Blood Histology: Red Blood Cells (RBCs)

• Histology of the Blood (RBCs)

Blood Composition

• Blood is viscous, slightly alkaline (pH 7.4), and bright to dark red.
• Blood makes up 7% of the body weight.
• The total blood volume in an average adult is about 5 liters.
• Blood is a specialized type of connective tissue.
• Blood consists of plasma (55%) and formed elements (45%).
• Formed elements include blood cells (various types) and platelets.
• Blood lacks fibers.

Components of Blood

• Red blood cells (RBCs, erythrocytes) transport oxygen.
• White blood cells (WBCs, leukocytes) are involved in immune defense.
• Granular leukocytes include neutrophils, eosinophils, and basophils.
• Agranular leukocytes include lymphocytes and monocytes.
• Blood platelets (thrombocytes) aid in blood clotting.

Composition of blood after centrifugation

• Plasma is the light-colored liquid portion, making up 55% of the blood volume.
• Plasma is approximately 90-92% water, 7-9% proteins, and 1% other solutes.
• Erythrocytes occupy 36-53% of the blood volume.
• Packed Erythrocytes is also called hematocrit.
• The buffy coat is a thin layer (1% of blood volume) containing platelets and WBCs.
• Buffy coat can contain Granular and Agranular leukocytes.

Plasma Explained

• Plasma is a yellowish fluid in which cells, platelets, and organic compounds are suspended.
• Water constitutes 90% of the plasma volume.
• Proteins make up 9% of plasma.
• Other solutes, including ions, nitrogenous compounds, nutrients, and gases, make up 1% of the plasma.
• Serum is plasma without the components.
• Plasma and its solutes maintain homeostasis, ensuring optimal pH and osmolarity.

Characteristics of RBCs

• RBCs have a biconcave disc shape, which increases their surface-to-volume ratio for gas exchange.
• RBC diameter is approximately 7.5 μm.
• RBC thickness is 2.6 μm at the rim and 0.75 μm in the center.
• RBCs lack a nucleus (anucleate).
• Hemoglobin occupies about 33% of the RBC volume and is concentrated at the periphery.
• Average RBC count in males is 5-5.5 million/mm3.
• Average RBC count in females is 4.5-5 million/mm3.
• Average lifespan of RBCs is about 120 days.
• Oligosaccharides are displayed on the surface when the cell ages.
• Macrophages from the spleen, liver, and bone marrow phagocytose old & deformed RBCs.

RBC Membrane

• The RBC membrane is supported by a well-developed cytoskeleton.
• The cytoskeleton is responsible for the flexibility of RBCs.
• The shape of RBCs is maintained via membrane proteins.
• The cell membrane is composed of a lipid bilayer.
• Functionally significant protein groups in the membrane are integral and peripheral proteins.
• Extracellular domains of integral membrane proteins are glycosylated and express specific blood group antigens.
• Glycophorin C plays a role in attaching the membrane to the cytoskeletal network.
• Band 3 protein provides an anchoring site for cytoskeletal proteins.
• Peripheral membrane proteins are located at the inner surface of the cell membrane.
• They are structured into a two-dimensional hexagonal lattice network.
• The lattice is made of cytoskeletal proteins called spectrin and actin.
• The unique cytoskeletal arrangement contributes to cell shape and stability.

ABO Blood Group System

• Blood type A has A antigens on RBCs and anti-B antibodies.
• Blood type B has B antigens on RBCs and anti-A antibodies.
• Blood type AB has both A and B antigens on RBCs and neither anti-A nor anti-B antibodies; it is a universal acceptor.
• Blood type O has neither A nor B antigens on RBCs and both anti-A and anti-B antibodies; it is a universal donor.

Functions of RBCs

• RBC's transport oxygen from the lungs to body tissues.
• RBCs collect carbon dioxide from tissues and transport it to the lungs for exhalation.
• Regulate pH balance via buffering.
• Unique shape contributes to the viscosity of blood.
• Specific blood group antigens determine blood compatibility during transfusions.

Bone marrow

• Bone marrow is a jelly-like substance that fills the cavities within the trabecular network of bone.
• It makes up approximately 5% of the total body weight.
• Bone marrow is located in the central cavity of long bones and cavities of cancellous bone.

Types of Bone Marrow

• Red bone marrow is active in hemopoiesis.
• Yellow bone marrow is inactive in blood cell formation due to excess fat cells.
• Yellow bone marrow can be reverted to red bone marrow in cases of hemorrhage and anemia.

Structure of Bone Marrow

• The bone marrow contains three main structures: Stroma, Blood sinusoids, and Hematopoietic cords/islands.
• Stroma consists of reticular fibers.
• Reticular fibers support myeloid cells and blood sinusoids.
• Stromal cells are the fixed cell population, included within are reticular cells.
• Reticular cells produce reticular fibers and release growth factors.
• Macrophages phagocytose aged RBCs.
• Store iron for reuse.
• Fat cells are one of the largest cells in bone marrow.
• Blood sinusoids are wide, irregular, specialized blood vessels that are lined by fenestrated endothelial cells.

Hematopoietic Cords or Islands

• Hematopoietic cords or islands are networks of hematopoietic stem cells.
• Two types of stem cells exist: pluripotential and multipotential.
• Pluripotential hemopoietic stem cells (PHSCs) can generate almost any cell type in the body.
• Multipotential hemopoietic stem cells (MHSCs) are partially differentiated with a limited differentiation range compared to PHSCS.
• Colony-forming unit-lymphocyte (CFU-Ly) cells are predecessors of the lymphoid cell lines.
• Colony-forming unit-granulocyte, erythrocyte, monocyte, megakaryocyte (CFU-GEMM) cells are predecessors of the myeloid cell lines.
• MHSCs aid the formation of various progenitor cells (Colony-forming units - CFUs).
• CFUs give rise to unipotent colonies
• CFU-erythrocytes, CFU-megakaryocytes, CFU-granulocytes-monocytes, CFU-lymphocytes.
• Precursor cells arise from progenitor cells.
• Hematopoiesis is the process of making new blood cells.
• Incapable of self-renewal but have a specific morphological characteristics, giving rise to mature cells.

Stages of Hematopoiesis

• All blood cells originate from a single pluripotent stem cell in bone marrow.
• Pluripotent stem cells can give rise to all blood cells.
• The cells proliferate and generate two major lineages of progenitor cells with specific potentials.
• Lymphoid cells migrate from the bone marrow to lymph nodes and the spleen.
• Myeloid cells develop in bone marrow.

Stages of Erythropoiesis

• Erythropoiesis is the process of forming RBCs, which is part of hematopoiesis.
• Stages include: PHSCs, MHSCs, and Colony-forming unit-erythrocytes (CFU-E).
• Proerythroblasts are the first recognizable erythrocyte precursors.
• Proerythroblasts are large cells with basophilic cytoplasm.
• Basophilic erythroblasts: The cytoplasm is strongly basophilic.
• Basophilic erythroblasts are where hemoglobin synthesis is most active.
• Polychromatophilic erythroblasts: Hemoglobin accumulates in large amounts.
• The cytoplasm shows eosinophilic areas alternating with basophilic spots.
• This is the last stage where cells undergo cell divisions.
• Subsequent phases involve morphological changes in erythrocytes.
• Orthochromatophilic erythroblasts complete hemoglobin synthesis.
• The nucleus becomes small, condensed, and extruded, then phagocytosed by macrophages.
• Reticulocytes are non-nucleated.
• Reticulocytes differ from mature RBCs by size.
• Last stage is a Mature RBC.