SCV 213: Lecture 4 WBC and Bone Marrow

What is leukopoiesis?

Production of white blood cells (leukocytes) in the bone marrow, though some mature elsewhere. Their main job is immune defense and fighting disease.

What are the two broad categories of leukocytes?

Granulocytes have segmented nuclei and visible cytoplasmic granules (neutrophils, eosinophils, basophils). Agranulocytes have unsegmented nuclei and no visible granules (lymphocytes, monocytes).

What are cytoplasmic granules for?

They're small pockets inside the cytoplasm that hold enzymes used to fight infection, manage allergic reactions, or for general storage. Different granulocyte types have different-colored, different-shaped granules.

What are the three branches of leukopoiesis?

Granulopoiesis makes neutrophils, eosinophils, and basophils. Lymphopoiesis makes T cells, B cells, and Natural Killer cells. Monopoiesis makes monocytes.

What are the two "pools" cells pass through during granulopoiesis?

The proliferation pool (myeloblast, promyelocyte, myelocyte), where cells can still divide. The maturation pool (metamyelocyte, band cell, mature segmented cell), where cells can no longer divide and are just maturing.

What does the Myeloblast look like?

It's the earliest granulocyte stage, larger than a rubriblast, with a round-to-oval nucleus and a visible nucleolus. The cytoplasm is grayish-blue with no granules yet.

What happens at the Promyelocyte stage?

The nucleolus disappears and red or purple granules first appear in the cytoplasm. At this point it's still unclear which type of granulocyte the cell will become.

What happens at the Myelocyte stage?

The cell gets smaller, the nucleus develops rope-like chromatin, and you can finally start to tell whether it's becoming a neutrophil, eosinophil, or basophil. It's rarely seen in peripheral blood unless there's severe infection or inflammation.

What happens at the Metamyelocyte stage?

It's the first stage in the maturation pool, meaning the cell can no longer divide. The nucleus starts to indent, and the granules specific to its type (light for neutrophil, pink/red for eosinophil, dark blue/purple for basophil) become more obvious.

What is a band cell?

It's the "teenager" stage, known for a smooth, horseshoe-shaped nucleus that isn't segmented yet. Band cells only show up in peripheral blood in low numbers when the body is healthy.

What distinguishes a mature segmented granulocyte?

Its nucleus has multiple distinct lobes, neutrophils usually have 3-5 lobes, while eosinophils and basophils tend to have just 2-3. Each type's granules also stain a distinct color (neutrophils light/lavender, eosinophils red/pink/orange, basophils dark blue/purple).

What is a lymphoblast?

It's the first stage of lymphocyte development, a small round cell with a large nucleus and barely any cytoplasm. It shouldn't normally be seen in healthy peripheral blood.

What's the difference between a small lymphocyte and a Natural Killer (NK) cell?

Both come from the same early lymphocyte pathway. Small lymphocytes go on to become B cells (antibody-producing) or T cells (cell-mediated immunity), while NK cells are larger and can destroy threats without needing to be activated first.

What is a monoblast and how does it develop into a monocyte?

The monoblast is a large cell with a round-to-oval nucleus, the earliest monocyte stage. It develops into a promonocyte (with folded chromatin) and then a mature monocyte, the largest WBC and a phagocytosis specialist.

What is the main job of a neutrophil and how common is it?

Neutrophils are the first responders, mainly fighting bacteria through phagocytosis. They make up about 50-60% of WBCs in most mammals (except cattle), and increased numbers usually mean infection or inflammation.

What does a mature neutrophil's nucleus look like?

It's "polymorphonuclear," meaning it's highly segmented into normally 3 to 5 lobes. Its cytoplasm has light-colored granules that can be hard to see.

What is a band neutrophil?

It's an immature neutrophil (also called a stab cell) with a smooth, horseshoe-shaped nucleus instead of lobes. Seeing more of them in blood usually means the body needs extra neutrophil help.

What is the main job of an eosinophil?

They can phagocytose, detoxify allergens, and damage parasite cell membranes. Higher numbers usually point to allergies or parasitic infections.

How do eosinophil granules differ between species?

Dog granules are round with light blue cytoplasm, cat granules are small and rod-shaped, horse granules are large and stain orange-red, and ruminant granules are smaller than a horse's and stain pink.

What is the main job of a basophil?

They act as mediators of the immune system, and their granules contain histamine for inflammatory responses. They're normally rare in healthy peripheral blood, and higher numbers point to inflammatory, allergic, infectious, or neoplastic conditions.

What are heterophils and where are they found?

They function like neutrophils but are found instead of them in small mammals (rabbits, guinea pigs, hamsters), birds, reptiles, amphibians, and some fish. They're large cells that stain deep orange or red with distinct granules.

What are azurophils and where are they found?

They're functionally similar to monocytes, but found only in reptiles. They're large cells with blue-gray cytoplasm and "dust-like" purple granules, and higher numbers point to bacterial infection or cell death.

What is the main job of lymphocytes, and what do they look like in dogs/cats?

Their major function is attacking specific pathogens and producing antibodies. In dogs and cats, they're small with a slightly indented nucleus, clumped chromatin, and no granules.

