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

2019
Views:

436

Pages:

21
Author:

Krista Conley

Spongy Bone • Spongelike appearance formed by rods and plates of bone called trabeculae – spaces among the trabeculae are filled with bone marrow • Light weight but strong – trabeculae develop along bone’s lines of stress Bone Marrow • Bone Marrow is a soft tissue that occupies the medullary cavity of long bones and the spaces among the trabeculae of spongy bone • Red Marrow – looks like thick blood – bone marrow is the adult hemopoietic tissue that produces blood cells and platelets – found in vertebrae, ribs, sternum, pelvic girdle and proximal heads of femur and humerus in adults • Yellow Marrow – fatty marrow in shaft of long bones in adults • Gelatinous Marrow – reddish, jelly-like marrow in old individuals Red Bone Marrow Two types of Bone Development • Intramembranous Ossification produces the flat bones of the skull. • Endochondral Ossification produces most of the other bones of the skeleton including the long bones of the extremities (arms and legs). The principal difference between intramembranous bone formation and endochondral bone formation is what is replaced during development. The tissue replaced during development in intramembranous ossification is embryonic CT. The tissue replaced in endrochondral ossification is hyaline cartilage. The two processes can occur side by side, and do, in the case of fracture healing and in the development of the the mandible and the clavicle which develop partially from intramembranous ossification and partially from endochondral ossification. Intramembranous Ossification • Produces flat bones of skull • Steps of the process: 1) Mesenchyme condenses into a sheet of vascular, soft tissue 2) Some mesenchymal cells develop into osteoblasts that form osteoid tissue (uncalcified bone matrix). Other mesencymal cells develop into fibroblasts that form the periosteum membrane. Calcium is deposited in the matrix and it hardens. Osteoblasts develop into osteocytes as they become surrounded by bone. 3) Bony trabeculae form spongy bone. 4) Spaces between trabeculae fill with bone in the surface layers forming compact bone that is covered with periosteum Intramembranous Ossification of the Skull Intramembranous Ossification periosteum bone periosteum This is a cross section of fetal skull bone developing by intramembranous ossification. The irregularly shaped pink areas are developing spicules of bone. Intramembranous Ossification Ob Oc HC T T Ocl ECT T HC Embryonic connective tissue (ECT) is gradually replaced by trabeculae (T) of bone that developes around blood vessels in Haversian canals (HC) . Osteoblasts (Ob) cover the developing trabecular bone (T) and become osteocytes (Oc) once completely surrounded by matrix. Multinucleated osteoclasts (Ocl) demineralize and remodel the bone matrix. Endochondral Ossification • Long Bones form from endochondral ossification as a hyaline cartilage model that is gradually replaced by bone. • Endochondral Ossification proceeds as follows: – a primary ossification center forms in the cartilage model – cartilage becomes hypertrophic (chondrocytes swell as they form the primary ossification center). – hypertrophic cartilage is removed by macrophages – primary marrow space is formed as cartilage is removed – blood vessels and nerves grow into the marrow space – osteogenic cells (mesenchymal-like fibroblasts) from the perichondrial/periosteal membrane invade the spaces created by the macrophages and transform into osteoblasts – osteoblasts deposit osteoid tissue and calcified matrix Ossification Centers • The primary ossification center forms as cartilage in the center of the cartilage model becomes hypertrophic from a growing blood supply. • Hypertrophic cartilage calcifies and dies. • Osteoclasts remove of the dead hypertrophic cartilage and then blood vessels, nerves and osteoblasts grow into that space. • The same process begins in each epiphysis forming secondary ossification centers that will develop into spongy bone. Osteoblasts in the Primary ossification center Secondary ossification center Langman’s Ossification Centers Perichondrium Zones of Ossification E Cartilage proliferates, becomes hypertrophic, calcifies, dies and is removed and is replaced with new bone tissue. Zones of Ossification A) Zone of Reserve Cartilage is a layer of resting hyaline cartilage B) Zone of Cell Proliferation is a layer of chondrocytes that multiply forming columns of flattened lacunae C) Zone of Cell Hypertrophy is a region of swollen chondrocytes D) Zone of Calcification is where the cartilage matrix mineralizes and the chondrocytes die E) Zone of Bone Deposition is where mineralized cartilage matrix is replaced by bone. Voids are filled with osteoblasts and blood vessels forming Haversian canals and osteons Secondary Ossification Centers develop in the Epiphyses Secondary Ossification Center Diaphysis Metaphysis Epiphysis Epiphyseal plate Metaphysis is cartilaginous tissue between the primary and secondary ossification centers. The epiphyseal plate (growth plate) is a temporary layer of cartilaginous tissue between the spongy bone of the medullary cavity (marrow cavity) and the spongy bone in the epiphyses that persists until growth of the bone is complete. Secondary Ossification Centers • Secondary ossification centers begin to form in the epiphyses near time of birth. • Bone development does not proceed all the way to the end of the bone. Hyaline cartilage remains over epiphysis as the articular cartilage of the joint surface. • The metaphysis is at the junction of the diaphysis and epiphysis and forms an epiphyseal plate (also called a growth plate) until growth ends. epiphyseal plate The Fetal Skeleton at 12 Weeks stained red for bone tissue. Cartilage is unstained. • Cranial ossification is incomplete at birth • Soft regions of the skull called fontanels are regions of dense irregular connective tissue. • There are usually 6 fontanels at birth • Most fontanels close by 12 months and all are usually closed by 24 months. Epiphyseal Plates