Skeletal System: Day Three Bone Formation, Growth, and Remodeling, Bone Diseases and Disorders

Bone Development and Growth CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display. Bone Development and Growth Bones form by replacing connective tissues in the fetus. Some form within sheetlike layers of connective tissue (intramembranous bones), while others replace masses of cartilage (endochondral bones). Ossification is another word for bone building.

Intramembranous Bones CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display. Intramembranous Bones The flat bones of the skull form as intramembranous bones that develop from layers of connective tissue. Osteoblasts deposit bony tissue around themselves. Once osteoblasts deposit bone and are located in lacunae, they are called osteocytes. Cells of the membranous connective tissue that lie outside the developing bone give rise to the periosteum.

Intramembranous Bone

Ossification: formation of bone 2 methods: 1. Intramembranous 2. Endochondral

The Major Players Osteoblasts – build bone Osteoclasts – break down bone tissue Osteogenic cells – “highly mitotic” bone stem cells in membranes Osteocytes – mature bone cells in lacunae

I. Intramembranous Ossification End goal: Production of flat bones of skull and most of the clavicle.

Formation of the Bony Skeleton Week 8 (of pregnancy) fibrous membranes and hyaline cartilage begin to ossify. Intramembranous ossification: development from fibrous membrane Endochondral ossification: development from hyaline cartilage

Intramembranous Ossification Forms most bones of the skull and clavicles All bones formed via this route ARE FLAT BONES Fibrous CT from mesenchymal cells is framework on which ossification begins

STEP ONE Four Major Steps: An ossification center appears in the fibrous CT membrane Mesenchymal cells cluster and become OSTEOBLASTS forming ossification center

Step Two Bone matrix (osteoid) is secreted within the fibrous membrane -osteoblasts secrete osteoid (fibers, GAGs, -trapped osteoblasts become osteocytes

Woven bone and periosteum form STEP THREE Woven bone and periosteum form -accumulating osteoid is laid down between embryonic blood, forming random network result is a network of trabeculae. -vascularized mesenchyme condenses on the external surface of the bone and forms periosteum

Step Four Bone collar of compact bone forms and red marrow appears -Trabeculae just deep to the periosteum thicken, forming a bone collar that will later be replaced by mature bone -Spongy bone persist internally and its vascular tissue becomes red marrow

Endochondral Bone Growth

CopyrightThe McGraw-Hill Companies, Inc CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display. Endochondral Bones Most of the bones of the skeleton fall into this category. They first develop as hyaline cartilage models and are then replaced with bone. Cartilage is broken down in the diaphysis and progressively replaced with bone while the periosteum develops on the outside. Cartilage tissue is invaded by blood vessels and osteoblasts that first form spongy bone at the primary ossification center in the diaphysis.

6. Secondary ossification centers appear later in the epiphyses. CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display. 5. Osteoblasts beneath the periosteum lay down compact bone outside the spongy bone. 6. Secondary ossification centers appear later in the epiphyses. 7. A band of hyaline cartilage, the epiphyseal plate, forms between the two ossification centers. 8. Layers of cartilage cells undergoing mitosis make up the epiphyseal plate. 9. Osteoclasts break down the calcified matrix and are replaced with bone-building osteoblasts that deposit bone in place of calcified cartilage.

CopyrightThe McGraw-Hill Companies, Inc CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display. 10. Epiphyseal plates are responsible for lengthening bones while increases in thickness are due to intramembranous ossification underneath the periosteum. 11. A medullary cavity forms in the region of the diaphysis due to the activity of osteoclasts.

Bone Formation: Ossification By birth, most hyaline cartilage have been converted to bone except for two regions: Articular cartilages Cover long bone ends (epiphysis) Persist for lifetime Reduce friction at the joint surfaces Epiphyseal Plate Allows for longitudinal growth of long bones during childhood Will eventually disappear leaving only the epiphyseal line

Bone Growth New cartilage is continuously formed Old cartilage is broken down and replaced by bony matrix Process of long-bone growth controlled by hormones: growth hormone and sex hormone Ends during adolescence when epiphyseal plate is converted to bone

New cartilage added on the external part of the articular cartilage and on the epiphyseal plate surface away from medullary cavity Old cartilage in broken down and replaced by bone on the internal part of the articular cartilage and on the epiphyseal plate near the medullary cavity

Homeostasis of Bone Tissue CopyrightThe McGraw-Hill Companies, Inc. Permission required for reproduction or display. Homeostasis of Bone Tissue Osteoclasts tear down and osteoblasts build bone throughout the lifespan with the processes of resorption and deposition, with an average of 3% to 5% of bone calcium exchanged annually.

Bone Remodeling Bone is very active tissue Bones are remodeled continuously in response to two factors: Calcium levels in the blood Pull of gravity and muscles on the skeleton

Bone Remodeling: Calcium levels in blood Blood calcium low Parathyroid glands release parathyroid hormone (PTH) PTH activate osteoclasts (destroy bone cells) Osteoclasts break down bone and release calcium into the blood Blood calcium high Thyroid gland releases calcitonin Calcitonin stimulates Calcium to be deposited from the blood into the bone

Bone Remodeling: Pull of gravity and muscles on the skeleton Bones become thicker to increase strength where bulky muscles attached Bones of physically inactive people tend to lose mass

Rickets Disease of children where bones fail to calcify Bones soften and a bowing of the weight-bearing bones of the legs occur Due to lack of calcium in the diet or a lack of Vitamin D (necessary to absorb calcium into the blood) Not seen much in U.S., but is a problem in other parts of the world.

Osteoporosis A loss in bone mass leading to thin, fragile bones Common consequence of aging, esp. in women Estrogen helps to maintain skeleton in women Lack of estrogen during menopause in women as well as a diet poor in calcium and protein, lack of Vitamin D, smoking, insufficient weight- bearing exercise can contribute to osteoporosis.