Chapter 2

Blood cell morphology and development

                  

2.1 Required reading assignments:

Schalm's Veterinary Hematology - Jain

2.2 Objectives and/or study questions: 

Students should be able to perform the following tasks upon completion of this section:

 

2.3 ORIGIN AND DEVELOPMENT OF RED BLOOD CELLS 

There are three types of cellular elements in the blood - erythrocytes (red cells), leukocytes (white cells), and thrombocytes (platelets). Each has its own functions and differs clearly from the others. In health, destruction is balanced by production and the quantity of circulating cells remain remarkably constant.

Most of the blood cells are formed in the bone marrow. Consequently, examination of marrow specimens often provides information of great value in the diagnosis and treatment of hematologic disorders. 

2.3.1 MYELOPOIESIS:

The prefix "myelo"- generally refers to all aspects of bone marrow activity and is not limited to granulocytic elements alone. The only exceptions to this rule are the terms myeloblast, myelocyte, and metamyelocyte, which refer to specific stages in granulocyte maturation.

Consists chiefly of erythropoiesis, granulopoiesis, and thrombopoiesis. Monocytes appear to be formed in the marrow as well as elsewhere. Lymphopoiesis occurs largely in extramedullary sites such as the spleen, thymus, and lymph nodes.

There is little doubt that a primitive, pluripotential reticulum cell or "stem cell" can undergo differentiation into a specific blast cell.

The marrow reticulum cells are those from which the differentiated blood cell precursors arise.

The reticulum cells in a Wright's stained preparation are relatively large (20-30 u in diameter) with abundant, light blue, opaque cytoplasm. The cell borders are typically irregular. These cells ordinarily have one to two nucleoli with the nucleus being round to oval, with a finely to coarse reticulated chromatin pattern. 

Figure II.1 lists the steps in the development of the various blood cells from the marrow reticulum cell.  

2.3.2 ERYTHROPOIESIS:

The nonnucleated red cells in the peripheral blood are derived from nucleated precursors in the bone marrow.

Distinct morphological and functional stages can be recognized in the development of an erythrocyte and serve as a useful outline of erythropoiesis.

As a group, all of the immature and mature erythrocytes in the body are designated the erythron.

Many names have been used for the developmental stages of an erythrocyte. The terminology used in this syllabus is preferred since accurate definitions are available. 

2.3.3 MATURATION SEQUENCE

As cells develop, their morphology changes. These morphologic alterations occur gradually; therefore, many cells are in transition from one named cell type to another. As a general rule, cells in transition are classified towards the more differentiated cell type. Thus, if there is a question whether a cell is a rubricyte or a metarubricyte, call it the latter.

Immature cells are generally larger and become smaller as they mature. Figure II.2 illustrates the relationship of cell size to maturation and division. The nuclei of the immature cells are relatively large in relation to the amount of cytoplasm and become smaller with maturity. The chromatin of the nucleus in immature cells is delicate, fine and stippled. As the cell matures, the chromatin becomes coarse, clumped and compact. Nucleoli are found in the nucleus of immature cells. Figure II.3 illustrates these maturation changes. 

2.3.4 ERYTHROCYTE MATURATION SEQUENCE:

2.3.4.1 Rubriblast

2.3.4.2 Prorubricyte

2.3.4.3 Rubricyte

2.3.4.4 Metarubricyte

2.3.4.5 Reticulocyte or polychromatophilic cell

2.3.4.6 Erythrocyte

 

Figure II.1 

 

  

Figure II.1 

The Relationship of Cell Size to Maturation and Division
 

 

FIGURE II.3

Morphologic Changes Occurring During Red Blood Cell Maturation  

 RUBRIBLAST:

  • First recognizable cell in the erythrogenic series.
  • A large round cell which contains a large round nucleus with a thin rim of royal blue cytoplasm.
  • The nuclear chromatin is delicate and stippled.
  • The nucleus contains one to several nucleoli.
  • The cytoplasm is somewhat scanty and stains very basophilic. 

