Session 3: Blood cells and haematopoiesis

What is haematopoiesis?

The process of producing mature blood cells from precursor cells

Where does haematopoiesis occur in the second trimester?

From the second trimester, haematopoiesis primarily occurs in the liver.

Where does haematopoiesis occur in the third trimester?

From the third trimester, haematopoiesis primarily occurs in the bone marrow

Where does haematopoiesis occur in adults?

Bone marrow of the sternum, pelvis, vertebrae, ribs and skull.

Where does haematopoiesis occur in the foetus?

The liver and spleen

Describe the features of erythrocytes (RBCs)

Anucleate cells densely filled with haemoglobin - have no organelles

- Flexible, biconcave discs with a diameter of ~7.2uM (x2 diameter of narrowest capillaries)

- Large surface area for gaseous exchange for delivering oxygen to tissues and returning carbon dioxide to lungs

What is the progenitor of erythrocytes?

Myeloid stem cell

What is the process of production of erythrocytes?

Erythropoiesis

What controls erythropoiesis?

- An imbalance in homeostasis of blood oxygen levels and acidity of the blood acts as a stimulus = hypoxia due to decreased RBC count, decreased amount of haemoglobin, or decreased availability of oxygen

- This leads to reduced oxygen in the blood

- The kidney (and liver) then releases erythropoietin in response

- The erythropoietin stimulates red bone marrow to enhance erythropoiesis and increase the RBC count as a result

- The increased RBC count leads to increase of oxygen carrying capacity of the blood

Describe the features of platelets?

- Platelets are anucleate 2-3uM cell fragments derived from bone marrow megakaryocytes.

- They contain a range of factors which are important for blood clotting and vessel wall repair

What is thrombopoiesis?

Thrombopoiesis is the formation of thrombocytes in the bone marrow

What is the main regulator of thrombopoiesis?

Thrombopoietin hormone (TPO)

Where is thrombopoietin produced?

By the liver and kidneys

How does thrombopoietin (TPO) work?

TPO acts in the bone marrow to stimulate megakaryocytes to increase in size = by undergoing DNA replication without dividing

- Platelets 'bud off' or ligate from enlarged cells

- TPO can bind to platelets, where it is destroyed = this reduces the bioavailability of the hormone as the platelet numbers rise.

What is innate immunity?

An immediate, yet non-specific and transient response to infection

What is adaptive immunity?

- Humoral responses involve secretion of immunoglobulins (antibodies) by B cells

- Cell-mediated responses involve the killing of infected cells by T cells

What are neutrophils?

Most common circulating leukocyte (40-70% of WBC).

Lifespan of 1-4 days

Lobed nucleus (polymorphonuclear cells)

Recruited to sites of infection to phagocytose pathogens

Granules inside them contain proteases, antibacterial peptides and oxidising agents

12-15 uM diameter

Where do neutrophils develop?

Neutrophils develop in the bone marrow

What are eosinophils?

Account for 1-6% of circulating leukocytes

Lifespan of 6-9 days

Red granules revealed with Wright's stain (appear eosinophilia with H&E stain)

Functions: combatting HELMINTH infection, mediating hypersensitivity reactions (allergies) and phagocytosis antigen-antibody complexes

In what tissue are abundant eosinophils found?

Eosinophils are abundant in the connective tissue of the intestinal lining and in the lungs of asthma patients.

What are basophils?

Least-common WBC (0.2-1% of circulating leukocytes)

Half-life 2.5 days

Purple-staining of basophilic granules

Cytoplasm contain histamine, heparin and pro-inflammatory cytokines

Function: importance in type 1 hypersensitivity reactions (e.g. re-exposure to allergens)

What are monocytes?

Account for 2-10% of total WBC

Lifespan varies (days)

Phagocytic and pinocytic

Monocytes are 'agranulocytes' that leave circulatory system by diapedesis and mature into macrophages in tissues

15-20uM diameter

Large, kidney-shaped nucleus

Fine cytoplasmic granules contain lysosomes

Function: respond to inflammation and act as antigen-presenting cells (APCs)

What are macrophages?

Lifespan of several years

Derived from circulating monocytes which migrate to loose connective tissue

Function: respond to local inflammation and phagocytic (degrade foreign organisms/cell debris)

Professional antigen presenting cells (APCs) which present foreign materials to T-lymphocytes

What is adaptive (or acquired) immunity is divided into two systems?

1. Humoral immunity: secretion of immunoglobulins by B lymphocytes into extracellular fluids (humours)

2. Cell-mediated immunity: T lymphocyte-mediated destruction of infected cells (via cytotoxic lysozymes).

What are lymphocytes?

White blood cells which constitute 20-50% of circulating leukocytes.

5-20uM in diameter

Appear spherical in histological preparations with thin strip of cytoplasm surrounding large but regularly shaped nucleus

B and T lymphocytes

How do erythrocytes survive if they have no organelles (mitochondria) to produce ATP via oxidative phosphorylation?

They get their energy from glycolysis in the cytoplasm (make ATP using glucose - does not involve mitochondria).

