1. Hemopoietic Connective tissue

2. Blood is a highly specialized, fluid tissue.
Human Red Blood Cells, Platelets and T-lymphocyte (erythocytes = red; platelets = yellow; T-lymphocyte = light green) (SEM x 9,900). This image is copyright Dennis Kunkel at used with permission.

3. Hemopoietic Connective tissue
Blood
Lymph
Bone marrow

4. Objectives: Identify the component of blood
Compare the different types of blood cells
Identify the different types of blood vessels in a tissue section and identify the 3 different layers present in the larger vessels.
Compare the components of the circulatory system in terms of structure and function.
Relate the wall structure of a blood vessel to its major function.

5. Blood
is traditionally classified as a specialized form of connective tissue, with:
no fibers, highly fluid ground substance, and mobile cells. Thus, blood is distinct from ordinary connective tissue.
In human adults about 5 liter of blood contribute 7-8 % to the body weight of the individual.

6. To appreciate the basic unity of blood and other varieties of connective tissue, consider the following.
All of the several blood cell types originate in the connective tissue of bone marrow.
Certain white blood cells, notably lymphocytes and monocytes, move freely back and forth between blood and other connective tissues.
The chemical composition of plasma is very similar to that of interstitial fluid in ordinary connective tissue.

7. Components of the human blood
Highly fluid ground substance: plasma (about 57%)
Mobile cells: (about 43%)
Erythrocytes (red blood corpuscles, RBCs)
Leucocytes (white blood cells, WBCs)
Thrombocytes (blood platelets, BP).
Erythrocytes and blood platelets perform their functions exclusively in the blood stream. In contrast, leukocytes reside only temporarily in the blood. Leukocytes can leave the blood stream through the walls of capillaries and venules and enter either connective or lymphoid tissues.

8. General aspects
Only the red blood cells, like trolley cars, are confined to the highways (i.e., blood vessels).  All other cell types in blood, and most plasma constituents as well, can circulate rather freely from blood to connective tissue and back again.  Thus, most of the mobile cellular components of ordinary connective tissue are interchangeable with those in blood.
Cell names may differ between blood and ordinary connective tissue: The cells which are called macrophages in ordinary connective tissue are called monocytes in blood.
Blood cells similar to tissue mast cells are called basophils.
From this point of view, the term "white blood cell" is not only very nonspecific but is also a misnomer.  "Circulating connective tissue cell" (still nonspecific) better fits the functional location of these mobile cell types.

9. The red blood cells In mature form they lack nuclei and most cytoplasmic structures; they are little more than discoid, flexible bags of hemoglobin. They are typically biconcave disks although their shape is influenced by osmotic forces. The average diameter of the disk is ~7 µm. Foetal erythrocytes (up to the 4th month of gestation) are larger than "adult" erythrocytes, and they are nucleated. The later feature they share with erythrocytes of other animal classes (e.g. amphibia and birds).
Blood Smear, human - Leishman stain How does the shape of the erythrocyte facilitate its function? How would you expect an erythrocyte to look like if it is in an extracellular fluid of very low or very high osmotic pressure?

10. Blood smear Most all the cells you see in this picture are red blood cells (erythrocytes), and the blue and green arrows are pointing to white blood cells (leukocytes). The matrix material, know as plasma in blood, can not be seen because it does not stain.
Blood is a good example of showing the distance between cells that is often associated with connective tissues.

11. Leucocytes Granulated, polymorphonuclear cell (PMN’s) neutrophils,
eosinophils basophils
Non-granulated mononuclear: lymphocytes monocytes

12. leukocyte
In healthy individuals the relative numbers of circulating leukocyte types are quite stable. A differential leukocyte count would typically produce the following cell frequencies (numbers in parentheses are the range of normal frequencies reported in different texts):
~ 60% neutrophils (50% - 70%)
~ 3% eosinophils (>0% - 5%)
~ 0.5% basophils (>0% - 2%)
~ 5% monocytes (1% - 9%)
~ 30% lymphocytes (20% - 40%)
Changes in their relative numbers indicate that something abnormal is happening in the organism. A larger than usual number of neutrophils (neutrophilia) would indicate e.g. an acute or chronic infection. The number of basophils and eosinophils may increase (eosinophilia or basophilia) as a consequence of e.g. allergic disorders.