PRORUBRICYTE:

  • Cell is similar to the Rubriblast but smaller in size.
  • Nucleoli are usually absent, but remnants of the nucleoli may still be present.
  • The chromatin material is somewhat coarser.
  • A perinuclear clear zone may be observed.
  • The cytoplasm stains basophilic. 

RUBRICYTE:

  • Cell is smaller than the Prorubricyte.
  • Nuclear chromatin material is coarsely clumped separated by light streaks giving the so-called cartwheel appearance.
  • Nucleus is round and stains very dark.
  • Cytoplasm stains very basophilic in the early rubricyte stage, but the blue color is diluted out by the pink color of hemoglobin as it matures toward the next stages.
  • This cell can be further subdivided according to the amount of hemoglobin in the cytoplasm into basophilic, polychromatophilic and normochromic rubricyte.
  • The cell is the last cell in the erythrocytic series capable of cell division under normal conditions. Cell division stops when a critical hemoglobin concentration is reached. A deficiency of hemoglobin in the cell can result in extra divisions and smaller cells. (EXAMPLE: Iron deficiency results in microcytes.) 

METARUBRICYTE:

The nucleus is small, pyknotic, and appears as a dark blue homogeneous mass without any distinct chromatin structure. The cytoplasm stains similarly to the mature erythrocyte.

The cell is not found in the peripheral blood of normal healthy animals. When observed in peripheral blood, it denotes a response to an anemic condition of at least 72-96 hours duration. The exception is the horse in that this species does not release nucleated red cells into the peripheral blood under any circumstances. 

RETICULOCYTE OR POLYCHROMATOPHILIC CELL:

  • This cell is larger than the mature erythrocyte and is non-nucleated.
  • Cell may contain a small round nuclear remnant called a Howell-Jolly body.
  • Cytoplasm stains slightly basophilic with Wright's stain. However, when stained with a supravital-stain such as new methylene blue or brilliant cresyl blue, precipitated ribrosomal RNA (reticulum) can be demonstrated within the cell.
  • Cells are not found in health in the peripheral blood of the horse, cow, sheep, and goat. This means that the reticulocyte ripens in the bone marrow in these species.
  • Dogs and cats may normally have 1-1.5% reticulocytes in the peripheral blood.
  • The cell does not participate in normal rouleaux formation or pathologic agglutination; is more resistant to crenation and lysis; is less susceptible to mechanical trauma; has a great excess of membrane in relation to its contents, but is able to synthesize hemoglobin.

 ERYTHROCYTE:

  • These are the mature non-nucleated red blood cells.
  • Cell stains buff or reddish color. 

2.3.5 CONTROL OF ERYTHROPOIESIS

Erythropoietin is accepted as the fundamental erythropoietic stimulus.It is produced by the kidney and/or liver in most species.Its release is determined by tissue oxygen levels. Erythropoietin also causes an increased release of reticulocytes into the circulation (reticulocytosis).

Endocrine glands such as the pituitary, thyroid, adrenals, and gonads affect erythropoiesis. These glands are responsible for general metabolic activity and cellular oxygen requirements. 

2.4 ORIGIN AND DEVELOPMENT OF GRANULOCYTES

Granulocytes develop in the bone marrow from undifferentiated stem cells. Granulocytes are readily differentiated from the nucleated erythrocytes by their fine, reticulated chromatin structure and bluish cytoplasm. 

2.4.1 MATURATION SEQUENCE

Maturation of the granulocytic series of cells is characterized by the development of granules. Granules are initially formed in the progranulocytes and are called "primary or azurophil" granules. These granules are peroxidase positive and develop from fusion of small proteinaceous material by the Golgi complex. As the maturation sequence continues, the primary or azurophil granules lose their acid mucopolysaccharide and, therefore, will not stain with Wright's stain. However, the myelocyte stage is characterized by the appearance of "specific" or secondary granules that persist throughout the maturation process. Cells which have granules with an affinity for blue or basic dye are identified as basophils; cells that are stained reddish-orange with the acid dye eosin are called eosinophils; and the cells with granules which do not stain intensely with either dye are called neutrophils. 

