Skeletal System

The skeleton is easy to dismiss as scaffolding, but for the MCAT it is a dynamic, hormonally regulated organ with roles in mechanics, mineral homeostasis, and blood cell production. This guide builds from why bone exists, to how it is built, to how the body controls it.

Priority labels: Must know = cold; Know the logic = mechanism not names; Passage-level = recognize, don't memorize; Optional = skippable.

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Functions of the Skeletal System

Must know

The skeletal system serves five functions (mnemonic SPMM-H):

1. Structural support – rigid framework against which muscles pull.
2. Protection – shields organs (skull → brain; ribs/sternum → heart and lungs; vertebrae → spinal cord).
3. Movement – bones act as levers; joints are fulcrums; muscles attach via tendons.
4. Mineral storage – the body's primary reservoir of calcium ($\ce{Ca^{2+}}$) and phosphate ($\ce{HPO4^{2-}}$), mobilized or deposited by hormonal signals.
5. Hematopoiesis – red bone marrow (in spongy bone of flat bones and epiphyses) houses hematopoietic stem cells that make all blood cells: erythrocytes, leukocytes, and platelets. This integrates the skeleton with the circulatory and immune systems — a frequent passage theme.

Passage-level

Bone also buffers blood pH slowly, releasing alkaline carbonate/phosphate salts to soak up excess $\ce{H+}$ when blood is acidic.

Quick check: A patient with multiple myeloma has plasma cells destroying spongy bone. Which skeletal function is most directly threatened beyond bone integrity itself?

Answer: Hematopoiesis — spongy bone contains red marrow, and its destruction leads to anemia, thrombocytopenia, and leukopenia.

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Endoskeleton vs. Exoskeleton

Must know

An exoskeleton (arthropods) is a hard external chitin covering: protective and rigid, but acellular, cannot remodel, and must be shed by molting (ecdysis) to grow.

An endoskeleton (vertebrates, echinoderms) is an internal mineralized structure surrounded by soft tissue. Because it is vascularized and made of living cells, it grows continuously, remodels in response to mechanical stress (Wolff's law), and is endocrine-regulated. That bone is living tissue is the conceptual cornerstone.

Quick check: Why can an endoskeleton respond to hormonal calcium demands whereas a chitin exoskeleton cannot?

Answer: Bone is vascularized and contains living cells (osteoclasts, osteoblasts) that dissolve or deposit mineral. Chitin is an acellular polysaccharide with no cellular machinery for remodeling or mineral mobilization.

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Skeletal Structure: Specialization of Bone Types

Axial vs. Appendicular Skeleton

Must know

• Axial skeleton — the central core: skull, vertebral column, rib cage (ribs + sternum). Role: protection and support.
• Appendicular skeleton — the limbs and girdles (pectoral and pelvic) that anchor them. Role: movement.

Classification by Shape

Know the logic

• Long bones (femur, humerus): leverage for movement — the most-tested type.
• Short bones (carpals, tarsals): gliding, weight distribution.
• Flat bones (skull, scapula, ribs): protection, muscle-attachment surface, red marrow.
• Irregular bones (vertebrae): mixed functions.

Regions of a Long Bone

Must know

• Diaphysis: shaft; compact bone around a central medullary cavity (yellow marrow in adults).
• Epiphysis: rounded ends; spongy bone under a thin compact shell.
• Metaphysis: between the two; in growing bone holds the epiphyseal plate (growth plate), hyaline cartilage where longitudinal growth occurs. After puberty it closes to the epiphyseal line.
• Articular cartilage: hyaline cartilage on joint surfaces; reduces friction.
• Periosteum: fibrous outer sheath with osteogenic cells, nerves, and blood supply; tendons/ligaments anchor here.
• Endosteum: thin membrane lining inner bone surfaces; also has osteogenic cells.

Quick check: A fracture that disrupts the periosteum will impair which two processes most directly?

Answer: Bone repair/remodeling (periosteal osteogenic cells become osteoblasts) and pain sensation/vascular supply (periosteum is richly innervated and vascularized).

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Bone Structure: Compact and Spongy Bone

Must know

Compact (cortical) bone forms the outer shell and diaphysis walls. Its unit is the osteon (Haversian system) — concentric rings of matrix around a central canal carrying blood vessels and nerves, with osteocytes embedded in the matrix.

Spongy (trabecular) bone lacks osteons. It is a lattice of trabeculae oriented along lines of stress — lightweight yet strong — with marrow in the spaces. Osteocytes here get nutrients by diffusion from marrow.

Quick check: What is the functional advantage of trabeculae aligning along lines of stress (Wolff's law)?

Answer: This orientation maximizes resistance to the forces the bone normally experiences while minimizing mass — implemented through continuous remodeling driven by mechanical loading.

