Histology, Development, & Growth of Bone Histology of bone Bone development Bone growth Role of bone in calcium homeostasis
Jan 03, 2016
Histology, Development, & Growth of BoneHistology, Development, & Growth of Bone
Histology of bone
Bone development
Bone growth
Role of bone in calcium homeostasis
Review of bone compositionReview of bone composition
Bone: 35% organic, 65% inorganic
- organic component is collagen: gives flexible strength
- inorganic component is calcium and phosphate
- calcium phosphate interacts with calcium hydroxide to form hydroxyapatite crystals
- gives compression strength
Result: strong, yet flexible structure
Bone Cells Bone Cells
Bone cells: osteoblasts, osteocytes, osteoclasts
- osteocytes are located in lacunae
- osteocytes are connected to each other by thin cytoplasmic projections with gap junctions
- the cell processes are enclosed in canaliculi
Types of bone: spongy Types of bone: spongy
Cancellous (spongy) bone- Cancellous bone consists of network of trabeculae (lighter than compact bone)- Trabeculae do not have direct blood supply, osteocytes receive nutrition through diffusion via canaliculi- Trabeculae are lined by osteoblasts- The orientation of trabeculae is influenced by stress on the bone- Found in areas without much bone stress, or where stress comes from several directions- Site of blood cell formation (red marrow)
Types of bone: compact Types of bone: compact
Compact bone- More dense than
spongy bone - Has a direct vascular
supply in matrix- osteons- circumferential lamellae
veincapillary
osteonsinterstitiallamellae
spongy boneperforatingcanal
central canal
Bone development (ossification) Bone development (ossification)
Ossification: the formation of bone by osteoblasts Two types: intramembranous and endochondral Intramembranous:- type of bone formation in bones of skull and clavicle- start with connective tissue membrane containing
osteoprogenitor cells- osteoblasts form, produce trabecullae (spicules) of woven
bone at centers of ossification
Bone formation Bone formation
Intramembranous (cont.): - cancellous bone forms as trabecullae radiate out of centers
of ossification- periosteum forms around bone- outer layer of compact bone is formed - as process continues, woven bone is removed and replaced
by mature bone
Growth of Bones Growth of Bones
There are two types of bone growth: endochondral and appositional
Endochondral growth: growth of cartilage, which is replaced by bone.
Endochondral Bone GrowthEndochondral Bone Growth
The epiphyseal plate is organized into four zones:
a) zone of resting cartilage (epiphyseal surface): chondrocytes don't divide rapidly
b) zone of proliferation: densely packed columns of chondrocytes, rapidly dividing
c) zone of hypertrophy: chondrocytes mature and enlarge
d) zone of calcification: thin layer of calcified matrix, chondrocytes die, vessels grow into the area bringing in osteoblasts, which form bone.
Endochondral Bone GrowthEndochondral Bone Growth
zone of resting cartilage
zone of proliferation
zone of hypertrophy
zone of calcification
Growth of Bones Growth of Bones
Appositional bone growth
- responsible for increased diameter (not length) of bones
- osteoblasts add matrix to surface of bone: rate varies
- osteoclasts remove bone at fixed rate
Growth of Bones Growth of Bones
Bone growth is influenced by many factors- nutrition: calcium availability & collagen synthesis
- calcium intake- role of vitamin D in calcium absorption- vitamin C required for collagen synthesis (scurvy)- vitamin A stimulates osteoblast activity
- stimulated by hormones - growth hormone
- thyroid hormone- estrogen and testosterone
- stimulated by weight-bearing stress/exercise
Role of Bone in Calcium Homeostasis Role of Bone in Calcium Homeostasis
Calcium plays important roles in:- bone formation- controlling permeability of cell membranes- contraction of muscle cells- hormone action (second messenger signaling)
Sources of calcium:- diet (main source)- bone reservoir
Regulation of bone calcium Regulation of bone calcium
Parathyroid hormone (PTH)- from parathyroid glands- reduces loss of calcium from the kidneys, small intestine- causes kidney to activate vitamin D- increases osteoclast activity- PTH levels increase in response to decreased blood Ca++
Calcitonin- from the thyroid gland- decreases osteoclast activity (calcium returned to bone)-increasing the rate of excretion of calcium ions at the kidneys- calcitonin levels increase in response to increased blood calcium levels