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Lesson 4-4 Thrombocytes


The general pattern of thrombocyte maturation is slightly different from that of leukocyte maturation. The cells of the megakaryocytic series tend to grow larger as they mature until there is cytoplasmic fragmentation (or breaking off) to form the cytoplasmic thrombocytes seen in the peripheral blood.

Azurophilic granulation begins to appear in the second stage of development and continues until it almost obscures the nuclear lobes. The nucleus develops from a fine single lobe to multiple ill-defined lobes. The stages in the normal maturation of the megakaryocytic series are: megakaryoblast, promegakaryocyte, megakaryocyte, and thrombocyte.


Megakaryoblast. See figure 4-37.

Figure 4-37. Megakaryocytic series: Megakaryoblast: bone marrow.

(1) 20 to 50 microns in diameter.

(2) Nucleus. One to two large oval or kidney-shaped nuclei are present. There is a fine chromatin pattern. Multiple nucleoli may be present which stain blue.

(3) Cytoplasm. The cytoplasm is blue, nongranular, and may have small, blunt pseudopods. It is usually seen as a narrow band around the nucleus.


(1) Size. 20 to 60 microns in diameter.

(2) Nucleus. One-to-two indented round or oval nuclei are present. They may show slight lobulation. The nuclear chromatin is purple, coarse, and granular. Multiple nuclei are present but may be indistinct.

Megakaryocyte. See figure 4-38.

Figure 4-38. Megakaryocytic series: Megakaryocyte

(1) Size. 40 to 120 microns in diameter.

(2) Nucleus. Two-to-sixteen nuclei may be visible or the nucleus may show multilobulation. No nucleoli are visible. The nuclear chromatin is purple, coarse, and granular.

(3) Cytoplasm. The cytoplasm is pinkish-blue in color and very granular. Numerous blue-purple granules begin to aggregate into small bundles that bud off from the cell to become platelets.

Thrombocyte (Platelet). See figure 4-39.

Figure 4-39. Megakaryocytic series: Thrombocytes.

(1) Size. 1 to 4 microns in diameter.

(2) Nucleus. None.

(3) Cytoplasm. The cytoplasm is light blue to purple and very granular. It consists of two parts:

(a) The chromomere, which is granular and located centrally.
(b) The hyalomere, which surrounds the chromomere and is nongranular and clear to light blue.

Exercises for Lesson 4

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