Histology

Intro to

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What is Histology?

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• The study of microscopic structures of tissues

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• Histological slides allow is to visualize tissues at a cellular level in both healthy and pathological states

Learning Objectives

Labi Sarrouh, Doctorials 2021-2022

1. Briefly outline what occurs to pathological specimens once they are sent to the lab and how slides are made
2. Explain, in depth, how to approach interpreting histological slides in terms of envisioning them as 3D tissues
3. Explain in depth the 4 basic tissue types, their key histological components, and how to differentiate them in histological images
4. Give pictorial examples of normal and diseased histological tissue

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Learning Objectives

Labi Sarrouh, Doctorials 2021-2022

1. Briefly outline what occurs to pathological specimens once they are sent to the lab and how slides are made
2. Explain, in depth, how to approach interpreting histological slides in terms of envisioning them as 3D tissues
3. Explain in depth the 4 basic tissue types, their key histological components, and how to differentiate them in histological images
4. Give pictorial examples of normal and diseased histological tissue

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How Slides are Prepared

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5 steps

1. Fixation
• Fixed with 10% neutral buffered formalin to prevent decay
2. Transfer to Cassettes
• Trimming of specimens to fit into a cassette until they’re ready to be put on a slide; stored in formalin
3. Processing
• Dehydration – using alcohol to remove water and formalin from tissue
• Clearing – organic solvents remove the alcohol
• Embedding – embedding agents like paraffin wax enclose the specimen in a block
4. Sectioning
• Thin tissue sections are sliced from the block and placed on a slide
5. Staining

How Slides are Prepared: Staining

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Components that CAN’T be Visualized

• Lost structures vs Artifacts
• Lost structures – depends on process
• Neutral lipids
• Glycogen
• Proteoglycans and glycosaminoglycans
• Artifacts – preparation errors
• Air bubble
• Lifting/folding of tissue

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Slides are not always perfect! Look at multiple slides

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Other Staining Methods

• Periodic Acid Schiff
• Starch
• Mucin granules
• Glycogen deposits

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• Masson’s Trichrome
• Red: keratin and muscle
• Blue/green: collagen and bone
• Light red/pink: cytoplasm
• Dark brown/black: nuclei

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• Silver
• Neurons

• Electron Microscopy
• Transmission
• Scanning

Pathological Specimens

• Gross examination
• Examining the specimen macroscopically, noting color, size, type of tissue, etc.
• Microscopic examination
• Same procedure outlined before

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Learning Objectives

1. Briefly outline what occurs to pathological specimens once they are sent to the lab and how slides are made
2. Explain, in depth, how to approach interpreting histological slides in terms of envisioning them as 3D tissues
3. Explain in depth the 4 basic tissue types, their key histological components, and how to differentiate them in histological images
4. Give pictorial examples of normal and diseased histological tissue

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Labi Sarrouh, Doctorials 2021-2022

Visualizing 3D as 2D

• Histological slides take 3D organs/specimens, and portray them as 2D on a slide
• Depending on how a specimen is cut, the same specimen tissue may appear different
• This concept is portrayed well is certain tissues e.g. skeletal muscles and blood vessels

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Visualizing 3D as 2D

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This Photo by Unknown Author is licensed under CC BY

This Photo by Unknown Author is licensed under CC BY-NC

• Cross Sectional Cut
• The fibers are coming out of the screen towards you, and into the screen away from you

• Longitudinal Cut
• The fibers are moving across the screen, left to right

Visualizing 3D as 2D

Labi Sarrouh, Doctorials 2021-2022

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• Cross Sectional Cut
• The fibers are coming out of the screen towards you, and into the screen away from you

• Longitudinal Cut
• The fibers are moving across the screen, left to right

Visualizing 3D as 2D

• Longitudinal

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• Cross Section

Learning Objectives

Labi Sarrouh, Doctorials 2021-2022

1. Briefly outline what occurs to pathological specimens once they are sent to the lab and how slides are made
2. Explain, in depth, how to approach interpreting histological slides in terms of envisioning them as 3D tissues
3. Explain in depth the 4 basic tissue types, their key histological components, and how to differentiate them in histological images
4. Give pictorial examples of normal and diseased histological tissue

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4 Types of Tissue

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Epithelial

• Lines the bodies cavities and surfaces

Connective Tissue

• Loose and dense CT

Muscle

1. Skeletal
2. Smooth
3. Cardiac

Nervous Tissue

• Neurons and neuroglia

1) Epithelial Tissue

• Covers bodies surfaces and cavities
• 3 KEY FEATURES
• Cell junctions connect the epithelial cells closely
• Demonstrate polarity (they have a top and bottom e.g. apical and basal surfaces)
• Basal surface attached to basement membrane
• Some Functions
• Transportation
• Secretion
• Absorption
• Protection
• Receptor function

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There are varying types of epithelial tissue located in different parts of the body, so let’s explore the differences

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Classification

• Simple – 1 layer
• Stratified - >1 layer
• Squamous – flat
• Cuboidal – cube
• Columnar – tall

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Other classifications

• Transitional
• Pseudostratified

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Comparisons

• Simple Squamous
• Simple – single layer
• Squamous – flat
• So it’s a single layer of flat cells

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• Simple Cuboidal
• Simple – single layer
• Cuboidal – cubes
• So it’s a single layer of cubed cells

• Simple Columnar
• Simple – single layer
• Columnar – tall
• So it’s a single layer of tall cells

Alveoli Kidney Intestinal

Comparisons

• Stratified Squamous
• Stratified – multiple layers
• Squamous – flat
• So it’s multiple layer of flat cells

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• Stratified Cuboidal and Stratified Columnar are rarer to see

Skin (Keratinized stratified squamous)

Pseudostratified Columnar with Cilia

• Single layer of irregular shaped and differently sized columnar cells
• Nuclei at different levels
• Respiratory epithelium

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Transitional Epithelium

• Found only in the urinary system (ureter, bladder wall, urethra)
• Can basically switch from cuboidal to squamous to accommodate for stretching of these structures
• Dome shaped cells in the top layer flatten when stretched

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2) Connective Tissue

• CT: tissue found within the extra cellular matrix
• It supports, protects, and gives structure to tissues and organs
• The ECM consists of elastin, collagen, reticular fibers, and ground substance (gel like substance that contains proteoglycans, glycosaminoglycans, glycoproteins)

2 Major Types

• Connective Tissue Proper: fibroblasts and fibers
1. Loose – few fibers but many nuclei
2. Dense – many fibers but few nuclei
1. Regular
2. Irregular
• Specialized
• Cartilage
• Bone
• Blood
• Lymphatic
• Adipose

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Dense vs Loose CT

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Dense CT

Loose CT

• Provides structural support and can be found under epithelial layers to attach them to underlying tissues

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Dense Regular CT

• Parallel bundles of collagen fibers
• Mostly fibers, not many fibroblasts (cells)
• Typically found in places where lots of tensile strength is needed like tendons and ligaments

Dense Irregular CT

• Fibers in random directions
• Many fibers, few nuclei
• Typically found in places that must resist stretching forces in different directions like the dermis layer of skin, and digestive tract

Specialized CT

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Cartilage

Fibrous Elastic

Hyaline

Bone

Adipose

Blood

3) Muscle

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Skeletal Muscle

• Striated
• Multi-nucleated
• Nuclei on periphery

Cardiac Muscle

• Striated
• Intercalated discs (contain gap junctions)
• Single, centrally located nucleus
• Can be branched

Smooth Muscle

• Non-striated
• Fusiform shaped
• Single, centrally located nucleus

4) Nervous Tissue

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Neuron

Peripheral Nerve

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Almost Done!!

Learning Objectives

Labi Sarrouh, Doctorials 2021-2022

1. Briefly outline what occurs to pathological specimens once they are sent to the lab and how slides are made
2. Explain, in depth, how to approach interpreting histological slides in terms of envisioning them as 3D tissues
3. Explain in depth the 4 basic tissue types, their key histological components, and how to differentiate them in histological images
4. Give pictorial examples of normal and diseased histological tissue

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Sickle Cell Anemia (SCA)

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Healthy blood smear

SCA

Emphysema

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Healthy alveoli

Emphysema

Atherosclerosis

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Healthy artery

Atherosclerotic artery

Take away points

• Histology is all about pattern recognition
• The way you become better at recognizing the different slides is through PRACTICE
• Try to identify the organ/tissue you’re looking at first
• Then get comfortable identifying the different parts o the tissue like the connective tissue, the specific epithelium that’s there, different types of muscle, etc.

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Labi Sarrouh, Doctorials 2021-2022

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• Professor Sheehan is the histology queen
• Everything you will need to know about histology should come from her

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• UWO slide box and Yale histology are good supplementary materials if you find yourself wanting to look at more images
• And of course feel free to reach out to me on Facebook, or email be at 20131542@studentmail.ul.ie if you find yourself having any questions or needing anything

Thanks and Good Luck!

Labi Sarrouh, Doctorials 2021-2022

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References

Ross & Pawlina Histology: A Text and Atlas with correlated cell and molecular biology

UWO Slidebox http://slides.uwo.ca/

Yale Histology http://medcell.med.yale.edu/histology/histology.php

Parry, Nicola. “Histology Slide Preparation: 5 Simple Steps.” Bitesize Bio https://bitesizebio.com/13398/how-histology-slides-are-prepared/

Yang, Jackie Doctorials 2019

Kaur, Manpreet Doctorials 2020

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Labi Sarrouh, Doctorials 2021-2022

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