13. Granular Leukocytes Granular Leukocytes
are all approximately the same size - about µm in diameter. Their nuclei form lobes, and nucleoli cannot be seen. The number of nuclear lobes varies according to cell type. All granulocytes are motile.
The term granulocytes refers to the presence of granules in the cytoplasm of these cells. The granules correspond to secretory vesicles and lysosomes. Specific granules are the granules which are only found in one particular type of granulocytes.

14. Neutrophils (also called polymorphonuclear neutrophilic leukocytes, PMNs, or polys): take their name from the staining properties of their cytoplasmic lysosomal granules (vesicles containing stored lysosomal enzymes). These granules are neutrophilic, meaning they show no special affinity for either acidic or basic stains but are stained mildly by both. (This is in contrast to the specific granules of eosinophils, which stain red with acidic stains such as eosin, and those of basophils, which stain with basic stains.)
They are about 12 µm in diameter in blood smear preparations (about twice the size of red blood cells). have a very characteristic nucleus. It is divided into 3-5 lobes which are connected by thin strands of chromatin. The number of lobes increases with cell age. Up to 7 lobes can be found in very old neutrophils (hypersegmented cells).
In darkly stained smears it is possible to see some faintly purple, very small granules in the cytoplasm. These granules represent the primary granules of neutrophils.

15. Eosinophil Their nucleus usually has only two lobes
Eosinophil Their nucleus usually has only two lobes. Almost all of the cytoplasm appears filled with the specific granules of the eosinophils. Their granules contain the enzymes histaminase and arylsufatase. These enzymes break down histamine and leukotrienes, which again may dampen the effects of their release by basophils or mast cells. Also, contain an electron-dense, proteinaceous crystal. This crystal is composed of major basic protein (MBP) function as a cytotoxin, and its release by eosinophils may be involved in the response of the body against parasitic infections, which are accompanied by an increase in the number of eosinophils.
In the center of the field is an eosinophil with a bilobed nucleus and numerous reddish granules in the cytoplasm. Just underneath it is a small lymphocyte.

16. Basophilic granulocytes: have a 2 or 3 lobed nucleus
Basophilic granulocytes: have a 2 or 3 lobed nucleus. The lobes are usually not as well defined as in neutrophilic granulocytes and the nucleus may appear S-shaped. The specific granules of basophils are stained deeply bluish or reddish-violet. The granules are not as numerous as those in eosinophils. The specific granules of basophils (about 0.5 µm) appear quite dark in EM pictures. They contain heparin, histamine lysosomal enzymes and leukotrienes (the later correspond to the slow-reacting substance of anaphylaxis or SRS-A).
There is a basophil in the center of the field which has a lobed nucleus (like PMN's) and numerous coarse, dark blue granules in the cytoplasm. They are infrequent in a normal peripheral blood smear, and their significance is uncertain.
A band neutrophil is seen on the left, and a large, activated lymphocyte on the right.

17. Eosinophils and basophils: are the only cell types present in normal blood which initially may be difficult to distinguish - in particular in darker smears. If you see them side by side in your drawing the difference between them should become apparent. Chances are 6:1 that the you find an eosinophil before you find a basophil. The two lobes of the nucleus of eosinophils are usually well-defined and of about equal size. The nucleus is embedded in a cytoplasm crowded with granules, which seem to form a solid mass in the cell. The 2-3 nuclear lobes of basophils are not as well defined as those of eosinophils, granules are not as numerous as in eosinophils, and pretty much all of them can be identified "as individuals" rather than the dense mass they form in eosinophils.
Blood Smear, human - Leishman stain Note that eosinophils and basophils are much easier to distinguish in B1. In B2, the difference in the staining of their of granules is not as pronounced, and the nuclei do not stand out as clear as in B1.

18. Lymphocytes These cells are very variable in size
Lymphocytes These cells are very variable in size. The smallest may be smaller than erythrocytes (down to ~5 µm in diameter) while the largest may reach the size of large granulocytes (up to 15 µm in diameter). How much cytoplasm is discernible depends very much on the size of the lymphocyte. In small ones, which are the majority of lymphocytes in the blood, the nucleus may appear to fill the entire cell. Large lymphocytes have a wider rim of cytoplasm which surrounds the nucleus. Both the nucleus and the cytoplasm stain blue (and darker than most other cell types in the blood). The typical lymphocyte only contains the usual complement of cellular organelles.
A normal mature lymphocyte is seen on the left compared to a segmented PMN on the right. An RBC is seen to be about 2/3 the size of a normal lymphocyte.

