bones

Human Skeletal System

Provides structure, protection, and movement for the body.

Bones

206 in adults; provide support, protection, and movement.

Cartilage

Flexible connective tissue that cushions joints and supports structures.

Joints

Points where bones meet, allowing movement.

Ligaments

Connect bones to other bones, stabilizing joints.

Tendons

Connect muscles to bones, enabling movement.

Bone Marrow

Found inside bones; produces blood cells and stores fat.

Osseous Tissue

Specialized connective tissue consisting of bone cells and matrix.

Osteoblasts

Build bone by secreting collagen & minerals.

Osteocytes

Maintain bone matrix, detect stress & damage.

Osteoclasts

Break down bone tissue for remodeling & calcium release.

Compact Bone

Dense, hard, smooth bone that provides strength and protection.

Spongy Bone

Porous, lightweight bone that absorbs shock and contains bone marrow.

Epiphysis

Ends of long bones filled with spongy bone for shock absorption.

Diaphysis

Shaft of the bone made of compact bone for structural support.

Medullary Cavity

Hollow center in diaphysis that stores yellow marrow.

Periosteum

Outer fibrous covering that protects bones and contains blood vessels.

Articular Cartilage

Hyaline cartilage at joint surfaces that reduces friction.

Hyaline Cartilage

Connective tissue that cushions joints and reduces friction.

Fibrocartilage

Strong connective tissue found in intervertebral discs and meniscus.

Intramembranous Ossification

Direct bone formation happening in flat bones like the skull.

Endochondral Ossification

Cartilage model replaced by bone, occurring in most bones.

Appositional Growth

Bone growth in width due to osteoblast activity in periosteum.

Osteoporosis

Condition where bone resorption outpaces formation, increasing fracture risk.

Growth Hormone (GH)

Stimulates bone lengthening.

Calcitonin

Lowers blood calcium levels by inhibiting osteoclasts.

Hormones Affecting Bone Growth

Include Growth Hormone, Parathyroid Hormone, Calcitonin, and sex hormones.

steps of intermembranous ossification

development of ossification center, calcification, formation of spongy bone, development of periosteum and compact bone

1- what happens in development of ossification center

mesenchymal cells differenciate into osteogenic cells-which become osteoblasts.-osteoblasts secrete osteoid, surround themselves with osteoid and become osteocytes when trapped in bone matrix- which leads to the signal to create more osteoblasts- leading to more bone growth.

2- what happens during calcification

nutrients (calcium and phosphate) move to soft osteoid and then accumulate and form hydroxyapatite crystals, more hydroxyapatite crystals form leading to completely hardened bone tissue with osteocytes in spaces called lacuna

3- formation of spongy bone

dense osteoid forms thin fibers called trabeculae, trabeculae connect and form spaces in between bone. blood vessels then fill those cavities as well as red bone marrow

4- creation of periosteum and compact bone.

mesenchymal cells gather and become dense at the outer layer of the bone-they become specialized cells, some of which turn into osteoblasts which then secrete osteiod. this forms periosteum. osteoblasts then secreting osteiod onto the surface of the bone until it forms spongy bone, then it forms compact bone with more build-up

what bones are formed from intermembranous ossification

flat bones. skull, clavicle, face.

how does bone grow in intermembraneous ossification

directly from mesenchymal cells, bone grows outward from ossification centers, centers expands and form bone. spongy bone remains in center compact bone forms outer layer. periosteum supports bone remodeling.

steps of endochondral ossification

formation of cartilage model, - growth of embryo cartilage model- development of primary ossification center - formation of medullar cavity, development of secondary ossification centers, bone growth at epiphyseal plate - epiphyseal plate closure

1- formation of cartilage model

mesenchymal cells turn into chondrocytes - chondrocytes secrete cartilage matrix (collagen and proteins) - then turns into soft cartilage in shape of future bone

2-carilage model grows

chondrocytes divide and expand making cartilage longer - central chondrocytes enlarge and trigger calcification- calcification leads to death of chondrocytes because it blocks oxygen and nutirents- this leaves cavities in cartilage

3- primary ossification center

blood vessels enter capillaries, osteoblasts arrive and replace cartilage with bone, osteoclasts shape bone, - bone grows from growth plate from diaphysis to epiphsyis

4- medullar cavity forms

osteoclasts remove inner bone tissue in diaphysis,- this creates the medullar cavity which fills with bone marrow

5- secondary ossification center

osteoblasts enter epiphysis and change cartilage to bone, some cartilage remains, articluar cartilage, compact bone forms outer lining

6- bone growth at epiphyseal plate

chondrocytes continue dividing, bone grows longer, osteoblasts replace old cartilage with bone, bone pushes epiphyseal plate away

epiphyseal plate closure

hormones signal bones to stop growing cartilage is completely replaced by bone,

interstitial growth

lengthwise growth of bone.

where does interstitial growth occur

epiphyseal plate of long bones

appositional growth

width growth of bone

where does appositional growth occur

surface of bones, osteoblasts in periosteum lay down bone layers increasing thickness

hormones responsible for bone growth

growth hormone- parathyroid hormone, calcitonin, growth hormone, thyroid hormone, sex hormones

calcitonin vs parathyroid

calcitonin- keeps in parathyroid- pulls out

1- hematoma formation

few hours after fracture, blood vessels break causing bleeding and clots (hematoma) stops bleeding and stabilizes fracture , triggers inflammation and brings cells to clean up dead tissue

2- fibrocartilage callus

few days to 2-3 weeks after, fibroblasts + chondroblasts enter hematoma , creates soft cartilage bridge, holds bone together but isnt very strong.

3- bony callus

3 weeks- several months. osteoblasts replace soft tissue/ callus with spongy bone (bony callus), strengthens fracture site but bone not fully restored

bone remodeling

(months- years) osteoclasts remove excess bone. osteoblasts rebuild stronger, organized bone, bone adapts to stress (wolffs law) to restore original bone

axial skeleton

80 bones- support. skull, spine, rib cage, sternum

appendicular skeleton

126 bones- movement. upper limbs, lower limbs, pectoral girdle, pelvic girdle