Histology and laboratory techniques

In short

Histology is the microscopic study of the structure of tissues; histological technique is the chain of steps - fixation, dehydration, embedding, sectioning, staining and mounting - that turns a piece of soft tissue into a permanent, stained, transparent section fit for a microscope. Applied zoology adds the wider laboratory toolkit: microscopy, micrometry, smears and squashes, histochemistry, cell counting, centrifugation, chromatography, electrophoresis and spectrophotometry. It is examined in every B.Sc Zoology core paper (UGC CBCS "Cell Biology / Techniques in Biology / Histology" and practical vivas), in CSIR-UGC NET and GATE Life Sciences, and in entrance tests for M.Sc. and for medical laboratory technology. The same skills earn jobs in Indian pathology and histopathology labs (Dr Lal PathLabs, SRL, Metropolis, Thyrocare), CSIR-CDRI/IICT and ICMR institutes, pharma toxicology and CRO units, fish and veterinary disease diagnostic labs, and forensic science laboratories.

Explained

Animal tissues - what a section is read for

• Four primary tissue types: epithelial (covering and lining), connective (support, binding, transport), muscular (contraction) and nervous (conduction).
• Epithelia by shape and layers: simple squamous (alveoli, Bowman's capsule), simple cuboidal (kidney tubules), simple columnar (intestine, with microvilli and goblet cells), pseudostratified ciliated columnar (trachea), stratified squamous keratinised (skin) and non-keratinised (oesophagus, buccal cavity), transitional (urinary bladder); glandular epithelium forms exocrine and endocrine glands.
• Connective tissue: loose (areolar, adipose), dense (regular - tendon, ligament; irregular - dermis), specialised - cartilage (hyaline, elastic, fibrous), bone (compact with Haversian systems, spongy), blood and lymph. Fibres: collagen, elastic, reticular; cells: fibroblast, macrophage, mast cell, plasma cell, adipocyte.
• Muscle: skeletal (striated, multinucleate, voluntary), cardiac (striated, uninucleate, intercalated discs, involuntary), smooth (unstriated, spindle, involuntary).
• Nervous: neurons (cell body, dendrites, axon, Nissl granules) and neuroglia (astrocyte, oligodendrocyte, microglia, Schwann cell).
• Reading a slide: identify shape of cells, number of layers, nucleus position, matrix, fibres, presence of a basement membrane and a lumen - the standard exam demand of "identify and give three reasons".

The paraffin method - step by step

1. Collection and killing: tissue taken immediately after death (autolysis begins within minutes); piece kept thin - 3-5 mm - so fixative penetrates (penetration roughly follows the square-root-of-time law).
2. Fixation: stops autolysis and bacterial decay, hardens tissue, preserves structure, makes cells receptive to stains. Volume of fixative ≥ 10-20× tissue volume, 6-24 h.
   • *Coagulant fixatives:* ethanol, acetic acid, picric acid, mercuric chloride. *Cross-linking (additive):* formaldehyde, glutaraldehyde, osmium tetroxide.
   • *Common fluids:* 10% neutral buffered formalin (NBF - 4% formaldehyde, routine), Bouin's fluid (picric acid + formalin + acetic acid - excellent for nuclei and glycogen-free work; yellow colour washed out with 70% alcohol), Zenker's and Helly's (mercuric - bone marrow, mitochondria), Carnoy's (ethanol-chloroform-acetic, fast, for nucleic acids), Karnovsky's (paraformaldehyde + glutaraldehyde) and 1-4% glutaraldehyde followed by osmium tetroxide for electron microscopy.
3. Washing: running water for formalin; 70% alcohol for picric and mercuric fixatives (plus iodine-thiosulphate to remove mercury pigment).
4. Dehydration: ascending alcohol grades 30 → 50 → 70 → 90 → absolute (two changes), each 30-60 min; sudden strong alcohol causes shrinkage and distortion. Acetone and dioxane are alternatives.
5. Clearing: alcohol replaced by a wax-miscible agent with a refractive index near that of protein, so tissue turns translucent - xylene (routine), toluene, benzene (carcinogenic, discouraged), chloroform, cedarwood oil (slow, gentle).
6. Impregnation (infiltration): 2-3 changes of molten paraffin wax (56-58 °C melting point) in an oven at about 2-5 °C above the melting point, 1-2 h each.
7. Blocking (embedding): tissue oriented in a mould (Leuckhart's L-pieces or plastic cassette) filled with wax and cooled rapidly for fine crystal structure.
8. Trimming and sectioning: block clamped in a microtome; ribbon of sections cut at 5-7 µm for routine light microscopy (3 µm for kidney and blood-forming tissue, 10-15 µm for nerve tissue).
9. Affixing: sections floated on warm water (about 45 °C) to flatten, picked up on a slide smeared with Mayer's egg albumen (albumen-glycerine) or poly-L-lysine/APES for immunostaining; dried at 37-60 °C.
10. Dewaxing and hydration: xylene (two changes) → descending alcohols → water.
11. Staining: usually H&E (below), then dehydration, clearing and mounting in DPX or Canada balsam under a cover slip. Mountant refractive index (~1.52) must match glass to avoid glare.
12. Alternative embedding: celloidin/nitrocellulose for large, hard, mixed-density tissue (eye, cochlea, brain) - no heat, minimal shrinkage, but very slow; resin (araldite, Epon, LR White) for 0.5-1 µm semi-thin and 50-90 nm ultra-thin sections; frozen/cryostat sections (−20 to −30 °C) for rapid intra-operative diagnosis and for enzyme and lipid histochemistry, since paraffin processing dissolves lipids and destroys most enzyme activity.