What is unique about lymphocytes in ruminants?

They're actually the most abundant WBC type in ruminants, unlike most mammals where neutrophils dominate, and their appearance varies a lot.

What does a monocyte look like and what's its main job?

It's the largest WBC in peripheral blood, with a variably shaped nucleus (kidney-bean, lobed, or amoeba-like) and blue-gray cytoplasm that may have vacuoles. Its main job is phagocytosis, and higher numbers usually mean chronic infection.

Why must a WBC differential be done even if you have an automated analyzer?

Automated machines can miss things like nucleated RBCs, toxic granulation, platelet clumping, and blood parasites. A manual differential from a blood smear catches what the machine can't.

How is a WBC differential performed?

Scan the whole slide first at low power (10-40x) for an overview and to spot things like clumps or parasites. Then move to 100x oil immersion at the feathered edge/monolayer and count 100 WBCs total, identifying each type.

How do you calculate a relative WBC percentage?

Out of the 100 WBCs counted, the number of each type becomes its percentage. Example: if 75 of the 100 cells counted were neutrophils, that's 75% neutrophils.

How do you calculate an absolute WBC value?

Multiply the total WBC count by the relative percentage for that cell type. Example: a total WBC count of 6000/µL with 80% neutrophils gives an absolute neutrophil value of 4800/µL.

Why can relative percentages alone be misleading?

If the total WBC count itself is abnormal, a normal-looking percentage can still represent too many or too few actual cells. That's why calculating the absolute value matters, not just the percentage.

What do the suffixes -OSIS/-TOSIS and -PENIA mean for leukocytes?

-(TO)SIS means an increased number, and -PENIA means a decreased number. So leukocytosis is too many WBCs overall, and leukopenia is too few.

What suffix is used for an increased granulocyte count, and what are the terms?

Granulocytes, ending in -phil, use -IA for an increase: neutrophilia, eosinophilia, basophilia. Neutrophilia is a common finding during bacterial infection.

What suffix is used for an increased agranulocyte count, and what are the terms?

Agranulocytes, ending in -cyte, use -TOSIS for an increase, same as leukocyte: lymphocytosis, monocytosis. Monocytosis commonly shows up with chronic infection or inflammation.

What suffix is used for a decreased count of a specific WBC type?

-PENIA is added regardless of whether the cell ends in -phil or -cyte: neutropenia, eosinopenia, basopenia, lymphopenia, monopenia.

What is pancytopenia?

A decreased number of all blood cell types at once, including RBCs, all WBCs, and platelets. It's a very serious finding.

What is a "left shift"?

It means more immature neutrophils, bands, or even earlier stages in severe cases, are showing up in circulating blood than normal. It signals the body urgently needs more neutrophils to fight infection or inflammation.

What's the difference between a regular left shift and a degenerative left shift?

A regular left shift mainly involves more band cells responding. A degenerative left shift is more severe, with even earlier immature stages like metamyelocytes and myelocytes being called into circulation because mature neutrophils are depleted.

What do hypercellular, hypocellular, and acellular mean for a bone marrow sample?

Normal marrow is about 50% nucleated cells and 50% fat. Hypercellular means more than 50% cells (less fat), hypocellular means fewer cells (more fat), and acellular means almost no cells, mostly fat.

What does "aplastic" mean?

It refers to absent or defective development of a certain cell type, or organ/limb. Aplastic anemia specifically means the bone marrow isn't producing blood cells properly.

What is leukemia?

It means neoplastic (cancerous) white blood cells are present in the bone marrow and/or peripheral blood, often with a predominance of immature blast cells. These extra WBCs can crowd out RBC and platelet production.

What's the difference between a lymphoproliferative and myeloproliferative bone marrow disorder?

Lymphoproliferative means an excess of lymphocytes or cells from the lymphoid cell line. Myeloproliferative means an excess of RBCs, platelets, or other WBCs from the myeloid cell line.

When would you want to evaluate bone marrow?

When there's a persistent cytopenia not responding to treatment, non-regenerative anemia, very high or abnormal cell counts in peripheral blood, suspected cancer, suspected systemic infection (like histoplasmosis), or an iron storage problem.

When should you NOT perform a bone marrow evaluation?

If the patient has a coagulopathy (clotting problem), is a poor anesthesia candidate, or has severe anemia that needs a transfusion first.

Where is bone marrow usually collected from?

Active (red) marrow is found in flat bones, most commonly the proximal humerus or femur. In large animals or large dogs, the sternum can also be used.

What's the difference between bone marrow aspiration and core biopsy?

Aspiration, more common, draws out a liquid sample using a syringe and pressure/rotation. A core biopsy removes an actual piece of bone, often done alongside aspiration, and better preserves tissue architecture and shows parasites.

How should bone marrow smears be prepared?

Make them immediately if not using EDTA, or within about an hour if EDTA is used, since marrow clots much faster than blood. About 12 slides are made using techniques like line, starfish, or compression smears, then air-dried and stained.

What do you need alongside a bone marrow sample to properly evaluate it?

A concurrent peripheral blood differential WBC count. You also look at overall cellularity, the M:E ratio (myeloid to erythroid), and the maturation index (left shift index).