What Is Hematopoiesis? Extramedullary Hematopoiesis System


Hematopoiesis is the process of blood cell production, differentiation, and development. The hematopoietic system consists of the bone marrow, liver, spleen, lymph nodes, and thymus. Before investigating the general maturational characteristics of cells, knowledge of blood cell development is useful.

Origin Of Hematopoiesis

Hematopoietic stem cells area unit the inspiration of the adult hematopoietic system. it’s currently widely accepted that the embryo produces the primary adult repopulating hematopoietic stem cells.

Types of Human Stem Cells

Functionally, 3 forms of human stem cells exist:

  1. Totipotential stem cells
  2. Pluripotential stem cells
  3. Multipotential stem cells

  1. Totipotential stem cells – These cells area unit gift within the initial few hours once Associate in Nursing egg cell is impregnated. Totipotential stem cells, the foremost versatile kind of stem cell, will be converted into any human cell kind, as well as development from Associate in Nursing embryo into a vertebrate.
  2. Pluripotential stem cells – These cells area unit gift many days once fertilization. Pluripotent stem cells will be converted into any cell kind, except they can not be converted into a vertebrate. 
  3. Multipotential stem cells – These cells area unit derived from pluripotent stem cells. they will be found in adults, however, they’re restricted to specific styles of cells to make tissues. for instance, bone marrow stem cells will manufacture every kind of blood cells, bone cartilage, and fat (fat) cells.

In tissues with a high cellular turnover, stem cell populations are essential for the long maintenance of organ operation. physical stem cells are known in many self-renewing organs, as well as the blood corpuscle system.

The hematopoietic stem cell area unit is ultimately accountable for the continual daily production of all mature blood corpuscle lineages. hematopoietic area unit traditionally the foremost totally characterized kind of adult stem cell.

Hematopoietic stem cell area unit functionally outlined at the single-cell level by their twin capability for self-renewal and multipotential differentiation. sign pathways area unit necessary to manage devices of hematopoietic fate. stem cell medical care focuses on embryonic somatic cell medical care, craniate stem cell medical care, wire stem cell medical care, and adult blood somatic cell medical care.

Development of Blood Cells (Hematopoiesis)

Embryonic blood cells, excluding the white blood corpuscle kind of white blood corpuscle, originate from the mesenchymal tissue that arises from the embryonic tissue, the germ layer.

The mesodermally derived intraembryonic region referred to as the arteria ductless gland mesonephros region or, at a rather earlier hematopoiesis stage, the paraaortic splanchnopleure—produces, severally, potent hematopoietic stem cells and potent ascendant cells before their look within the food sac. The class embryo contains a minimum of 2 spatially separated sources of hematopoietic cells.

The anatomical sites of the blood corpuscle development area unit as follows:

 Following biological process and germ layer formation, the primary hematopoietic cell area unit generated within the food sac. the primary blood cells area unit primitive red blood cells (erythroblasts) fashioned throughout the primary a pair of to eight weeks of life.

The onset of circulation between the food sac and embryo correct confounds the precise determination of hematopoietic sources. Mesenchymal stem/progenitor 
cells and hematopoietic flow into along within the peripheral blood throughout the primary trimester to the secondary ontogenic sites of hematopoiesis, the liver, and bone marrow.

Gradually, the liver becomes the positioning of blood corpuscle development. By the second month of gestation, the liver becomes the key website of hematopoiesis, and granular styles of leukocytes have created their initial look. The liver predominates from concerning the second to fifth months of craniate life.

In the fourth month of gestation, the bone marrow begins to operate within the production of blood cells. once the fifth craniate month, the bone marrow begins to assume its final role because of the primary website of hematopoiesis.

Bone Marrow Site And Function

Bone marrow is found at intervals in the cavities of all bones and will be a gift in 2 forms: yellow marrow, which is generally inactive and composed principally of fat (adipose) tissue, and bone marrow, which is generally active within the production of most styles of leukocytes, erythrocytes, and thrombocytes.

