelium Histology Laboratory -‐ Year 1, Fall Term Dr. Heather Yule ([email protected]) September 30, 2014

Slides for study: 119 (Kidney), 44 ( Skin), 116 (Gallbladder), 107 (Colon), 60 (Epiglottis), 93 (Auditory Tube), 123A (Human Urinary Bladder), 123B ( Pig Urinary Bladder), 63A (Trachea), 121 (Kidney, PAS)

Electron m icrographs for study: EM: Stratified squamous keratinized epithelium in thin skin EM: Striated (brush) border in intestinal epithelium EM: Two adjoining epithelial cells with their basal lamina

Main Objective: Using key histological features, name, identify and locate the different types of lining epithelia and relate structure to function.

Specific Objectives:

1. Describe the nomenclature for and identify different types of lining epithelia.

2. Explain where you would find simple vs. stratified epithelia, keratinized vs. non-‐keratinized epithelia, pseudostratified epithelia and transitional epithelia.

3. Describe why epithelial tissue is more susceptible to malignant transformation than other tissues.

4. Differentiate between secretory and ductal portions of an eccrine sweat gland.

5. Identify and describe functions for cell specializations found on apical, lateral and basal borders of epithelial tissue.

Keywords: simple squamous, simple cuboidal, simple columnar, mucous cells, stratified squamous keratinized, keratinocyte, stratified squamous non-‐keratinized, stratified columnar, stratified cuboidal, eccrine sweat gland, myoepithelial cell, pseudostratified ciliated columnar epithelium with goblet cells/respiratory epithelium, transitional epithelium/urothelium, umbrella cell, basement membrane, basal lamina

“This copy is made solely for your personal use for research, private study, education, parody, satire, criticism, or review only. Further reproduction, fixation, distribution, transmission, dissemination, communication, or any other uses, may be an infringement of copyright if done without securing the permission of the copyright owner. You may not distribute, e-mail, or otherwise communicate these materials to any other person.” (Full statement from http://copyright.ubc.ca/faq/digital-classroom/) Today’s lab will take you on a journey through the body examining various types of lining epithelia. We will stop and admire the view at only the best locations and we will examine only specific points of interest on each slide. You will revisit these slides over the course of the next two years. For example, slide 107 of the colon will be examined in detail in Lower the “ Gastrointestinal Tract” lab in the fall term of second year.

Let’s start with simple epithelia. In general, they are located in areas where there is diffusion or transport of material across a lining. In order to facilitate this movement, the lining is thin (single layer) and cell surface specializations may be present.

Simple Squamous – 119 Kidney – Bowman’s capsule (UBC, CPS Virtual Slidebox, Slide #119) Renal corpuscles lie in the outer cortex of the kidney. They are composed of a tuft of capillaries surrounded by Bowman’s c apsule. The outer parietal layer of Bowman’s capsule is an excellent example of simple squamous epithelium. Can you find the parietal layer of a Bowman’s capsule in slide 119 and identify the squamous epithelial cells?

Simple Cuboidal – 119 Kidney – Renal Tubules Surrounding the renal corpuscles is a vast network of renal tubules. There are two types of tubules in the cortex, proximal and distal convoluted , tubules both lined by simple cuboidal epithelium. Promixal convoluted tubules have an extensive brush border composed of microvilli facing the lumen (Please see the EM “Striated (brush) border in intestinal epithelium” for brush border ultrastructure. ‘EM on Medicol’ and found in ‘Netter’s Essential Histology, Ovalle and Nahirney, Fig .2.5’ Used with permission. © Copyright 2008 Elsevier Inc. All rights reserved.). In contrast, distal convoluted tubule cells lack a brush border and thus individual cells appear smaller and the lumen of the tubule often appears larger. We will look at renal tubules in more detail in the “Urinary System” lab later in the term. Please do NOT spend a lot of time today differentiating each type of renal tubule! Can you locate a renal tubule and find the nucleus of one cuboidal epithelial cell?

Simple Columnar – 116 Gallbladder and 107 Colon (UBC, CPS Virtual Slidebox – Slides #116 and 107) The lining epithelium of the gallbladder is located on the right side of slide 116. At high power, tall columnar epithelial cells are visible with oval shaped i nucle at their base. In addition to lining the lumen of the gallbladder, what is the function of this epithelium? Hint: Can you see the brush border?

Another good example of simple columnar epithelium is found lining the lumen of the colon. If you look closely, you will notice that some cells in this epithelium stain poorly with H&E. These are mucous cells and you will learn more about them in the “Glandular Epithelium” lab.

