What is Bone Marrow?

The spongy tissue within the bones, like the pelvic and leg bones, is called bone marrow. The bone marrow consists of stem cells. These cells can differentiate into red blood cells that transport oxygen across the body, white blood cells that combat infections, and platelets that aid in blood clotting.

Where is Bone Marrow found in Living Organisms?

The bone marrow is needed to form new blood cells in birds and other well-developed creatures. The hematopoietic cells, marrowfat tissue, and stable stromal cells make up the structure of the bone marrow. The bone marrow is primarily found in the ribs, vertebrae, sternum, and pelvic bones in adults.

Humans deliver roughly 500 billion platelets each day with the help of the bone marrow. The bone marrow joins the entire course through porous vascular sinusoids inside the medullary hole.

A wide range of hematopoietic cells, including myeloid (blood cells) and lymphoid genealogies, originate from the bone marrow. The lymphoid cells should relocate to other lymphoid organs (for example, the thymus) to finish development. 

The Function of Bone Marrow in the Immune System

The immune system protects the body from diseases. It kills unwanted pathogens, which include microorganisms and viruses that could invade the body. It produces lymphocytes that enter the lymph nodes. Through the lymphatic channels, lymphocytes journey between each node. These lymphatic channels meet at huge ducts that lead to blood vessels or blood cells. Through these ducts, the lymphocytes enter the bloodstream.

Lymphocytes are of two primary types, each playing a crucial function in the body:

• B lymphocytes (B cells)
• T lymphocytes (T cells)

Structure of the Bone Marrow

The marrow is dynamic because of the mixture of cellular and non-cellular elements (connective tissue).

The infant's bones best comprise haematopoietically dynamic "crimson" marrow, and there may be a reformist exchange in the direction of "yellow" marrow with age. In grown-ups, a high concentration of red marrow is observed in the bones of the pelvis, sternum, skull, ribs, vertebrae, and scapulae.

1. Redbone marrow

The red bone marrow produces around 60 to 70 percent of lymphocytes. Other lymphocytes begin dividing within the red marrow and grow inside the lymphatic tissues, together with the thymus, spleen, and lymph nodes.

Together with the liver and spleen, the red bone marrow performs an additional task of disposing of the old erythrocytes.

2. Yellow bone marrow

It helps keep the best surroundings for the bone to function and stores fat globules. However, under particular conditions, including severe blood loss, or fever, the yellow marrow converts into red marrow.

It tends to be located at the center of the long bones. A red marrow layer generally surrounds it with lengthy beam-like structures (trabecular) inside the sponge-like reticular framework.

Bone Marrow structure is divided into two components

Hematopoietic component

• At the cellular level, the primary practical segment incorporates the precursor cells, bound to develop in the blood and lymphoid cells.
• The pluripotent hematopoietic stem cells have two major properties:
  • Renewal: They can reproduce every other cell of the same type.
  • Differentiation: They can generate one or multiple subsets of mature cells.
• The process of development of various blood cells from these pluripotent stem cells is called hematopoiesis. Stem cells give rise to all other types of cells like red blood cells and white blood cells.
• The bone marrow contains hematopoietic stem cells that bring about the three classes of blood cells:
  • Leukocytes
  • Erythrocytes
  • Thrombocytes

Stroma

• It incorporates all tissues not straightforwardly engaged with hematopoiesis.
• Stromal cells might be in a roundabout way associated with hematopoiesis by providing a microenvironment that impacts the development of hematopoietic cells.
• Cell types that establish the stroma include:
  • Fibroblasts (reticular connective tissue)
  • Macrophages, which contribute particularly to erythrocyte creation as they convey iron for producing hemoglobin.
  • Adipocytes (fat cells)
  • Osteoblasts (form bone)
  • Osteoclasts (resorb bone)
  • Endothelial cells, which make the sinusoids. These originate from the endothelial stem cells.

The Function of Bone Marrow

Mesenchymal stem cells (MSCs)

The stroma of the bone marrow contains MSCs. These are multipotent stem cells that can separate into several cell types. MSCs have appeared to separate, in vitro or in vivo, into osteoblasts, chondrocytes, myocytes, adipocytes, and beta-pancreatic islets cells.

Bone marrow barrier

The veins hinder the movement of immature blood cells out of the membrane proteins such as aquaporin and glycophorin. These proteins are present in mature cells and facilitate the movement of blood cells across the blood vessel. Hematopoietic stem cells may likewise cross the barrier and may accordingly be gathered from the blood.

Lymphatic role

The red marrow is a vital component of the lymphatic framework that creates lymphocytes from immature hematopoietic progenitor cells.

The bone marrow and thymus establish the essential lymphoid tissues engaged in creating and determining the lymphocytes. Moreover, the bone marrow acts as a valve to prevent lymphatic fluid from flowing backward into the lymphatic system.

Compartmentalization

Certain cells tend to occupy a specific space within the bone marrow through a process called compartmentalization. For example, erythrocytes, macrophages, and their antecedents tend to accumulate around the blood vessels, while granulocytes assemble at the boundaries of the bone marrow.

Diseases related to Bone Marrow

• Leukemia- It is a cancer of blood associated with bone marrow white blood cell production, where the white blood cells are produced at a higher rate. In leukemia, the body cannot fight the normal infection the way a healthy body does. Leukemia is a very serious condition with no treatment; only bone marrow transplantation is known.
• Aplastic anemia- the bone marrow produces an insufficient number of blood cells.
• Multiple myeloma- cancer of the plasma cells.

Bone Marrow Transplant

A bone marrow transplant is a surgical procedure that replaces the unhealthy cells in the bone marrow with healthy cells.

A stem cell transplant or, more precisely, a hematopoietic stem cell transplant is another name for a bone marrow transplant. The bone marrow transplant can be used to treat leukemia, myeloma, lymphoma, and other blood and immune system disorders.

How is a Bone Marrow Disease Diagnosed?

A bone marrow aspiration is investigated for the determination of a disease associated with it. The process regularly includes using an empty needle to get a redbone marrow sample from the peak of the ilium under anesthesia.

Pre-Transplant Tests

X-ray

Plain film x-rays go through the delicate tissues like bone marrow but do not help in its visualization. However, any defect can be easily detected in the procedure.

CT (computed tomography) scan

CT imaging has a fairly good limit for evaluating the site cavity of bones, despite low specificity and sensitivity. For instance, ordinary greasy "yellow" marrow in the long bones of grown-ups have low density. If any cancerous cell or "red" marrow is present within the medullary cavity, the density will increase.

Radiation therapy

Greasy marrow change, the opposite of red bone marrow hyperplasia, can happen with typical aging, though it can likewise be seen with specific therapies like radiation treatment.

Coronavirus Disease (COVID-19) after-effects on the Bone Marrow

In response to COVID-19, the bone marrow cannot produce monocytes properly, which produces inflammatory cytokines and phagocytic macrophages. It is a proven study based on the severely affected patients of COVID-19.

Common Mistakes

• Differentiation of hematopoietic stem cells: learning the chart will help remember which mother cell is forming which daughter cells; for example, the myeloid cell form thrombocyte from megakaryocyte.
• Red and yellow bone marrow differentiation: write and learn in a tabular form.

Context and Application

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for

• Bachelors of Science in Immunology/ Microbiology/ Applied Sciences
• Master of Science in Immunology/ Microbiology/ Applied Sciences
• Health care department (World Health Organization (WHO) and other organization)

Related Concepts

1. Immune system
2. Production of red blood cells, white blood cells
3. Stem cells production system
4. Stem cell transplantation
5. Pathogenesis
6. Genetic disorders