[MUSIC PLAYING]
NARRATOR: SOME HAVE DESCRIBED
THE SKELETAL SYSTEM AS THE
INFRASTRUCTURE OF THE HUMAN
BODY, PROVIDING A RIGID
FRAMEWORK THAT OFFERS PROTECTION
AND SUPPORT, AS
WELL AS ATTACHMENT SITES FOR THE
TENDONS AND MUSCLES THAT
ARE ESSENTIAL FOR LOCOMOTION.
THE SKELETAL SYSTEM IS COMPRISED
OF 206 BONES, EACH
OF WHICH IS FORMED THROUGH A
PROCESS KNOWN AS OSSIFICATION.
BONE DEVELOPMENT AND REMODELING
TAKES PLACE ON A
CONTINUAL BASIS, FROM THE
PRENATAL PERIOD AND EARLY
CHILDHOOD THROUGH ADULTHOOD, AS
WELL AS DURING THE HEALING
PROCESS THAT OCCURS FOLLOWING
BONE FRACTURES.
OSSIFICATION CAN TAKE ONE OF
TWO FORMS, DEPENDING ON THE
TYPE OF BONE THAT IS FORMING.
IN THE CASE OF FLAT BONES,
LIKE THOSE OF THE RIBS,
SCAPULA, PELVIS, AND SKULL, THE
OSSIFICATION PROCESS IS
KNOWN AS INTRAMEMBRANOUS
OSSIFICATION.
THE PROCESS IS INITIATED BY
MESENCHYMAL STEM CELLS, WHICH
ARE CELLS THAT HAVE YET TO
BE DIFFERENTIATED INTO A
PARTICULAR CELL TYPE.
EVENTUALLY, CENTRALLY LOCATED
MESENCHYMAL CELLS WITHIN THE
EMBRYONIC FIBROUS CONNECTIVE
TISSUE DIFFERENTIATE INTO
BONE-FORMING CELLS
OR OSTEOBLASTS.
THIS PROCESS CONSTITUTES THE
FORMATION OF WHAT ARE CALLED
OSSIFICATION CENTERS.
IN THE MATRIX FORMATION PHASE
THAT FOLLOWS, THE OSTEOBLASTS
BEGIN TO LAY DOWN OSTEOID, WHICH
IS THE ORGANIC PART OF
THE BONE, CONSISTING
OF COLLAGEN FIBERS.
SOME OF THE OSTEOBLASTS ARE
ENTRAPPED WITHIN THE OSTEOID
AND ARE THEN SAID TO
BE OSTEOCYTES.
THE OSTEOID THEN CALCIFIES AND
FORMS SLENDER NEEDLE-LIKE
STRUCTURES OF SPONGY BONE
CALLED SPICULES, WHICH
AGGREGATE IN THE FORM OF SMALL
SUPPORTING BEAMS OR
TREBECULAE.
MEANWHILE, THE BLOOD VESSELS ON
THE OUTSIDE OF THE SPONGY
BONE CONDENSE AND FORM THE
PERIOSTEUM, THE FIBROUS SHEET
THAT COVERS BONE.
AS THE TREBECULAE THICKEN, AN
INTERCONNECTED NETWORK FORMS,
RESULTING IN WHAT'S CALLED
WOVEN BONE, WHICH IS
CHARACTERIZED BY A SOMEWHAT
HAPHAZARD ORGANIZATION OF
COLLAGEN FIBERS AND IS
MECHANICALLY WEAK.
EVENTUALLY, THIS LEADS TO THE
FORMATION OF LAMELLAR BONE,
AROUND A NEWLY FORMED
SPONGY BONE.
THIS LAMELLAR OR COMPACT BONE
HAS REGULAR, PARALLEL
ALIGNMENT OF COLLAGEN IN THE
FORM OF SHEETS OR LAMELLAE AND
IS MECHANICALLY QUITE STRONG.