19. Note the many small smudgy blue platelets between the RBC's.
Monocytes These cells can be slightly larger than granulocytes (about µm in diameter). Their cytoplasm stains usually somewhat stronger than that of granulocytes, but it does not contain any structures which would be visible in the light microscope using most traditional stains It is slightly larger than a lymphocyte and has a folded nucleus. Monocytes can migrate out of the bloodstream and become tissue macrophages under the influence of cytokines.
Note the many small smudgy blue platelets between the RBC's.

20. Blood Platelets (or thrombocytes)
Blood platelets do not contain a nucleus. Unlike erythrocytes, which also lack a nucleus, the blood platelets of mammals have never been nucleated cells. Instead, blood platelets are fragments of the cytoplasm of very large thrombocyte precursor cells, megakaryocytes. Like other cells involved in the formation in blood cells, megakaryocytes are found in the bone marrow.
Platelets are about 3 µm long but appear somewhat smaller in the microscope. This is because their cytoplasm is divided into two zones: and outer hyalomere, which hardly stains, and an inner granulomere, which contains bluish staining granules. These granules are usually not individually visible with the highest magnification on your microscope, and the granulomere appears more or less homogeneously blue. In addition to different types of vesicles (i.e. the granules), mitochondria, ribosomes, lysosomes and a little ER are present in the thrombocyte granulomere. Different types of vesicles contain either serotonin (electron-dense delta granules; few) or compounds important for blood coagulation (alpha granules - they also contain platelet-derived growth factor (PDGF) which may play a role in the repair of damaged tissue). The hyalomere contains cytoskeletal fibres, which include actin and myosin.

21. blood platelets in blood smear, human - Leishman stain
In lightly stained smears, blood platelets will appear like light blue, fairly ill-defined specks between the other blood cells.

22. Bone marrow
Yellow bone marrow, which harbours mainly adipocytes, dominates in the hollow of the diathesis of adult long bones.
Red bone marrow, which is typically found between the trabeculae of spongy bone in the epiphysis of adult long bones.
Haemopoiesis occurs in the red bone marrow.

23. Red Bone Marrow, rabbit - H&E H&E is not the method of choice for looking at haemopoietic cells, but a few of the numerous named types or broader groups can actually be recognized. Precursors of platelets are the haemopoietic cells easiest to find in red bone marrow. Note that adipocytes are present also in the red bone marrow.

24. Normal bone marrow
This is the appearance of normal bone marrow at medium magnification. Note the presence of megakaryocytes, erythroid islands, and granulocytic precursors.
This marrow is taken from the posterior iliac crest in a middle aged person, so it is about 50% cellular, with steatocytes admixed with the marrow elements.

25. Normal bone marrow at high magnification: Note the presence of megakaryocytes, erythroid islands, and granulocytic precursors. This marrow is taken from the posterior iliac crest in a middle aged person, so it is about 50% cellular, with steatocytes admixed with the marrow elements.

26. Normal bone marrow smear at high magnification: Note the presence of erythroid precursors and granulocytic precursors.

27. Normal bone marrow smear at high magnification: Note the presence of an eosinophilic myelocyte, a basophilic myelocyte, and a plasma cell.

28. Normal bone marrow smear at high magnification: Note the presence of megakaryocytes, erythroid precursors, and granulocytic precursors.

30. Blood vessels Arteries, Veins, Capillaries, sinusoids
Arteries conduct away from the heart and have proportionately more smooth muscle and elastic tissue than veins of comparable size.
Arteries are commonly sub-categorized into elastic arteries (large), muscular arteries (medium sized ), and arterioles (small).  
Veins return blood to the heart and have proportionately less smooth muscle and elastic tissue than arteries of comparable size.
Note that arteries and veins tend to travel together, so histological sections often provide opportunities to compare an adjacent vein/artery pair.
Veins: large, medium sized and small.
Capillaries and sinusoids communicate between arteries and veins.  Their lining generally consists only of endothelium and underlying basement membrane.
Note that capillaries are found in connective tissue (or stroma) in nearly every organ (cartilage is an exception, an avascular connective tissue) but are seldom noticable in routine histological sections..

