Human Anatomy & Physiology I Skeletal System Human Anatomy & Physiology I

Introduction Bone tissue is a complex and dynamic living tissue. It continually engages in a process called remodeling – building new bone tissue and breaking down old bone tissue. Bone is made up of several different tissues working together: bone, cartilage, dense connective tissues, epithelium, adipose tissue, and nervous tissue.

Support The skeleton serves as the structural framework for the body by supporting soft tissues and providing attachment points for the tendons of most skeletal muscles.

Protection The skeleton protects many internal organs from injury. For example, cranial bones protect the brain, vertebrae protect the spinal cord, and the ribs protect the heart and lungs.

Movement Because skeletal muscles attach to bones, when muscles contract, they pull on bones. Together, bones and muscles produce movement.

Mineral homeostasis Bone tissue stores several minerals, especially calcium and phosphorus. On demand, bone releases minerals into the blood to maintain critical mineral balance (homeostasis) and to distribute the minerals to other parts of the body.

Blood cell production Within certain bones, a connective tissue called red bone marrow produces red blood cells, white blood cells, and platelets, a process called hemopoiesis.

Triglyceride storage Triglycerides stored in the adipose tissue of yellow bone marrow are an important chemical energy reserve.

Histology Compact and spongy bone Osteons Bone cells: osteoblast, osteoclast, osteocytes Hematopoetic Tissue Chemical Composition of Bone

Compact bone tissue Contains few spaces! It forms the external layer of bones and makes up the bulk of the diaphyses of long bones. Provides protection and support and resist the stresses produced by weight and movement.

Osteon Compact bone is arranged in units called osteons or Harvesian systems. Osteons in compact bone tissue are aligned in the same direction along lines of stress.

Spongy bone Spongy bone does NOT contains osteons. It consist of trabeculae, an irregular latticework of thin columns of bone. The macroscopic spaces between the trabeculae of some bones are filled with red bone marrow. Within each trabeculae are osteocytes that lie in lacunae.

Bone cells Osteogenic cells Osteoblasts Osteocytes Osteoclasts

Bone cells Osteogenic cells: The only bone cell that undergo division; the resulting daughter cells develop into osteoblasts. They are going to be found along the inner portion of the periosteum, in the endosteum, and in the canals within the bone that contain blood vessels.

Bone cells Osteoblasts: (buds or sprouts) are bone building cells. They synthesize and secrete collagen fibers and other organic components needed to build the matrix of bone tissue, and they initiate calcification. As osteoblasts surround themselves with matrix, they become trapped in their secretion and become osteocytes.

Bone cells Osteocytes: mature bone cells, are the main cells in bone tissue and maintain its daily metabolism, such as exchange of nutrients and wastes with the blood. Like osteoblasts, osteocytes do not undergo cell division.

Bone cells Osteoclasts: (clast = break) are huge cells derived from the fusion of as many as 50 monocytes (a type of white blood cell) and are connected to the endosteum. One the side of the cell that faces the bone surface, the osteoclast’s plasma membrane is deeply folded into a ruffled border.

Ruffled border Here the cell releases powerful lysosomal enzymes and acids that digest protein and mineral components of the underlying bone matrix. This breakdown of bone matrix, termed resorption, is part of the normal development, growth, maintenance, and repair of bone.

Bone cells

Hematopoetic Tissue Red bone marrow is a highly vascularized tissue located in the microscopic spaces between the trabeculae of spongy bone.

Red bone marrow Consists of developing blood cells, adipocytes, fibroblasts, and macrophages within a network of reticular fibers. It is present in developing bones of the fetus and in some adults bones (i.e. pelvis, ribs, skull, vertebras, epiphyses of long bones).

Red bone marrow

Bone markings: Projections & Depressions

Long & Short

Irregular & Flat

Sesamoid & Wormian

Typical Long Bone Epiphysis & Diaphysis Bone Marrow Periosteum & endosteum Articular cartilage Medullary Cavity Epiphyseal Plate or Line

Bone formation The process by which bone forms is called ossification or osteogenesis. Two (2) types: Intramembranous Endochondral

Bone formation In the first type of ossification, called intramembranous ossification, bone forms directly on or within loose fibrous connective tissue membranes. In the second type, endochondral ossification, bone forms within hyaline cartilage.

Intramembranous ossification Development of the center of ossification Calcification Formation of trabeculae Development of the periosteoum

Intramembranous Ossification Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Mesenchymal cell Osteocyte Osteoblasts Sheet of condensing mesenchyme Trabecula Calcified bone Osteoid tissue Blood capillary Fibrous periosteum 1 Condensation of mesenchyme into soft sheet permeated with blood capillaries 2 Deposition of osteoid tissue by osteoblasts on mesenchymal surface; entrapment of first osteocytes; formation of periosteum Osteoblasts Fibrous periosteum T rabeculae Osteoblasts Osteocytes Spongy bone Marrow cavity Compact bone Figure 7.8 3 Honeycomb of bony trabeculae formed by continued mineral deposition; creation of spongy bone 4 Surface bone filled in by bone deposition, converting spongy bone to compact bone. Persistence of spongy bone in the middle layer. produces flat bones of skull and clavicle 7-31

Intramembranous Ossification Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Periosteum: Fibrous layer Osteogenic layer Osteoid tissue Osseous tissue (bone) Osteoblasts Osteocytes © Ken Saladin Figure 7.9 note the periosteum and osteoblasts. 7-32

Endochondral ossification Development of the cartilage model Growth of the cartilage model Development of the primary ossification center Development of the secondary ossification centers Formation of articular cartilage

Stages of Endochondral Ossification Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Articular cartilage Spongy bone Epiphyseal line Perichondrium Hyaline cartilage Secondary marrow cavity Periosteum Epiphyseal plate 1 Early cartilage model Epiphysis Nutrient foramen Secondary ossification center Marrow cavity Metaphysis Compact bone Enlarging chondrocytes Blood vessel Diaphysis Bony collar Primary marrow cavity Primary ossification center Metaphysis Secondary ossification center Periosteum Cartilage 2 Formation of primary ossification center, bony collar, and periosteum 3 Vascular invasion, formation of primary marrow cavity, and appearance of secondary ossification center 4 Bone at birth, with enlarged primary marrow cavity and appearance of secondary marrow cavity in one epiphysis 5 Bone of child, with epiphyseal plate at distal end 6 Adult bone with a single marrow cavity and closed epiphyseal plate Figure 7.10 7-34

Bone homeostasis Bone is the body’s major calcium reservoir, storing 99% of total body calcium. The level of calcium in the blood can be regulated by controlling the rate of calcium resorption from the bone into the blood and the calcium deposition from blood into bone.

Calcium homeostasis Parathyroid hormone (PTH) Calcitriol Calcitonin (CT) Just a note: Miacalcin (calcitonin harvested from salmon) is an effective drug for the treatment of osteoporosis because it slows bone resorption!

© Robert Calentine/Visuals Unlimited Design of Spongy Bone Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Greater trochanter Head Trabeculae of spongy bone Compact bone Lines of stress Shaft (diaphysis) Figure 7.6 7-37 © Robert Calentine/Visuals Unlimited

Bone growth vs. bone loss

Fractures that involves the growth plate!