Clinical Point Epithelial tissues (both lining and constantly glandular) undergo cell division to regenerate or repair themselves . Therefore, these tissues are common sites of tumor formation. In general, umors t arising epithelium from are called carcinomas , whereas tumors specifically arising from glandular epithelium are called adenocarcinomas.

Now let’s examine the stratified epithelia. In general, they are located in areas exposed to abrasion . To help withstand stress and protect underlying structures, these epithelial linings are thick (two or more layers) and epithelial cells are often tightly linked together by cell junctions.

Stratified Squamous Keratinized – 44 Thick Skin (UBC, CPS Virtual Slidebox – Slide #44) Thick skin, found on the palm of the hand and the sole of the foot, is an excellent example of stratified squamous keratinized epithelium. The outermost keratinized layer is composed of dead cells packed full of the protein keratin. The inner living layer is composed of maturing keratinocytes. Keratinocytes arise from a basal layer of stem cells (adjacent to the underlying connective tissue) and immediately begin to differentiate, produce vast quantities of keratin and move towards the outer surface. Eventually, they die and form the keratinized layer. The keratinized layer provides protection from abrasion, movement and pathogen entry and makes the surface of the skin dry in order to help conserve water. Can you identify the living and keratinized layers of thick skin?

EM “Stratified squamous keratinized epithelium” is a low power EM illustrating ultrastructural features of thin skin. Thin skin is found on the external surface of the body in locations other than palm of the hand and the sole of the foot. It is structurally similar to thick skin in that keratinocytes arise from a basal layer of stem cells, differentiate, move towards the surface and die, leaving a dry, keratinized protective layer. Both the living and keratinized layers are thinner in thin skin compared to thick skin. Can you find the keratinized layer in this EM?

Stratified Squamous Non-‐Keratinized – 60 Epiglottis – Lingual Surface (UBC, CPS Virtual Slidebox – Slide #60) The epiglottis guards the entrance to the larynx. Its superior (lingual) surface is covered by the lining same as the oral cavity, stratified squamous non-‐keratinized epithelium. At high power, observe how thick this epithelium is and look at the different morphologies of cells at the basal and apical surfaces. Why is this epithelium called stratified squamous? Once again, cells arise from a basal layer, differentiate and move towards the lumen of the pharynx where they will slough off. This process is called desquamation.

Stratified Columnar – 60 Epiglottis – Laryngeal Surface (UBC, CPS Virtual Slidebox – Slide #60) The inferior surface of the epiglottis is covered by an epithelium undergoing change . Stratified squamous non-‐keratinized epithelium extends from the superior surface around the tip of the epiglottis to the proximal third or half of its inferior surface. At this point, the epithelium undergoes a dramatic change to stratified columnar epithelium (shown here) and then to pseudostratifed epithelium (none present on slide 60). Can you locate the transition point between stratified squamous epithelium and stratified columnar epithelium? The distribution of stratified columnar epithelium is limited in the body. Where else can you find it?

Stratified Cuboidal – 44 Thick Skin – Duct of Eccrine Sweat Gland (UBC, CPS Virtual Slidebox – Slide #44) Deep in the connective under tissue thick skin are eccrine sweat glands. They are simple coiled tubes important for thermoregulation. The distal end of the tube produces sweat which travels up a duct to the surface of skin (often the duct connecting the gland to the surface is NOT seen). The secretory portion of the sweat gland is composed of a simple cuboidal epithelium. These cells pump sodium ions (and other materials) into the lumen of the gland, followed by passive diffusion of water. At high power, surrounding eosinophilic myoepithelial cells are also visible. Contraction of myoepithelial cells helps expel sweat into the ducts. The ducts of the sweat glands are composed of stratified cuboidal epithelium. The first part of the duct is visible in the cluster along with the secretory portion of the gland. Can you find a sweat gland and differentiate between the secretory and ductal portions? Can you identify myoepithelial cells?

Finally, let’s take a look at two unique types of epithelia: pseudostratified epithelium found in the respiratory system and transitional epithelium found in the urinary system.

