Blood Formation
The blood consists of a fluid part called plasma and the formed elements called cells. The blood cells are of three types: red blood cells (RBC), white blood cells (WBC) and platelets (Plt). White blood cells are further divided into three main groups, granulocytes (neutrophils, eosinophils, and basophils), monocytes and lymphocytes.
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 in healthy people. The production of blood cells is termed hemopoiesis.
The formation of each type of blood cell is named after the cell line. For RBCs, it is called Erythropoiesis, for granulocytes, it is called Granulopoiesis, for platelets, it is called Thrombopoiesis & for lymphocytes, it is called Lymphopoiesis. The formation of blood cells and their delivery into the bloodstream involves three processes, as described below:
1. Multiplication/Proliferation which takes place by the successive division of stem and progenitor cells by the process of mitosis.
2. Maturation/Differentiation that occurs by the progressive development of specific structural and functional cell characteristics.
3. The release of mature cells from the marrow into the bloodstream. Some maturation normally occurs after the release of cells, e.g. maturation of reticulocytes to RBCs. Immature forms may be released into circulation under conditions of stress
ERYTHROPOIESIS
Decrease in cell size Haemoglobinisation Extrusion of the nucleus. The time for maturation from pronormoblast to mature red cell is about 7 days. The various stages in the development of a RBC are
1. Pronormoblast: It is a round cell with a diameter of 12-20 μm. It has a large nucleus surrounded by a small amount of cytoplasm. The cytoplasm is deep blue in colour. The nucleus is round and consists of a network of uniformly-distributed chromatin strands. It is reddish purple in colour and contains several nucleoli. It divides and matures to basophilic or early normoblast.
2. Basophilic (Early) Normoblast: It is 10-16 μm in diameter. It has a large nucleus with thick chromatin strands and no nucleoli. The cytoplasm is blue like the pronormoblast‘s. It divides and matures into a polychromatic or intermediate normoblast.
3. Polychromatic (Intermediate) Normoblast: It is 8-14 μm in diameter. The nucleus occupies a smaller part of the cell and stains deeply. The cytoplasm gives a reddish tinge and is not so blue in colour, due to the formation of haemoglobin. It divides and matures into Orthochromatic or late normoblast.
4. Orthochromatic (Late) Normoblast: It varies from 8 to 10 μm in diameter. The cytoplasm is acidophilic (red) due to haemoglobinisation. The nucleus is small & appears as a deeplystaining, blue-black homogeneous mass (pyknotic). It becomes eccentric in position and is finally extruded out from the cell. Late normoblasts cannot divide and only mature into reticulocytes by extrusion of the nucleus.
5. Reticulocyte: The reticulocyte is a flat discshaped cell. It has no nucleus and is slightly larger than the mature red cell. It has a diffuse, basophilic (bluish) tinge (polychromatic). With supravital stains such as Brilliant Cresyl Blue, the basophilic material, which is RNA, appears in the form of a reticulum. The reticulocyte becomes a mature red cell in about 1-4 days. Half of this time is spent in the spleen.
6. Red Blood Cell (RBC): The mature RBC is a non-nucleated cell. It is a biconcave disc that is about 7.2 μm in diameter. The cytoplasm is pink due to the presence of haemoglobin. There is no nucleus, no mitochondria and no ribosome.
GRANULOPOIESIS
The earliest recognisable cell of the granulocytic series in the bone marrow is the myeloblast. It divides and matures into various granulocytes in stages. The process is characterised by: Change in the size of the cell Maturation and lobulation of the nucleus Production of specific granules in the cytoplasm. The time for maturation from myeloblast to mature granulocyte is about 4 days. The various stages in the development of a granulocyte are:
1. Myeloblast: This is the first recognisable cell of this series. It has a large round or oval nucleus which occupies most of the cell and contains 2-4 nucleoli. The cytoplasm is nongranular and deep blue in colour. It divides and matures into a Promyelocyte.
2. Promyelocyte: This is the next cell in the white-cell series. It resembles a myeloblast, but is larger, has more cytoplasm, which contains purplish-red granules (azurophilic granules). The nucleus still contains some nucleoli or their remnants. It divides and matures into a Myelocyte.
3. Myelocyte: The next stage in granulopoiesis is a myelocyte, which differs from the promyelocyte in two respects. First, the cytoplasmic granules develop their specific character (purplish for neutrophils, eosinophilic for eosinophils, basophilic for basophils). Second, the nucleus has no nucleoli. The diameter of a myelocyte may be up to 25 μm. The cytoplasm is light blue in the early stages and acquires a pinkish colour with maturation. A myelocyte does not divide and only matures into a metamyelocyte.
