Bone Marrow and Haematopoiesis

Question 1

What is haematopoiesis?

Answer 1

The process of production of mature blood cells from pluripotent stem cells and haematopoietic stem cells.

A restricted number of blood stem cells give rise to a differentiated progeny of at least 10 distinct lineages, while maintaining a population of haematopoietic stem cells capable of sustaining blood formation throughout the lifespan of the organism.

Question 2

When and where do the first traces of blood formation occur in the embryo?

Answer 2

Occur around day 17 in the extraembryonic splanchnic mesoderm surrounding the yolk sac.

Question 3

What induces the formation of haemangioblastic aggregates?

Answer 3

Mesoderm association with the yolk sac

Question 4

Haemangioblastic aggregates differentiate into 2 cell lineages. What are they?

Answer 4

• Endothelial precursor cells
• Primitive haematopoietic stem cells (develop in 2 waves)

Question 5

What do endothelial precursor cells (angioblasts) differentiate into?

Answer 5

• Endothelial cells
  
  • Through vasculogenesis, these form capillaries

Question 6

Describe primitive haematopoiesis

Answer 6

• First wave
• Emerge at mid to late primitive streak stage
• Gives rise to:
  
  • Primitive erythropoietic cells
  • Primitive macrophages
  • Primitive megakaryocytes

Question 7

Describe difinitive haematopoiesis

Answer 7

• Second wave
• Definitive haematopoietic stem cells and multipotent
• Arise in the placenta and aorta-gonad-mesonephros region.
• From there, cells migrate into the fetal livel, spleen and eventually (just before birth) bone marrow.
• Finished by day 40.

Question 8

Draw a rough diagram to illustrate the pathway of definitive haematopoietic stem cells

Answer 8

Question 9

From which type of cell do erythroblast progenitors develop?

Answer 9

Common myeloid precursors

Question 10

Where to primitive erythrocytes mature?

Answer 10

In the blood stream

Question 11

How many generations are there between the erythroid stem cell and the erythrocyte?

Answer 11

At least 5 generations

Question 12

Where do difinitive erythrocytes mature?

Answer 12

In the fetal liver

Question 13

What are erythropoietic stem cells formed of?

Answer 13

Nucleated erythrocytes containing embryonic haemoglobin

Question 14

What is the difference between embryonic, fetal and adult haemoglobins?

Answer 14

The stability of the subunit interference

Question 15

Describe the primitive red cells

Answer 15

• 6 times larger than the adult red blood cell
• Express genes for embryonic and adult haemoglobin

Question 16

What is the principal site of blood production of all mature circulating blood?

What is the one exception to this?

Answer 16

• The bone marrow = principal site
• Exception = T cells. These require the specialised microenvoronment of the thymus to complete their development.

Question 17

Samokhvalov et al. (2007) on haematopoietic stem cells

Answer 17

• Some adult haematopoietic stem cells have extraembryonic origin.
• Samokhvalov et al. (2007) demostrated the migration of haematopoietic stem cell progenitors from the yolk sac to the fetal liver and thymus.
  
  • ​In turn, showed that yolk sac blood island contains precursors to adult haematopoietic stem cells.
• Conclusion: yolk sac normally contributes to the adult haematopoietic system, but the extent of its contribution is yet to be determined.

Question 18

What happens to haematopoietic stem cells in the fetal liver and why?

Answer 18

HSCs expand in the fetal liver so as to make up the number of stem cells necessary to sustain haematopoiesis in the adult.

Question 19

What is the mark of a true haematopoietic stem cell?

Answer 19

Capacity to long-term reconstitute the haematopoietic system of the adult.

Question 20

What happens to haematopoietic stem cells after their expansion in the fetal liver?

Answer 20

They eventually migrate to the bone marrow cavities of the axial skeleton perinatally, where adult haematopoiesis becomes definitively established.

