a-short-review-of-histology

Platelets:

Platelets, also known as thrombocytes are small ovoid "cells" which are about 3-4 microns in length ie about half the diameter of a red blood cell
They are not normally counted as part of a "white blood cell count"
Platelets consist of a central part - known as the granulomere, and a peripheral part known as the hyalomere
The granulomere contains granules
In a Romanovsky-type stain of a blood smear these are the only parts of the platelet which stain
Thus on examination of a blood smear, platelets appear to be very much smaller than they really are
The enzymes located within the granules are important in triggering the cascade reaction which results in blood clotting
The hyalomere of the platelet lies peripherally and contains microtubules which form the skeleton of the platelet to which contractile proteins attach
These contractile proteins function in clot retraction
Platelets are formed by budding off from a large cell in the bone marrow known as the megakaryocyte
They circulate in the blood for about 9-10 days

Platelets - 2:

In this image of a blood smear there are numerous red blood cells and three white blood cells (A = neutrophil, B = basophil, C = small lymphocyte)
If you look closely you will also see some very small stained granules (two examples are arrowed)
These are stained granules within platelets
Most of the platelet in a Romanovsky stained blood smear does not bind any of the stain
Note that in reality a platelet is about 3-4 microns in diameter, about half the diameter of a red blood cell

Platelets - 3:

In this electron micrograph a small clot has formed in a blood vessel
Red blood cells (labelled B) are trapped within the clot
The bundles of fibrous material (labelled X) are fibrin
Formation of this fibrin to form a blood clot is triggered by release of enzymes from the granules of platelets