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Calcium–Protein Matrix of Bone

Must know

Bone is a composite whose two components cover each other's weaknesses:

• Organic (~35%): type I collagen → tensile strength and flexibility. Without it, bone is brittle.
• Inorganic (~65%): hydroxyapatite, $\ce{Ca10(PO4)6(OH)2}$ → compressive strength and hardness. Without mineral, bone is rubbery.

Analogy: reinforced concrete (collagen = steel rods; hydroxyapatite = concrete).

Osteoid is newly synthesized, unmineralized organic matrix. When osteoid fails to mineralize, bones become soft — rickets (vitamin D deficiency in children) and osteomalacia (adults).

Quick check: A patient has a defect in type I collagen synthesis. Would you expect their bones to fracture more easily under tension, compression, or both — and why?

Answer: Primarily tension (and bending/twisting forces that generate tension), because collagen provides tensile strength while hydroxyapatite provides compressive strength. Collagen defects (osteogenesis imperfecta) make bone brittle, fracturing under minor force.

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Cellular Composition of Bone

Must know

Four cell types govern bone biology:

Cell	Origin	Function
Osteogenic cells	Mesenchymal stem cells	Divide → osteoblasts (in periosteum/endosteum)
Osteoblasts	Osteogenic cells	Secrete osteoid, initiate mineralization; become osteocytes
Osteocytes	Trapped osteoblasts	Maintain matrix; mechanosense; communicate via canaliculi
Osteoclasts	Monocyte/macrophage (hematopoietic)	Resorb bone via acid and enzymes; multinucleated

The balance of osteoblast formation vs. osteoclast resorption sets bone mass; PTH, calcitonin, vitamin D, and estrogen tip it (endocrine section below).

Bone remodeling is continuous: osteoclasts resorb old bone, then osteoblasts refill the cavity — repairing microfractures and adjusting architecture to mechanical demands.

Quick check: Why would an osteoclast-activating factor (as in multiple myeloma) cause hypercalcemia?

Answer: Osteoclasts resorb matrix, releasing stored calcium into blood. Excess activity outpaces renal excretion, raising serum $\ce{Ca^{2+}}$.

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Ossification: How Bone Forms

Know the logic

• Intramembranous ossification: mesenchyme → osteoblasts directly, no cartilage intermediate. Forms the flat bones of the skull (and the clavicle, an exception).
• Endochondral ossification: a hyaline cartilage model is laid down first, then replaced by bone (cartilage does not "turn into" bone). This builds long bones and most of the skeleton, and continues at the epiphyseal plate for longitudinal growth. Growth hormone drives chondrocyte proliferation; sex hormones (estrogen) eventually close the plate at puberty.

Quick check: A child has a mutation causing premature fusion of the epiphyseal plates. What is the expected outcome, and which hormone likely drove it?

Answer: Premature closure → shortened stature. Excess sex steroids (estrogen/androgens), as in precocious puberty or exogenous steroids.

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Cartilage: Structure and Function

Must know

Cartilage is avascular, alymphatic, and aneural. Because it lacks blood vessels, chondrocytes rely on diffusion — so cartilage heals slowly and poorly.

Know the logic

• Hyaline (type II collagen): the most abundant; glassy. Articular surfaces, epiphyseal plates, costal cartilage, trachea/larynx/nose. Smooth movement and the endochondral template.
• Fibrocartilage (abundant type I collagen): toughest, least compressible. Intervertebral discs, pubic symphysis, knee menisci. Shock absorption under compression/shear.
• Elastic (type II + elastic fibers): flexible and resilient. External ear, epiglottis. Holds shape while bending.

Quick check: Why does articular (hyaline) cartilage damaged in osteoarthritis fail to self-repair effectively?

Answer: It is avascular — chondrocytes have no blood supply to deliver progenitor cells or growth factors fast enough for repair.

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Joint Structures

Must know

A joint is a point of contact between bones. Classified by structure and movement.

Structural classes:

• Fibrous joints: dense fibrous tissue, immovable. E.g., cranial sutures.
• Cartilaginous joints: joined by cartilage, slightly movable. E.g., pubic symphysis, intervertebral discs.
• Synovial joints: fluid-filled cavity, freely movable. Most limb joints.

Synovial joint anatomy: a fibrous joint capsule; a synovial membrane secreting synovial fluid (hyaluronic-acid-rich, lubricates and nourishes articular cartilage); articular cartilage on the bone ends; and reinforcing ligaments. Some also have bursae (cushion friction) and menisci (fibrocartilage pads improving fit, as in the knee).

Passage-level

Functional types: ball-and-socket (hip/shoulder, multiaxial), hinge (elbow/knee, flexion/extension), pivot (C1–C2, rotation), plus saddle, gliding, and condyloid. Recognize, don't memorize the full table.

