Skeletal System: Structure, Function, and Key Landmarks

Skeletal Tissue and Structure

Bone and Cartilage Histology

The skeletal system is composed of various tissues, primarily bone and cartilage, each with distinct structural and functional properties. Understanding their microscopic anatomy is essential for grasping how bones grow, repair, and interact with other tissues.

• Osteon (Haversian System): The fundamental structural unit of compact bone, consisting of concentric lamellae surrounding a central canal.

• Lamella: Layers of bone matrix within the osteon.

• Osteocyte: Mature bone cell residing in a lacuna, responsible for maintaining bone tissue.

• Lacuna: Small cavity within the bone matrix housing an osteocyte.

• Canaliculi: Tiny channels connecting lacunae, allowing for nutrient and waste exchange between osteocytes.

• Central Canal: The central passage in each osteon containing blood vessels and nerves.

• Chondrocyte: Cartilage cell found in lacunae within the matrix of cartilage tissue.

• Matrix: The extracellular substance in cartilage, providing support and flexibility.

Example: Compact bone (ground bone) and hyaline cartilage (e.g., monkey trachea) are commonly studied slides in anatomy labs.

Bone Types and Regions

Bones are classified based on their density and location within the body. Understanding these classifications is crucial for identifying bone function and pathology.

• Compact Bone: Dense, strong bone forming the outer layer of all bones.

• Spongy (Cancellous) Bone: Porous bone found at the ends of long bones and within flat bones; contains red marrow.

• Diaphysis: The shaft of a long bone.

• Epiphysis: The ends of a long bone; subdivided into proximal and distal epiphyses.

Bone Composition Experiments

Laboratory experiments often involve altering bone composition to study its properties. The following table summarizes the effects of removing specific components from bone:

Bone Treatment	Component Removed	Component Remaining	Characteristics
Bones in Acid	Mineral (inorganic) component	Organic (collagen) component	Flexible, bends easily
Baked Bones	Organic (collagen) component	Mineral (inorganic) component	Brittle, breaks easily

Axial Skeleton

Skull Bones and Landmarks

The skull is composed of several bones, each with unique landmarks that serve as attachment points, passageways, or structural features.

• Frontal Bone: Forms the forehead; includes the supraorbital foramen and frontal sinus.

• Lacrimal Bone: Contains the lacrimal fossa for the nasolacrimal duct.

• Ethmoid Bone: Features the perpendicular plate, cribriform plate, middle nasal conchae, and crista galli.

• Zygomatic Bone: Forms the cheek; part of the zygomatic arch.

• Maxillary Bone: Contains infraorbital foramen and palatine process.

• Sphenoid Bone: Includes lesser and greater wings, optic foramen, sella turcica, and sphenoid sinus.

• Mandible: Features mental foramen, mental protuberance, mandibular condyle, coronoid process, and mandibular ramus.

• Palatine Bone, Nasal Bone, Vomer: Contribute to the structure of the nasal cavity and palate.

• Parietal Bone: Forms the sides and roof of the cranium.

• Temporal Bone: Contains zygomatic process, mandibular fossa, styloid process, mastoid process, petrous ridge, internal acoustic meatus, carotid canal, and jugular foramen.

• Occipital Bone: Features foramen magnum, occipital condyle, and external occipital protuberance.

Sutures and Fetal Skull

Sutures are immovable joints between skull bones. The fetal skull contains fontanels, which are soft spots that allow for growth and flexibility during birth.

• Coronal Suture: Between frontal and parietal bones.

• Squamous Suture: Between parietal and temporal bones.

• Lambdoid Suture: Between parietal and occipital bones.

• Sagittal Suture: Between the two parietal bones.

• Fontanels: Anterior, posterior, anterolateral (sphenoidal), and posterolateral (mastoid) fontanels.

Vertebral Column and Thorax

Vertebrae Types and Features

The vertebral column consists of different types of vertebrae, each with characteristic features.