The bone marrow is one of the body’s largest organs. It represents more or less three.5% to six of total weight and averages around one,500 g in adults, with the hematopoietic marrow being organized around the bone vasculature. The bone marrow consists of hematopoietic cells (erythroid, myeloid, lymphoid, and megakaryocyte), fat (adipose) tissue, osteoblasts, and osteoclasts, and stroma. hematopoietic cell colonies area unit compartmentalized within the cords.

Following maturation within the hematopoietic cords, hematopoietic cells cross the walls of the sinuses, specialized in vascular areas, and enter the current blood. throughout the primary few years of life, the marrow of all bones is red and cellular. The red bone marrow is at the start found in each external body part and therefore the skeletal structure in young persons however increasingly becomes confined to the skeletal structure and proximal ends of the long bones in adults.

By age of eighteen, bone marrow is found solely within the vertebrae, ribs, sternum, os bones, pelvis, and to some extent the proximal epiphyses of the thighbone and arm bone. Insure abnormal circumstances, the spleen, liver, and humor nodes revert back to manufacturing immature blood cells.

In these cases, enlargement of the spleen and liver is often noted on physical examination. this example suggests that uniform primitive blood cells area unit gift in these area unit Associate in Nursingd are able to proliferate if an acceptable input is a gift.

This situation happens under the following conditions: 

  • When the bone marrow becomes dysfunctional in cases like aplastic anemia, infiltration by malignant cells, or overproliferation of a cell line (example, leukemia).
  • When the bone marrow is unable to fulfill the stress placed thereon, as within the anemia (full discussions of the anemia area unit presented).


Progenitor Blood Cells

The pluripotent stem cell is that the initial during a sequence of steps of hematopoietic cell generation and maturation. The ascendant of all blood cells is termed the multipotential hematopoietic stem cell. Stem cells perform the last word burden of generating multilineage mature blood cells over the life of the organism.

During this span of your time, the stem cell population could bear quantitative and qualitative changes Stem cells have the capability for self-renewal in addition to proliferation and differentiation into ascendant cells. Recent analysis has incontestable that blood, the brain, and plenty of alternative regions of the body have their own specialized stem cells that area unit capable of constructing replacement cells.

Some of these stem cell area units surprisingly variable, an inspiration said as “stem cell malleability,” Associate in Nursingd area unit ready to generate an assortment of on the face of it unrelated varieties of cells. This analysis suggests that adults carry a reservoir of “master cells” within their bone marrow that’s capable of reconstruction virtually any broken tissue. These “master cells” area unit being known as strong adult ancestor cells (MAPCs). 

Multipotent adult ancestor cells categorical Associate in Nursing catalyst known as an enzyme that keeps cells from aging. In vitro, strong adult ancestor cells may be coaxed into changing into muscle, cartilage, bone, liver, or different kinds of neurons and brain cells.

Hematopoietic Cells may be Divided Into 3 Phases 

According to cell maturity:

Primitive, multipotential cells. the foremost immature cluster capable of self-renewal and differentiation into all somatic cell lines.
Intermediate cells. This cluster consists of committed ancestor cells destined to transform distinct cell lines.
Mature cells. the foremost developed cluster with specific functions.

The multipotential vegetative cell is that the ancestor of 2 major ancestral cell lines: white blood corpuscle and nonlymphocytic cells. The body fluid vegetative cell is that the precursor of either mature T-cells or B cells/plasma cells. The nonlymphocytic (myeloid) vegetative cell progresses to the ancestor colony-forming unit, leukocyte corpuscle leucocyte bone cell (CFU-GEMM). The descriptor CFU is employed as a prefix to record the amount of colony-forming units of various ancestor cells that area unit is known through in vitro being assayed.

The unit colony of the colony-forming unit(CFU) leukocyte corpuscle leucocyte bone cell ends up in the event of distinct subsets of committed ancestor cells. The colony-forming unit, leukocyte corpuscle leucocyte bone cell will cause the formation of colony-forming unit-granulocyte macrophage/monocyte (CFU-GM), CFU-eosinophil (CFU-Eo), CFU-basophil (CFU-B), and colony-forming unit –megakaryocyte (CFU-Meg). In hematopoiesis, the colony-forming unit -granulocyte corpuscle leucocyte bone cell differentiates into the burst-forming unit-erythroid (BFU-E).

