The Skeletal System: An organ system composed primarily of a variety of connective tissues. Bone makes up most of the skeleton system, but also includes.

The Skeletal System:

• An organ system composed primarily of a variety of connective tissues.

• Bone makes up most of the skeleton system, but also includes the cartilage, joints and ligaments

• make up about 20% of the total body mass of the body

Characteristics of Osseous Tissue:

• Mostly intercellular material with a few cells (about 2/3 minerals/proteins and 1/3 cells)– Matrix: rich in mineral salts. Calcium and

phosphorous are the primary minerals in osseous tissue. Fibers: collagen and elastin

– Cells: Osteocytes, Osteoblasts, Osteoclasts

Basic types of osseous tissue:

–Compact (or dense bone): dense, looks smooth, homogenous

Spongy (or cancellous bone)

• Found in interior of bone• Has a good deal of open space filled with marrow

– Trabeculae: small needle-like or flat pieces of bone that make up the interior of bone.

Trabeculae

Functions of Bones:

• Provide support, protection, movement: Framework that supports and protects soft organs, functions as levers to move the body.

• Serve as the body’s mineral bank– Can be released as ions into blood

• Serves as the body’s blood cell factory– Hematopoiesis: blood cell formation• Occurs in the marrow cavities of certain bones

• Stores fat

Classification of bone:

• The 206 bones of the skeleton are classified by their shape; long, short, flat and irregular.

Long Bones:– Considerably longer than they are wide. They

have a central shaft and 2 ends or extremities– Primarily compact bone, but may contain

substantial amounts of spongy bone.– Includes the bones of the legs, thigh, arms,

forearms, fingers, toes, bones of the hand and foot, and the collar bone.

Gross Anatomy of Long Bone

• diaphysis: The tubular central shaft of compact bone:– Surrounds the medullary cavity

• Medullary cavities in long bones contain fat (yellow marrow), with blood vessels and nerve cells.

• Epiphyses: The bone ends or extremities.– more expanded than the diaphysis– surrounded by thin layer of compact bone– interior compose of spongy bone

• In adults, the spongy bone in the head of the femur and humerus are the only areas in long bones that routinely produce new blood cells.

Epiphyseal line: remnant of the epiphyseal plate (or growth plate: where long bones

lengthen)

• Periosteum: double layered membrane covering the outer surface of the diaphysis– Outer layer: connective tissue w/ blood vessels,

lymphatic vessels and nerves– Inner layer: elastic fibers, blood vessels,

osteoblasts and osteoclasts (bone destroying cells) • Growth, development, repair of bone is initiated from

this double membrane. • Provides an insertion of anchoring point for tendons

and ligaments.

• Endosteum: Delicate connective tissue membrane covering the internal bone surfaces– Covers trabeculae of spongy bone – Lines the canals that pass through the compact

bone– Contains both osteoblasts and osteoclasts

• Articular cartilage: cartilaginous covering of the epiphyseal surfaces where long bones articulate.– Glassy textured hyaline cartilage– Cushions bone ends and absorbs stress during

joint movement.

Short Bones:

• cube-like bones whose external surfaces are covered with periosteum and internal surfaces with endosteum

– mostly spongy bone, with compact bone making up their thin surface layer.

– Sesamoid bones are a special type of short bone found embedded within a tendon. The patella is a sesamoid bone.

Flat Bones • Thin, flattened and usually slightly curved

bones; (external surfaces periosteum, internal surfaces endosteum).

• Have 2 roughly parallel compact bone surfaces with a layer of spongy bone between.

• Include the sternum, ribs and most skull bones• flat bones such as the sternum are the most

significant areas of blood cell formation.

Irregular Bones

• All have complicated shapes: bones that don’t fit into the other classes. (external surface: periosteum, internal surface: endosteum)

• mostly of spongy bone enclosed by thin layers of compact bone

• Include the vertebrae, the hip bones, and some skull bones.

• some irregular bones, such as the hip, also make blood cells.

Surface features of bone

• The external surfaces of bone are rarely smooth and featureless. They display projections, depressions, and holes.

Projections:• A.K.A. processes; help form joints or are the

sites of muscle and ligament attachment.

Processes that help form Joints

• Head: large rounded articular process• Facet: smooth, nearly flat articular surface• Condlye: rounded, knuckle-like process where

one bone articulates with another• Ramus: arm-like bar of bone

Processes that serve as sites for muscle and ligament attachment

• Neck: narrow part of bone between “head” and “shaft”• Spine: sharp, slender process• Crest: ridge of bone that is unusually narrow• Trochanter: large projection for attachment of muscles

(only example is on the femur)• Tuberosity: large, rounded projection; may be

roughened• Line: narrow ridge of bone; less prominent than a crest• Tubercle: small rounded projection or process• Epicondyle: raised area on or above a condyle

Scapula

Femur

Rib

Depressions and openings: allow blood vessels and nerves to pass

• Meatus: canal-like passageway• Sinus: cavity within a bone, filled with air and lined with

mucous membrane• Fossa: shallow, basin-like depression in a bone, often serving as

an articular surface• Fissure: narrow, slit-like opening• Foramen: round or oval opening through a bone

sinuses

Meatus

Foramen

Fissure

Microscopic Structure of Bone

• Compact Bone: – Although it appears very dense, compact bone is

riddled with canals/passageways that serve as conduits for nerves, blood vessels and lymphatic vessels

– The osteon or Haversian system is the structural unit of compact bone• Functionally each osteon is a tiny weight-bearing pillar• Each osteon is an elongated cylinder oriented along the

long axis of the bone and consists of a group of hollow tubes of bone matrix (called lamella); one place inside the next.

Osteons– All the collagen fibers in a lamella run in a single direction– Collagen fibers in adjacent lamella run in different directions.– Lamella reinforce each other and make units called osteons– A central or Haversian canal runs through the core of each

osteon; containing blood vessels and nerve fibers

– Perforating or Volkmann’s canals run at right angles to the long axis of the bone connecting the vascular and nerve supplies of the pariosteum to those of the Haversian canals and the medullary cavity

– Both the Haversian/Volkmann’s are lined with endosteum

– Osteocytes lie in small concavities called lacunae at the junctions of lamellae

– Canaliculi connect lacunae to each other and to the Haversian canal and also tie all osteocytes together, permitting nutrients and wastes to be easily relayed from one osteocyte to another.

Osteon

Spongy Bone– Consists of trabeculae • Trabeculae contain irregularly arranged lamellae and

osteocytes interconnected by canaliculi• There are no osteons: nutrients reach the osteocytes by

diffusing through the canaliculi from the marrow spaces between the trabeculae