Histology Fact Sheet: Blood

There are four basic tissue types: epithelium, connective tissue, muscle and nervous tissue. Connective tissue is the most diverse. Blood is considered a type of connective tissue.

Blood is composed of the formed elements and plasma. Plasma is the liquid portion of blood. The formed elements consist of the cells and cell fragments. The erythrocytes (red blood cells), platelets, and leukocytes (white blood cells) are all considered formed elements.

When a sample of blood is centrifuged, the percentage occupied by the formed elements is the hematocrit. An average hematocrit is about 45%. The buffy coat is seen in a sample of centrifuged blood. It is the thin layer above the red blood cells, but below the plasma. It consists of the leukocytes and platelets.

Wright's stain is a common histology stain to visualize a peripheral blood smear.

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Histology of Red Blood Cells

Erythrocytes are red blood cells. Erythrocytes (red blood cells) are the most abundant formed element seen when looking at a histology slide of a peripheral smear of blood. Erythrocytes do not contain a nucleus. During their development within the bone marrow, they have a nucleus. However, prior to their entering into the blood stream, the nucleus is ejected. Erythrocytes (red blood cells) are described as biconcave discs. When a histology stain is used, this phenomenon results in the central portion of the cell staining lighter, thus the "central pallor".

Red blood cells Courtesy CDC/ Janice Carr

Histology hint from Sarah Bellham: A biconcave disc can be visualized by holding two Frisbees together so that the middle portion is thinner than the top or bottom.

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Histology of Platelets

A thrombocyte is a platelet. Platelets are cell fragments. They are fragments from megakaryocytes within the bone marrow. When examining the histology of platelets, it is apparent that they also do not contain a nucleus.

Histology hint from Sarah Bellham: The prefix "mega" is from the Greek "megas" which means great or big. For example: megaphone, megalomaniac, megabyte, and megakaryocyte.

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Histology White Blood Cells

Leukocytes are the white blood cells. Leukocytes are cells with a nucleus. The leukocytes consist of neutrophils, lymphocytes, monocytes, eosinophils, and basophils. The order of frequency of the leukocytes is: neutrophil, lymphocyte, monocyte, eosinophil, basophil. Neutrophils are the most abundant leukocyte; basophils are the least abundant leukocyte.

Histology hint from Sarah Bellham: The order of frequency of the leukocytes can be remembered by the mnemonic: "Never Let Monkeys Eat Bananas".

Histology of Granulocytes

The granulocytes are named because of the presence of visible cytoplasmic granules which are visible on a histology slide. The granulocytes consist of neutrophils, eosinophils, and basophils.

Histology hint from Sarah Bellham: The suffix (or prefix) "phil" comes from the Greek word meaning love. It is used to specify an attraction or affinity towards something. It is seen in such words as philosophy, philanthropy and bibliophile. This suffix is used in naming the three granulocytes: eosinophils, basophils, and neutrophils. Eosinophils "love" or are attracted to the eosin histology dye; thus the granules in an eosinophil are orange/pink. Basophils "love" or are attracted to the basophilic histology dye; thus the granules in a basophil are blue. Neutrophils "love" or are attracted to the neutral histology dye; thus the granules in a neutrophil are neutral colored.

Histology of Neutrophils

Neutrophils are granulocytes. Neutrophils have a multi-lobed nucleus. The lobes are separated by a thin strand. Because of the shape of the nucleus, a neutrophil is also called "polymorphonuclear neutrophil", ""poly" "PMN", or "polymorph". "PMN" is an abbreviation for polymorphonuclear neutrophil. Thus, it is a neutrophil, which is a granulocyte.

Histology hint from Sarah Bellham: Poly is from the Greek "polys", which means many. "Poly" is also sometimes used as a nickname for polymorphonuclear leukocyte.

Histology of Eosinophils

Eosinophils are granulocytes. On a histology slide of peripheral blood, eosinophils have prominent orange pink granules. The nucleus of eosinophils is usually bi-lobed.

Histology of Basophils

Basophils are granulocytes. On a histology slide of peripheral blood, basophils have large blue granules, which often obscure the nucleus. The granules are basophilic, therefore they are blue.

Histology of Agranulocytes

The agranulocytes are lymphocytes and monocytes.

Histology of Lymphocytes

Lymphocyte Courtesy: Dr. Triche. National Cancer Institute

Lymphocytes are agranulocytes. Lymphocytes can be categorized by three sizes: small, medium, and large. The small lymphocytes are the smallest leukocyte, being only slightly larger than an erythrocyte. T cells (T lymphocytes) gain their immunocompetence in the thymus. Plasma cells are derived from B lymphocytes.

Histology of Monocytes

Monocytes are agranulocytes. They are the largest leukocyte.

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Sources:

Histology: A Text and Atlas
Michael H. Ross/Edward J. Reith

Manual of Hematology (A Scope Publication)
Paul Richard Reich, M.D.

Morphology of Human Blood Cells
L.W. Diggs, M.D., Dorothy Sturm, and Ann Bell, M.S