Bone (2) Chapter 8. The bone is surrounded by the periosteum, the periosteum consists of two layers: a fibrous outer layer and an innercellular layer.

Transcription

1 Bone (2) Chapter 8 The bone is surrounded by the periosteum, the periosteum consists of two layers: a fibrous outer layer and an innercellular layer. The innercellular layer contains osteoprogenitor cells, and the doctor named them as undifferentiated cells, because osteoprogenitor means that it's going to differentiate an osteoblast, while if the stimulus is different these cells are able to change into other cells. For example they can change to chondroblasts, fibroblasts so the proper name id undifferentiated cells. But remember if they differentiate into osteoblast they were If we are looking for an embryo at a stage of pregnancy where all the bones are seen as a model and this model is hyaline cartilage. Hyaline cartilage is similar to the bone, it's surrounded by Perichondrium, and the perichondrium also has an outer fibrous and innercellular layer which is similar to the periosteum. In some places the perichondrium and periosteum are continuous with each other, for example when we look at the ribs there will be the rib itself which is a bone, and then there is a costal cartilage, so the perichondrium is continuous with the periosteum, then we have the sternum which is surrounded by periosteum. So we have periosteum>> perichondrium >> periosteum (all of them are continuous with each other.) At a stage this embryo was a hyaline cartilage and when we talked about the cardiomuscular system we said that it s the first system that develop in the human body at the intrauterine life, in this stage we have: blood vessels, mesodermal cells connected together, the ulcocell is differentiated into endothelium, while the cells in the center are differentiated into endothelium, while the cells in the center are differentiated into hematopoietic cells (cells which are going to form the hematopoietic system). As if the blood vessels are going to take them from their site of formation towards the future bone marrow cavity. The muscles are mesodermal in origin. The contents of the blood are epidermal in origin.

2 The contents of the blood vessels are mesodermal in origin Now the muscles originated from one side and they fuse with the wall o of the blood vessel and they started contracting. When this structure is formed (the muscles fusion with blood vessels and they started contracting) and this is the feature of the heart, because the heart makes these contractions. The blood vessels start distributing all over the growing mass of cells, this mass was depending on the diffusion in the mother s endometrium. So the blood vessels now are, before the development of the cardiomuscular system the nutrients are arriving to the cells by diffusion, they were under low oxygen tension and all the cells had low metabolic activity, it is a muscle and that needs a high amount of oxygen which is not affordable in that stage from the embryonic development. When the blood vessels are propagated all over the body and in all directions almost all the cells will get a sort of blood supply, one of the blood vessels go towards the cartilage, the cartilage is similar to the epithelium in one aspect at least they are Avascular. So this blood vessel goes to the central part of the shaft of the hyaline cartilage (perichondrium of the shaft). When the blood vessel inter through the perichondreum it pass deeper towards the cellular layer, the cell present in the center got a message that this blood vessel is carrying oxygen and the oxygen is at a high tension so they start to differentiate into bone. So the bone cells were undifferentiated cells in the perichondreum and that s why we call these cells undifferentiated even if its cartilage, and if they get the proper stimulus they will be differentiated into cells needed in that place. So now these cells differentiate into osteoblasts, and the osteoblasts start laying down/forming bone the blood vessels contain bone marrow inside them, and they start looking for a space (safe area) to put the bone marrow inside it.