Each of the colony-forming units successively will manufacture a colony of 1 haematogenic lineage below acceptable growth conditions. The formation and development of mature blood cells from the bone marrow multipotential vegetative cell is controlled by growth factors and inhibitors furthermore because of the microenvironment. The microenvironment or scene influences behavior and controls the proliferation of multipotential cells. Bone appears to supply the microenvironment most acceptable for the proliferation and maturation of cells.

Hematopoietic ancestor cells (HPCs) may be mobilized from the bone marrow to the blood by a large form of stimuli, as well as haematogenic growth factors and chemokines. Individual haematogenic cytokines may be lineage-specific or will regulate cells in multiple lineages, and for a few cell varieties, e.g. stem cells, the concurrent action of multiple cytokines is needed for proliferative responses.

Hematopoietic ancestor cells within the bone marrow exist in an exceedingly extremely organized, three-dimensional microenvironment composed of a various population of stromal cells Associate in Nursingd a living thing matrix wealthy in fibronectin, collagens, and varied proteoglycans. haematogenic ancestors may be found in canal blood (UCB) furthermore. UCB haematogenic cells are utilized with success as a therapeutic supply of autologous and allogeneic transplants for quite twenty years. Cryopreservation prolongs the storage time of UCB.

Erythropoiesis In Hematopoiesis

Erythropoiesis happens in distinct anatomical sites known as biological process islands, specialized niches within which blood corpuscle precursors proliferate, differentiate, and enucleate. every island consists of a phagocyte enclosed by a cluster of erythroblasts. among blood corpuscle niches, cell, and cell-extracellular matrix adhesion, positive and negative regulative feedback, and central phagocyte performance occur. blood corpuscle cells account for five to thirty-eighth of organelle cells in traditional bone.

Granulopoiesis In Hematopoiesis

Myeloid cells account for twenty third to eighty-fifth of the organelle cells within the traditional bone marrow. Granulopoiesis will be recognized as a hematopoiesis unit. Early cells are set within the cords and round the bone trabeculae. Neutrophils within the bone marrow reside within the proliferating pool and therefore the maturation storage pool. Maturing cells pay a mean of three to six days within the proliferating pool. If needed, cells from the storage pool will exit into the circulation quickly and can have a mean lifetime of six to ten hours.

Lymphopoiesis In Hematopoiesis

Unlike different cell lines, lymphocytes and plasma cells are created in bodily fluid follicles. Lymphocytes are indiscriminately spread throughout the cords. bodily fluid follicles may additionally be discovered, particularly once the age of fifty. Plasma cells are set on the tube-shaped structure wall. bodily fluid cells usually account for I Chronicles to five of the organelle cells within the traditional bone marrow.

Megakaryopoiessis In Hematopoiesis

Megakaryopoiesis takes place adjacent to the sinus epithelium. Megakaryocytes protrude through the tube-shaped structure wall as tiny cytoplasmatic processes to deliver platelets into the curving blood. Megakaryocytes grow to be platelets in or so five days.

Other Cells Found in Bone Marrow

Marrow Stromal Cells

The network of stromal cells consists of reticulum cells, histiocytes, fat cells, and epithelial tissue cells. This structure is wherever the hemopoietic cells are suspended in an exceedingly delicate semifluid state. Stromal cells turn out an associate degree animate thing matrix composed of collagens and proteins (for example, glycoproteins and proteoglycans). The animate thing matrix is essential for the upkeep of traditional renewal and differentiation of bone marrow cells.

Mast Cells 

Tissue mast cells, an animal tissue cell of mesenchymal origin, is generally discovered within the bone marrow. The thick blue-purple granules that typically obscure the spherical or oval latticelike nucleus contain polyose, histamine, serotonin, and chemical process enzymes. multiplied numbers of mast cells will be seen in an exceeding sort of abnormal conditions (for example, chronic lymphoproliferative disorders or chronic infections).