Microtomes, microscopes and measurement

• Microtome types: *rotary (Minot)* - paraffin, ribbons, the workhorse; *rocking (Cambridge)* - simple, student labs; *sliding (base-sledge)* - celloidin and hard tissue; *freezing microtome and cryostat* - unfixed tissue; *ultramicrotome* - glass or diamond knife, ultra-thin sections for TEM; *vibratome* - thick live slices for physiology.
• Knife profiles and angles: plane-concave for paraffin, biconcave for rocking microtomes, wedge for hard/celloidin; knife tilt about 15° - excess tilt scores sections, too little compresses them.
• Compound microscope: total magnification = objective × eyepiece; resolving power (limit of resolution) d = 0.61λ/NA (Rayleigh criterion; Abbe's form is λ/2NA) - about 0.2 µm for light microscopy with an oil-immersion 100× (NA 1.25-1.40). Immersion oil (RI ≈ 1.515) raises NA by preventing refraction loss.
• Special microscopes: *dark-field* (scattered light - spirochaetes, live protozoa), *phase contrast* (converts phase differences to amplitude - unstained living cells), *fluorescence* (excitation filter, dichroic mirror, barrier filter; fluorochromes - FITC, DAPI, rhodamine), *confocal laser scanning* (optical sections, 3-D reconstruction), *polarising* (birefringence - muscle, collagen, amyloid), *TEM* (electron beam, resolution ~0.2 nm, ultra-thin sections on copper grids, contrast by uranyl acetate and lead citrate) and *SEM* (surface topography, specimen dried by critical-point drying, sputter-coated with gold).
• Micrometry: an ocular (eyepiece) micrometer is calibrated against a stage micrometer (ruled in 0.01 mm = 10 µm divisions); the calibration factor changes with every objective. Camera lucida and drawing tubes give scaled drawings; magnification of a drawing = drawing size / actual size.
• Haemocytometer (Neubauer chamber): depth 0.1 mm; central ruled area 1 mm² divided into 400 smallest squares; RBC counted in 80 of them (5 groups of 16, each small square 1/400 mm²), WBC in the 4 corner 1 mm² squares. Diluting fluids: Hayem's for RBC (1:200), Turk's (1-2% acetic acid + gentian violet) for WBC (1:20), Toisson's for both.