Question 21

Describe the haematopoietic capacity of the adult liver

Answer 21

• Haematopoietic potential of the liver is retained in the adult
• Demonstrated by the extramedullaet erythropoiesis that occurs in the liver or spleen at times of severe bone marrow dysfunction.
  
  • Extramedullary erythropoiesis in the adult is rare.
• Schlitt et al. (1995) - extramedullary erythropoiesis in the liver and reconstitution of multilineage haematopoiesis by donor-derived cells has been reported following liver transplantation in adult humans with normal bone marrow function.

Question 22

Desribe the bone marrow

Answer 22

• Primary site of haematopoiesis in adults
• Primary lymphoid tissue
• One of the biggest organs in the human body
• Interstices of trabecular bone form the medullary cavity, which contains the bone marrow.
• Found in the central parts of long bones and some bones of the axial skeleton.
• Accounts for ~5% of body weight

Question 23

What are the 3 components of bone marrow?

Answer 23

• Red marrow (haematopoietically active)
• Yellow marrow (inactive)
• Osseous

Question 24

Describe the bone marrow in the fetus

Answer 24

• In the neonate, virtually the entire bone marrow cavity is occupied by proliferating haematopoietic cells.
• Haematopoiesis at this stage even occurs in the phalanges!

Question 25

What happens to the bone marrow as a child ages?

Answer 25

Haematopoietic marrow contracts centripitally, being replaced by fatty marrow.

Question 26

In which parts of the adult skeleton is haematopoiesis confined to?

Answer 26

* Skull * Vertebrae * Ribs * Clavicles * Sternum * Pelvis * Proximal half of humeri * Proximal half of femora * **Large amount of variation between indviduals** * **The volume of the marrow cavity occupied by haematopoietic tissue expands in response to demand.**

Question 27

Describe the structure of bone marrow

Answer 27

* Haematopoietic tissue islands and adipose cells surrounded by vascular sinuses, interspersed within a meshwork of trabecular bone. * Extensive blood supply. * 2 compartments: * **Stroma** - framework of mesenchymal stem-cell-originated adipose cells, stromal cells, fibroblasts, macrophages and blood vessels interspersed with trabeculae. * **Parenchyma** - spongy network of haematopoietic cells.

Question 28

Describe pathology of the bone marrow of the intervertebral disc

Answer 28

* Vertebral bone marrow is involved in IV disc nutrition * Fat conversion is suspect to participate in disc degeneration

Question 29

Describe the vascular supply to the bone marrow

Answer 29

Question 30

Describe red marrow

Answer 30

* Abundant in neonates * Contains haematopoietic cells * Mainly localised in metaphysis of long bones * Conversion from red → yellow marrow occurs throughout childhood * Conversion starts in the limbs and continues proximally, then proceeds into the axial skeleton. * 40% water * 40% fat * 20% protein

Question 31

Describe yellow marrow

Answer 31

* Largely adipose tissue * Found in appendicular skeleton of adults * Mainly localised in diaphysis and epiphyses of long bones * Accounts for ~7% of all body fat * 80% fat * 15% water * 5% protein

Question 32

Describe the function of bone marrow adipocytes

Answer 32

* Like other adipocytes, bone marrow adipocytes are secretory cells. * They have not only a simple function of storage, but are involved in secreting numerous **adipokines** in the bone marrow environment (particularly leptin and adiponectin). * Bone marrow adipocytes and osteoblasts have a common origin (mesenchymal stem cell). * Common origin results in competition between alternative differentiation towards adipogenesis or osteogenesis

Question 33

What do common lymphoid progenitor cells give rise to?

Answer 33

* T cells * B cells * Natural killer cells

Question 34

What do common myeloid progenitor cells give rise to?

Answer 34

* Red blood cells * Basophils * Neutrophils * Eosinophils * Maacrophage * Mast cells * Platelets

Question 35

Draw a flow chart to illustrate the production of mature blood cells from pluripotent stem cells and haematopoietic stem cells.

