Development of the Skeletal System PDF

Lecture 3 Development of the skeletal system  Development of the skeletal system The skeletal system defines an individual’s structure It undergoes considerable change over the life span It reflects the influence of both genetic and external factors.  Prenatal Development of the S...

Lecture 3 Development of the skeletal system  Development of the skeletal system The skeletal system defines an individual’s structure It undergoes considerable change over the life span It reflects the influence of both genetic and external factors.  Prenatal Development of the Skeletal System In embryonic life, the skeletal system exists as a “cartilage model” of the bones Ossification centers appear where bone is deposited Ossification = conversion of cartilage or other connective tissue into bone  The upper-extremity limb bud appears slightly before the lower-extremity limb bud. Its formation starts in the 4th week Each limb develops in proximal-distal sequence Primary ossification centers appear in the mid- portions of the long bones  It begins to form bone cells (osteogenesis) starting at the fetal age of 2 months The bone shafts ossify outward in both directions from these primary centers By birth, the entire shafts are ossified  The secondary ossification centers, or epiphyseal plates where new bone cells are formed and deposited so that the bones grow in length. In contrast to the long bones, small round bones such as those in the wrist and ankle simply ossify from the center outward.  developing long bone consists of a diaphysis, epiphyses, and metaphysis A growth plate is formed between each epiphysis and metaphysis These regions are established by the middle of the embryonic stage and undergo proportional changes in size until skeletal maturity.  By the time the fetal skeleton is fully formed Cartilage only remains at the joint surface as articular cartilage and between the diaphysis and epiphysis as the epiphyseal plate A baby's body has about 300 bones at birth Then They eventually fuse to form the 206 bones that adults have.  Factors affecting prenatal growth of bones 1. Genetic 2. Maternal Nutrition: Calcium and Vitamin D 3. Ultraviolet B (UVB )exposure 4. maternal smoking  Post natal growth and development of the skeletal system  While the long bones are growing in length they also increase in girth, a process called appositional bone growth Girth is increased by the addition of new tissue layers under the periosteum There are also epiphyses at the sites where the muscles’ tendons attach to bones. They are called traction epiphyses.  During childhood, the cartilage grows and is slowly replaced by bone By the time at age of 25, this process will be complete. After this happens, there can be no more longitudinal bone growth All of these bones are very strong and very light.  How Bones Grow in Length The epiphyseal plate is the area of growth in a long bone. It is a layer of hyaline cartilage where ossification occurs in immature bones. On the epiphyseal side of the epiphyseal plate, cartilage is formed. On the diaphysis side, cartilage is ossified, and the diaphysis grows in length.  Bones continue to grow in length until early adulthood. The rate of growth is controlled by hormones When the chondrocytes in the epiphyseal plate cease their proliferation and bone replaces the cartilage, longitudinal growth stops All that remains of the epiphyseal plate is the epiphyseal line  How Bones Grow in Diameter While bones are increasing in length, they are also increasing in diameter Growth in diameter continues even after longitudinal growth ceases  Osteoclasts resorb old bone that lines the medullary cavity Osteoblasts produce new bone tissue beneath the periosteum This increases the diameter of the diaphysis and the diameter of the medullary cavity. This process is called modeling  Bone Remodeling Occurs in adult life Resorption of old or damaged bone takes place on the same surface where osteoblasts lay new bone to replace it Injury, exercise, and other activities increase the remodeling rate. About 5 to 10 percent of the skeleton is remodeled annually without Injury or exercises  Head growth Infant’s head growth reflects the growth of his brain. During the development of the fetus, the skull is developed with fibers linking the cranial bones The skull is growing faster during the first four months than at any other time in life. A year after birth these fibers disappear and the cranial bones fuse together  The pliable head is responsible for enabling normal human development during the first 18 months of a child’s life. The permanent skull is fused between the ages of 20 months and two years. An adult skull consists of 22 bones There are 8 cranial bones and 14 facial bones The lower jaw (mandible) is the only movable bone in the skull  Head circumference Is a measurement of a child's head around its largest area. It measures the distance from above the eyebrows and ears and around the back of the head. The average newborn’s head circumference measures about 35 cm, growing to about 38 cm by one month.  Between birth and 2 months, the average head growth in 1 week is 0.5 cm Then slows to 0.25 cm between 2 and 6 months. The average total head circumference growth from birth to 3 months is 5 cm Between 3 and 6 months is 4 cm  Between 6 and 9 months, head circumference increases by 2 cm Between 9 and 12 months, head circumference increases only by 1 cm During the second year, a child’s head circumference increases by 2.5 cm Attainment of 90% of adult head size occurs by the end of the 2nd year. Head Fontanelles THANK YOU