2.4.2 GRANULOCYTE MATURATION SEQUENCE

Several distinct developmental stages of granulocytes can be recognized morphologically. The stages are:

2.4.2.1 Myeloblast

2.4.2.2 Progranulocyte

2.4.2.3 Myelocyte -- Basophil, eosinophil, neutrophil

2.4.2.5 Metamyelocyte -- Basophil, eosinophil, neutrophil

2.4.2.6 Band Cell -- Basophil, eosinophil, neutrophil

2.4.2.7 Segmented Cell -- Basophil eosinophil, neutrophil 

 

2.4.2.1 MYELOBLAST

  • The first cell that can be recognized in the granulocytic series.
  • It is the most immature granulocytic precursor.
  • The cell possesses a relatively large, round to oval nucleus, with one to several nucleoli.
  • The chromatin material of the nucleus is finely stippled or has a light ground glass appearance.
  • The cytoplasm is somewhat scanty, basophilic, and does not contain granules.  

2.4.2.2 PROGRANULOCYTE

  • The nuclear chromatin material is coarser and slightly more clumped than that of a myeloblast.
  • Remnants of the nucleoli may still be present.
  • The cytoplasm is less basophilic than the myeloblast and contains darkly stained non-specific granules called "primary or azurophilic granules." These granules are peroxidase positive. 

2.4.2.3 MYELOCYTE

  • The nucleus of the myelocyte remains somewhat round to oval and the chromatin material is more closely clumped.
  • This cell contains "secondary" or specific granules that are identified by their staining properties as neutrophils, eosinophils, and basophils. These granules are peroxidase negative.
  • Therefore, the myelocyte and all subsequent cells of the granulocytic series should be characterized as neutrophils, eosinophils and basophils.
  • These granules vary greatly in shape, size and concentration in domestic species.
  • As maturation of the granulocytes continue, the "specific granules" will increase and the "azurophilic granules" will not take the Wright's stain.
  • The myelocyte stage is the last stage of cell division and the first cell capable of phagocytosis. 

2.4.2.4 METAMYELOCYTE

  • This cell closely resembles the myelocyte.
  • The nucleus is indented and often resembles a kidney bean.
  • Nucleoli are not present and the nuclear chromatin material is coarser and clumped.
  • Cytoplasmic granules are also present.

2.4.2.5  BAND CELL

  • This cell has a horseshoe shaped nucleus.
  • The opposite sides of the nucleus are more or less parallel.
  • This cell may be differentiated from the metamyelocyte by the nuclear shape and the tendency for the nuclear sides to become parallel.
  • The nuclear chromatin material is markedly clumped. 

2.4.2.6 SEGMENTER CELL

The nucleus may be mono-lobed with clumped chromatin material, or may consist of several lobes separated by constrictions or by filaments. The cytoplasm stains very faintly. 

2.4.3 LEUKOCYTE LYSOSOMES

The term lysosome is used to describe intracellular membranous sacs containing acid hydrolytic enzymes. When leukocytes phagocytize, there is a release of lysosomal contents. Primary granules (azurophil granules) contain acid phosphatase, acid hydrolytic enzymes, basic protein and one-third of the lysozyme. Secondary granules (specific granules) contain alkaline phosphatase, lactoferrin and two-thirds of the lysozyme. 

2.5 ORIGIN AND DEVELOPMENT OF AGRANULOCYTES

The agranulocytic series is comprised of leukocytes devoid of specific granulation. These cells generally originate in the lymphatic system, but like the granulocytic series, they may be produced elsewhere in the body. This series includes the lymphocytic and monocytic groups.

