[5] Human marrow produces approximately 500 billion blood cells per day, which join the systemic circulation via permeable vasculature sinusoids within the medullary cavity.

[7] The composition of marrow is dynamic, as the mixture of cellular and non-cellular components (connective tissue) shifts with age and in response to systemic factors.

MSCs have been shown to differentiate, in vitro or in vivo, into osteoblasts, chondrocytes, myocytes, marrow adipocytes and beta-pancreatic islets cells.

[citation needed] The red bone marrow is a key element of the lymphatic system, being one of the primary lymphoid organs that generate lymphocytes from immature hematopoietic progenitor cells.

[24] The bone marrow and thymus constitute the primary lymphoid tissues involved in the production and early selection of lymphocytes.

[citation needed] Biological compartmentalization is evident within the bone marrow, in that certain cell types tend to aggregate in specific areas.

[26] Furthermore, exposure to radiation or chemotherapy will kill many of the rapidly dividing cells of the bone marrow, and will therefore result in a depressed immune system.

This typically involves using a hollow needle to acquire a sample of red bone marrow from the crest of the ilium under general or local anesthesia.

Plain film x-rays pass through soft tissues such as marrow and do not provide visualization, although any changes in the structure of the associated bone may be detected.

For example, normal fatty "yellow" marrow in adult long bones is of low density (-30 to -100 Hounsfield units), between subcutaneous fat and soft tissue.

Tissue with increased cellular composition, such as normal "red" marrow or cancer cells within the medullary cavity will measure variably higher in density.

MRI enables assessment of the average molecular composition of soft tissues and thus provides information regarding the relative fat content of marrow.

[citation needed] When "yellow" fatty marrow becomes replaced by tissue with more cellular composition, this change is apparent as decreased brightness on T1-weighted sequences.

Falsely normal marrow on T1 can be seen with diffuse multiple myeloma or leukemic infiltration when the water to fat ratio is not sufficiently altered, as may be seen with lower grade tumors or earlier in the disease process.

Bone marrow examination is used in the diagnosis of a number of conditions, including leukemia, multiple myeloma, anemia, and pancytopenia.

For individuals who have sustained egg and follicle damage, IVF has been found to be more effective following bone marrow stem cell transplantation.

[44] One human clinical case has shown improvements of uterine lining thickness and overall endometrium repair following bone marrow stem cell transplantation.

Additional repairs following bone marrow stem cell transplant to the endometrium include increased vascularity and iron levels, with egg implantation clustering around areas with high blood flow.

[44][46] Using quantitative polymerase chain reaction (qPCR) and next-generation sequencing (NGS) a maximum of five DNA viruses per individual have been identified.

[47] The earliest fossilised evidence of bone marrow was discovered in 2014 in Eusthenopteron, a lobe-finned fish which lived during the Devonian period approximately 370 million years ago.

[48] Eusthenopteron is closely related to the early tetrapods, which ultimately evolved into the land-dwelling mammals and lizards of the present day.