A myeloid cell is any cell of the myeloid family, which is one of the two major groups of blood cells. All blood cells develop from hematopoietic stem cells in the bone marrow, which gives rise to two cell lineages; the myeloid cell line, and the lymphoid cell line.
Cells of the myeloid lineage develop from myeloid progenitor cells and include the monocytes, macrophages, dendritic cells, erythrocytes, and platelets.
What is a Myeloid Cell?
All blood cells develop from multipotent hematopoietic stem cells (HSC) in the bone marrow. HSCs give rise to two main lineages of blood cells; these are the myeloid cells, and the lymphoid cells.
HSCs first differentiate to form the common myeloid progenitor and the common lymphoid progenitor cells. The progenitor cells then differentiate further to produce all the cells of the myeloid and lymphoid families. Myeloid progenitor cells give rise to monocytes, macrophages, Dendritic cells, granulocytes, erythrocytes, and platelets.
The process by which blood cells develop from a common progenitor cell in the bone marrow is called hematopoiesis.
What is Myeloid Tissue?
Myeloid tissue is also known as bone marrow. It is the spongy material inside the bones that produces blood cells. Myeloid tissue contains hematopoietic stem cells and is also known as hematopoietic tissue.
Functions of Myeloid Cells
The myeloid cells are a diverse group of cells with a wide range of functions. Cells in the myeloid family include the monocytes, granulocytes, macrophages, dendritic cells, erythrocytes, and platelets.
Monocytes account for around 5% of circulating blood cells and are a key component of the innate immune system.
In the event of infection or inflammation, monocytes migrate into the affected tissues and differentiate into macrophages and dendritic cells. Both cells function as phagocytes, and rapidly clear pathogens and other foreign agents from the infected region. Macrophages and dendritic cells are also antigen-presenting cells (APCs) and present antigens on their surface to activate T cells. Therefore, they also have important functions in the stimulation of the adaptive immune response.
The three types of granulocytes are basophils, neutrophils, and eosinophils.
Granulocytes are phagocytic white blood cells that are categorized by the presence of granules in their cytoplasm, which often contain enzymes for the digestion of engulfed pathogens. They may also contain inflammatory factors (such as histamine), which granulocytes release into the bloodstream in response to allergen exposure.
Macrophages arise from monocytes, which differentiate when they migrate into infected tissues from the bloodstream. In the tissues, macrophages function as phagocytes and as antigen-presenting cells (APCs). They are found throughout the body, and their main function is to capture and destroy pathogens and other harmful agents in their resident tissues. They also present antigens to T cells and release proteins called cytokines to activate other cells of the immune system. Therefore, macrophages play important roles in the activation of the adaptive immune response.
Dendritic Cells are important antigen-presenting cells and, like macrophages, they are produced by monocyte differentiation. They have long, branching structures (called dendrites) which make them highly efficient at capturing and engulfing pathogens. Dendritic cells display the antigens of infectious agents on their surface for the activation of T cells and, in doing so, play a central role in the initiation of the adaptive immune response.
Erythrocytes (AKA red blood cells) also develop from myeloid progenitor cells. Their primary function is to deliver oxygen from the lungs to all the tissues of the body and to carry carbon dioxide from the tissues to the lungs. Erythrocytes contain the protein hemoglobin, which binds to oxygen in the lungs and releases it to the cells of the body.
Platelets are produced by megakaryocytes; large cells that also develop from the myeloid cell line. Megakaryocytes reside in the bone marrow, where they grow into giant cells before fragmenting to form over 1000 platelets.
The platelets are then released into the bloodstream, where they play a central role in blood clotting. They prevent bleeding by clumping together at the site of blood vessel injuries and are vital for wound healing.
What Are Lymphoid Cells?
The two major blood cell lineages are the myeloid cells and the lymphoid cells. Lymphoid cells develop from lymphoid progenitor cells and include the T cells, B cells, and natural killer (NK) cells.