Staining - principles and the standard repertoire

• Dye chemistry: a dye = chromophore (colour-bearing group) + auxochrome (ionisable, binds tissue). Basic (cationic) dyes (haematoxylin's lake, methylene blue, safranin, basic fuchsin) stain acidic basophilic structures - nuclei, RNA, cartilage matrix; acidic (anionic) dyes (eosin, acid fuchsin, orange G, light green) stain basic acidophilic/eosinophilic structures - cytoplasm, collagen, muscle.
• Progressive vs regressive: progressive staining is stopped when correct; regressive deliberately overstains and then differentiates (1% acid alcohol) and blues (tap water, Scott's or dilute ammonia) - the routine H&E route.
• Mordant and lake: haematoxylin itself is a poor stain; oxidised to haematein and combined with a metal mordant (alum, iron) to form a positively charged lake. Variants: Harris's, Mayer's, Ehrlich's, Delafield's (alum) and Heidenhain's iron haematoxylin, Weigert's (iron, resists acid - used with picric mixtures).
• H&E result: nuclei blue-black (chromatin), cytoplasm, muscle and collagen shades of pink; it shows general architecture but not specific chemistry.
• Special stains (what each demonstrates):
  • PAS (periodic acid-Schiff): glycogen, mucins, basement membrane, fungal walls - magenta; glycogen confirmed by loss after diastase digestion (PAS-D control).
  • Masson's trichrome / van Gieson: collagen vs muscle (fibrosis grading).
  • Verhoeff / orcein: elastic fibres. Silver impregnation (Gomori, Golgi, Cajal): reticular fibres, neurons, Golgi apparatus.
  • Alcian blue: acid mucopolysaccharides. Oil red O / Sudan black on frozen sections: lipids. Feulgen: DNA (specific - acid hydrolysis then Schiff's). Methyl green-pyronin: DNA green, RNA red. Toluidine blue: metachromasia of mast-cell granules. Giemsa / Leishman / Wright: blood films and blood parasites (*Plasmodium*). Ziehl-Neelsen: acid-fast bacilli. Gram stain: bacteria. Congo red: amyloid (apple-green birefringence under polarised light). Perls' Prussian blue: ferric iron. Von Kossa: calcium salts. Janus green B: mitochondria in living cells (vital stain, along with neutral red and trypan blue for dye exclusion).
• Histochemistry and enzyme histochemistry: localising chemicals in situ - Gomori's lead method for acid and alkaline phosphatase, tetrazolium methods for dehydrogenases; needs frozen or cold-fixed sections.
• Immunohistochemistry (IHC): antigen located by a specific antibody tagged with peroxidase (visualised with DAB - brown) or a fluorochrome; direct (one labelled antibody) and indirect (unlabelled primary + labelled secondary, more sensitive); needs antigen retrieval (heat in citrate buffer pH 6) after formalin fixation and blocking of endogenous peroxidase. In situ hybridisation / FISH localises nucleic acid sequences.

Non-sectioning preparations and allied techniques

• Smear: thin film of fluid or soft tissue on a slide, air-dried or wet-fixed - blood films, bone marrow, buccal smear, vaginal smear, Pap smear (cervical cytology, fixed in 95% alcohol, stained by Papanicolaou).
• Squash (crush) preparation: soft tissue flattened under a cover slip - grasshopper testis or onion anther (young flower bud) for meiosis, onion root tip for mitosis, *Drosophila* salivary gland for polytene chromosomes (acetocarmine or aceto-orcein, 1 N HCl pre-treatment).
• Whole mount: entire small organism fixed, stained and mounted - *Hydra*, liver fluke, *Taenia* proglottids, *Daphnia*; mounted in DPX or glycerine jelly.
• Maceration: soft tissue dissolved with 10% KOH to expose skeleton; alizarin red S stains bone and alcian blue cartilage in cleared whole-mount skeletons.
• Decalcification: bone treated with 5-10% nitric or formic acid or neutral EDTA (slow but preserves antigens) before sectioning; end point checked chemically or by radiography, never by needle alone.
• Chromosome preparation / karyotyping: blood lymphocytes cultured with phytohaemagglutinin, arrested in metaphase with colchicine, swollen in hypotonic KCl, fixed in Carnoy's, dropped, air-dried and G-banded with trypsin-Giemsa; idiogram drawn from measured arm ratios.
• Separation and estimation: differential and density-gradient centrifugation (RCF = 1.118 × 10⁻⁵ × r × N²); paper, thin-layer, column and gas chromatography; gel electrophoresis (agarose for DNA, SDS-PAGE for protein); spectrophotometry by Beer-Lambert law; ELISA and PCR for antigen and DNA detection; microtomy quality control by section thickness and staining controls.
• Tissue culture and allied: primary and established cell lines in laminar flow hoods, CO₂ incubators, trypsinisation, cryopreservation in liquid nitrogen with DMSO.