3 In this case the bone at which it enter through the perichondreum and elicited the changes when the oxygen tension increases, consequently the undifferentiated cells changed into osteoblast, that bone specifically is known as primary ossification center. The first ossification center in the body develops in the clavicle. The second ossification again develops in the clavicle when the embryo is at the 6 th week. So the first two ossification centers develop in the body appears in the clavicle, the reason is that after a short period of time the embryo starts to move its upper limb and this means that he is transmitting the weight of the upper limb towards the axial skeleton, so there should be bone to tolerate the force in these regions. The third ossification center appearance is followed by primary ossification centers all over the long bones in the whole body. By the 8 th week, all the long bones have primary ossification centers. The blood vessels as it attempted to penetrate through the cartilaginous model but now it's much harder (its bone), which is difficult to be penetrated by blood vessels, so the monocytes which are situated inside them (the blood vessels) join together to form a osteoclast, and this osteoclast start resolving the area to allow the penetration of the blood vessels inside. Consequently, the bone is forming and the osteoblasts are resolving for the blood vessel s penetration till it reaches almost the central part of the hyaline cartilaginous model, which is now a small area of bone. If we have an area of perichondreum and at certain points there are changes towards the bone, this means that what is overlying this area is a periosteum not perichondreum, remember that the periosteum and perichondreum are continuous with each other. Now the blood vessels are in the central part, while they are entering to the central part of the cartilaginous model, they carry with them some of the undifferentiated cells from the deeper layer, So they enter the blood vessel and inside there are stem cells for the hematopoietic system and around the blood vessels there are undifferentiated cells from the deeper layer of the

4 periosteum or what was the perichondreum. Now the blood vessel is in the center. Part of the contents of the blood vessels is calcium, the area where the chondrocytes are situated is the lacunae, and it start to precipitate calcium around the lacunae in the area of the immediate facility with the blood vessel. When there is calcium precipitation around the lacunae, the diffusion or chondrocytes around the lacunae is changing, and when the deposition of calcium increases the diffusion towards the chondrocytes decreases, till it reaches a stage where there is no diffusion what so ever the chondrocytes, in this case the chondrocytes will degenerate. The chondrocytes enter the lacunae and the calcium come all around, there is no diffusion in this case, so the chondrocytes inside the lacunae will degenerate in this case, as if I'm saying that the lacunae is now empty because the chondrocytes which were inside degenerated. Now the osteoclasts inside the blood vessel create a sort of bore inside the lacunae and a cell from the undifferentiated ones that enters with the blood vessels passes through the bore to occupy the lacunae, which contained previously the chondrocytes. This means that the chondrocytes then don t change into osteocytes; instead they are replaced by osteocytes. The undifferentiated cells enter the empty lacunae and occupy it. The blood vessels don't branch till it reaches the central part of the bone. All the bones possess foramen for the entrance of the blood vessels and these foramina are known in anatomy as nutrient canal or nutrient foramen, the blood vessels enter through them till they reach the bone marrow cavity and when they reach that point they start branching and the branches return on the opposite direction. Remember, that the blood vessels don t branch till they reach the bone marrow cavity, the blood vessel is present now in a certain place, and it calcified the chondrocytes inside the lacunae, and these chondrocytes

5 AGAIN: degenerated and replaced by osteocytes, so the blood vessels now divides into two branches one of them goes proximally or superiorly and the other goes distally or inferiorly. Gradually the changes start in the cartilaginous model during the intrauterine life, and they will continue till the bone stop growing at a certain age which is probably 20 years. Now the chondrocytes t one side: the cartilage is avascular and the diffusion must be from all sides, so in this case in the calcified side the diffusion amount will decrease and consequently the cell will decrease its metabolic activity. The area which is just above the calcification area is the hypertrophy zone. The area where the blood vessels make precipitation of calcium is the calcification zone. The area where the cells swell because of lowering their metabolic activity in response to the decreased diffusion is the hypertrophy zone. Part of the homeostasis in the body that the number of the cells must remain constant, in this case the cells when they lost their cartilage cells some of them start multiplying -they undergo mitosis-. Now the area that started the process of increased mitotic activity is just above the hypotrophy zone is named as the proliferation zone. The cells that are at the upper part of the shaft are still not aware that there is something going on inside the tissue so the area is named as resting or reserve zone. There was a blood vessel entered and it is formed by the osteocyte during its movement deeper >> resorption zone. The area high above where the blood vessels are precipitating calcium around the chondrocytes in their lacunae >> calcification zone. The area above where these cells low their metabolic activity in response to the decreased diffusion >> hypertrophy zone. The area where the cells are actively mitotic dividing is >> the proliferation zone.