WHILE FLAT BONES ARE FORMED
THROUGH INTRAMEMBRANEOUS
OSSIFICATION, MOST BONES IN
THE BODY, SUCH AS THE LONG
BONES OF THE LEGS AND ARMS,
ARE FORMED IN A SOMEWHAT
DIFFERENT MANNER KNOWN AS
ENDOCHONDRAL OSSIFICATION.
THIS TYPE OF BONE DEVELOPMENT
INVOLVES CARTILAGE MODELS,
WHICH ARE EVENTUALLY REPLACED
WITH BONY TISSUE.
THE PROCESS BEGINS IN THE
DEVELOPING FETUS AND CAN BE
DIVIDED INTO SEVERAL STAGES.
AROUND THE EIGHTH WEEK OF
DEVELOPMENT, CHONDROBLASTS
BEGIN SECRETING A CARTILAGINOUS
MATRIX THAT WILL
FORM THE HYALINE CARTILAGE MODEL
FOR BONE DEVELOPMENT.
AS WITH OTHER HYALINE
CARTILAGE TISSUES,
CHONDROCYTES ARE TRAPPED IN
LACUNAE AND A PERICHONDRIUM
SURROUNDS THE MODEL.
THE NEXT STAGE BEGINS WHEN
CHONDROCYTES WITHIN THE CENTER
OF THE MODEL HYPERTROPHY AND
BEGIN TO RESORB SOME OF THE
SURROUNDING CARTILAGE MATRIX.
AS THESE CHONDROCYTES ENLARGE,
THE MATRIX BEGINS TO CALCIFY
AND CHONDROCYTES BEGIN TO DIE,
AS NUTRIENTS CANNOT DIFFUSE
THROUGH THE NEWLY-FORMED
CALCIFIED MATRIX.
NEXT, STEM CELLS WITHIN THE
PERICHONDRIUM DIVIDE TO FORM
OSTEOBLASTS AND A COMPACT BONE
COLLAR IS FORMED AROUND THE
CALCIFIED CARTILAGE SHAFT.
A PERIOSTEAL BUD CONSISTING OF
CAPILLARIES AND OSTEOBLASTS
INVADES THE CORE OF THE
CARTILAGINOUS SHAFT, FORMING A
PRIMARY OSSIFICATION CENTER.
THE REMAINS OF THE CALCIFIED
CARTILAGE SERVE AS A TEMPLATE
ON WHICH OSTEOBLASTS
CAN BUILD BONE.
BONE DEVELOPMENT EXTENDS TOWARDS
THE EPIPHYSES FROM THE
PRIMARY OSSIFICATION CENTER.
MOST PRIMARY OSSIFICATION
CENTERS ARE FORMED BY THE 12TH
WEEK OF DEVELOPMENT.
THE SAME BASIC PROCESS OF
PRIMARY OSSIFICATION IS
REPEATED IN THE EPIPHYSES, AS
SECONDARY OSSIFICATION CENTERS
ARE FORMED, REPLACING THE
CALCIFIED CARTILAGE WITH
MOSTLY SPONGY BONE.
AS THE SECONDARY OSSIFICATION
CENTERS ARE FORMED,
OSTEOCLASTS RESORB BONE WITHIN
THE DIAPHYSIS, CREATING A
HOLLOW MEDULLARY CAVITY.
WHEN SECONDARY OSSIFICATION IS
COMPLETE, THE CARTILAGE IS
TOTALLY REPLACED BY BONE,
EXCEPT IN TWO PLACES.
A REGION OF CARTILAGE REMAINS
OVER THE SURFACE OF THE
EPIPHYSES AS THE ARTICULAR
CARTILAGE.
AND ANOTHER AREA OF CARTILAGE
REMAINS BETWEEN THE EPIPHYSES
AND DIAPHYSIS.
THIS IS KNOWN AS THE GROWTH
REGION OR EPIPHYSEAL PLATE AND
IS THE POINT FROM WHICH ALL
GROWTH IN BONE LENGTH OCCURS.
AT ADULTHOOD, EPIPHYSEAL
CARTILAGE PLATES ARE
COMPLETELY OSSIFIED AND
BONE GROWTH STOPS.