Laboratory safety, ethics and Indian practice

• Hazards: formaldehyde (sensitiser, classified carcinogenic - fume hood, closed containers), xylene and benzene (neurotoxic; benzene avoided), picric acid (explosive when dry - stored wet), mercuric chloride and osmium tetroxide (highly toxic; mercury waste must not go to the drain), ethidium bromide and acrylamide (mutagenic/neurotoxic), UV from transilluminators.
• Biosafety levels BSL-1 to BSL-4 as per the Department of Biotechnology's biosafety guidelines; Institutional Biosafety Committee (IBSC) clearance under the Rules for Manufacture, Use, Import, Export and Storage of Hazardous Micro-organisms/GMOs, 1989 (Environment (Protection) Act, 1986).
• Animal use: regulated by the Prevention of Cruelty to Animals Act, 1960 through CPCSEA; every institution needs an IAEC (Institutional Animal Ethics Committee) and must follow the 3Rs (replacement, reduction, refinement); CPCSEA rules now restrict dissection of vertebrates in UG/PG teaching, so models, charts, simulations and permanent slides are used. Wild animal tissue needs permission under the Wild Life (Protection) Act, 1972.
• Waste: Bio-Medical Waste Management Rules, 2016 (as amended) - yellow bag for anatomical and chemical waste, red for contaminated plastics, white (puncture-proof) for sharps, blue for glass and metallic implants; discharge standards and lab effluent under CPCB norms.
• Human samples: informed consent and ICMR *National Ethical Guidelines for Biomedical and Health Research Involving Human Participants* (2017); Institutional Ethics Committee registration with CDSCO under the New Drugs and Clinical Trials Rules, 2019. Diagnostic labs follow NABL accreditation to ISO 15189 and, where applicable, the Clinical Establishments (Registration and Regulation) Act, 2010.
• Good practice: labelled reagents with dates, a bound laboratory notebook, positive and negative staining controls, calibration records for microtome and balance, and traceability of every block and slide by accession number.

Key points

• Histology - microscopic study of tissue structure; histopathology - the same applied to diseased tissue; cytology - study of individual cells and smears.
• Fixation - arrests autolysis and putrefaction, hardens tissue and improves stainability; 10% neutral buffered formalin is the routine fixative, Bouin's for nuclear detail, glutaraldehyde + OsO₄ for EM.
• Order of the paraffin method - fix, wash, dehydrate, clear, infiltrate, block, section, affix, dewax, stain, dehydrate, clear, mount.
• Dehydration uses ascending alcohols; clearing uses xylene to make tissue wax-miscible and translucent; skipping a grade causes shrinkage and brittle blocks.
• Paraffin melting point 56-58 °C; routine section thickness 5-7 µm; ultra-thin TEM sections 50-90 nm.
• Rotary microtome for paraffin ribbons, sliding for celloidin and hard tissue, cryostat for rapid frozen sections, ultramicrotome for TEM.
• Haematoxylin is basic and needs a metal mordant (alum or iron) to form a lake; eosin is acidic and counterstains cytoplasm pink.
• Regressive staining = overstain, then differentiate in acid alcohol and blue - the standard H&E route.
• PAS demonstrates glycogen and mucin (diastase control), Feulgen is specific for DNA, Sudan/Oil red O needs frozen sections because paraffin processing dissolves lipids.
• Resolution d = 0.61λ/NA; light microscope limit ≈ 0.2 µm, TEM ≈ 0.2 nm; magnification without resolution is empty magnification.
• Phase contrast images living unstained cells; fluorescence and confocal localise specific molecules; SEM shows surfaces, TEM internal ultrastructure.
• Ocular micrometer must be calibrated against a stage micrometer separately for each objective.
• Neubauer chamber depth 0.1 mm; RBC in 80 small squares (5 groups of 16) with Hayem's fluid (1:200), WBC in 4 corner squares with Turk's fluid (1:20).
• Decalcification with acid or EDTA precedes bone sectioning; EDTA is slower but preserves antigens for IHC.
• Artefacts - chatter, knife scores, folds, air bubbles, formalin pigment, ice-crystal holes - must be told apart from true structure.
• CPCSEA/IAEC clearance governs animal experiments in India; BMW Rules, 2016 govern colour-coded waste segregation; diagnostic labs are accredited by NABL to ISO 15189.

