What is Bone Marrow?
Bone marrow is the soft tissue filling the internal cavities of bones, best known as the body's blood cell factory. It comes in two types with different roles across a lifetime.
Bone marrow is the soft, vascular tissue inside bone cavities that produces blood cells (hematopoiesis) in its red form and stores fat in its yellow form, with red marrow found mainly in flat bones and the ends of long bones in adults.
- •Hematopoietic — produces red cells, white cells, and platelets
- •Found in flat bones (pelvis, sternum, ribs, skull) and epiphyses
- •Dominant marrow type in infants and children
- •Rich blood supply, contains hematopoietic stem cells
- •Mostly fat cells (adipocytes), stores energy
- •Fills the medullary cavity of long bone shafts in adults
- •Can convert back to red marrow if the body needs more blood cells (e.g. severe blood loss)
- •Contains some mesenchymal stem cells
Step-by-step worked examples
Explain why a bone marrow biopsy is usually taken from the hip (iliac crest).
The pelvis is a flat bone that retains active red marrow throughout adult life Red marrow is where blood cell production actually happens, so it reflects marrow health accurately The iliac crest is close to the skin and easily accessible without risking major organs Sampling the femur shaft instead would mostly collect fatty yellow marrow, which is less useful diagnostically
A patient with chronic blood loss needs more red blood cells. How does bone marrow respond?
Low oxygen delivery triggers the kidneys to release more erythropoietin (EPO) EPO stimulates hematopoietic stem cells in red marrow to produce more red blood cells If red marrow reserves are insufficient, some yellow marrow in the long bones converts back to red marrow This expands the body's blood cell production capacity to compensate for the loss
Why do infants have almost all red marrow while adults have significant yellow marrow?
Infants are growing rapidly and need high rates of blood cell production, so red marrow fills nearly every bone cavity As a child grows into adulthood, blood cell demand per unit of marrow decreases relative to skeletal size Red marrow in long bone shafts gradually converts to fat-storing yellow marrow starting in childhood By adulthood, red marrow is mostly confined to flat bones, vertebrae, and the ends of long bones
Flashcards
Quick quiz
Q1.What is the main function of red bone marrow?
Q2.Where does yellow marrow mainly accumulate in adults?
Q3.What hormone stimulates red marrow to make more red blood cells?
Q4.Which bone is commonly biopsied to sample active red marrow?
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Common mistakes
Bone marrow and the periosteum are the same structure. — Correct: Bone marrow fills internal cavities and makes blood cells or stores fat; the periosteum is the outer membrane covering bone.
All bone marrow produces blood cells. — Correct: Only red marrow is actively hematopoietic; yellow marrow mainly stores fat.
Adults have the same marrow distribution as infants. — Correct: Red marrow is widespread in infants but becomes mostly confined to flat bones and epiphyses by adulthood.
Yellow marrow can never turn back into red marrow. — Correct: Yellow marrow can revert to red marrow when the body urgently needs more blood cells.
FAQ
What is bone marrow?
Bone marrow is the soft tissue inside bone cavities that produces blood cells (red marrow) or stores fat (yellow marrow).
What is the difference between red and yellow bone marrow?
Red marrow actively produces blood cells and is found in flat bones and epiphyses; yellow marrow is mostly fat and fills long bone shafts in adults.
Where is bone marrow located?
Inside the spongy bone of flat bones and epiphyses (red marrow), and inside the medullary cavity of long bone shafts (yellow marrow).
What does bone marrow produce?
Red bone marrow produces red blood cells, white blood cells, and platelets through hematopoiesis.