cardiac muscle
endo-, myo-, and epicardium
elastic artery
internal and external elastic laminae
Purkinje fiber
tunica intima, media, and adventitia
vas vasi (pl. vasa vasorum)


Slide #170 (Artery, vein, and nerve) There are several features that allow one to distinguish between arteries and veins. Arteries have a thicker more regular tunica media. In histological preparations they more often retain their round profiles when cut in cross section. Veins tend often to look "collapsed" or flattened under the same circumstances. In addition, the tunica media of veins is much thinner than in arteries, and the adventitia is the thickest layer.

Identify the three layers of both arteries and veins and note the differences in thickness.





Slide #174 In very large vessels, the adventitial layer (outer layer) will contain blood vessels (vasa vasorum) that supply the vessel wall. Autonomic nerves are found in all vessels but are not evident without special staining techniques. (i.e. don't look for nerves!)

Slide #90 (Elastic artery) This is a section of just part of a human aorta. Notice the large constituent of elastic fibers in the tunica media (middle layer) as compared to the muscular artery on the previous slide. Which side of the section is the luminal side? [hint: identify tunica intima (inner layer)]




Slides #27 & #28 & Slides #177 & #179 Test yourself on these slides until you feel comfortable distinguishing arteries and arterioles from veins and venules. Remember, their appearance will depend upon whether they've been sectioned lengthwise, crosswise, obliquely, or tangentially (see figure immediately after table of contents).








This image is of an arteriole in longitudinal section.



Arteriole in cross section.

Various texts call vessels arterioles if the have two, three, or up to seven layers of smooth muscle. This vessel has aboout five layers of smooth muscle so would be an arteriole in one text and a small muscular artery in another.

We will call any small artery with up to five layers of smooth muscle an arteriole.











As seen in the transmission electron microscope (TEM)








Slide #107 Review morphology of cardiac muscle and Purkinje fibers.





myelocyte (nutri-, eosino-, or basophil)
metamyelocyte (nutri-, eosino-, or basophil)
band (stab) cell (nutri-, eosino-, or basophil)
segmented (mature) granulocyte (nutro-, eosino-, or basophil)
specific granules
basophilic erythroblast
polychromatophilic erythroblast
pyknotic nucleus
peripheral Blood

Slide #15 Human blood smear, Wright stain. While scanning the smear, it becomes obvious that erythrocytes are the most numerous type of cell in peripheral blood. Based on morphological criteria, use the high-dry objective to differentiate the following blood cells (then, if you'd like, take a closer look with your oil immersion lense):

Erythrocytes (RBCs)- 7 to 9 u diameter, no nucleus, rounded.
The peripheral zone of erythrocytes stains deep orange-red whereas the central zone appears pale and at times, colorless. The size of the central zone and the overall staining intensity of the erythrocyte is proportional to the content of hemoglobin.

Platelets - 1 to 3 u diameter.
Platelets are cytoplasmic fragments derived from megakaryocytes. They usually stain light blue and contain red or purple granules. These fragments tend to form small aggregates randomly dispersed throughout the blood smear.



Photomicrographs from Bloom & Fawcett, Textbook of Histology, Eleventh Edition


Photomicrographs by Allen L. Bell


I. Granulocytes
All cells in this category contain secondary (specific) granules. The staining characteristics of secondary granules are very specific for each cell type, thereby facilitating identification of the three different types of granulocytes. Another distinguishing feature is the presence of a multi-lobed nucleus. There are three types of granulocytes.





Neutrophils (polymorphonuclear leukocytes; PMNs)-- 10 to 15 u diameter.
These cells constitute 70% of leukocytes and usually have 2 to 5 nuclear lobes connected by fine filaments of chromatin. Neutrophils are the most numerous of all leukocytes, therefore, easiest to identify. The cytoplasm is pink to grey because of the neutral staining of specific granules (i.e. they don't stain). Neutrophils function as scavengers within extravascular tissue, destroying bacteria or other infectious organisms that invade the body. Neutrophils are also called Polymorphonuclear Leukocytes (PMNs) in some laboratories even though the following two cell types also have multi-lobed nuclei.










Eosinophils-- 10 to 15 um diameter
Eosinophils constitute 2.0 to 4.0% of leukocytes. These cells usually contain a bilobate (two lobes) nucleus and a cytoplasm full of brightly stained eosinophilic (orange-red) specific granules. Eosinophils function specifically as phagocytes to destroy larvae of parasites that have invaded tissues i.e. in trichinosis, schistosomiasis, and appear to play a role in allergic responses. Other functions of eosinophils include phagocytosis of antigen antibody complexes.














Basophils - 10 to 15 u diameter.
Basophils constitute approximately 0.5 to 1.0% of leukocytes and are therefore more difficult to find in a routine blood smear. The nucleus consists of 2 to 3 lobes but is usually not as lobulated as neutrophils. The cytoplasm is full of dark purple specific granules. Basophils are also phagocytic, but function largely like mast cells, (i.e. their granules contain histamine and heparin which play an important role in initiation of the acute inflammatory response).









II. Agranulocytes
As the name suggests, these leukocytes usually have no distinguishable cytoplasmic granules.


Lymphocytes - 9 to 14 um diameter
Lymphocytes constitute 20 ­ 25% of agranulocytes and may be small, medium or large in size. The nucleus is rounded or oval, and usually the same size as an erythrocyte. The chromatin is densely packed with no apparent nucleoli. When compared with nuclei of other cells, the lymphocyte nucleus almost always appears smudged. The cytoplasm is scanty and stains pale blue. Lymphocytes are composed of two subpopulations: T lymphocytes & B lymphocytes, which have distinctive functional differences. However, they appear morphologically identical in the stained blood smears you are using. Some lymphocytes migrate into the connective tissues and become Plasma Cells.








Monocytes - 12 to 20 um diameter
Monocytes comprise 3 ­ 8% of agranulocytes. This large cell has a lightly stained nucleus that often appears horseshoe or kidney shaped. The chromatin appears lacy and nucleoli are usually not apparent. The nucleus looks a bit like a "brain." The abundant cytoplasm stainsnd quite often contains vacuoles (small, clear areas). These areas help to distinguish monocytes from large lymphocytes. Small dense granules are frequently present. (Yes! even though they are called agranulocytes.) Monocytes migrate into connective tissue and become Macrophages.













Blood Development

Slides #99 & 172 (Red bone marrow smear, human). You should be able to identify the stages of the granulocytic (myelocytic) and erythrocytic developmental series.


All stages of cells in the granulocytic series, except myeloblasts, will have some type of granule in the cytoplasm. Developing neutrophils are the most numerous. The developing eosinophils and basophils are less common and more difficult to locate.


Myeloblast - 16 to 20 u diameter
These cells are rarely seen and are difficult to distinguish from proerythroblasts of the erythrocytic series. Prominent nucleoli and scanty basophilic cytoplasm are two distinguishing features. Do not try to identify this cell.

Promyelocyte - 15 to 25 u diameter
The outstanding features of cells in the promyelocyte stage are (1) the large size, (2) rounded, indented nucleus and (3) numerous azurophilic (deep blue, purple) or primary granules in the cytoplasm. Do not try to identify this cell.





Modified drawing from your text. Note differences in labeling for Basophilic Erythroblast, Early and

Late Metamyelocyte (Band or Stab), Normoblast, and Reticulocyte





Myelocyte - 15 to 30 u diameter
This is the first stage at which one can distinguish between neutrophils, eosinophils and basophils by the presence of specific granules in the cytoplasm. The shape of the nucleus is variable, but is usually indented and displaced from the center. The cytoplasm is spotted with azurophilic and specific granules. Most of the cells from this stage are neutrophilic since neutrophils are the most numerous type of mature granulocyte. Cells in the myelocyte stage may also have clear halo bordering the indentation of the nucleus representative of an extensive Golgi apparatus.









Metamyelocyte - 10 to 16 u diameter
These cells can also be of the neutrophilic, eosinophilic or basophilic variety and are similar to cells in the myelocyte stage except they (1) are smaller in size and (2) demonstrate a deeply indented, kidney shaped nucleus.




Band Form ("Stab") - 10 to 15 u
Cells in this stage have a long, thinned-out nucleus with parallel sides: i.e. it resembles a thick bar stretched across the center of the cell, or, more often, a horseshoe. The rest of the cell still shows other characteristics of a mature neutrophil.








Segmented Form - (mature cell)
This is the mature form seen in the previous peripheral blood slide. The distinguishing feature is the presence of several nuclear lobes connected by thin filamentous strands of chromatin.
















Erythrocytic Series

The distinguishing characteristics of cells in this series are the lack of cytoplasmic granules and, except for the first and last stages, a centrally placed, well rounded nucleus..Again, just identify the underlined cells.

Proerythroblast ­ Do not try to identify this cell.



Basophilic erythroblast - 12 to 18 u diameter
The most outstanding feature of cells in this stage is the deeply basophilic cytoplasm. The nucleus is round with clumped chromatin and is centrally located. These cells may be hard to distinguish in some of your slides.

Polychromatophilic erythroblast - 10 to 12 u diameter
These cells are smaller and the nuclear material is more condensed than in the previous stage. Due to increased synthesis of hemoglobin, the cytoplasm is now staining blue-grey to grey-green. Again, note the well-rounded shape of the nucleus.

Normoblast (orthochromatophilic erythroblast) - 9 to 11 u diameter
Called normoblasts because the cytoplasm approaches the "normal" staining pattern of mature erythrocytes: i.e. red-orange due to the concentration of hemoglobin. The nucleus is much smaller and denser (pyknotic) at the periphery of the cell ready to be extruded.

Reticulocyte - 8 to 9 u diameter
These cells are anucleate and are indistinguishable from mature erythrocytes by routine Wright's stain. If treated with supravital stain, cytoplasmic ribosomes and residual reticular framework are evident.(You cannot find these cells but should know what they are.)

Erythrocyte - 7 to 8 u diameter
These mature, functional cells are biconcave, anucleate and eosinophilic. They contain a small area of central pallor (pale staining in the center).

Other cells that maybe seen in red bone marrow preparations:

Megakaryocytes - very large cells (approximately 50 to 100 u) with a convoluted nucleus. Pieces of cytoplasm break off from these cells and are released into the circulation as platelets. Due to their size, these cells are most apparent when scanning the slide at low power.


Lymphocyte and monocyte precursors - do not concern yourself with these cells at this time.

Plasma Cells and Macrophages - See lab on connective tissue.





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