Formulas and examples

• Micrometry. On a 40× objective, 50 ocular divisions coincide with 12 divisions of a stage micrometer ruled in 0.01 mm.
  • Length of 12 stage divisions = 12 × 10 µm = 120 µm.
  • 1 ocular division = 120/50 = 2.4 µm.
  • A nematode egg measuring 25 ocular divisions = 25 × 2.4 = 60 µm long.
• Resolution and useful magnification. Oil-immersion objective, NA 1.25, green light λ = 550 nm.
  • d = 0.61 × 550 / 1.25 = 335.5/1.25 ≈ 268 nm ≈ 0.27 µm.
  • Useful magnification ≈ 1000 × 1.25 = 1,250×; a 100× objective with a 10× eyepiece gives 1,000× - within the useful range, while adding a 20× eyepiece (2,000×) would be empty magnification.
• RBC count (Neubauer). Blood diluted 1:200 in Hayem's fluid; 475 cells counted in 80 small squares (5 groups of 16; each small square 1/400 mm², depth 0.1 mm).
  • Area counted = 80 × (1/400) = 0.2 mm²; volume = 0.2 × 0.1 = 0.02 mm³.
  • Cells per mm³ = 475 × 200 / 0.02 = 95,000/0.02 = 4,750,000 ≈ 4.75 × 10⁶ per mm³ (normal adult male range about 4.5-5.5 × 10⁶, so this is normal).
  • Shortcut check: RBC/mm³ = N × 10,000 = 475 × 10,000 = 4.75 × 10⁶ ✓ (the factor 10,000 holds only for 1:200 dilution and 80 small squares).
• WBC count. 1:20 dilution in Turk's fluid; 160 cells in the 4 corner squares (4 mm², depth 0.1 mm).
  • Volume = 4 × 0.1 = 0.4 mm³; count = 160 × 20 / 0.4 = 3,200/0.4 = 8,000 per mm³ (normal 4,000-11,000).
• Sections from a block and slide yield. A 4 mm long piece of mouse intestine cut at 5 µm.
  • 4 mm = 4,000 µm → 4,000/5 = 800 sections; at 4 sections per slide, about 200 slides (before trimming and ribbon loss).
• Spectrophotometry. A protein solution gives A = 0.42 at 595 nm (Bradford) in a 1 cm cuvette; the standard curve has slope 0.70 absorbance per mg mL⁻¹.
  • Concentration = 0.42/0.70 = 0.60 mg mL⁻¹; in 2 mL of extract that is 1.2 mg of protein.
• Centrifugation. Rotor radius 8 cm at 10,000 rpm: RCF = 1.118 × 10⁻⁵ × 8 × (10,000)² = 1.118 × 10⁻⁵ × 8 × 10⁸ = 8,944 × g - enough to pellet mitochondria (about 10,000 × g), not microsomes (about 100,000 × g).

Where it is used

• Diagnostic histopathology and cytology labs - Dr Lal PathLabs, SRL, Metropolis, Thyrocare, AIIMS/PGIMER/Tata Memorial departments: roles such as histotechnologist, cytotechnologist and lab technician; grossing, embedding, microtomy, H&E, special stains and IHC under NABL/ISO 15189 systems.
• Pharma and CRO toxicology - Sun Pharma, Cipla, Dr Reddy's, Syngene, Vimta, Aurigene: preclinical toxicity studies need organ histopathology of rodent tissue under CPCSEA and GLP, with slide archives retained for the study record.
• Fisheries and veterinary disease diagnosis - ICAR institutes (ICAR-CIFA, CIFE, CIBA, IVRI), state fish and animal disease labs: gill, liver and kidney histopathology for white spot disease, EHP in shrimp and parasite load - directly linked to aquaculture health certification.
• Research institutes and universities - CSIR (CDRI, IICB, CCMB), ICMR, DBT institutes, NCBS, IISc: histology, histochemistry, IHC, confocal and electron microscopy facilities; JRF/SRF and technical officer posts.
• Forensic and food/environment labs - State Forensic Science Laboratories (histology of tissue in post-mortem material, diatom test in drowning), FSSAI-notified labs (muscle and hair identification, meat speciation), pollution labs (gill and liver histopathology as a biomarker of water toxicity).
• Teaching and products - permanent slide, model and charts manufacturers, school/college lab supply (microtomes, stains, mountants) and biology teaching posts where slide preparation and demonstration are part of the job.

