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Essay on Bone Marrow
Essay Contents:
- Essay on the Introduction to Bone Marrow
- Essay on the Types of Bone Marrow
- Essay on the Functions of Bone Marrow
- Essay on the Vascular Arrangement in the Bone Marrow
- Essay on the Method of Examining Bone Marrow
Essay # 1. Introduction to Bone Marrow:
The terms ‘bone marrow’ and ‘myeloid tissue’ are often used synonymously. Myelos meaning marrow is confined in the cavities of the bone in the postnatal life.
Bone marrow is the cellulovascular tissue occupying the medullary cavities and the cancellous spaces of the bone. Active marrow in the adult is estimated at from 3.5-6.0 percent of body weight. The volume of the marrow is 70ml at birth and about 4000ml in the adult. In the adult only about half the marrow is in active state, known as red bone marrow, the remainder being inactive -yellow bone marrow.
Though the yellow bone marrow is inactive, yet it has the capacity of resuming its activity to produce blood cells during urgent need. Under such condition a certain portion of the yellow bone marrow is converted into the red bone marrow. Although only half the marrow is active in the adult, yet it has enormous functional capacity and considerable room for expansion.
Essay # 2.
Types of Bone Marrow:
i. Red Bone Marrow:
Red cells are actively manufactured here, hence the colour. In foetal stage most of the bones contain red bone marrow. But with the advancement of age and in postnatal life the red bone marrow is only located in the upper ends of humerus and femur, the bones of skull and thorax, the vertebrae and the innominate bones of the pelvis.
ii. Yellow Bone Marrow:
It is made up of fat and a little reticular tissue with blood vessels. Here, red cells are not manufactured. In the adult life they occupy the spaces where red marrow is absent.
i. From birth up to the fourth year all the bones contain red marrow.
ii. By seven years the marrow becomes less active and is pale red in color.
iii. Between ten to fourteen years a patch of yellow marrow appears in the distal ends of the shafts of the long bones and gradually extends on both sides.
iv. At the age of twenty, entire red marrow of the long bones is replaced by yellow marrow except the upper ends of femur and humerus. Throughout adult life this distribution persists. As age advances yellow marrow proportionally increases.
v. By seventy years more than half the ribs and half of sternum contains yellow marrow.
Essay # 3. Functions of Bone Marrow:
1. Haematopoietic (Haemopoietic) Function (Production and Release of Blood Cells):
Production of myeloid elements is the important function of bone marrow. It has been described that red bone marrow is active and has the capacity of forming red cells as well as other blood cells. In the embryo and even in the new-born, only red bone marrow is formed, but in the adult stage nearly 50% of the red bone marrow is converted into yellow bone marrow.
This ratio is not constant and is changed with the advancement of age and also with the degree of the need of haemopoietic elements. All the blood cells like erythrocytes, granulocytes, platelets, monocytes and lymphocytes are formed in the red bone marrow. It has been studied that the marrow contains about 5.6 × 109 erythroid precursors per Kg body wt. and 11.4 × 109 neutrophilic precursors per Kg body wt.
Mechanisms by which the blood cells are released in the blood are not clear. Under certain urgent need and in case of anaemia, mature and even immature cells may be released in the circulation.
2. Erythroclasia or Destruction of R.B.C:
In the bone marrow not only the blood cells are formed but also the abnormal, imperfect, damaged and aged R.B.C., are destroyed. These cells are sequestered or trapped and phagocytised in the macrophages of the bone marrow. Iron portion is stored as haemosiderin arid ferritin in the liver, spleen, R.E. cells and bone marrow and the rest of heam is ultimately converted into the bile pigments.
3. Storage Functions:
Bone marrow is an important site for storage of iron in the form of ferritin and of haemosiderin coming from food sources as transferrin and also from destruction of R.B.C. through phagocytosis. These stored irons are easily utilized for the synthesis of haemoglobin.
4. Reticuloendothelial Function:
Bone marrow plays an important role in the inactivations of toxins or other toxic substances of the body. The free macrophages of the bone marrow are increased during the invasion of toxins or during rapid haemolysis.
5. Immunological Function:
Regarding its immunological function, the marrow is not so competent as it is found in spleen and lymph nodules. Presence of lymph nodules in the bone marrow has been reported by many.
6. Osteogenic Function:
The cellular elements which take part is the formation of bone are formed in the marrow. The osteoclast, osteoblast, osteocyte, endosteum blood vessels are found within the marrow.
7. Connective Tissue Functions:
Due to its different connective tissue contents, the bone marrow performs several functions associated with the connective tissues.
Essay # 4. Vascular Arrangement in the Bone Marrow:
The nutrient artery breaks up into smaller branches which widens out and becomes blood sinuses. These sinuses are lined by single layer of endothelium, the cells of which divide and give rise to red cells. These sinuses, where active erythrogenesis is going on, remain collapsed, thus creating the anoxic condition favourable for red cell formation.
When the red cells are sufficiently mature, these collapsed sinuses open up, blood stream enters and the newly born cells are washed away into the circulation. There is some evidence that marrow activity depends to some extent upon the sympathetic system. Turnbull (1936) and later Gilmour (1942) demonstrated that erythropoiesis is also extravascular.
Essay # 5. Method of Examining Bone Marrow:
In animals, the bone is taken out and the marrow is collected after breaking the bone. A smear is prepared in the same way as drawing a blood film on a glass slide. In the human beings the sternum is punctured with a special needle, the marrow is drawn out and the slides are prepared just like blood smears.
Staining:
(a) For staining the smear, Jenner’s stain or Leishman’s stain may be used in the same way as in staining blood film. For staining the reticulocytes cresyl blue should be used,
(b) Vital staining of the living cells of the bone marrow can be carried out by injecting suitable preparation of Janus green and Neutral red in the circulation of living animal.
It is to be noted that in the circulation the average ratio between white and red cells is 1: 700. In other words, the red cells are much more in proportion to the white cells. But in the red bone marrow, the relation is reversed. Myeloid cells are more in number than the erythroid cells.
The proportion between the cells of the myeloid series and the erythroid series varies from 8: 1 to 2: 1. This reversed relation is due to the fact that the life of the white cells is much shorter in the circulation than that of the red cells. Consequently, the white cells should be more speedily manufactured than the red cells.
Due to this reason, myeloid cell count is much higher than the erythroid cells in the bone marrow. The red bone marrow also contains giant cells known as megakaryocytes having a diameter of about 40µ. Each cell contains a ring of lobed nuclei. From these megakaryocytes platelets are formed.