Question

17). Describe the stages of endochondral ossification and name at least one (1) bone that is formed using this process.

18). What effect does the hormone calcitonin have on blood calcium levels & how does it produce this effect? Explain.

19). What effect does parathyroid hormone have on blood calcium levels & how does it produce this effect? Explain.

20). How is vitamin D synthesized and what effect does vitamin D have on blood calcium concentrations? Explain.

Answer 1

17)

Endochondral ossification steps:

1) Bone Collar Formation:
The primary ossification center develops in the center of the bone, and is the source of bone development. Osteoblasts found in the bone will secrete osteoid against the walls of the diaphysis. The diaphysis ,made of hyalin cartilage, is now encased in compact bone. This bone collar gives the developing bone structural support to begin hardening.

2) Cavitation
The chondrocytes go through hypertrophication or enlargement and signal the hyalin cartilage to harden into bone. This hardening into bone is called calcification. The calcified hyalin cartilage is impermeable to the diffusion of nutrients. Since the chondrocytes cant receive any nutrients they begin to die and leave small cavities. These small cavities leave room in the hardened bone for blood vessels to travel through.

3) Periosteal Bud Invasion:
Periosteal bud invasion is the introduction of a nutrient highway to the bone. Previously nutrients were delivered through the cartilage by simple diffusion. To support the capacity of the hardening bone, the periosteal region is invaded by a bud containing blood vessels and nerves. This allows not only nutrients, but osteoblasts and osteoclasts cells to enter into the cavities that were previously inhabited by chondrocytes. The osteoblasts secrete osteoid into the remaining hyalin cartilage and give rise to early spongy bone.

4) Diaphysis Elongation:
After the nutrient source is delivered to the center of the bone, the diaphysis region has the resources to elongate. The elogated region is powered by cells dividing in the primary center of ossification. This elongated region is known as the medullary cavity. The medullary cavity is where the bone marrow is contained.

5) Epiphyseal Ossification
Just before birth the ends of the bone or Epiphysis will develop their own centers of ossification. They cant stay made of cartilage or they would be structurally weak. These centers are known as the secondary centers of ossification. They go through the same process as the primary center of ossification: hypertrophication, calcification, cavitation, and periosteal bud invasion.

Bone formed by endochondral ossification are long bones such as femur, tibia etc.

18) Calcitonin decrease the blood calcium level by stimulating the osteoblastic activity of the bone and inhibiting the osteoclast.

19) Parathyroid harmones has action opposite to Calcitonin. It increases the bone resorption by stimulating the Osteoclastic activity thereby decreasing calcium deposition in the bone and increasing the calcium concentration of the blood.

20) Vitamin D increases the absorption of Calcium form Kidney and intestine and therefore increase the blood calcium level. It also responsible for deposition of calcium in the bone. Vitamin D is synthesized in the kidney by enzyme 1,25 hydroxylase.