Hematology

Hematology is the study of blood and its components.

Hemopoiesis:

The formation of blood cells is called Hemopoiesis.

The blood consists of a fluid medium, Plasma in which blood cells are suspended. The blood cells are of three types

1.      White blood cells (WBCs)

2.      Red blood cells (RBCs)

3.      Platelets

WBCs are further divided into 3 main groups

1.      Granulocytes (Neutrophils, Eosinophils, and Basophils)

2.      Monocytes

3.      Lymphocytes

The blood cells are continuously destroyed either by aging or as a result of their functional activities and are replaced by new cells. There is a fine balance between the rates of formation and destruction of these cells.

Sites of blood formation:

Mesoblastic Phase:

In first 19-20 days of embryonic stage, blood cells are formed in the wall of the yolk sac. These cells are mesodermal in origin hence this phase is called Mesoblastic phase. Mesoblastic phase only produces RBCs. The hemoglobin in these RBCs is called Embryonic Hemoglobin.

Hepatic Phase:

The liver is the main site of hemopoiesis in the fetus from 5^th to 30^th week of intra-uterine life. This is termed as hepatic phase. All type of blood cells are produced in later part of this phase .

RBCs produced at this stage are larger than adult RBCs. The RBCs contain hemoglobin at this stage is called fetal hemoglobin.

Medullary Hemopoiesis:

The bone marrow gradually takes over hemopoietic functionfrom the 5^th month until when the bone marrow is the only major site for blood cells formation. Lymphocytes precursors are formed in liver and bone marrow but the main sites for lymphocytes development are spleen, lymph nodes and other lymphoid tissues.

Initially hemopoiesis takes place in the marrow of all bones but after birth it slowly and gradually reduces to get confined to marrow of flat bones and vertebrae.

Origin of blood cells:

All blood cells formed from the undifferentiated primitive cell which resembles a large lymphocyte and is called Plueripotent stem cell or totipotent stem cell.

It gives rise to lineage specific stem cells termed as colony forming units (CFU). The stem cells maintain their numbers by self replication or self renewal. All of this takes place under the influence of certain proteins called hemopoietic growth factors such as Interleukins (IL) and colony stimulating factors (CSF).

Self renewal and cell cycle

Erythropoiesis:

The formation of red blood cells is called as Erythropoiesis. The process of erythropoiesis is characterized by following progressive changes.

1.      Decrease in cell size

2.      Hemoglobinization

3.      Extrusion of the nucleus

Stages of Erythropoiesis

Pronormoblast:

Ø  It is a round cell with a diameter of 12-20u.

Ø  It has a large nucleus surrounded by a small amount of cytoplasm

Ø  Cytoplasm is deep blue in color with small ear shaped projections

Ø  The nucleus is round and consist of a network of uniformly distributed chromatin strands, which is reddish purple in color and contain several nucleoli.

Basophilic Normoblast:

Ø  It is 10-16u in diameter

Ø  It has a large nucleus with thick chromatin strands and no nucleoli

Ø  Cytoplasm is basophilic in color

Polychromatic Normoblast:

Ø  It is 8-14u in diameter

Ø  The nucleus occupies a smaller part of the cell and stains deeply

Ø  Hemoglobin synthesis start at this stage which turns cytoplasm into a slight reddish color

Orthochromatic Normoblast:

Ø  It varies in size from 8-10u in diameter

Ø  The cytoplasm is red due to hemoglobinization

Ø  The nucleus is small and appears as deeply staining blue black homogeneous mass called pyknotic in appearance

Ø  It becomes eccentric in position and is finally extruded out from the cell.

Reticulocytes:

Ø  It is a flat disc shaped cell

Ø  It has no nucleus and is slightly larger than mature RBCs

Ø  It has threads of remnants of RNA in cytoplasm which can be visible after staining with supravital stains e.g Brilliant Cresyl Blue stain.

Ø  The reticulocytes becomes a mature RBCs in about 24 hours in circulation

Red Blood Cells:   

Ø  The mature RBC is a non-nucleated cell

Ø  It is a biconcave disc with average diameter of 7.2u

Ø  The cytoplasm is pink-red due to hemoglobin

Ø  Anaerobic glycolysis occurs in RBCs as source of energy

Granulopoiesis

The formation of granulocytes is called as granulopoiesis. The earliest recognizable cell of granulopoiesis is myeloblast. The process of granulopoiesis is characterized by

1.      Change in the size of cell

2.      Maturation and lobulation in nucleus

3.      Production of specific granules in cytoplasm

Myeloblast:

Ø  It has a large round or oval nucleus, which occupies most of the cell and contains 2-4 nucleoli

Ø  The cytoplasm is non-granular and deep blue in color

Promyelocyte:

Ø  It resemble myeloblast but is larger in size

Ø  It has more cytoplasm which contains purplish red granules (azurophilic granules)

Ø  The nucleus still may contain some nucleoli or their remnants

Myelocyte:

Ø  The granules develop their specific character (purplish for neutrophils, orange and large for Eosinophils and Basophilic for basophils)

Ø  Nucleus has no nucleoli

Ø  The diameter of myelocyte  may be upto 25u

Metamyelocyte:

Ø  The nucleus of this cell is small, eccentric and slightly indented

Ø  The cytoplasm contains specific granules

Ø  The cell is smaller in size than myelocyte

Band/Stab Form:

Ø  It is a mature metamyelocyte, which has a band like nucleus, adapted to a U shape

Mature Granulocytes

Neutrophils:

Ø  It is 12-14u in diameter

Ø  The nucleus is lobulated having 2-5 lobes that are connected by thin chromatin strands

Ø  The cytoplasm is pink and contains numerous fine purplish granules

Eosinophils:

Ø  The mature eosinophil is about 16u

Ø  The nucleus usually has two lobes

Ø  The cytoplasm is packed with relatively larger granules which do not overlap the nucleus

Ø  These granules stain reddish orange with Romanowsky stains

Basophils:

Ø  Basophils rarely contain more than two lobes

Ø  The cytoplasm is pink and contain a number of large oval or round deeply stained basophilic granules

Ø  They do not pack the cytoplasm as eosinophils but they overlap the nucleus

Monopoiesis

Monocytes are formed mainly in the bone marrow and migrate to the spleen, lymphoid and other tissues.

Monoblast:

Ø  It is the earliest recognizable cell of the series

Ø  It is a large cell similar in structure to the myeloblast

Pro-Monocyte:

Ø  It is a large cell about 20u in diameter

Ø  It has abundant cytoplasm, gray blue in color and may contain fine azurophilic granules

Ø  The nucleus is usually round or kidney shaped giving folded appearance

Monocyte:

Ø  It is slightly smaller than pro-monocyte

Ø  Its cytoplasm has typical ground glass appearance

Ø  The nucleus is like a band folded upon itself to assume a spherical shape

Lymphopoiesis:

Mature lymphocytes develop mainly in the lymphoid tissue of the body namely lymph nodes, spleen, appendix and tonsils. Bone marrow makes only a smaller contribution to lymphocyte produntion. Colony Forming Unit Lymphoid (CFU-L) probably migrates to lymphoid tissue early in life. The maturation of lymphocyte is characterized by

Ø  Maturation of nucleus and cytoplasm

Ø  Adaptation to their function by expression of specific proteins

Lymphoblast:

Ø  It is the earliest recognizable cell of series

Ø  It measures 15-20u in diameter

Ø  It contains a large, round or oval nucleus

Ø  Nucleoli are present usually 1-2 in number

Ø  The cytoplasm is non-granular and deep blue in color forming a narrow rim around the nucleus

Pro-Lymphocyte:

Ø  It is the next stage in formation of lymphocytes

Ø  Nucleus contains a prominent nucleolus usually centrally placed

Ø  Cytoplasm is variable

Large Lymphocyte:

Ø  It is about 12-16u in diameter

Ø  Cytoplasm is sky blue in color and may contain few granules, which stain purplish red

Ø  The nucleus is slightly indented or round

Ø  Nucleoli are absent

Small Lymphocyte:

Ø  The large lymphocyte matures into small lymphocytes

Ø  It is about 9-12u in diameter

Ø  The cytoplasm is scanty and stains blue

Ø  The nucleus is round or slightly indented

Ø  The size of nucleus is roughly equal to mature normal RBC

Thrombopoiesis

Platelets are formed from the cytoplasm of a large cell in the bone marrow known as megakaryocyte

Megakaryoblast:

Ø  It is a large cell of 20-30 u in diameter

Ø  It has a large, oval or indented nucleus that contain several nucleoli

Ø  The cytoplasm is blue, small in amount and contains no granules

Ø  Cytoplasm may show budding

Pro-Megakaryocyte:

Ø  It is formed from the megakaryoblast

Ø  It is larger than megakaryoblast

Ø  It has deep blue cytoplasm that contains azurophilic (pinkish) granules

Ø  The nucleus is non-lobulated or partly lobulated

Ø  From this stage to onward only nucleus divides while the cell enlarges without division (endomitosis)

Megakaryocyte:

Ø  It is a large cell from 30-90u in diameter

Ø  It contains a single multi-lobulated or indented nucleus

Ø  The cytoplasm is abundant and stains light blue

Ø  It contains fine azurophilic granules

Platelets:

Ø  It is a smaller discoid structure 1-2u in size

Ø  These are formed by partition of cytoplasm of megakaryocytes into numerous structures that separate to form platelets