Know the logic

Bones as levers: bones are levers pivoting at joints (fulcrums), so the MCAT pairs the skeleton with torque. Three classes by the order of fulcrum (F), effort (E), load (L): first-class (E–F–L, seesaw), second-class (F–L–E, gives mechanical advantage, e.g., tiptoe), third-class (F–E–L, most common in the body, trades force for speed/range, e.g., biceps flexing the forearm).

Quick check: A patient dislocates their shoulder (glenohumeral joint). What type of joint is this, and why is it more prone to dislocation than the hip?

Answer: Both are ball-and-socket, but the shallow glenoid fossa makes the shoulder depend on muscles (rotator cuff) and ligaments rather than bony congruity (the hip has a deep acetabulum) — making it the most commonly dislocated joint.

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Ligaments and Tendons

Must know

• Tendons connect muscle to bone, transmitting contractile force. Mostly type I collagen in parallel bundles (maximal tensile strength along the pull axis). Poor blood supply → slow healing.
• Ligaments connect bone to bone, stabilizing joints and limiting range of motion. Also mostly type I collagen but interwoven (resist multidirectional force), with more elastic fibers than tendons.

Mnemonic: Ligament = Links bones; Tendon = muscle To bone.

Quick check: An ACL tear heals poorly compared with a muscle strain. What structural feature explains this?

Answer: Ligaments are poorly vascularized; the ACL is especially avascular (fed mainly by synovial fluid diffusion), so healing is slow and often requires surgical reconstruction.

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Endocrine Control of Bone and Calcium Homeostasis

Must know

Blood calcium (~8.5–10.5 mg/dL) is tightly regulated because $\ce{Ca^{2+}}$ is essential for muscle contraction, nerve conduction, and clotting. This is a classic negative feedback loop sensed by the parathyroid's calcium-sensing receptor (CaSR).

Parathyroid Hormone (PTH)

Must know

Secreted by parathyroid chief cells when serum calcium falls; the primary calcium-raising hormone. Three actions:

1. Bone: stimulates osteoclast resorption → releases $\ce{Ca^{2+}}$ and phosphate.
2. Kidney: ↑ calcium reabsorption, ↓ phosphate reabsorption (phosphaturia), and activates vitamin D.
3. Intestine (indirect): via vitamin D, ↑ calcium absorption.

Net effect: serum calcium rises, serum phosphate falls.

Vitamin D (Calcitriol)

Know the logic

Activated in steps: skin (UV → cholecalciferol, D₃) → liver (→ calcidiol) → kidney (1α-hydroxylase, PTH-stimulated → calcitriol, the active form). Its main action: ↑ intestinal absorption of calcium and phosphate; it also supports bone mineralization. Deficiency → rickets (children) / osteomalacia (adults).

Calcitonin

Must know

From parafollicular C cells of the thyroid when serum calcium is high. Opposes PTH: inhibits osteoclasts → lowers serum $\ce{Ca^{2+}}$.

Passage-level

Minor in adult humans physiologically, but used pharmacologically (salmon calcitonin) for osteoporosis/Paget's.

Other Hormones

Must know

Estrogen inhibits osteoclasts and preserves bone mass — its loss after menopause drives osteoporosis. Growth hormone (via IGF-1) stimulates osteoblasts and growth-plate chondrocytes. Glucocorticoids inhibit osteoblasts and GI calcium absorption → secondary osteoporosis.

Calcium Homeostasis: The Big Picture

Must know

$\underbrace{\text{Low } [\text{Ca}^{2+}]}_{\text{stimulus}} \rightarrow \underbrace{\text{PTH}^{\uparrow}}_{\text{parathyroid}} \rightarrow \begin{cases} \text{Bone resorption} \uparrow \\ \text{Renal Ca}^{2+} \text{reabsorption} \uparrow \\ \text{Vitamin D} \uparrow \rightarrow \text{GI absorption} \uparrow \end{cases} \rightarrow [\text{Ca}^{2+}] \text{ restored}$

$\underbrace{\text{High } [\text{Ca}^{2+}]}_{\text{stimulus}} \rightarrow \underbrace{\text{Calcitonin}^{\uparrow}}_{\text{C cells}} \rightarrow \begin{cases} \text{Bone resorption} \downarrow \\ \text{Renal Ca}^{2+} \text{excretion} \uparrow \end{cases} \rightarrow [\text{Ca}^{2+}] \text{ restored}$

Quick check: A patient has a parathyroid adenoma secreting excess PTH. Predict serum calcium, serum phosphate, and urine calcium.

