Connective tissue, bone anatomy and biomechanics, and calcium homeostasis

Reticular fibers

Thin, randomly-oriented collagen strands forming a delicate supportive mesh

Collagen

Triple-helix protein; most abundant protein in the body; provides tensile strength, flexibility, ductility, and toughness

Elastin

Protein that allows tissue to stretch and then recoil back to its original shape

Ehlers-Danlos Syndrome (EDS)

Disorder caused by abnormal collagen, making tissue MORE elastic and LESS able to resist deformation; affects skin, joints, vessels, organs; no cure

Tendon

Dense regular connective tissue attaching bone to muscle

Ligament

Dense regular connective tissue attaching bone to bone

Loose connective tissue

Connective tissue with fewer protein fibers and more ground substance (e.g., adipose tissue)

Blood

Fluid connective tissue with a liquid matrix; contains erythrocytes, leukocytes, platelets, and plasma (incl. albumin)

Brown Adipose Tissue

Specialized fat that generates heat (thermogenesis) via UCP1 in mitochondria, activated by norepinephrine; critical in infants

Uncoupling Protein 1

Mitochondrial protein in brown fat that uncouples the electron transport chain from ATP synthesis, releasing energy as heat instead

Skeletal muscle

Striated, voluntary, multinucleated muscle attached to the skeleton

Cardiac muscle

Striated, involuntary muscle with single nucleus, branching Y-shaped cells joined by intercalated discs

Smooth muscle

Non-striated, involuntary muscle with single nucleus, short fusiform cells; found in walls of internal organs

Intercalated discs

Strong gap-junction-containing connections joining cardiac muscle cells, enabling rapid electrical conduction

Neuron

Nervous tissue cell that initiates and conducts electrical activity

Neuroglia

Support cells of nervous tissue

Dendrite

Neuron structure that receives input signal

Axon

Neuron structure that conducts signal away from cell body to the axon terminal

Osteoblast

Bone cell responsible for depositing/building new bone matrix

Osteoclast

Bone cell responsible for resorbing (breaking down) bone matrix

Osteocyte

Mature from osteoblasts and reside in the lucnae of most bones.

Hydroxyapatite

Mineral ground substance of bone $\text{Ca}_{10}(\text{PO}_4)_6(\text{OH})_2$; provides strength, stiffness, elasticity, hardness

Cortical bone (compact)

Dense, solid outer layer of bone forming the external layer of long and flat bones

Cancellous bone (spongy)

Open interior lattice of bone made of trabeculae; behaves mechanically like an elastic beam

Osteon (Haversian system)

Functional unit of compact bone; concentric cylinders of lamellae around a central (Haversian) canal, running parallel to the bone's long axis

Lamellae

Concentric, hollow tubes of bone matrix that make up an osteon

Haversian canal

Central channel of an osteon carrying blood vessels and nerves

Lacunae

Small spaces within bone matrix that house osteocytes

Canaliculi

Tiny channels connecting osteocytes to each other and to the central canal

Diaphysis

Elongated, cylindrical shaft of a long bone

Epiphysis

Knobby region at each end of a long bone; site of tendon/ligament attachment

Metaphysis

Region of a long bone between the diaphysis and epiphysis; site of the growth plate

Epiphyseal plate/line

The "growth plate" of a long bone; at the end of a long bone

Articular cartilage

Thin hyaline cartilage on epiphyseal surfaces that reduces joint friction

Medullary cavity

Hollow space in the diaphysis containing yellow marrow (fat) in long bones or red marrow (blood cell production) in flat bones; the central space.

Periosteum

Dense irregular connective tissue covering the external surface of bone (except at joints); anchors tendons/ligaments via perforating fibers; the outer membrane of the bone basically

Endosteum

Vascular connective tissue membrane covering the internal surface of bone

Stress

Force per unit area ($\text{Force}/\text{Area}$); "load" or "pressure" applied to a material

Strain

Change in length divided by original length ($\Delta L / L_0$); "deformation" of a material

Elasticity

Ability of a material to return to its original shape after a force is removed

Plasticity

Propensity of a material for non-reversible (permanent) deformation

Yield point

Point on a stress-strain curve where a material shifts from elastic to plastic deformation

Failure point

Point on a stress-strain curve where a material fractures or buckles

Stiffness

Resistance to deformation; steeper slope = more of this

Flexibility

Readiness to deform; a flatter stress-strain slope means more of this

Strength

Amount of stress a material withstands and still recovers its shape; determined by the yield point

Brittle

Material property showing little plastic deformation before failure (e.g., glass)

Ductile

Material property showing a large plastic deformation region before failure (e.g., copper)

Toughness

Ability to absorb energy before failure; equals the area under the stress-strain curve

Young's Modulus

The slope of the elastic region of a stress-strain curve; a measure of stiffness

Hysteresis

Historical dependence of a material's behavior; whether bone returns to original shape depends on whether loading stayed in the elastic or plastic region

Anisotropy

bone is strongest along its long axis and weakest transverse to it

Spiral fracture

Bone fracture pattern caused by a twisting (shear/torsional) force

Transverse fracture

Bone fracture pattern caused by axial (straight compression) loading

Oblique fracture

Bone fracture pattern caused by offset axial + bending loading; the fracture/bend happens at an angle

Butterfly fracture

Bone fracture pattern caused by opposing transverse (bending) forces

Viscoelasticity

Property where a material's mechanical behavior is time/rate-dependent; Quick forces make it act more rigid, while slow forces allow it to flow or stretch more

Cartilage

Semi-rigid, avascular connective tissue that is flexible and provides support; less strong than bone but more resilient

Fibroblast

Cell that secretes collagen types I and III

Chondroblast

Dividing cartilage cell that begins matrix production

Chondrocyte

Mature cartilage cell that continues matrix production and sits within a lacuna, largely secreting collagen II

Hyaline cartilage

Most abundant cartilage type; translucent with no visible fibers; found in respiratory tree, joints, fetal skeleton; has a perichondrium

Fibrocartilage

Cartilage type with thick collagen fibers acting as a shock absorber; found in knee joint, vertebral discs, pubic symphysis; NO perichondrium

Elastic cartilage

Least abundant cartilage type; highly branched elastic fibers; found in the ear and epiglottis; has a perichondrium

Interstitial growth

Cartilage growth from within, via chondrocyte mitosis inside lacunae, each daughter cell secreting its own matrix

Appositional growth

Cartilage growth along the periphery (perichondrium), via stem cells differentiating into new chondroblasts that secrete matrix, making cartilage thicker

Perichondrium

Connective tissue membrane surrounding cartilage (present in hyaline and elastic cartilage, absent in fibrocartilage)

Calcitonin

Hormone released by parafollicular (C) cells of the thyroid in response to HIGH plasma calcium; lowers blood calcium by decreasing bone resorption and increasing renal calcium excretion

Parathyroid hormone (PTH)

Hormone released by chief cells of the parathyroid in response to LOW plasma calcium; raises blood calcium by increasing bone resorption, renal calcium reabsorption, and (via calcitriol) intestinal calcium absorption

Parafollicular (C) cells

Cells of the thyroid gland that release calcitonin in response to high plasma calcium

Chief cells

Cells of the parathyroid gland that release parathyroid hormone in response to low plasma calcium

Calcitriol

Active form of vitamin D, stimulated by PTH, that increases intestinal calcium absorption

Hypocalcemia

Condition of low plasma calcium (with high phosphate); can follow parathyroidectomy; causes muscle hyperexcitability/tetany

Osteoporosis

Condition of long-term imbalance favoring high osteoclast activity over bone deposition; results in loss of bone mass, weaker/less tough bone