DAT skeletal system

2 types of skeletons

exoskeleton - many invertebrates & all arthropods

endoskeleton - vertebrates

exoskeleton

hard covering on outer surface

- a jointed exoskeleton is found in insects & other arthropods

- made of hard chitin, which necessitates molting (ecdysis) for insect to grow

endoskeleton

vertebrate skeleton comprised of internal skeleton under soft tissue

2 major components of endoskeleton

cartilage & bone

cartilage

avascular connective tissue that is soft + flexible (more so than bone)

- distinguished from bone, which is heavily innervated by nerves and vasculature

- also found in other body parts, like joints (provides cushioning there)

how does cartilage develop?

mesenchyme tissue differentiates into chondrocytes --> chondrocytes secrete molecules that form cartilaginous matrix of collagen & proteoglycans

final cartilage is made of collagen embedded in condroitin sulfate

most abundant protein in vertebrates

collagen

what is bone?

living connective tissue that is hard + strong while also elastic & lightweight

- store Ca2+, produce blood cells, energy storage in adipose cells in bone marrow (yellow bone marrow)

- e.g. has hemapoetic stem cells --> become all types of blood cells

immature vs mature bone

immature bone = woven bone = weak, less mineralized

- first bone to form during development & in fracture repair

- eventually replaced by mature bone

mature bone = lamellar bone, strong

axial skeleton

basic framework of skeleton along central axis of body: skull, spinal column, and ribcage

- functions in protection

appendicular skeleton

bones of appendages (arms & legs), pectoral & pelvic girdles; everything that isn't in axial skeleton

- functions in movement

types of joints

immoveable (sutures) & moveable

sutures

immoveable joints (where 2 bones meet) that hold tgr bones of skull

moveable joints (name 2 types of connections)

bones that move relative to each other

- 2 types are ligaments & tendons

- diff joints for diff movement types

- e.g. ball & socket joint in shoulder, hinge joint in elbow

joints

meeting points between two or more bones

ligaments

bone-bone connectors that strengthen joints (e.g. ACL ligament)

tendon

dense connective tissue that connect muscle to bone --> bend skeleton at moveable joints

- when muscles contract, pull on bone they're attached to --> move tgr

mnemonic for ligament & tendon

ligament: ligament starts with L = like. It connects two things that are alike (bone to bone) -- can't be referring to muscle since no connector between muscles

tendon: Muscles tend to be on bone

t/f: bone forms once & remains constant thruout rest of life

FALSE

bone remodeling exists; bone is a living tissue & constantly being remodeled

older bone broken down & replaced by new bone to keep skeleton strong & durable

- when bones experience lots of force, eventually become more dense via bone remodeling

- this allows bones to build up on specific muscle attachment sites that are frequently used

3 joint types (by material bones are connected via)

1. fibrous (no movement)

- made of fibrous connective tissue

- super rigid

- sutures in skull, e.g.

2. cartilaginous (semi moveable)

- connected entirely by cartilage

- limited movement, cartilage provides shock absorption

3. synovial joints (freely moveable)

- typical joint allowing for significant movement

- filled w/ synovial fluid, lubricates joint & allows mobility

- still have cartilage b/w them just like cartilaginous, but diff type

how can muscle allow movement of bones

when muscle connects to 2 diff bones, 1 stationary & the other mobile --> allows for movement since when muscle contracts, it moves bone w/ it

origin of muscle

where muscle attaches to stationary bone that doesn't move during contraction

insertion

where muscle is attached to bone that does move

osteoprogenitor cells

bone stem cells associated w/ mesenchyme stem cell lineage; undifferentiated cells that can differentiate into osteoblasts

- serve as reservoir for osteoblasts, like when bone is remodeling or healing from fracture

osteoblasts (b for build)

responsible for building bone & eventually become part of it

- do this by secreting organic parts of bone, like collagen --> build organic bone matrix (aka the osteoid)

- as they secrete collagen, eventually get trapped within it --> differentiate into osteocytes

what happens if stress is put on bone?

bone becomes denser

osteoblasts deposit collagen & release Ca3PO42 to strengthen bone (basically hydroxyapatite)

osteocytes

most abundant type of bone cell

- helps maintain bone by exchanging nutrients & materials w/ blood

- can communicate w/ neighboring bone cells & are critical to bone homeostasis since they constantly monitor stresses on bone to regulate remodeling

***incapable of mitosis

- b/c embedded in a mineralized matrix called bone matrix, limits the space available for osteocytes to divide

osteoclasts (c for crush)

break down bone (aka bone resorption)

- derived from a WBC, monocytes (hematopoietic stem cell lineage)