Answer: Serum $\ce{Ca^{2+}}$ elevated (bone resorption + renal reabsorption); serum phosphate decreased (phosphaturia); urine calcium elevated (filtered load so high that excretion rises despite reabsorption → kidney stones).

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Common Confusions & Tricks

Osteoblasts vs. osteoclasts: "B for Build" (deposit matrix), "C for Chew" (resorb). Osteoclasts are multinucleated and derived from monocytes — not mesenchyme.

Tendons vs. ligaments: Both are mostly type I collagen. Ligament = Links bone to bone; Tendon = muscle To bone (more parallel organization). The single most common skeletal mix-up.

Calcitonin = thyroid; PTH = parathyroid: distinct glands. Thyroid surgery can remove parathyroids → hypocalcemia → tetany.

PTH raises calcium three ways: bone resorption AND renal reabsorption AND (indirectly via vitamin D) intestinal absorption. Students forget the renal/GI parts.

PTH vs. Vitamin D on phosphate: PTH lowers serum phosphate (phosphaturia); vitamin D raises it (intestinal absorption). PTH's phosphaturia dominates → primary hyperparathyroidism gives hypercalcemia + hypophosphatemia.

Rickets vs. Osteomalacia vs. Osteoporosis: Rickets = unmineralized osteoid in children; osteomalacia = same in adults (qualitative defect). Osteoporosis = normal composition but reduced bone mass (quantitative defect).

Hyaline vs. fibrocartilage: Hyaline (articular, growth plate) = type II collagen. Fibrocartilage (intervertebral discs, menisci) = type I, tougher. "Intervertebral disc" → fibrocartilage.

Intramembranous vs. endochondral: skull flat bones = intramembranous; long bones and most others = endochondral. The clavicle is intramembranous despite being a long bone.

Epiphyseal plate closure: Estrogen (both sexes) is the primary signal — anabolic steroids that aromatize to estrogen cause premature closure and shortened stature.

Synovial fluid nourishes articular cartilage — how avascular cartilage survives; immobilization reduces this diffusion and degrades cartilage.

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Key Takeaways

Skeletal Functions

• SPMM-H: Support, Protection, Movement, Mineral storage, Hematopoiesis
• Red marrow (flat bones, epiphyses) → blood cells; yellow marrow (adult medullary cavity) → fat

Bone Architecture

• Long bone: diaphysis (compact, medullary cavity) → metaphysis (growth plate) → epiphysis (spongy)
• Compact bone unit = osteon (Haversian system); spongy = trabeculae along stress lines, no osteons, holds marrow

Bone Matrix

• Organic (~35%): type I collagen → tensile strength
• Inorganic (~65%): hydroxyapatite → compressive strength
• Unmineralized = osteoid; failure to mineralize = rickets/osteomalacia

Bone Cells

Cell	Origin	Function
Osteogenic	Mesenchyme	Stem cells; divide
Osteoblast	Osteogenic	Build bone; become osteocytes
Osteocyte	Osteoblast	Maintain; mechanosense
Osteoclast	Monocyte	Resorb bone; multinucleated

Cartilage

• Hyaline: type II; articular surfaces, growth plates, trachea, costal
• Fibrocartilage: type I; intervertebral discs, menisci, pubic symphysis
• Elastic: type II + elastic fibers; ear, epiglottis
• All cartilage: avascular, heals poorly

Connective Tissues

• Tendon: muscle → bone; parallel type I collagen
• Ligament: bone → bone; interwoven type I collagen + elastic fibers

Joints

• Fibrous (sutures), Cartilaginous (discs, symphysis), Synovial (most limb joints)
• Synovial components: capsule, synovial membrane, synovial fluid (hyaluronic acid), articular cartilage
• Key synovial types: ball-and-socket (hip/shoulder), hinge (elbow/knee), pivot (C1–C2)

Endocrine Control of Calcium

Hormone	Source	Stimulus	Serum Ca²⁺	Mechanism
PTH	Parathyroid	Low Ca²⁺	Raises ↑	Bone resorption ↑; renal reabsorption ↑; activates Vitamin D
Calcitonin	Thyroid C cells	High Ca²⁺	Lowers ↓	Inhibits osteoclasts; renal excretion ↑
Calcitriol	Kidney (via PTH)	Low Ca²⁺	Raises ↑	Intestinal absorption ↑; bone mineralization ↑
Estrogen	Ovary/adipose	—	Preserves	Inhibits osteoclasts
Glucocorticoids	Adrenal cortex	Stress	Bone loss	Inhibits osteoblasts; ↓ GI absorption

• Vitamin D: skin (D₃) → liver (calcidiol) → kidney (calcitriol, active)
• Osteoporosis = ↓ bone mass, normal mineralization; Rickets/Osteomalacia = unmineralized osteoid