What is the function of B lymphocytes?

to shuffle DNA encoding their immunoglobulins to create a range of antibodies able to recognise a range of antigens.

What does a B cell do if presented to a foreign antigen it recognises?

The B cell will proliferate under the control of T-helper cells - to form a population of plasma cells that will produce antibodies specific for that antigen.

What is the name given to B lymphocytes which are able to expand again following re-exposure to the same antigen?

Long-lived memory B lymphocytes

What are T lymphocytes?

Lymphocytes that originate in the bone marrow but mature in the thymus (or spleen in adults). They undergo rearrangement of their T cell receptor genes. T lymphocytes are made up of CD4+ (T helper) cells and CD8+ (T cytotoxic) cells

What is the function of CD4+ (T helper) cells

CD4+ T-helper cells induce proliferation and differentiation of T and B cells and activate macrophages

What are mast cells?

They appear like basophils but are derived from a DIFFERENT precursor

Components of the innate immune system but also play role in allergy

Granules contain: heparin (anticoagulant), histamine (increase blood vessel wall permeability) and cytokines (to attract eosinophils and neutrophils)

Where can mast cells be found?

Connective tissue

What is the function of CD8+ (T cytotoxic) cells

• induce apoptosis in virally-infected cells by creating holes in the plasma membrane with perforin

• injecting granzymes which degrade the cell

What is the name given to the pathological response resulting from mast cell hypersensitivity?

Type 1 hypersensitivity.

This is stimulated by allergens.

Responses can be localised (allergy) or systemic (anaphylaxis).

Describe the process of type 1 hypersensitivity resulting from mast cell hypersensitivity?

Following initial exposure, mast cells become coated with immunoglobulin E (IgE) molecules that specifically bind allergens

When an allergen cross-links these surface-bound IgE molecules, the contents of the granules are rapidly released from the cell.

Describe structure/function of RBC (erythrocytes)

Structure: anucleate cells, specialised for mechanical flexibility, 120-day lifespan, contain concentrated haemoglobin

Function: oxygen and carbon dioxide carriage

Describe structure/function of neutrophils

Structure: end cells; cannot re-enter blood.

• Numerous granules with proinflammatory and antibacterial products.

• Phagocytose and kill bacteria.

Function: inflammation and defence against bacteria

Describe structure/function of eosinophils

Structure: end cells; cannot re-enter blood. Numerous granules with proinflammatory and anti-parasite products.

Function: Inflammation, allergy defence and defence against parasites

Describe structure/function of basophils

Structure: end cells; cannot re-enter the blood. Numerous granules with proinflammatory products.

Function: Inflammation, allergy and defence against parasites.

Describe structure/function of monocytes

Structure: can mature into macrophages - including becoming long-term tissue macrophages. Ingest organisms (phagocytosis) and debris. Major cytokine producers

Function: Inflammation and defence against infections

Describe structure/function of lymphocytes

Structure: after formation can proliferate in tissues and lymph nodes and recirculate through the blood

Function: adaptive immune system

Describe structure/function of platelets (thrombocytes)

Structure: cell fragments produced by fragmentation of megakaryocyte cytoplasm - major source of growth factors at sites of injury

Function: blood clotting (haemostasis)

Which one of the following most accurately describes the functions of mast cells?

A) Phagocytosis

B) Immunoglobulin production

C) Killing of virally-infected cells

D) Local inflammation, innate immunity, tissue repair

E) Antigen presentation

D) Local inflammation, innate immunity, tissue repair

What is the function of 'chemotactic factors' produced by mast cells?

Recruiting immune cells, particularly eosinophils and neutrophils

What is the function of 'histamine' produced by mast cells?

Increase vascular permeability

What is the function of 'cytokines' produced by mast cells?

Modulators of immune cell activity

What is the function of 'heparin' produced by mast cells?

Anti-coagulant

From which immune cell type do macrophages develop?

Monocytes

Where do macrophages mature?

In organs such as the skin and liver

Arrange these blood components according to life span length with 1 being the shortest and 4 being the longest.

RBCs, Neutrophils, Platelets, Memory B cells

1: Neutrophils (shortest life-span)

2: Platelets

3: RBCs

4: Memory B cells (longest life-span)

What are the names of the cells which detect reduced pO2?

Peritubular cells of the kidney

What is the name of the hormone which is released by peritubular cells of the kidney to promote maturation of erythrocyte precursors?

Erythropoietin (EPO)

Where are erythrocyte precursors found?

Bone marrow

What is the name of the hormone which regulates platelet homeostasis?

Thrombopoietin

The hormone _______ stimulates ______ cells in the bone marrow to enlarge.

The hormone thrombopoietin stimulates megakaryocyte cells in the bone marrow to enlarge.

Thrombopoietin is produced constitutively by the ______ and ______.

Thrombopoietin is produced constitutively by the kidney and liver.

Match the circulating leukocyte to the differential count.