Pseudostratified Ciliated Columnar Epithelium with Goblet Cells – 93 Auditory (Eustachian) Tube (UBC, CPS Virtual Slidebox – Slide #93) The auditory tube connects the middle ear with the nasopharynx and it is important in pressure equalization. It is lined with psuedostratified ciliated columnar epithelium with goblet cells, the same epithelium that lines most of the nasal cavity and upper respiratory tract. This epithelium is also known as Respiratory Epithelium. The auditory tube can be found in the upper right portion of slide 93. Look for a partially collapsed tube lined with what appears to be a stratified epithelium. Nuclei of different cell types composing this epithelium are located at different heights which gives the appearance of stratification. However, electron microscopy reveals that all cells contact the basement membrane, thus it is truly a simple epithelium. If you examine respiratory epithelium at high power, you will see it is made up of at least three different cell types: basal stem cells, mucus producing goblet cells and tall columnar ciliated cells. Can you identify these three cell types? What is the function of cilia in this epithelium?

Transitional Epithelium (Urothelium) – Urinary Bladder – 123A Human, 123B Pig (UBC, CPS Virtual Slidebox – Slides #123A and 123B) The urinary bladder and parts of the urinary system are subject to distention as urine accumulates and/or passes through various structures. When the bladder is empty, the epithelium contracted is = thick epithelium (5-‐7 cell layers) and the apical u mbrella cells are rounded. When the bladder is full, the epithelium is distended = thin epithelium -‐ (2 3 cell layers) and the umbrella cells flatten.

Which of these images shows contracted transitional epithelium? Where are the umbrella cells?

UBC, CPS Virtual Slidebox, Slide 123B – Urinary Bladder, Pig Fig.2.10 ‘Netter’s Essential Histology, Ovalle & Nahirney’ Used with permission. Copyright © 2008 Elsevier Inc. All rights reserved. Epithelial Cell Specializations: Epithelial cells can have specializations on their apical, lateral and basal borders in order to facilitate the overall function the epithelium. You have already studied specializations on the apical (Cell Cytoplasm Lab) and lateral borders (Cell Nucleus and Junctions Lab). e W will review this information and introduce the basement membrane, a basal border specialization.

Apical and Lateral Border Specializations EM “Striated border in intestinal epithelium” has two notable features, an apical brush border with microvilli and a lateral junctional In complex. this columnar epithelial cell, what is the function of microvilli? Can you recall two other types of apical specializations and their functions? In the lateral junctional complex, can you locate a tight junction, a zonula adherens and a desmosome?

Basal Border Specialization – Basement Membrane – 63A Trachea and 121 Kidney (PAS) (UBC, CPS Virtual Slidebox – Slides #63A and 121) The basement membrane lies between epithelia and connective tissue. It is different composed of proteins that help secure the epithelial lining to the connective tissue. In addition to attachment, basement membranes also serve other functions such as a selective barrier, regulation of epithelial cell differentiation in growth and tissue repair. For more detailed information about the composition and functions of the basement membrane, please see a histology textbook.

Basement membranes are poorly visible with H&E except for a few locations such as the trachea. On slide 63A, look for the respiratory epithelium lining the airway on the right side of this slide. Can you see the basement membrane at the boundary between the epithelium and the connective tissue?

Special stains such silver as and periodic acid-‐Schiff (PAS) stain carbohydrates in the basement membrane magenta. Can you find the basement membranes in the cortex of the kidney on slide 121?

Basal Lamina With electron microscopy, individual layers of the basement membrane become visible. Epithelial cells produce the lamina lucida and lamina densa and connective tissue cells produce the reticular lamina. The term basal lamina refers to the lamina cida lu and lamina densa only.

The EM “ Two adjoining epithelial cells with their basal lamina” shows basal portions of two epithelial cells along with their lamina lucida, lamina densa . and reticular lamina The lamina lucida is an electron lucent (light) layer directly under the epithelial cell membrane. The lamina densa is an electron dense (dark) layer under the mina la lucida. reticular The lamina is a layer of collagen fibrils (seen in cross-‐section) under the lamina densa. Can you locate these three layers in this EM? Hint: The quiz will help you answer this question.

NB: Please ignore the arrows on this image. They indicate lateral interdigitations between the epithelial cells and they are not associated with the basal lamina.

NOTE: You may have noticed that there is some confusing terminology here! The term basement membrane is a light microscopic term and refers to all three layers visible under the electron micrograph (lamina lucida, mina la densa and reticular lamina). Whereas, the term basal lamina is an electron microscopic term and refers to only the lamina lucida and lamina densa.

Lining Epithelium Histology Laboratory -­‐ Year 1, Fall Term Dr

Lining Epithelium Histology Laboratory -‐ Year 1, Fall Term Dr