BLOOD. LYMPHOID TISSUE

Department Of General Histology

INTRODUCTION Blood is a specialized connective tissue in which cells are suspended

in fluid extracellular material called plasma. Propelled mainly by rhythmic contractions of the heart, about five liters of blood in an average adult moves unidirectionally within the closed circulatory system. The so-called formed elements circulating in the plasma are erythrocytes (red blood cells), leukocytes (white blood cells) and platelets.

When blood leaves the circulatory system, either in a test tube or in the ECM surrounding blood vessels, plasma proteins react with one another to produce a clot, which includes formed elements and a yellowish liquid called serum. Serum contains growth factors and other proteins released from platelets during clot formation, which confer biological properties very different from those of plasma.

Collected blood in which clotting is prevented by the addition of anticoagulants (eg, heparin, citrate) can be separated by centrifugation into layers that reflect its heterogeneity (Figure 12–1). Erythrocytes make up the bottom layer and their volume, normally about 45% of the total blood volume in healthy adults, is called the hematocrit.

COMPOSITION OF WHOLE BLOOD

PREPARING A BLOOD SMEAR

NORMAL HUMAN ERYTHROCYTES

Colorized SEM of normal erythrocytes with each side concave.

b): Diagram of an erythrocyte giving the cell's dimensions. The biconcave shape gives the cells a very high surface-to-volume ratio and places most hemoglobin within a short distance from the cell surface, both qualities which provide maximally efficient O2 transport. Erythrocytes are also quite flexible and can easily bend to pass through small capillaries. (c): In small vessels red blood cells also often stack up in aggregates called rouleaux.

FIVE TYPES OF HUMAN LEUKOCYTES

NEUTROPHILS

In blood smears neutrophils can be identified by their multilobulated nuclei, with lobules held together by thin strands. With this feature the cells are often called polymorphonuclear leukocytes, or just polymorphs. The cells are dynamic and the nuclear shape changes frequently. X1500. Giemsa.

b): Other identifying features of neutrophils include overall diameter of 12–15 m, approximately twice that of the surrounding erythrocytes. The cytoplasmic granules are relatively sparse and heterogeneous in their staining properties, although generally pale and not obscuring the nucleus. X1500. Giemsa.

(c): Micrograph shows a neutrophil from a female in which the condensed X chromosome appears as a drumstick appendage to a nuclear lobe (arrow). X1500. Wright.

NEUTROPHIL ULTRASTRUCTURE

EOSINOPHILS

Eosinophils are about the same size as neutrophils but have bilobed nuclei and abundant coarse cytoplasmic granules. The cytoplasm is often filled with brightly eosinophilic specific granules, but also includes some azurophilic granules.

(a): Micrograph shows an eosinophil next to a neutrophil for comparison with its nucleus and granules. X1500. Wright.

(b): Even with granules filling the cytoplasm, the two nuclear lobes of eosinophils are usually clear. X1500. Giemsa.

(c): TEM of a sectioned eosinophil clearly shows the unique specific granules, as oval structures with disk-shaped electron-dense crystalline cores (EG). These along with lysosomes and a few mitochondria (M) fill the cytoplasm around the bilobed nucleus (N). X20,000.

BASOPHILS

(a, b, c): Basophils are approximately the same size as neutrophils and eosinophils, but have large, strongly basophilic specific granules which usually obstruct the appearance of the nucleus having two or three irregular lobes. a and b: X1500, Wright; c: X1500, Giemsa.

(d): TEM of a sectioned basophil reveals the lobulated nucleus (N), appearing as three separated portions, the large specific basophilic granules (B), mitochondria (M), and Golgi complex (G). Basophils exert many activities modulating the immune response and inflammation and share many functions with mast cells, which are normal, longer term residents of connective tissue.

LYMPHOCYTES

(a): The most numerous small lymphocytes shown here are slightly larger than the neighboring erythrocytes and often have only a thin rim of cytoplasm surrounding the spherical nucleus. X1500. Giemsa.

(b): Medium lymphocytes are distinctly larger than erythrocytes. X1500. Wright.

(c): Large lymphocytes, much larger than erythrocytes, may represent activated cells that have returned to the circulation. X1500. Giemsa.

(d): Ultrastructurally a medium-sized lymphocytes is seen to be mostly filled with a euchromatic nucleus (N), with a nucleolus (Nu), surrounded by cytoplasm containing mitochondria (M), free polysomes, and a few lysosomes (azurophilic granules).

MONOCYTES

Monocytes are large agranulocytes with diameters from 12 to 20 m that circulate as precursors to macrophages and other cells of the mononuclear phagocyte system. (a, b, c, d): Micrographs of monocytes that show their eccentric nuclei indented, kidney-shaped, or U-shaped. a: X1500, Giemsa; b–d: X1500, Wright.

(e): TEM of the cytoplasm of a monocyte shows a Golgi apparatus (G), mitochondria (M), and lysosomes or azurophilic granules (A). Rough ER is poorly developed and there are some free ribosomes

PLATELETS

In a blood smear, platelets (arrows) are often found as aggregates. Individually they show a lightly stained hyalomere region surrounding a more darkly stained central granulomere containing membrane-enclosed granules. X1500. Wright.

Ultrastructurally a platelet typically shows a system of microtubules and actin filaments near the periphery to help maintain its shape and an open canalicular system of vesicles continuous with the plasmalemma. The central granulomere region contains glycogen and secretory granules of different types. X40,000.

TEM section shows platelets adhering to collagen (C). Upon adhesion to collagen, platelets exocytose their granules into the canalicular system, which allows the very rapid secretion of factors involved in blood coagulation. Degranulating platelets (arrows) remain as an aggregate until their contents are exhausted.

FIBRIN CLOT

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