31. Histology of the blood vessel: Blood vessels consist of three concentric layers or "tunics" of different tissue types. The thickness of the three layers varies greatly depending upon the size and type of vessel
The tunica intima is the inner lining, consisting of endothelium and a relatively thin layer of supporting connective tissue. In arteries it also includes the internal elastic lamina which is often seen as a thick wavy band surrounding the lumen of the vessel. Endothelial cytoplasm is inconspicuous in routine light microscopy.  Typically only the nuclei are visible, at the boundary between the lumen and the wall of a vessel.  
The tunica media is the middle muscular and/or elastic layer, containing smooth muscle and elastic tissue in varying proportions.
The tunica adventitia - is the outer layer made up of loose connective tissue (collagen fibres, fibroblasts) along with some smooth muscle cells. It is the thickest layer in veins (particularly the larger veins).

32. The tunica adventitia
The adventitial connective tissue is usually more or less continuous with the stromal connective tissue of the organ in which the vessel is found.  That is, there is not distinct outer boundary to the tunica adventitia. 
Nevertheless, the fibers of adventitial connective tissue tend to be more concentric around the vessel and often somewhat denser than the surrounding connective tissue (fascia).

33. Sustenance of the blood vessels
All blood vessels are embedded in connective tissue.  The only cells which receive their sustenance directly from the blood are the endothelial cells lining the vessels themselves.  All other cells are supplied via diffusion through intermediary connective tissue.

34. The Vascular Smooth Muscle : In most vessels, the smooth muscle fibers are arranged circumferentially, so the plane of section across an artery can be deduced by the orientation/shape of smooth muscle nuclei.   Elongated smooth muscle nuclei indicate longitudinally-cut muscle fibers and hence a transversely-sectioned vessel. Tiny round smooth muscle nuclei indicate transversely-cut muscle fibers and hence a longitudinally-sectioned vessel.
Smooth Muscle in Arterial Wall The vascular smooth muscle is of great importance in maintaining our bodies and adapting to minute-to-minute changes. Arteries have a generous supply of smooth muscle. It relaxes to allow more blood to flow to an area, and dilates to restrict the local blood flow. Smooth Muscle in venous Wall: Veins have less smooth muscle, but it is also of great importance. It is the venous smooth muscle that gently pumps blood back to the heart. Except when the heart is failing, this is the way cardiac output is regulated.

35. Smooth Muscle in Arterial Wall

36. Artery and Vein You will often be able to distinguish arteries and veins, especally when they run together, as they usually do.
Arteries have thicker walls and tend to have narrower lumens. Because of the large amount of muscle in their walls, they are usually round when cut in cross-section.
Veins have thinner walls and tend to have wider lumens. Because the walls are collapsable, they may change shape depending on the surrounding tissue conditions.
The thickness of arterial walls is typically not much less than the diameter of the lumen.
The thickness of vein walls is typically much less than the diameter of the lumen (i.e., proportionately much thinner than arteries carrying a similar volume). 
In arteries a continuous layer of elastic tissue, called the internal elastic lamina, forms the boundary between the intima and the media and the external elastic lamina, forms the boundary between the media and the adventitia.
The media of arteries is generally thicker than the media of veins of comparable diameter.
The tunica adventitia is the thickest layer in veins

37. Arteries and veins in the pancreas:

38. Arteries and veins in the pancreas:

39. Muscular arteries (medium sized ),

40. A medium-sized (muscular) artery, showing the typical 3 wall layers:
(1) tunica intima, consisting of endothelium lining the lumen, and a small amount of areolar c.t., a black and wavy inner elastic membrane divides this layer from the tunica media. (Only the elastic membrane shows up at this magnification.) (2) tunica media = smooth muscle (bluish color here) and some elastic fibers (black). (3) tunica adventitia = dense irregular c.t. (pink collagenic fibers, black elastic fibers). There is fat outside the adventitia.
Most arteries are muscular arteries, with a media dominated by smooth muscle.

41. A medium-sized (muscular) artery, higher magnification

42. medium artery

43. medium artery This is a fairly big medium artery that has a lot of elastic fibers in it.

44. Internal Elastic Lamina
In routine histological sections, the internal elastic lamina of transversely-sectioned arteries typically displays a distinctive sinusoidal appearance, resulting from postmortem contraction of the artery's smooth muscle in the absence of normal blood pressure.

45. MEDIUM SIZED MUSCULAR ARTERY AND VEIN This is a piece of transversely sectioned skeletal muscle with a piece of overlying connective tissue fascia.

46. Medium Sized Muscular Artery External Elastic Lamina

47. External Elastic Lamina: Note, the external elastic lamina is thinner and more fragmented than the inner elastic lamina visible to the far right. To the inside of the outer elastic lamina is the tunica media composed of smooth cells and some faintly visible elastic fibers. To the outside of
the outer elastic lamina is the tunica adventitia composed of pale thick collagen fibers, some fragmented elastic fibers and fibroblast/cyte cells.