Answer 35

Question 36

Describe erythrocytes

Answer 36

* Life span - ~120 days * Anucleated * Biconcave disc * Main function is gas exchange * Have heme-containing protein: **haemoglobin**, which binds the O₂ and CO₂ * The shape of erythrocytes provides a high level of flexibility required for passage through the small diameter capillaries.

Question 37

What aspect of the structure of an erythrocyte withstands shear during circulation?

Answer 37

* In addition to the lipids, proteins and carbohydrates, erythrocyte plasma membrane has a cytoskeleton. * **Cytoskeleton** formed of **cross-linked** proteins which **withstand shear during circulation.**

Question 38

Describe leukocytes

Answer 38

* White blood cells * Responsible for immunity: * Defend against pathogens * Remove damaged cells * Remove toxins * Contain nucleus and other organelles * All leukocytes (except lymphocytes) are non-specific * Classified into granulocytes and agranulocytes * Both types contain secretory vesicles and lysosomes

Question 39

List the types of granulocytes

Answer 39

* Eosinophils * Neurtophils * Basophils

Question 40

List the types of agranulocytes

Answer 40

* Monocytes * Lymphocytes

Question 41

Describe eosinophils

Answer 41

* Bi-lobed nucleus * Granules stain bright red * Attack foreign bodies by releasing toxins - **nitric oxide, cytotoxic enzymes,** or by phagocytosing foreign bodies. * These cells are also able to resist the inflammatory actions of neutrophils and mast cells.

Question 42

Describe basophils

Answer 42

* Contain heparin and histamines * Functionally related to the mast cell * The main function of basophils is to release heparin and histamines at the site of injury. * Released agents cause vasodilation and prevent blood clotting, ensuring blood supply to the site of injury. * The least numberous white blood cells.

Question 43

Describe neutrophils

Answer 43

* Most abundant white blood cells * Polymorphous nucleus * Life span of ~10 hours * Induce inflammation by releasing prostaglandins. * Engulf foreign material * Vesicle of engulfed bacterium fuses with enzyme lysosomes destroying the foreign body. * Cytoplasm contains many **small, specific granules** and l**arger azurophilic granules**, which contain enzymes.

Question 44

Describe monocytes

Answer 44

* Kidney-shaped nuclei * Largest leukocyte * Life span in the blood of ~3 days * Develop into **macrophages** after activation * Macrophages can phagocytose large cells and debris * Release signals to attract other leukocytes to the site of injury.

Question 45

Describe lymphocytes

Answer 45

* Most abundant agranulocyte * Responsible for specific immune system response to infection * Regulates inflammation * Types of lymphocytes: * T cells * B cells * Natural killer cells

Question 46

Describe the different types of T cells

Answer 46

* T lymphocyte subgroups differ in the antigenic markers they have and their function. * **Cytotoxic T cells** - attack foreign cells * **Helper T cells** - activate B cells * **Suppressor T cells** - inhibit T and B cell activity

Question 47

What do active B cells do?

Answer 47

Differentiate into plasma cells to produce and release antibodies.

Question 48

Describe natural killer cells

Answer 48

* Recognise foreign cells and attack by attaching onto the target cell. * NK cells release vesicles that cover the cell membrane. * The release of the perforin from the vesicles completely destroys the membrane of a foreign cell as well as breaking the cell apart.

Question 49

Describe platelets

Answer 49

* Fragments of megakaryocytes * Life span is ~10 days * Change shape after activation from round discs to a sphere with dendritic extensions. * Contain secretory granules which secrete various proteins, reinforce platelet aggregation and platelet-surface coagulation reactions.

