Normal cells of blood as seen in a blood smear. This slide shows many red blood cells and one neutrophil (or polymorphonuclear leukocyte). Neutrophils characteristically have a multi-lobed nucleus and very fine, neutral-stained cytoplasmic granules. These cells migrate out into the connective tissue and become phagocytic and provide a first line of defense in acute infections.
Basophil -- with very dark, coarse, basophilic (purple-blue) granules in the cytoplasm surrounding the lobed nucleus. The granules contain principally histamine and heparin. Basophils are activated in response to immunologically mediated hypersensitivity reactions.
Small lymphocyte - only a little larger than a red blood cell, it has only a thin rim of pale cytoplasm around a darkly stained round nucleus. Its function is related to the body's immunological defenses. Scattered among the r.b.c.'s are some very small clumps of platelets, which are necessary for the clotting of blood.
Monocyte - the largest of the leukocytes, it has quite a bit of bluish cytoplasm, surrounding a typically kidney-bean-shaped nucleus. When out in connective tissue, this cell becomes a macrophage (histiocyte).
EM of eosinophil cutting through bilobed nucleus. Notice the typical "cat's-eye" appearance of the cytoplasmic granules with the dark crystalloid band in the middle of each one. (Such bands do not appear in human eosinophils.) These granules, banded or not, contain hydrolytic enzymes and are lysosomal in nature.
EM of basophil showing dense granules reminiscent of those of mast cells. At one time it was thought that the basophil of the blood became the mast cell of connective tissue, but most work now indicates that these are two different cell lines ... though their granules contain basically the same secretory substances.
EM of lymphocyte -- rather a nondescript looking cell considering its great functional importance. Notice the cytoplasmic process to the right and relate it to the appearance of lymphocytes in the next two pictures.
Scanning electron micrograph of lymphocyte with relatively smooth surface. Differences in cell surface presumably represent differences in cell activity at the moment. At one time such visible differences were thought to provide a distinction between B cells and T cells, but recent work does not substantiate this.
Longitudinally cut capillaries running in the connective tissue between cardiac muscle cells. Note the very thin endothelial lining of the vessels. Notice too that the capillary diameter is essentially that of the red blood cell. Several r.b.c.'s can be seen in transit here. Their shape is plastic, responding to surrounding pressures, but cells are traveling independently. Compare their appearance with the stacked cells on the previous slide.
Capillaries in the connective tissue supporting cardiac muscle cells, this time cut in cross-section. Good examples lie in the upper left and lower left of the field. Look for a small thin-walled circle with a dark, crescent-shaped endothelial nucleus on one side. The rest of the thin circle of wall is composed of endothelial cytoplasm.
Small blood vessels of various sizes in areolar connective tissue. The two cross-cut capillaries at center contain erythrocytes and show an endothelial nucleus at the rim. The largest vessel, at extreme center right, is a venule. All of the vessels shown here are thin-walled and capable of fluid and ion exchange with the surrounding connective tissue fluid. In addition, leukocytes can squeeze between endothelial cells of the walls of such vessels (by diapedesis) and enter the connective tissue. Only when they leave the bloodstream do they assume their active roles.
EM of cross-cut capillary lying between skeletal muscle cells. Note a peripheral muscle nucleus at the top of the micrograph. A thin basal lamina surrounds the endothelium as well as the muscle cells. CL=capillary lumen; CJ=cell junction; G=glycogen particles; M=mitochondria; N=nucleus of endothelial cell; PV=pinocytotic vesicles.
Two EM views of fenestrated endothelium. In the section at left, through the cytoplasm of an endothelial cell, fenestrations are represented whenever the inner and outer cell surfaces meet in a thin line. In the picture at right a tangential cut through the surface of an endothelial cell shows multiple round fenestrations.