Macrophages conjointly known as reticulum cells or histiocytes, seem like giant cells within the bone marrow. the looks of the cytoplasms can vary, betting on what the cell has eaten (for example, siderophages are macrophages containing iron-rich pigment and ferritin). Gaucher cells are macrophages are full of uncatabolized glucocerebrosides.

Bone Cells

Osteoblasts are bone matrix–synthesizing cells that gibe plasma cells and are typically discovered in teams. though these cells are solely often seen in traditional adult bone marrow aspirates, an associate degree multiplied the variety of cells is characteristically seen in aspirates from kids and from patients WHO have metabolic unwellness. Osteoclasts gibe megakaryocytes. These are bone reworking cells.


A comprehensive examination of bone marrow involves the examination of each bone marrow smears and histologic tissue sections. ancient paraffin n-reactive immunohistological reagents not to mention newer molecular techniques have improved the study of bone marrow diagnostic assay specimens. The examination of a stained peripheral blood smear is a very important part of the whole corpuscle count (CBC) procedure, which is habitually performed within the hematology laboratory.
In this procedure, white blood cells are examined, identified, and counted. Red blood cells and platelets also are rigorously examined throughout this procedure. to spot the traditional cells that seem on a blood smear and acknowledge immature cells that will seem in numerous disorders or within the bone marrow, it’s vital to grasp the sequences of cellular development by name additionally because of the general characteristics hematopoiesis of blood cells. 

General Cellular Characteristics

The identification and stage of maturation of stained blood cells will be guided  by a spread of systematic options. 2 vital characteristics to watch ab initio in cell identification are:
  1. Overall cell size
  2. Nuclear-cytoplasmic quantitative relation

Overall Cell Size

The overall size of a corpuscle is sometimes compared with the scale of a mature RBC. aside from the bone cell hematopoiesis series, erythrocytes and leukocytes decrease in overall size as maturation progresses.

Nuclear-Cytoplasmic Quantitative Relation

The amount of house occupied by the nucleus in relevance the house occupied by the protoplasm is that the nuclear-cytoplasmic (N: C) quantitative relation. the scale of the nucleus typically decreases as a cell matures. Consequently, the N: C quantitative relation decreases in several cell sorts with maturation. Blast varieties of erythrocytes, leukocytes, and megakaryocytes have a high (4:1) N: C quantitative relation.
As these cells mature, the quantitative relation is reduced to 2:1 or 1:1 in most cells, except in thrombocytes, mature erythrocytes, and therefore the WBC style of the corpuscle. Thrombocytes and erythrocytes lack a nucleus (anuclear), and mature lymphocytes oftentimes retain the first 4:1 to 3:1 N: C quantitative relation.

Nuclear Characteristics

Nuclear characteristics play a very important role in cell identification. vital options of the nucleus embrace
  • Chromatin pattern
  • Nuclear form
  • Presence of nucleoli

Chromatin Patterns

The chromatin granule arrangement demonstrates characteristic patterns. These patterns are the foremost distinctive nuclear feature of a cell in terms of maturity and cell sort recognition. In general, the general pattern progresses from a loose-looking arrangement to a lot of clumped pattern as a cell matures. The terms wont to describe numerous patterns embrace the following: sleek or homogenized, fine, delicate, lacy or thready, smudged, clumped, or pycnotic (dense or compaction. 
Examples of common chromatin granule options for every cell sort are given here.
  •  Lymphocytes exhibit a sleek or homogenized pattern of chromatin granule throughout development till the mature stage once clumped heterochromatin is a lot obvious.
  •  Granulocytes progress from having a fine to an extremely clumped pattern.
  •  Monocytes have a lacy pattern, that becomes finer because the cell matures.
  •  Erythrocytes still develop a lot of clumped pattern as maturation progresses, till the very dense (pyknotic) nucleus is lost (extruded) from the mature cell.

Nuclear Shape

The shape of the nucleus in young cells is either round or oval; however, monocytes may have a slightly folded nuclear shape. In the cells that retain their nucleus as they mature, nuclear shapes become very distinctive for particular cell types.