- release minerals stored in bone, mostly Ca2+, back into blood

**Have multiple nuclei (multinucleated)** unique characteristic

1 thing osteoblasts & osteoclasts share

- no mitosis

hematopoietic stem cell

immature cells that can develop into all types of blood cells

spongy bone

- found within the ends of long bones

- porous; has lots of blood vessels moving thru it

- very flexible compared to compact bone

- has yellow & red bone marrow

- instead of osteons, contains trabeculae which contribute to strength and flexibility

compact bone (cortical bone)

- forms outer layer of all bones, more dense & solid; provides support to skeletal system

- composed of circular layers of bone called lamellae

- osteoblasts secrete lamellae --> become embedded later as osteocytes

- lamellae surround hollow central opening called Haversian canal --> contains nerves, blood vessels, lymphatic vessels that run thru bone

- formed by burrowing of osteoclasts

- Haversian canals are interconnected via Volkmann's canal

- osteon = structural unit of compact bone; composed of Haversian canal & surrounding lamellae layer

Periosteum is an outer membrane surrounding bones that distribute nutrients around the bone. The Volkmann's canals link the periosteum to the Haversian canals in osteons.

--

medullary cavity

compact bone surrounds medullary cavity, which is filled w/ yellow bone marrow w/ adipose cells for fat storage

how is bone remodeled / reformed

osteoclasts burrow tunnels that form haversian canals

then osteoblasts come in & lay down new matrix onto tunnel walls --> form lamellae (concentric rings)

osteocytes trapped b/w lamellae reside in lacunae & exchange nutrients via small canals, canliculi

where are osteocytes located?

within lacunae within lamellae of osteon

- can extend parts of cell outward into canaliculi to communicate w/ other osteocytes, exchange nutrients/waste, regulate bone homeostasis

long bone structure

- found in femur of upper leg

- periosteum

- endosteum

- epiphysis (proximal, distal)

- metaphysis

- epiphyseal plate

- diaphysis

spongy on inside & compact on outside = allows bone to be strong but lightweight

periosteum

periosteum = covers outside of whole bone; outermost bone layer

- contains blood vessels & nerves that supply bone tissue within

- also home to osteoprogenitor cells + osteoblasts, ready to replace old bone

endosteum

endosteum = another membrane on inside of bone, similar to periosteum

epiphysis

ends of bone are called epiphysis

- spongy bone inside but compact bone outside

- contains red bone marrow for hematopoiesis (blood cell synthesis)

- proximal epiphysis = closer to center of body; distal epiphysis = farther away

metaphysis

- very similar to epiphysis; has spongy bone & red bone marrow

epiphyseal plate

- "Growth plate" located between epiphysis and metaphysis

- source of bone growth

- when we're young (still growing), epiphyseal plate is filled w/ cartilage cells that can divide & make epiphysis longer --> grow taller

- over time, cartilage cells are replaced by bone cells --> epiphyseal plate is no longer capable of vertical growth

- at this point, becomes epiphyseal line

diaphysis

- Long hollow shaft in center of bone

- also has compact bone on outside & spongy bone inside. this spongy bone has yellow bone marrow

what is bone made of (i.e., what do osteoblasts secrete?) - 2 things

embryonic ossification -- how is bone formed? 2 ways

1. endochondral ossification

2. intramembranous ossification

endochondral ossification -- goes thru cartilage intermediate

- cartilage is replaced by bone tissue due to osteoblast & osteocytes moving into area & depositing bone tissue around them

- this is what happens after bones grow long when cartilage cells divide at epiphyseal plates --> those cartilage cells are replaced w/ bone via endochondral ossification

- this type of bone formation occurs in most bones of body -- spines, limb, hands/feet

intramembranous ossification -- no cartilage intermediate

- start w/ undifferentiated connective tissue called mesenchyme --> differentiate into osteoblast --> begin to make bone

- NO cartilage intermediate

- less common; found in flat bones of skull & clavicles

shoulder has what type of joint

ball & socket joint

knee has what type of joint

hinge joint

foramen

opening in bone that allows passage of nerves

- e.g. foramen magnum in skull allows for passage of spinal cord

osteoarthritis

when cartilage that covers bone ends of freely moveable joints begins to wear away due to aging

rheumatoid arthritis

degenerative cartilage disorder w/ genetic basis

osteoporosis

bone density decreases & bone is more prone to break & fracture

- estrogen can help maintain bone density, but increases female's risk of blood clots, heart disease, & cancer

- to prevent osteoporosis, ensure high calcium & vit D intake, regular exercise

why is vit D so important

helps the body absorb calcium, which is essential for keeping bones strong and healthy

compact bone is also called

cortical bone

spongy bone is also called

cancellous bone

Dense Regular Connective Tissue

Tissue with well-organized, parallel collagen fibers

Dense Irregular Connective Tissue

Tissue with scattered, non-parallel collagen fibers