6 The highest area which has nothing to do with the changes below is the resting or reserve zone. Now if the blood vessel is directed upwards, as if this blood vessel is shifting these areas upwards. When the blood vessel goes upwards that means that the resorption increased and allowed it to pass, and then the calcification will be shifted upwards and the hypertrophy will also shift upwards this will continue till the whole resting area receives all the changes, in this case the shaft will be completely ossified. On the other hand or if I am looking for it from another side: ( from the top too the center) The upper area is a typical cartilaginous model >> the resting zone. The cells are actively dividing >> the proliferation zone. The cells are swollen >. Hypertrophy zone. There is precipitation of calcium >> calcification zone. In the central part there is a blood vessel and the area is almost empty and this blood vessel formed its own contents of hematopoietic cells inside >> the resorption zone which is going to be the future bone marrow cavity.

7 blood vessel which is the response for all the previous changes, it arrived the perichondrium and the undifferentiating cells they differentiate into osteoblast. The point which the blood vessel inter the perichondrium "which going to be periosteum " called Primary Ossification Center, meanwhile this blood vessel when it's interring the perichondrium it takes sleeve midi cells undifferentiation around it also it contains bone marrow cells. Osteogenic bud : the blood vessel with the surrounding cells and the cells that it contains them. The beginning of ossification is in the shaft of the bone, after short period ( starting from 12th week) there is another blood vessel inters through the proximal epiphysis; epiphysis is a swelling area so the blood vessel is not going to divide into proximal (superior) and distal (inferior), but it starts to branch in a radial way, after a while all changes that mentioned (feeresolution, hypertrophy, proliferation..) are still exist. What remained of the cartilage in this cartilaginous model is just a plate (22:00) this plate is laying between epiphysis and diaphysis (the area which the Dr called it metaphysic) and this plate described as the epiphyseal growth plate. In the same bone we have 2 metaphysis (proximal and distal) so we have also 2 epiphyseal growth plate (proximal and distal). These epiphyseal growth plates remain as "22: ", then they grow more and more, after a period of time they closure. *Closure of epiphyseal growth plate means that this area has been transported into bone and this bone is unable to grow in length any more. All bones in the human body they continue to elongate just when the epiphyseal growth plate existent; either proximal, distal or commonly both. If the epiphyseal growth plate is not their the bone stop its growth.

8 Ex. Look to the near picture there is a person sits in squatting position. *At the upper limb: -The bones which directed upward are the lower ends of the radius and ulna, and the upper end of the humerus. -The bones which directed down word they are the upper ends of radius and ulna, and lower end of humerus. *At the lower limb: -The bones which directed upward are the lower end of the femur and the upper ends of the tibia and fibula. -The bones which directed downward are the lower ends of the tibia and fibula, and the upper end of the femur. All the bones directed upward they end their ossification in male when he becomes 20 years old, and when the female becomes 18 years old. All the bones directed downward they end their ossification in male when he becomes 18 years old, and when the female becomes 16 years old. -There is a difference in the age of ossification of bones between males and female; The females are more mature than males. (bayen jddan :P) The reason: the estrogen accelerates the closure of the epiphysis growth plate, while the testosterone is delays the ossification of the epiphyseal growth plate. * The average of females height less than the males. You have X-ray of humerus of a person and you want to know his/her age.(we can estimate the age by looking at the epiphyseal growth plate)

9 If there is epiphyseal growth plate at the lower end of humerus that mean: -If the person was male ( ), he is below the age of 18. -If the person was female ( ), she is below the age of 16. If there is a closure of lower epiphysis growth plate and the upper one is still there that mean: -If the person was male ( ), he is bigger than 18 and younger than 20 years. -If the person was female ( ), she is bigger than 16 and younger than 18 years. When the lower epiphyseal growth plate is present the upper must be present; because the lower closures before the upper one. But when the upper starts closure the lower is completely closure. The closure is not at 16 or 18 precisely but it's round these ages. The closure is not complete (homogenesis) by the same way, you could find some areas that start to closure so you can say that the person age around 18 or 20 (depending on the sex) 26:53