4. Metamyelocyte: The nucleus of this cell is small, eccentric and slightly indented. The cytoplasm is pinkish and contains specific granules. This cell is slightly smaller in size than the myelocyte. The specific granules are more abundant.
5. Band (Stab) form: It is a mature metamyelocyte, which has a band-like nucleus adapted to a U shape. The specific granules are abundant.
6. Mature Granulocyte: Depending upon the type of specific granules, these are of three types:
a. Neutrophil: It is 12-14 μm in diameter. The nucleus is lobulated having two to five lobes that are connected by thin chromatin strands. The cytoplasm is pink and contains numerous fine, purplish granules.
b. Eosinophil: The mature eosinophil is slightly larger than the mature neutrophil. Its average diameter is about 16 μm. The nucleus usually has two lobes. The cytoplasm is packed with relatively larger granules, which do not overlap the nucleus. The granules stain reddishorange with Romanowsky Stains.
c. Basophil: The mature basophil has a lighter-staining nucleus than the neutrophil. It seldom contains more than two lobes. The cytoplasm is pink and contains a number of large oval or round, deeply-staining basophilic granules. They do not pack the cytoplasm as do eosinophilic granules, but overlie the nucleus
MONOPOIESIS
Monocytes are formed mainly in the bone marrow and migrate to the spleen, lymphoid and other tissues and organs of the body where these are transformed into macrophages. The various stages in its development are as follows:
1. Monoblast: It is the earliest recognisable cell of the series. It is a large cell similar in structure to the myeloblast. Its nuclear outline is, however, not as regular as in myeloblasts and it may show indentation or convolutions.
2. Promonocyte: It is a large cell about 20 μm in diameter. It has abundant cytoplasm, greyblue in colour and may contain fine azurophilic granules. The nucleus is usually round or kidney-shaped, giving a folded appearance, but it may be lobulated.
3. Monocyte: It is slightly smaller than a promonocyte.The other features are similar. Its cytoplasm has a typical ‗ground glass‘ appearance. The nucleus is like a band folded upon itself to assume a spherical shape.
LYMPHOPOIESIS
Mature lymphocytes develop mainly in the lymphoid tissues of the body, namely the lymph nodes, spleen, gastrointestinal tract and tonsils. Bone marrow makes only a small contribution to lymphocyte production. CFU-L probably migrates to lymphoid tissue early in life. These also develop through stages. The maturation of lymphocytes is characterised by:
Maturation of the nucleus and cytoplasm, Adaptation to their function by an expression of specific proteins.
1. Lymphoblast: It is the earliest recognisable cell of the series. It measures 15-20 μm in diameter and contains a large, round or oval nucleus. Nucleoli are present, usually 1-2 in number. The cytoplasm is non-granular and deep blue in colour, forming a narrow rim around the nucleus.
2. Prolymphocyte: This is the next stage in the formation of lymphocytes. The nucleus contains a prominent nucleolus, usually centrally placed. Cytoplasm is variable.
3. Large lymphocyte: It is about 12-16 μm in diameter. The cytoplasm is sky blue in colour and contains few granules, which stain purplish red. The nucleus is round or slightly indented. Nucleoli are absent.
4. Small lymphocyte: The large lymphocyte matures into a small lymphocyte and is 9-12 μm in diameter. The cytoplasm is scanty and stains blue. Purplish-red granules may be present. The nucleus is round or slightly indented. Nucleoli are absent.
THROMBOPOIESIS
Platelets are formed from the cytoplasm of a large cell in the bone marrow known as megakaryocyte. This also passes through various stages of development in the bone marrow. These are
1. Megakaryoblast: It is a large cell about 20-30 μm in diameter. It has a large oval or indented nucleus that contains several nucleoli. The cytoplasm is blue, small in amount and contains no granules. It may show budding.
2. Promegakaryocyte: This is formed from the megakaryoblast and is larger than the megakaryoblast. It has a deep-blue cytoplasm that contains azurophilic granules. The nucleus is non-lobulated or partly lobulated. From here onwards, only the nucleus divides while the cell enlarges without division (Endomitosis).
3. Megakaryocyte: It is a large cell, from 30-90 μm in diameter. It contains a single multilobulated or indented nucleus. The number of nuclear lobes varies from 4-16, depending upon the number of divisions it has undergone. The cytoplasm is abundant and stains light blue. It contains fine azurophilic granules. The margin is irregular and may show pseudopod formation.
4. Platelet: It is a small discoid structure, 1-2 μm in size. These are formed by the partitioning of the megakaryocyte‘s cytoplasm into numerous structures that separate to form platelets.