Monocytes

Neutrophils

Lymphocytes

Basophils

Eosinophils

Basophils: 0-0.75% of WBCs

Eosinophils: 1-3% of WBCs

Monocytes: 3-7% of WBCs

Lymphocytes: 25-33% of circulating WBCs

Neutrophils: 57-67% of WBCs

Which cell provides defence against HELMINTH infection?

Eosinophils

Which cell provides phagocytosis against bacteria in particular

Neutrophils

Which cells provide adaptive immunity?

T and B lymphocytes

What component of blood is important for clotting?

Platelets

What cells are important for oxygen transport?

Red blood cells

Which cells are mildly phagocytosis APCs that differentiate into macrophages in tissues?

Monocytes

Which cells are important for allergic reactions?

Basophils

Order the following stages of neutrophil maturation, with the earliest being 1 and the latest being 6

Band cell

Myelocyte

Neutrophil

Myeloblast

Promyelocyte

Metamyelocyte

Myeloblast (earliest)

Promyelocyte

Myelocyte

Metamyelocyte

Band cell

Neutrophil (latest)

Where are dust cells found?

Lung

Where are Langerhans cells found?

Epidermis

Where are sinusoidal macrophages found?

Spleen

Where are Kupffer cells found?

Liver

Which lymphocyte kills "stressed cells" in an MHC-independent fashion?

Natural killer (NK) cells

Which lymphocyte are 'lymphoid progenitor cells'?

Lymphoblastic

Which lymphocyte produces immunoglobulins?

Plasma cells

Which lymphocyte suppresses immune functions?

CD4+ CD25+ (regulatory) T-cells

Which lymphocyte regulates immune functions e.g., in activating B cells?

CD4+ (helper) T-cells

Which lymphocyte kills virally-infected cells in an MHC-dependent fashion?

CD8+ cytotoxic T-cells

Identify the blood cells/components in the following high-power photomicrograph of a Giemsa-stained blood smear?

A: Eosinophil

B: Basophil

C: Neutrophil

D: Monocyte

E: Platelet

F: Lymphocyte

What stain is typically used when investigating WBC count in a bone marrow biopsy to assist in the diagnosis of a condition like chronic myeloid leukaemia (CML)?

Wright's stain

Which cell type would you NOT expect to see in a normal peripheral blood film?

A) Band cells

B) Lymphoblasts

C) Reticulocytes

D) Megakaryocytes

E) Macrophages

A) Band cells

Where would the band cells visible in this blood film below normally be found?

In the bone marrow

From which site would bone marrow samples most likely be collected?

A) Rib

B) Vertebra

C) Head of the femur

D) Superior iliac crest

E) Sternum

D) Superior iliac crest

By which method would a bone marrow sample most likely be collected?

Trephine biopsy

What class of drugs is used to treat Chronic Myeloid Leukaemia (CML)?

Tyrosine kinase inhibitors

Jaundice results from the deposition in the skin of which product of haemoglobin catabolism?

Bilirubin

What cells in the reticuloendothelial system is responsible for destroying damaged RBCs?

Macrophages

In which organ are platelets stored?

Spleen

Activation of WHICH receptors on the surface of basophils (and mast cells) leads to degranulation, and is important in hypersensitivity reactions?

IgE

Which tissue-resident immune cell shares many features with basophils?

Mast cells

Which T-cell population is depleted in HIV infection?

Helper-T cells (CD4+)

Which immune structure monitors blood, as opposed to lymph, for infectious agents?

Spleen

In which of the following bones of adults does relatively LITTLE haematopoiesis take place in adults?

A) Pelvis

B) Femur

C) Scapula

D) Vertebrae

E) Skull

F) Ribs

B) Femur

Which one of the following interleukins is primarily involved in the development of myeloid lineages?

A) IL-2

B) IL-3

C) IL-6

D) IL-7

E) IL-12

B) IL-3

Which one of the following metabolic processes does NOT occur in RBCs?

A) Pentose phosphate pathway

B) Glycolysis

C) TCA cycle

D) Lactic acid fermentation

C) TCA cycle

Why is sickle cell anaemia not manifested in utero?

Beta chains progressively replace gamma chains from birth onwards

In sickle cell anaemia a ______ valine residue replaces a ______ glutamic acid at position 6 of the beta-globin gene as a consequence of a point mutation. Under hypoxic conditions, the altered haemoglobin ______, leading to a rigid, sickle-shaped cells that block microvasculature.

In sickle cel anaemia a non-polar/hydrophobic valine residue replaces a negatively-charged glutamic acid at position 6 of the beta-glob in gene as a consequence of a point mutation. Under hypoxic conditions, the altered haemoglobin polymerises, leading to a rigid, sickle-shaped cells that block microvasculature.

Which organ most commonly sustains damage due to the accumulation of damaged RBCs in an anaemic state?

Spleen

What is stercobilin?

breakdown product of conjugated bilirubin in the gut that gives stool brown color

What is bilirubin?

a orange-yellow pigment formed in the liver by the breakdown of hemoglobin and excreted in bile

What is biliverdin?

a green pigment resulting from hemoglobin breakdown

What is urobilin?

yellowish pigment that is responsible for the color of urine