49. The largest arteries (elastic arteries ), such as the aorta and its larger branches,
This is a piece of the aorta that is stained for elastic fibers. At low power, the tunica intima (endothelial lining plus thin layer of underlying c.t.) is on the right edge and tunica adventitia (fairly dense c.t. carrying small blood vessels, the vasa vasorum) is somewhat fragmented on the left. The vessel lumen is to the right.
Tunica media (alternating layers of elastic membranes and smooth muscle, bound together by areolar c.t.) This is by far the thickest layer.

50. Aorta at low power have a tunica media dominated by elastic tissue.
The elasticity conferred by elastin allows these elastic arteries to smooth out the sharp changes in blood pressure resulting from the pumping heart.

51. Aorta:
The elastic fibers show up most prominently at medium and high power in the middle zone, or tunica media, as darker reddish fibers and actually obscure the view of smooth muscle cells.

52. Aorta:
The elastic fibers show up most prominently at medium and high power in the middle zone, or tunica media, as darker reddish fibers and actually obscure the view of smooth muscle cells.

53. Medium Sized Veins have a wall similar to that of arteries but with a thinner tunica media. 
Also note that even though the lumen of the vein is collapsed the vein has a larger diameter than the artery

54. Medium Sized Veins at high magnification
The pointer is between the tunica media and adventitia of the vein. Although some of the vein fibers are elastic, there is no distinct external elastic membrane but a thin inner elastic membrane is visible.

55. The large vein:
Note the thinner wall thickness compared to the aorta. While the media and adventitia layers are about equal, there is evidence of only a few smooth muscle cells in the tunica media

56. The large vein:

57. The large vein, at high magnification

58. 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.

59. This is the connective tissue layer of the intestinal tract using the 40x objective lens, observe the small blood vessels within the connective tissue at the end of the pointer.
at the pointer is an arteriole. to the right and left of the arteriole are venules. Note the wall of the arteriole is mostly tunica media with 2-3 layers of smooth m cells while the wall of the venule is mostly adventitia.

60. Small Blood Vessels: Arterioles are the smallest arteries
Small Blood Vessels: Arterioles are the smallest arteries.  Note that gross anatomists and surgeons may use the term arteriole for any very small artery. 
Histologists tend to use the term arterioles only for terminal arterial vessels (i.e., those immediately preceding a capillary bed) which are characterized by having only a single layer of smooth muscle cells, outside the endothelium, . The walls of the smallest veins (sometimes called "venules") do not include smooth muscle.

61. SMALL BLOOD VESSELS
Toward the center of this field is a round, cross-cut arteriole with just one or two layers of muscle in the media. To the right is the more irregular, wider shape of a venule with only a thin adventitial wall.

63. In histological preparations,
capillaries tend to be quite inconspicuous unless they contain red blood cells.  Capillary endothelial nuclei are commonly difficult to distinguish from fibroblasts and other connective tissue cells, unless the preparation is fine enough to resolve the thin endothelial cytoplasm surrounding a small lumen.

64. In most parts of the body, capillaries and sinusoids are more or less "leaky".   The degree of "leakiness" is determined by variations in the endothelial lining of the capillaries. 
In most ordinary connective tissue, adjacent endothelial cells are incompletely sealed together. 
In many visceral organs, notably the kidney and the liver, each endothelial cell is full of patent holes called fenestrations (L., windows). 
In the brain and a few other places, endothelial cells are closely attached to one another, forming a tight seal so that any transfer of materials can occur only by passing across the endothelial cells (i.e., crossing endothelial membranes and cytoplasm).  

66. Capillaries in the connective tissue supporting cardiac muscle cells, 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.

67. CAPILLARY cut transversely

69. This sinusoid, like a capillary, has only an endothelial wall, but its lumen is characteristically considerably wider. Also, in some locations in the body (such as bone marrow, liver, and spleen) the endothelial cells of sinusoids are rather loosely joined together, thus permitting passage of blood cells between them.
Lymphatic vessel with a c.t. wall even thinner than a vein. There is a cut leaflet of a valve across the lumen. Material in the lumen contains no r.b.c.'s, mostly just structureless lymph and some lymphocytes. There are some fat cells and lymphocytes in the surrounding connective tissue.