  1. Lymphocytes usually continue to have a round or oval nucleus. Some cells may have a small cleft in the nucleus.
  2.  Monocytes have a kidney bean-shaped nucleus but folded or horseshoe shapes are common.
  3.   Mature neutrophils, eosinophils, and basophils have segmented nuclei attached to one another by fine filaments. The number of distinctive lobes ranges from two to five depending on the cell type.

Presence Of Nucleoli 

The presence or absence of nucleoli is vital to the identification of cells. The 3 cell lines of erythrocytes, leukocytes, and megakaryocytes all have nucleoli within the earliest cell stages. As cells mature, nucleoli are typically not visible. These changes within the look of the nucleoli are associated with the 
rate of synthesis of the ribosomal polymer.
The number of nucleoli varies counting on the cell kind, as is shown within the following examples:
  •  Lymphoblasts have one or 2 nucleoli.
  • Myeloblasts have one to 5 nucleoli.
  • Monoblasts typically have one or 2 nucleoli however sometimes might have 3 or four.
  • Erythroblasts might not have any nucleoli or might have up to 2 nucleoli that will stain darker than in alternative sorts of blast cells.
  •  Megakaryoblasts usually have one to 5 nucleoli.

Cytoplasmic Characteristics

A variety of living substance options aid within the microscopic identification of cell maturity and kind. These options include:
  1. Staining color and intensity
  2. Granulation
  3. Shape
  4. Quantity of living substance
  5. Vacuolization
  6. Inclusion bodies

Staining Color and Intensity

The overall color and intensity of staining in an exceedingly Wright-stained blood smear vary with cell maturity and kind. In general, living substance color progresses from darker blue (indicating active macromolecule synthesis) in younger cells to lighter blue or pink in mature cells.
Most early cells have a medium-blue living substance. Immature erythrocytes have an awfully distinctive blueish living substance that becomes paler and grey wanting because the cell synthesizes hemoprotein. As mature cells, lymphocytes are typically noted for his or her pale sky-blue living substance color. Variations in living substance color develop in several cells owing to abnormalities or the presence of granules.


The presence, size, and color of granules are vital in cellular identification. In general, granulation progresses from no granules to nonspecific granulation to specific granulation.
The earliest, blast sorts of leukocytes and megakaryocytes don’t have granules, and erythrocytes ne’er exhibit granulation throughout their life cycle. The white corpuscle cell line of leukocytes is noted for distinctive granulation. the whole development of granules in leukocytes is mentioned.
Granules vary in many ways:
  • In size, starting from terribly fine to coarse
  • In color, together with red (azurophilic), blue (basophilic), and orange (eosinophilic)
  • In the quantity of granulation per cell

Cytoplasmic Form

The living substance define or form is beneficial in cellular identification. the foremost distinctive variation in living substance form happens in some blast forms, monocytes, and megakaryocytes. Pseudopods are also ascertained in mature monocytes and in some white corpuscle blast forms. The bone cell develops a lot of irregular defines because the cell matures.

Quantity of living substance

In some cell sorts, the particular amount of living substance will increase with age. The bone cell, particularly, develops in-depth quantities of living substance. Abnormalities of lymphocytes oft demonstrate accrued amounts of living substance.


Monocytes are oft-noted for having vacuoles throughout their life cycle and beneath traditional conditions. apart from the white cell, the condition of the living substance is usually seen in older cells and in abnormal conditions. Anticoagulants may also manufacture vacuoles as artifacts if the blood is held on for an extended than an acceptable amount. Severe microorganism infections, infective agent infections (e.g., infectious mononucleosis), and malignancies might manufacture a motivating range of vacuoles in numerous white corpuscle sorts.

Inclusion Bodies

Cytoplasmic inclusions like Auer bodies or Auer rods in myelocytic or monocytic blast forms or eaten particles are vital to look at as a result of the aid within the identification of cell sorts. numerous erythrocytic inclusions and leukocytic inclusions are indicative of specific diseases.
Some sorts of inclusions are also seen on a Wright-stained blood smear, however, alternative inclusions (such as iron particles) need special staining techniques.

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