Blood is a type of connective tissue that contains red blood cells, white blood cells and platelets. The contents of blood cells are identified in lab by using cell staining techniques and evaluated under a light microscope. Some cell contents are barely visible and thus require an electron microscope to clearly view the microanatomy of the cell according to "Histology: A Text and Atlas, Fourth Edition."
Red Blood Cells
Bone marrow produces young red blood cells called reticulocytes. Electron microscope analysis of reticulocytes reveals membrane-bound organelles including the Golgi apparatus, lysosomes and mitochondria. Ribosomes and remnants of the nucleus are also present. As reticulocytes mature into red blood cells, organelles are removed from their cytoplasm. The maturation process results in smaller red blood cells with no nucleus or organelles providing additional space to store hemoglobin according to the "Journal of Cell Science."
White Blood Cells
White blood cells, also called leukocytes, are divided into granulocytes and agranulocytes. Granulocytes are a category of cells that contain specific granules or vesicles and include neutrophils, eosinophils and basophils. Agranulocytes are cells without specific granules and include lymphocytes and monocytes. All leukocytes contain nonspecific azurophilic granules according to "Histology: A Text and Atlas, Fourth Edition."
Neutrophils are identified by their multi-lobed nucleus, which gives them their name of polymorphonuclear neutrophils or polymorphs. The nucleus has two to four lobes connected by narrow nuclear strands. Extending off one nuclear lobe is a drumstick-shaped structure, which is present in women called the Barr body.
Neutrophils contain three types of granules including specific granules, azurophilic granules and tertiary granules. Specific granules contain enzymes called type IV collagenase, phospholipase and lysozyme. Azurophilic granules are lysosomes and contain myeloperoxidase enzymes and proteins called defensins. Tertiary granules come in two types and either contain phosphatase or metalloproteinase enzymes. In the center of the neutrophil is the Golgi apparatus and in the cytoplasm are a few scattered mitochondria.
According to "Histology: A Text and Atlas, Fourth Edition," eosinophils contain a bi-lobed nucleus. The cytoplasm contains two types of granules: specific granules and azurophilic granules. The specific granules contain a crystal-like or crystalloid body, which consists of a protein called major basic protein, and in the surrounding area are other proteins called eosinophil cationic protein, eosinophil peroxidase, and eosinophil-derived neurotoxin. Specific granules also contain enzymes including arylsulfatase, histaminase, collagenase and cathepsins. The azurophilic granules contain hydrolytic enzymes.
Basophils contain a lobed nucleus and two types of granules in the cytoplasm, specific granules and azurophilic granules. Specific granules contain substances called heparin sulfate, leukotrienes and histamine. Azurophilic granules contain hydrolytic enzymes.
According to "Histology: A Text and Atlas, Fourth Edition," agranulocytes include lymphocytes and monocytes. There are three types of lymphocytes, T and B cells and natural killer cells. Lymphocytes are characterized by their function and not by their shape or size.
T and B lymphocytes are distinguished by their cell surface receptors since they cannot be distinguished by cell staining techniques. Lymphocytes come in different sizes of small, medium and large. Electron microscope analysis of small lymphocytes reveals free ribosomes, occasional lysosomes, few scattered mitochondria and a small Golgi apparatus.
Medium to large cells have more cytoplasm, a larger nucleus, more polysomes and mitochondria, a rough endoplasmic reticulum, and a more developed Golgi apparatus. Natural killer cells can be identified under a light microscope by their kidney-shaped nucleus and large cytoplasmic granules.
Monocytes contain several cell components. The Golgi apparatus and centrioles are located near the nuclear indentation. Despite the agranular designation, monocytes contain azurophilic granules but no specific granules. Other cytoplasmic contents include smooth and rough endoplasmic reticulum, free ribosomes and small mitochondria according to "Histology: A Text and Atlas Fourth Edition."
Platelets
Platelets originate from larger mature cells within the bone marrow called magakaryocytes. These large cells have a single nucleus but appear multi-lobed because the nucleus folds upon itself. The cytoplasm periphery appears "foamy" indicating areas where the cytoplasm will fragment to produce smaller platelet cells. Electron microscope shows the presence of glycogen deposits, mitochondria, peroxisomes and three types of granules. The most abundant granule is the so-called alpha granule, which contains numerous substances that aid in platelet aggregation, blood clotting and vessel repair according to "Histology: A Text and Atlas, Fourth Edition."
References
- "Journal of Cell Science" ; Maturation of the Reticulocyte In Vitro; Gronowicz et al.; 1984
- "Histology a Text and Atlas Fourth Edition"; Kaye et al.; 2003
