P 104 Histology and Cell Biology Read G&H chap. 10

October 6, 2003

Blood

Blood is a connective tissue.

It is specialized by being liquid and confined to an endothelial-lined chamber.

Its functions are obvious; primarily transport of small molecules, proteins and cells.

We are concerned here mainly with its cells which:

- are called “formed elements” because it is arguable whether they are “cells”

- come from red bone marrow (and its proteins from the liver, except for antibodies)

Erythrocytes = red blood cells (RBC)

- make up about 45% of the blood volume (= the “hematocrit”).

An insufficiency of erythrocytes is called anemia

- are biconcave disks 7.5 microns in diameter

This is slightly larger than small capillaries.

- have no cellular organelles or RNA

- contain mainly hemoglobin but also enzymes for producing ATP and NADPH.

- have a plasmalemma supported by an underlying web of fibrous proteins.

The life cycle of erythrocytes

Erythrocytes develop in the bone marrow under control of the hormone erythropoietin.

They enter the blood as reticulocytes with traces of RNA in their cytoplasm.

It takes about a day for reticulocytes to become fully mature RBC’s

Anemias can cause maturing RBC’s to leave the bone marrow prematurely and

increase the numbers of circulating reticulocytes (normal about 1%).

After circulating for 120 days aged RBC’s are removed by macrophages in the spleen.

Changes in the carbohydrates on the cell surface signal them for phagocytosis.

The most interesting features of erythrocytes are biochemical instead of histological

Nucleated blood cells = white blood cells = leukocytes are grouped into two categories

Granulocytes contain secretory granules called specific granules.

These are specific for neutrophils, acidophils and basophils.

Granulocytes are postmitotic with distinctively shaped nuclei

See G&H Table 10-3

Agranulocytes (lymphocytes and monocytes) lack secretory granules.

Note, both granulocytes and some agranulocytes contain lysosomes.

These can be stained and are called “non-specific” or azurophilic granules.

There are usually a total of 6,000 – 10,000 leukocytes per cubic mm of blood.

Neutrophils = polymorphs make up ~60-70% of total leukocytes in blood.

They phagocytize bacteria (but not larger cellular debris, as do macrophages)

Their nuclei are divided into 3-5 lobes, separated by thin strands of chromatin.

No other cell type has nuclei at all similar to them.

Neutrophils have two types of granules that are hard to see because

they stain a pale pink and are only 0.1 microns across.

Their abundant lysosomes are about 0.5 microns in diameter.

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Important terms are in bold font

Life history of neutrophils

See G&H Table 10-7 and read p 246

Neutrophils stay in the blood for only 6-7 hours.

They spend about half of this time attached to the walls of capillaries and venules

“marginating” instead of circulating.

They go back and forth between these two compartments.
During infections the numbers of circulating neutrophils can rise rapidly.

Major infections can cause neutrophils to leave bone marrow prematurely as band cells.

Neutrophils leave the blood stream by diapedesis.

Proteins on the surface of endothelial cells of postcapillary venules cause neutrophils

to adhere, become active and push their way between endothelial cells.

Various chemical messengers induce endothelial cells to synthesize these proteins (eg selectins) and therefore control where neutrophils will accumulate.

In the connective tissue

Neutrophils become mobile and release the contents of one type of granule to facilitate movement through tissues.

They are attracted to sites of bacterial infection (chemotaxis).

There, they release the contents of their other secretory granules

which contain bactericidal proteins and enzymes.

Neutrophils phagocytize bacteria into phagosomes

These they kill with H₂O₂ and O₂ (-).

Then they fuse lysosomes with the phagosome to digest the bacteria.

The used up neutrophils and killed bacteria form pus.

Neutrophils remain in connective tissue for only several days.

Eosinophils = acidophils have more obscure functions.

Allergic reactions and helminth worm infections increase their circulating numbers.

Eosinophils also are known to take up antibody-antigen complexes.

Basophils have functions similar to those of mast cells

Their granules contain histamine, heparin and chemotaxic attractants for neutrophils and eosinophils.

All three types of granulocytes also produce chemical messengers that initiate or enhance inflammation.

Monocytes

- have large bean shaped to horseshoe shaped nuclei and pale bluish cytoplasm that

contains numerous lysosomes (nonspecific granules)

- enter C.T. to become macrophages.

They are not phagocytic until they reach the C.T. and become activated.

The main functions of activated macrophages are to:

1. ingest cellular debris and bacteria

2. secrete a range of chemical messengers involved in inflammation, wound healing and hemopoiesis.

3. turnover tissues and cells and remodel bone

When faced with large particulate matter monocytes fuse together to form multinucleate giant foreign body cells (or osteoclasts in bone)

We described the properties of macrophage in the second lecture on C.T.

Lymphocytes are abundant, easily recognized cells with round nuclei.

They vary in size:

Small lymphocytes have small, dark nuclei surrounded by a narrow rim of cytoplasm visible only along one side

Large lymphocytes have larger, paler nuclei completely surrounded by cytoplasm (and possibly containing scattered large azurophilic granules.)

Lymphocytes represent a collection of distinct cell types that look similar but with different, important functions in immunity and hemopoiesis.

We shall discuss this complex family of cells in a future lecture.

Platelets

- are small fragments of cells (megakaryocytes) ~2-4 microns across that promote blood clotting and plug holes in endothelium, to prevent hemorrhages.

- occur as 2-400,000 per microliter and circulate for about 10 days.

Often platelets are clumped together in blood smears

- have a complex internal structure.

They consist of a small dark central granulomere surrounded by a pale hyalomere.

The granulomere contains 3 types of granules: alpha, delta and lambda.

See G&H Table 10-4 and Fig. 10-10

Because blood clotting will be so important to your work in dentistry I recommend that you definitely read pp 233 – 236 in your text even though you will review the actions of platelets in several other courses.