Histology of Blood

4 Functions

Transport O2 and CO2
Transport hormones and molecules
Body buffer = CO2 regulates overall pH in the body via lungs and kidney. CO2 can leave cells in soluble form, is taken up by RBCà add water to form Carbonic acid→ can be secreted by RBCs in exchange for Cl- or carried with Hb.
Produce and transport antibodies. Done by WBCs.

Components of blood:

Liquid 92%, whole blood 8%

Whole blood:

Plasma = 55%à 7% proteins 91.5% water, 1.5% other
Formed elements = 45% → platelets, RBCs, WBCsà mostly neutrophils >lyphocytes > monocytes > eosinophils > basophils.

Hematocrit exam: RBC bottom layer, then buffy coat, then plasma on top.

Buffy coat = WBC and platelets. Plasma layer should be 55%

Plasma: 90% water

7% plasma proteins – created in liver, confined to blood stream
Albumin maintains osmotic pressure
Globulins (aka immunoglobulins) are formed by WBCs. They are antibodies that bind to foreign substances (antigens = bacteria, fungus, parasites, toxins etc.). They form antigen-antibody complexes.
Fibrinogen = for clotting. Made by liver.
2% other = electrolytes, nutrients, hormones, waste, gasses

Formed elements of blood:

RBCs
WBCs

Granular leukocytes = Neutrophils, eosinophils, basophils
Agranular leukocytes = 1. lymphocytes = T / B/ natural killer cells, and monocytes = precursor for MACs.

3.Platelets

RBC membranes = easy to come by.

RBCs are deformable, have diameter larger than smallest capillary.
Glycoproteins are in cell membrane: Band 3 = anion transporter as well as an anchor for Ankyrinà acts with cytoskeletal actin / band 4.1. All these components form scaffold to allow deformation.

Reticulocytes: the first form of RBC that leaves the bone marrow. They have no nucleus and can be seen in peripheral blood as maintaining some ER and some RNA (thus are still synthesizing proteins like carbonic anhydrase).

Retic count is important.
Anemia = normochromic anemia = hereditary spherocytosis (bad spectrin can’t act with band 4.1), sickle cell (point mutation in beta chain gluàval). The color of the cells is normal, no effect on Hb.
Hypochromic anemia = palor due to lack of Hb resulting from Fe deficiency. RBCs are not being produced due to this Fe deficiency.
Polycythemia = erythrocytosis. Increased hematocrit. This will increase the blood viscosity which puts strain on the heart.

In sickle cell anemia:

Within a biconcave RBCà deoxy state causes HbS tetramers to formà form deoxy HbS polymes which reversibly sickle the RBCà may lead to irreversible sicklingà causes damage to small vessels leading to tissue damage.

Leukocytes: Differential white cell count

Granulocytes:

Neutrophils 60-70%
Eosinophils 2-4%
Basophils less than 1%

Agranulocytes:

Lymphocytes 20-25%
Monocytes 3-8%

All granulocytes share common granules = Azurophilic granules (develop first) as well as have some individually specialized granules which define each group.

Neutrophils, Eosinophils, Basophils each have their own specialized granules.

Neutrophils have 3 types of granules. Tertiary granules have gelatinase and cathepsins (important note)

Neutrophils:

First defense against bacterial infection
Are active phagocytes
Contain 3 kinds of granules
Release granulesà leukotrienes, histamine, eosinophil chemoattractant. All released at site of bacterial infection to act as chemoattractants for eosinophils.
In peripheral blood: will see multilobular (usually 5) nucleus in mature cell. These lobes are connected by ridges of nucleoplasm. Will not see other granulocytes with this pattern. Can’t see granules at LM level.
If see band cell stage in peripheral blood = is a sign of disease

Eosinophils:

Defense against parasites
Granules contain: major basic protein (most prominent), eosinophil cationic protein, eosinophil-derived neurotoxin.
Granules are released when encounter parasite→ put holes in membranes of parasite→ destroyed
Inhibit histamine released by basophils and mast cells. Must get rid of histamine to avoid damage that may ensue by prolonged presence.
Phagocytose antigen-antibody complexes.
Chemoattraction for eosinophils are inhibited by steroids→ lowered immune function.
Appear as bilobed nucleus with horseshoe shape→ small ridge connects. Has prominent granules. These granules are very red under normal staining.

Basophils:

Inflammatory response: release histamine, heparin, seratonin.
Are mobilized to sites of inflammation due to same chemoattractants for eosinophils released by neutrophils.
Carry IgE on surface = important in allergic responses. These tags on basophils bind when first exposed to substance. Upon next exposure→ antigen binds to IgE→ basophils release their granules→ massive release of histamine may cause anaphylactic shock due to homeostatic disruption.
Vasoconstriction, respiratory distress, leaky vesselsà anemia due to fluid loss into tissue→ severe drop in blood pressure (shock).
Also have antiparastic activity
Granules are large and dark→ appear rocky and jagged.

Monocytes:

Precursor of macrophages. Has no other function as a monocyte.
MACs = phagocytosis, inflammatory response, antigen presentation. Are present in all tissues.
Monocytes leave blood streamà differentiate into tissue specific macrophage
Kupffer cells = liver
microglia = CNS
Some MACs are antigen presenting cellsà ingest material and place portion (antigen) on surface which will act to assist antibody formation against that material.
Osteoclasts = bone remodeling.
Appear very lavender with smooth cytoplasm. Bilobed nucleus, each lobe has a nucleolus. Appears as one structure folded on itself, no ridge present.

