Introduction to Pathology Flashcards
What is disease?
Consequence of failed homeostasis with consequent morphological and function disturbances
What is the importance of a microscopic diagnosis?
- Definitive diagnosis
- Before major surgery to remove a lesion a microscopic diagnosis is required (this guides the type and extent of surgery)
Examples of histology?
- Core biopsies
- Cancer resection specimens
- excised skin lesions
- Endoscopic biopsies
What are the benefits of histology?
- Often therapeutic as well as diagnostic
- Can assess architecture as well as cellular atypia
- Can differentiate invasive from in situ disease by determining if it has reached the cell membrane
- Can provide information on completeness of excision and more complete information on grading and staging
- Better for immunohistochemical and molecular testing
Examples of cytology?
- Fine needle aspirates (FNA) of breast, thyroid, salivary glands, lymph nodes, lung
- Effusions
- Cervical smears
- Sputum
- Urine
What are the benefits of cytology?
- Faster and cheaper
- Non-invasive or minimally invasive and safe
- Can be used for cells in fluid
- Sometimes a preliminary test before other investigations or more tissue taken for histology
- Highly inadequate and error rates
- Generally used to confirm/exclude cancer/dysplasia and not to diagnose any other condition with accuracy
How does a histopathologist arrive at a diagnosis?
Pattern recognition
1) Is it normal or not?
2) Is this inflammatory or neoplastic?
3) Is this benign or malignant?
4) Is this a primary tumour or a metastasis?
What else can be determined by histology?
- Type of cancer
- Grade of cancer
- Completeness of excision and if margins are involved which ones
- Stage of cancer
Likely efficacy of further treatments (molecular pathology HER2, ER/PR, EGFR, PDL1 status) - All of which influence decisions on further treatment and management of the patient
What is autolysis and how can it be prevented?
- Tissue autolysis begins when the blood supply is cut off
- It destroys cells and tissue architecture
- Can be prevented with fixatives
What are fixatives?
- Inactivate tissue enzymes and denature proteins
- Prevent bacterial growth
- Harden tissue
What is fixation?
- Hold tissue in ‘suspected animation’
- Usually use formalin (formaldehyde in water)
- Penetrates tissue at approximately 1mm/hour
- Usually fix for 24-48 hours
How are samples cut up?
- Samples cut into size of a stamp and placed in cassettes
- Cassettes have holes in to allow formalin to infiltrate
What is the process of embedding?
- In order to be able to cut very thin sections the tissue has to be surrounded and impregnated with a hardening agent, usually paraffin wax
- Tissue needs to be dehydrated with alcohol in a vacuum to remove water.
- Alcohol is then replaced with xylene whuch can mix with the wac
- Xylene is then replaced with molten paraffin wax
- Usually takes place in a processor overnight
- Tissue is taken out of cassettes by hand and placed into metal blocks which are filled with more paraffin wax
- The wax is allowed to harden and the metal tray is removed
What is microtomy?
- Need to cut very thin slices from the block (3-4 micron sections) using a microtome
- Must be cut this thin so can be seen through a microscope
- Extra thin slices are floated in water to prevent damage
What is the process of staining?
- Colouring the tissue so it can be seen under a microscope
- Staining usually with H&E
What does H&E stain?
Haematoxylin stains nuclei purple
Eosin stains cytoplasm and connective tissue pink
What is the process of mounting?
Preserving and protecting the slice of tissue:
- Mounting medium is applied to the slide
- Coverslip is placed on top
- Mounting medium dries and hardens preserving the tissue and attaching the coverslip
What is immunohistochemistry?
- Demonstrates substances in/on cells by labelling them with specific antibodies
- Usually the antibody is joined to an enzyme (eg. peroxidase) that catalyses a colour-producing reaction
What can be used in immunohistochemistry?
Any substance that is antigenic can be demonstrated:
- Contractile protein actin (identifies smooth muscle cell)
- Cadherins (cell adhesion molecules, deficient in some carcinomas, eg. lobular breast carcinoma)
- Hormone receptors, eg. ER, PR
- HER2 receptor (growth factor receptor, predicts response of breast cancer to Herceptin)
- Microorganisms, eg. CMV, HPV, Herpes simplex
How do Cytokeratins work with immunohistochemistry?
- Family of intracellular fibrous proteins
- Present in almost all epithelia
- At least 20 known
- Markers for epithelial differentiation and show tissue-specific distribution in epithelia
- Can give information about the primary site of the carcinoma, particularly when used in combination
- –> CK7+/CK20: lung, breast, endometrium, ovary, thyroid
- –> CK7-/CK20+: large bowel, some gastric carcinomas