2.5.1  LYMPHOCYTIC MATURATION SEQUENCE

The lymphocytic series refers to the development of the lymphocytes. The cells arise mainly from the reticular tissue of the lymph nodes and lymphoid tissue from which they derive their name. Lymphocytes are the most undifferentiated of all blood cells normally found in the peripheral blood. The nucleus does not become segmented and specific granules do not develop. Therefore, lymphocytes are "peroxidase negative". The lymphocyte cytoplasm does not develop past the blue stage. The stages in lymphocytic development are:

  • Lymphoblast
  • Prolymphocyte
  • Lymphocyte 

    2.5.1.1 LYMPHOBLAST

    • Cell is similar to other blast cells. It is round or oval, very large, with a large round to oval reddish-purple nucleus.
    • The nuclear chromatin material is fine and well distributed but perhaps more coarse than in myeloblasts.
    • The nucleus contains one or two nucleoli.
    • The cytoplasm is bluish and nongranular and forms a thin rim around the nucleus. 

    2.5.1.2 PROLYMPHOCYTE

    • The nucleus is round or oval in shape but smaller than the lymphoblast.
    • The nuclear chromatin is coarse and slightly clumped. 
    • Nucleoli or remnants of nucleoli may be present.
    • There is an abundant amount of light blue cytoplasm around the nucleus. Also, there may be a few azurophilic granules in the cytoplasm. 

    2.5.1.3 LYMPHOCYTE

    This is the mature cell of the lymphocytic series and the only cell form found in the peripheral blood in health.

    Lymphocytes vary greatly in size and may be classified as small, medium or large. However, size does not determine age of these cells.

    The cells are easily distorted and often appear in irregular shapes in stained preparations. The nuclear chromatin is condensed to form large, discrete almost solid clumps, with thickening of the nuclear membrane. Nucleoli are absent. Non specific granules may be observed in the cytoplasm of these cells. 

2.5.2 MONOCYTIC MATURATION SEQUENCE

The monocytic series refer to the stages of development of the monocyte. These cells may be formed from RE cells. One of the most important sites of origin is the spleen. The stages in the monocytic development are:

  • Monoblast
  • Promonocyte
  • Monocyte 

    2.5.2.1 MONOBLAST

    The cell is large with a round or oval nucleus. A nucleolus is present. The nuclear chromatin material is fine and well-distributed. There is a thin rim of clear blue cytoplasm around the nucleus. There are no granules present in the cytoplasm. 

    2.5.2.2 PROMONOCYTE

    The cell is somewhat smaller than the monoblast with the nucleus being irregularly-shaped.The nuclear chromatin material is fine and spongy. There may be a nucleolus or a remnant of the nucleolus present. The cytoplasm is grayish blue and may contain non-specific granules. 

    2.5.2.3 MONOCYTE

    The cell is larger than a neutrophil in the thin portions of a smear. 

    The shape of monocytes is variable.The nuclei are usually round or kidney-shaped, but may be deeply indented or have two or more lobes connected by narrow bands. Blunt pseudopods and digestive vacuoles may be present. Monocytes are most difficult to identify and to differentiate from other cells. They are frequently mistaken for immature neutrophils and large lymphocytes.The three most characteristic features of the monocytes and the most helpful in diagnosis are the dull grayish-blue color of the cytoplasm, blunt pseudopods and the brain-like convolution of the nucleus.

2.6  ORIGIN AND DEVELOPMENT OF Thrombocytes/platelets

Cells of the megakaryocytic system are peculiar in that the nucleus undergoes multiple mitotic divisions without cytoplasmic separation, thus producing giant polyploid cells. The multiple nuclei usually remain attached to each other and are often superimposed giving a lobular appearance. The cytoplasm undergoes maturation changes characterized by the development of granules and membranes, culminating in platelet differentiation and liberation. The stages in thrombocyte development are: