8.1.5 Cells of the Blood

• Blood is a tissue composed of a number of important specialised cells
• Red blood cells, monocytes, neutrophils and lymphocytes all have distinguishable structures which enable them to be recognised on microscope slides, in photomicrographs and in electron micrographs

Red blood cells

Red blood cell

• There are approximately 5 million red blood cells per mm³ of blood
• Red blood cells contain haemoglobin, a protein with a quaternary structure that contains haem iron groups which can bind reversibly to oxygen
• Distinctive features of erythrocytes when viewed under a microscope, are their distinctive biconcave disc shape (caused by their lack of nucleus)

Red blood cell micrograph

Monocytes

Monocyte

• Monocytes are identifiable by their size – they are the largest of the leukocytes and have a nucleus shaped like a kidney or a bean
• The nucleus of monocytes tends to appear lighter after staining than other leukocytes

  The nucleus should appear a light blue colour, while the chromatin inside is distinct and fine

Monocyte micrograph

Neutrophils

Neutrophil

• Neutrophils are distinguished by their multi-lobed nuclei
• Up to 70% of all leukocytes are neutrophils – this makes them easy to spot on a micrograph
• The granules of neutrophils typically stain pink or purple-blue

Neutrophil micrograph

Lymphocytes

Lymphocyte

• Lymphocytes are small leukocytes that are identifiable by their very large nuclei, which typically stains a dark colour
• Lymphocytes constitute around 20-25% of all leukocytes
• Lymphocytes are around the size of red blood cells

Lymphocyte micrograph