Megakaryocotes:

Give rise to platelets. Not present in the blood, resides in bone marrow.
Has a complex multilobed nucleus.
Ploidy can increase to 16-64N
Have azurophilic granules very prominent

Platelets: as megakaryocite extrudes a portion of its cytoplasm into sinusoid of bone marrow→ platelets bud off and enter blood stream.

important in clotting, vascular repair.
Give off chemoattractants for neutrophils and basophils, these will go to areas of clotting.
Have series of granules with clotting factors, repair factors, Ca+ mobilizing agents.
On outside: microtubules form a sheath for structural integrity that enable platelet to move. This area = hyalomere→ contains 2 sets of channels:
- Surface-opening tubule = through hyalomere which allows molecule release / allows communication with external space to detect clotting factors.
- Dense tubular network = where Ca+ is sequestered.
Remainder of platelet = Granulomere, where all granules are.

Clinical correlations of platelets:

Thromboembolism = platelet clot, causes vascular blockage.
Coagulation disorders: Vitamin K deficiency. Needed in liver for cofactor to synthesize enzymes for clotting. Hemophilia = difficulty clotting, leads to blood loss.
Thrombocytopenia = reduction of platelets due to megakaryocyte malfunction.

Lymphocytes:

T lymphocytes:

90% in the blood
Have a long life
Cell medated immunity = cells release perforins which destroy invaders.
Responsive to HIV

B lymphocytes:

4-10 % in blood
Respond to foreign antigen by making antibodies which will circulate to fight infections. Must interact with T cells in order to work.
Variable life span
Mediated humoral immunity

Lymphocytes may have apparent golgi sitting near nucleus. Cytoplasm is bluish.

Hematopoesis:

Embryo: First sight of production = Yolk sac

2 months: liver takes over for yolk sac for production

2.5 months: Spleen acts along with liver, but ceases at 7 months.

4 months: bone marrow begins to engage in hematopoeisis which continues throughout life but with different sites being emphasized at different times:

Vertebrae continue to act strongly through whole life, is the strongest
Sternum is next strongest and acts in whole life
Ribs function lesser so, but still contribute through whole life
Femur and tibia taper in production at about 25 yrs.

Bone Marrow:

Divided into hematopoetic cords = endothelial lining beneath which have adventitial cell→ their end feet form a loose syncitium. Red marrow in young people, lacks adipose cells. As age increases→ yellow marrow forms in place of red marrow as fat
Within cords: Hematopoetic stem cells give rise to all cells of blood.
Also have growth factors within cord→ forms pockets of growth factor islands→ cells in that area will form specific subtypes of cells.
Erythroblastic islands: form around “nurse cell” which is a macrophage. Precursors gather around this MAC in response to erythropoeitin released by kidney. Precursor becomes reticulocyte→ MAC takes nucleus away before cell is released as reticulocyte into stream.
Endosteum separates bone forming osteoblasts / clasts from the hematopoeitic tissues (marrow).

Differentiation:

Totipotential cell: Self renewing, not abundant→

Lymphoid multipotential cell→ B/T cells
Myeloid multipotential cell→ all other cells

Assymetric division: when totipotential cell divides, one daughter is identical to parent, the other daughter is of one of the two subtypes above.

Stem cell→ progenitor cells→ precursor cells (blasts)→ mature cells.

Growth factors most influential at transition from progenitors to precursors.

Mitotic activity tapers off as approach mature cell stage.

Erythropoiesis:

Proerythroblast = large cell with large nucleus. Has lacy chromatin. No Hb, high RNA, 3 nucleoli.

Reduce in size and in RNA content
Nucleus reduces and moves to side of cell to be extruded
Will go through 3 divisions before cessation of mitosis.
Hb levels increase as RNA levels decrease. Causes pink color to dominate. In the middle of pathway have polychromatic erythroblast due to blue and red color being present. After final mitotic divisionone color present (orthochromatophilic).
EPO is released in response to oxygen content and is thus an intersection between lung, heart, and kidney function. Kidney failure causes anemia due to EPO reduction.
Following nucleus→ extrusion→reticulocyte→within blood stream

Granulopoiesis:

Precursor = myeloblast. Have prominent nucleus with 2 nucleoli. No granules. →becomes larger promyelocyte in which granules can be seen.
→acquire specific granules in response to specific factors to become a myelocyte (of different types). These cells no longer divide
Nucleus loses nucleoli and becomes kidney shaped within metamyelocyte.
Band form cell shows horseshoe stage. This is a stage specific for neutrophils. → become segmented neutrophil with segmented horseshoe nucleus.
In other cell types: metamyelocyte nucleus changes shape to phenotype specific for either eosinophil or basophil. Round / central nucleus becomes kidney shape and more peripheral.

Monopoiesis:

Monoblasts→promonocytes with well developed golgi, abundant RER , mitochondria, and lysosomes. →differentiate ito MACs

Lymphopoiesis:

Lymphoblast→prolymphocytes either
T cell progenitorà goes to thymus to mature. or
B cell progenitor→remains in the bone marrow and leaves as B cell competent to differentiate into anbibody producing cell.

Clinical correlations

Leukemia – Malignant proliferation of white cell precursors in bone marrow.
1. Chronic leukemia = Slowly progressive, proliferating cells are partly or completely differentiated myelocytes, metamyelocytes, band cellsà chronic granulocytic leukemia
2. Acute leukemia = Rapidly progressive, proliferating cells are virtually undifferentiated precursor cells.

Platelets: Megakaryoblast→magakaryocyteà platelets

Megakaryocyte has demarcation channels which are invaginations into sinusoids into which portions of the MKC will pinch off to form platelets.

Thrombocytopenia: