University:
Glendale Community College
Course:
BIO160 | Introduction to Human Anatomy and Physiology
Academic year:

2019
Views:

259

Pages:

21
Author:

Omezzollifm

Bone Growth and Remodeling • Bones grow and remodel throughout life – exercise or manual labor increases density and mass of bone – pulling of muscle on periosteum stimulates bone growth on that side of the bone – osteoclasts are stimulated by increased pressure – osteoblasts are stimulated by decreased pressure • Dental braces reposition teeth by creating greater pressure on the bone on one side of the tooth socket and less pressure on the other side of the tooth socket which remodels the jaw bone as the tooth moves Factors Affecting Bone Growth • 20 or more hormones, vitamins and growth factors affect bone and not all the mechanisms are known. • Bone growth is especially rapid at puberty – growth hormones and sex hormones stimulate proliferation of osteogenic cells and chondrocytes in growth plate – adolescent girls grow faster than boys and reach their full height earlier (estrogen has strong effect on bone growth) – males grow for a longer time due to sustained high levels of testosterone resulting in larger average stature • Growth ceases when epiphyseal plate “closes” – anabolic steroids may cause premature closure of growth plate producing short adult stature Achondroplastic Dwarfism • Achondroplastic dwarfism is a genetic disorder resulting in short stature but normal-sized head and trunk • Long bones of the limbs stop growing in childhood but other bones are unaffected. Gigantism • A pituitary giant can result from an excess of growth hormone during skeletal growth. 8’ tall adult Gigantism Robert Pershing Wadlow in 1938, at age 20 with actresses Maureen O’Sullivan, left, and Ann Morris. Smithsonian August 2005 p. 76. Robert Pershing Wadlow, with his mother at his side, had grown to a record height of 8’11” by the time of his death at age 22. 7 pounds at age 2 There are over 200 different types of dwarfism, all of which involve bone growth disorders (osteodysplasia) that result in short stature (adult height less than 4 ft. 10 in. tall). Primordial Dwarfism is a group of disorders in which growth is proportional but severely delayed, beginning in the womb. This results in some of the smallest people in the world. The individual pictured has Majewski osteodysplastic primordial dwarfism (MOPD) Type II. Only about 100 individuals worldwide have been identified as having MOPD type II. Both males and females of all ethnic backgrounds are affected. Some families have more than one child with MOPD Type II, which suggests that the disorder is inherited in an autosomal recessive manner. Majewski osteodysplastic primordial dwarfism type II (MOPD II): Natural History and Clinical Findings. Hall, Flora, Scott, Pauli, Tanaka, Am J. Med Genet A. 2004 Sep 15 ; 130A(1):55-72. Mineralization of Bone Matrix • Mineralization is a process in which ions (mostly calcium and phosphate) are removed from blood plasma and are deposited in the matrix of bone tissue. • Steps of the mineralization process: – osteoblasts produce collagen fibers – fibers become encrusted with minerals Image taken using a Scanning Electron Microscope showing collagen fibers produced by osteoblast cells on a polyurethane foam scaffold mimicking the bone matrix production and mineralization processes. http://www.sciencewords.net/images/main.php?g2_itemId=157 Mineral Resorption • Bone is dissolved by the osteoclast “ruffled border” and dissolved calcium and phosphate are released into the blood • pumps in the cell membrane secrete hydrogen ions into the space between the osteoclast and the bone and chloride ions follow forming hydrochloric acid (HCl) • hydrochloric acid with a pH of 4 dissolves bone minerals • an enzyme is also released that digests the collagen Calcium Balance • Calcium levels outside of normal values cause: – hypocalcemia (deficiency of blood calcium) • causes excessive excitability of nervous system by increasing the gradient for diffusion of cations out of the cell which leads to: – muscle spasms, tremors or tetany – laryngospasm may cause suffocation – hypercalcemia (excessive of blood calcium) • depresses nervous system activity by decreasing the ionic gradient across the neuron membrane • Calcium phosphate homeostasis depends on a balance of calcitriol, calcitonin and parathyroid hormone (PTH) Calcitriol (Activated Vitamin D) • UV radiation of epidermal keratinocytes converts a cholesterol derivative into Vitamin D3 (cholecalciferol) which is absorbed into the blood. • Liver and kidney enzymes convert Vitamin D3 into the hormone calcitriol. • Actions of calcitriol include: – stimulates the intestines to absorb calcium and phosphate – stimulates the kidneys to retain calcium – stimulates osteoclasts to release calcium from bone Calcitriol Synthesis and Action Calcitonin • Secreted by C cells of the thyroid gland when calcium is abundant in the blood • Functions: – reduces blood calcium levels – reduces osteoclast activity – increases the number and activity of osteoblasts • Calcitonin has a powerful effect in children, but it is less effective in adults – calcitonin deficiency is not known to cause any disease in adults, but it may be useful in reducing bone loss in osteoporosis Parathyroid Hormone (PTH) • Secreted by the parathyroid glands when the calcium level is too low in the blood. • Functions – promotes calcitriol synthesis – stimulates osteoclast multiplication and activity – inhibits collagen and bone matrix synthesis by osteoblasts – promotes calcium resorption by the kidneys – promotes phosphate excretion by the kidneys Osteoporosis • Most common bone disease • Bones lose mass and become brittle due to loss of both organic matrix and minerals – increases risk of fracture of hip, wrist and vertebral column – hump results from deformed spine • Best treatment is prevention: exercise and a recommended calcium intake of 1000 mg/day between ages 25 and 40. Effects of Osteoporosis Healing of Fractures • Normally healing takes 8 - 12 weeks (longer in elderly) • Stages of healing: see next slide 1. fracture hematoma is a blood clot resulting from broken blood vessels 2. soft callus of collagen and fibrocartilage is formed by fibroblasts and infiltrated by capillaries at site of break 3. a soft callus is gradually replaced by a hard callus of spongy bone in about 6 weeks 4. remodeling occurs over 6 months as spongy bone is replaced with compact bone Healing of Fractures 1 3 2 4 Fractures and Repairs Fibrodysplasia Ossificans Progressiva