What are the two types of bone marrow?

Bone marrow is an indispensable substance in the human body. It mainly exists in the bone marrow cavity. According to its structure and organization, it is divided into red bone marrow and yellow bone marrow. Red bone marrow is the main hematopoietic organ, mostly distributed in reticular cells, and also has certain repair functions; yellow bone marrow is mainly composed of adipose tissue, and its function is very small, but if an adult suffers from severe anemia, yellow bone marrow can be transformed into yellow bone marrow.

1. What are the two types of bone marrow?

Bone marrow exists in the spongy bone cavities and the medullary cavities of long bones. It is composed of various types of cells and reticular connective tissue. According to its structure, it is divided into red bone marrow and yellow bone marrow.

2. Introduction to Red Bone Marrow

Red bone marrow is the human body's hematopoietic organ, distributed in the bone marrow cavity. A small amount is also contained in the Haversian tubules. It is mainly composed of sinusoids and hematopoietic tissue. Sinusoids are sinusoidal cavities formed after the arterial capillaries entering the red bone marrow branch off. They have irregular shapes and varying diameters. The sinusoidal wall is lined with endothelial cells, with a basement membrane and pericytes attached to the outside. The hematopoietic tissue is located between the sinusoids. Its matrix is ​​reticular fibers and reticular cells, which form a network. The pores in the network are filled with various free cells, such as various blood cells and mesenchymal cells at different developmental stages. During the newborn period, the bones are filled with red bone marrow, which has active hematopoietic function. In adulthood, red bone marrow is mainly found in the epiphyses of some flat bones, irregular bones and long bones. It is most abundant in the vertebrae, sternum and ilium, and has the most active hematopoietic function.

In addition to the hematopoietic function, red bone marrow also has multiple functions such as defense, immunity and trauma repair. Its wound repair function is mainly due to the immature mesenchymal cells therein, which retain the potential to differentiate into fibroblasts and osteoblasts. Some scholars have used bone marrow stromal cells cultured from red bone marrow and implanted them into fractures and bone defects, proving that they can promote bone tissue formation, which is beneficial to fracture healing and defect repair.

2. The bone marrow cavity of young children is entirely red bone marrow. After the age of 5, the red bone marrow in the long bones is gradually replaced by fat tissue and becomes yellow bone marrow. After the age of 18, the shafts of long bones throughout the body are almost filled with yellow bone marrow. In a normal adult, red bone marrow and yellow bone marrow each account for half. Red bone marrow is distributed in the spongy bone of flat bones (skull, sternum, ribs, ilium), vertebrae, clavicle, scapula, and epiphyses of long bones. The bone marrow of the elderly becomes a fat-deficient, mucous, gelatinous bone marrow. Yellow bone marrow contains a large amount of adipose tissue. Although it has no hematopoietic function, it still contains a small number of immature hematopoietic cell clusters and maintains hematopoietic potential. Under certain pathological conditions, yellow bone marrow can be reconverted into red bone marrow with hematopoietic function.

3. Introduction to Yellow Bone Marrow

Yellow bone marrow is mainly composed of adipose tissue, that is, a large number of bone marrow stromal cells turn into adipocytes, and there are only a small number of immature cell clusters, and its hematopoietic function is weak.

The red bone marrow of an adult can produce red blood cells, granulocytes, platelets and some lymphocytes.

Many factors are involved in the regulation of bone marrow hematopoietic function: the decrease of oxygen in the blood stimulates the formation of red blood cells; similarly, living at high altitude increases the number of red blood cells; blood loss enhances the mitosis of cells in the bone marrow; protein in food promotes the formation of red blood cells; and vitamin B2 is necessary for the normal process of red blood cell formation.

In adults with severe anemia and blood loss, yellow bone marrow can turn into red bone marrow.