Interview questions

• Q: Why is fixation the most critical step in histology? A: It stops autolysis and bacterial decay within minutes of death, hardens tissue for sectioning and makes components receptive to dyes - no later step can repair a badly fixed tissue.
• Q: Why is a clearing agent needed between dehydration and wax? A: Alcohol is not miscible with paraffin, so a wax-miscible agent such as xylene must replace it; it also raises the refractive index and makes the tissue translucent.
• Q: What is the ideal thickness of a routine paraffin section and why? A: About 5-7 µm - thick enough to hold together and show cell relations, thin enough that cells are not superimposed and light passes through.
• Q: Why does haematoxylin need a mordant? A: Oxidised haematein has little affinity for tissue; a metal mordant such as potassium alum or iron forms a positively charged lake that binds the acidic phosphate groups of chromatin.
• Q: Differentiate progressive and regressive staining. A: Progressive staining is stopped at the right intensity and not removed; regressive deliberately overstains and then removes excess by differentiation in acid alcohol, as in routine H&E.
• Q: Which fixative would you choose for electron microscopy and why? A: Buffered 2-4% glutaraldehyde (or Karnovsky's) followed by osmium tetroxide - glutaraldehyde cross-links proteins finely and OsO₄ fixes lipids and adds electron density; pieces must be under 1 mm because penetration is poor.
• Q: Why are frozen sections used when paraffin sections look better? A: Because they are ready in minutes for intra-operative diagnosis and because lipids and most enzyme activities survive freezing but are destroyed by heat, alcohol and xylene.
• Q: How would you prove a PAS-positive magenta material is glycogen? A: Run a duplicate section digested with diastase/amylase - loss of magenta staining in the digested section confirms glycogen.
• Q: A slide shows parallel ridges and alternate thick and thin bands. What went wrong? A: Chatter, from a loose block or knife, a blunt knife, a wrong knife angle or too hard a block - tighten the clamps, change the knife edge and soften or re-block the tissue.
• Q: What is the difference between magnification and resolution? A: Magnification enlarges the image; resolution is the smallest separation at which two points are still seen as two (d = 0.61λ/NA). Enlarging beyond about 1000 × NA gives empty magnification.
• Q: Why is immersion oil used with the 100× objective? A: Its refractive index (~1.515) matches glass, so light rays are not lost by refraction at the air gap; this raises numerical aperture and thus resolution.
• Q: What permissions does a student need before working on rat tissue in India? A: Institutional Animal Ethics Committee approval under CPCSEA (Prevention of Cruelty to Animals Act, 1960), with the 3Rs followed; tissue from wild species also needs clearance under the Wild Life (Protection) Act, 1972.
• Q: How is a histology lab's waste disposed of? A: Under the Bio-Medical Waste Management Rules, 2016, in colour-coded containers - yellow for anatomical and chemical waste, red for contaminated plastics, white for sharps, blue for glass - handed to an authorised common treatment facility; mercury and osmium waste go through hazardous-chemical routes, never the drain.

Common mistakes

• Writing the sequence wrongly - putting clearing before dehydration, or staining before dewaxing; the order of the paraffin method is a standard marks-losing question.
• Using too little fixative or too thick a block, then blaming the microtome for soft, mushy centres - fixative must be 10-20× the tissue volume and the piece 3-5 mm thick.
• Jumping straight into absolute alcohol or 100% xylene; abrupt changes shrink and harden tissue and make blocks crumble.
• Expecting to see lipids, glycogen after Bouin's, or enzyme activity in a routine paraffin H&E section - these need frozen sections or specific fixatives.
• Calling haematoxylin an "acidic" stain and eosin "basic"; haematoxylin's lake is basic/cationic and stains basophilic nuclei, eosin is acidic/anionic and stains acidophilic cytoplasm.
• Forgetting to recalibrate the ocular micrometer after changing the objective, and quoting measurements in ocular divisions instead of micrometres.
• Dropping the 0.1 mm chamber depth or the dilution factor in haemocytometer arithmetic, giving counts wrong by orders of magnitude.
• Reporting artefacts - folds, air bubbles, knife scores, formalin pigment, ice-crystal holes - as structures, and never running positive and negative controls for special stains and IHC.

Related topics

• Cell biology: cell structure and organelles · Animal physiology and organ systems · Parasitology · Genetics and cytogenetics · Biochemical and molecular techniques (PCR, electrophoresis, ELISA) · Zoology careers.