Question 50

Describe reconversion

Answer 50

* **Replacing of yellow marrow with haematopoietic cells.** * The reverse of the natural conversion process * In response to the haematopoietic needs of the body: * Non-medical conditions: * Smoking cigarettes * Doing sports with a high oxygen debt * Medical conditions: * Obesity and related respiratory disorders * Diabetes * Chronic conditions related to anaemia * Patients treated with haemtopoietic growth factors * **Occurs when the haematopoietic capacity of existing red marrow stores is exceeded.**

Question 51

Poulton et al. (1993) on bone marrow reconversion in heavy smokers

Answer 51

* Study found significant differences between marrow reconversion in smokers and non-smokers. * Hyperplasia of erythroid cells is found in smoking patients with polycythemia. * Hypothesis: this accounts for reconversion phenomenon in heavy smokers. * Explanation: process occurs as a result of tissue hypoxia from increased carboxyhaemoglobin and resultant stimulation of erythrocyte production. * Limitation of study: any red marrow in atypical locations was considered reconversion. Did not take into account the possibility that not all conversion had occurred by age 25.

Question 52

Describe neoplastic marrow infiltrate disorders

Answer 52

* Fatty marrow is replaced with neoplastic tissue * Marrow can be infiltrated as a result of neoplastic diseases: * Lymphoma * Leukaemia * Multiple myeloma * Metastases * Metastases localise in the red marrow becasue it has a richer blood supply than fatty marrow. * **Most common site of metastatic diseases is the vertebral column (69%).**

Question 53

Describe fibrotic marrow infiltrate disorders

Answer 53

* Fatty marrow is replaces with fibrotic tissue * Infiltration occurs as a result of fibrosis: * **Osteomyelitis:** infiltration of the marrow by inflammatory cells * **Marrow infarction:** obstruction of the medulla * Results from either **malignant infiltration of the marrow** with consequent elevation of intraosseous pressure or it can be **secondary** to **chemotherapy** or **steroid** administration. * Can also be seen in patients with **sickle cell disease**.

Question 54

Describe myeloid depletion

Answer 54

* Loss of normal red marrow * Pathologically, the marrow is acellular or hypocellular. * Yellow marrow fills the marrow space. * Initial pathological changes as a result of myeloid depletion are: * Edema * Vascular congestion * Diminished haematopoiesis * Caused by: * Viral infections * Some medications * Chemotherapy / radiation therapy * In many cases, cause is unknown

Question 55

Describe myelofibrosis

Answer 55

* **Replacement of normal marrow cells with fibrotic tissue** * Usually caused by chemotherapy or radiation therapy * Occasionally occurs as a primary disorder

Question 56

What is hyperplasia?

Answer 56

* Abnormal increase in number of cells in an organ or tissue. * In the bone, it is a process of repopulation of yellow marrow by red marrow. * Mechanism of reconversion, caused by an increased demand for haematopoiesis. * Hyperplasia may involve cell lineages or just an individual cell line.

Question 57

What effect can hyperplasia have on the vertebral column?

Answer 57

Can cause increased numbers of all haematopoietic cell lineages (erythrocytes, myeloid and megakaryocitic cells) by 50-95%, which can disrupt the trabeculae of the bone and result in **osteoporosis.**

Question 58

What is hypoplasia?

Answer 58

* **The underdevelopment of a tissue or organ.** * In the bone, hypoplasia is a replacement of haematopoietic marrow with yellow marrow. * Mechanism of myeloid depletion * Caused mainly by chemotherapy and radiotherapy

Question 59

Describe thalassaemia

Answer 59

* Results from diminished or absent production of one or more globin chains. * Causes imbalanced globin chain production. * Excess globin chains form tetramers and precipitate within erythrocytes. * Leads to chronic haemolysis in bone marrow and peripheral blood. * Severity varies based on type of mutation or deletion. * α or β subgroups of thalassaemia. * Extramedullary haematopoiesis occurs in diseases with chronic overproduction of erythrocytes.

Question 60

Describe the possible effect of thalassaemia on the vertebral column

Answer 60

Spinal cord compression by epidural extramedullary haematopoietic tissue.