• Typical Vertebra: Body, vertebral foramen, transverse process, spinous process, superior/inferior articular surfaces, lamina, pedicle.

• Cervical Vertebrae: C1 (Atlas) – supports the skull, no body; C2 (Axis) – has the dens (odontoid process).

• Thoracic Vertebrae: Costal facets for rib articulation.

• Lumbar Vertebrae: Large body, superior/inferior articular surfaces.

• Sacrum: Sacral promontory, sacral foramina.

• Coccyx: Tailbone, composed of fused vertebrae.

Ribs and Sternum

The thoracic cage protects vital organs and supports respiration.

• Ribs: True (vertebrosternal), false (vertebrochondral & floating).

• Features: Head, tubercle, shaft.

• Sternum: Manubrium, body, xiphoid process.

Appendicular Skeleton

Pectoral Girdle and Upper Limb

The appendicular skeleton includes bones of the limbs and girdles that attach them to the axial skeleton.

• Scapula: Acromion, coracoid process, glenoid cavity, margins, fossae, spine, angles.

• Humerus: Greater/lesser tubercle, head, necks, deltoid tuberosity, capitulum, trochlea, fossae.

• Radius: Head, neck, radial tuberosity, styloid process.

• Ulna: Coronoid process, olecranon process, trochlear notch, radial notch, styloid process.

Pelvic Girdle and Lower Limb

• Coxal Bones (Os Coxae): Ilium (crest, ASIS), ischium (tuberosity, spine), pubis (symphysis), acetabulum, obturator foramen, greater sciatic notch.

• Femur: Head, neck, trochanters, gluteal tuberosity, linea aspera, patellar surface, condyles.

• Patella: Kneecap.

• Fibula: Head, lateral malleolus.

• Tibia: Condyles, tuberosity, medial malleolus.

• Foot: Tarsals (talus, calcaneus), metatarsals, phalanges.

Key Bone Landmarks and Functions

Important Landmarks and Their Functions

Bone landmarks serve as attachment points, passageways, and structural features essential for movement and protection.

• Acromion: Shoulder bone landmark.

• Atlas: First cervical vertebra, supports the skull.

• Bony Orbit of Eye: Formed by frontal, lacrimal, ethmoid, zygomatic, maxillary, sphenoid, and palatine bones.

• Cribriform Plate: Passage for olfactory nerves.

• Crista Galli: Attachment for meninges.

• Deltoid Tuberosity: Insertion for deltoid muscle.

• Fontanels: Soft spots in fetal skull for growth and birth.

• Foramen Magnum: Passage for spinal cord.

• Groove for Radial/Ulnar Nerve: Pathways for nerves in humerus.

• Hard Palate: Formed by palatine and maxilla bones.

• Intervertebral Discs/Foramina: Cushion and passageways in vertebral column.

• Ischial Spines/Tuberosities: Pelvic landmarks, important for childbirth and sitting.

• Jugular Fossa: Landmark for jugular vein.

• Lacrimal Fossa: Location of nasolacrimal duct.

• Occipital Condyles: Articulate with atlas.

• Odontoid Process (Dens): Pivot for atlas.

• Olecranon Process: Tip of elbow.

• Olfactory/Optic Foramina: Passage for nerves.

• Paranasal Sinuses: Air-filled spaces in skull bones.

• Radial Tuberosity: Biceps muscle attachment.

• Sacral Promontory: Landmark in sacrum.

• Sella Turcica: Houses pituitary gland.

• Spina Bifida: Congenital defect in vertebral arch.

• Tibial Tuberosity: Quadriceps muscle attachment.

• Transverse Foramina: Passage for vertebral arteries.

• Zygomatic Arch: Cheekbone structure.

Joints and Articulations

Major Joints

Joints are classified by their structure and movement. The following are key joints in the human body:

• Shoulder

• Elbow

• Hip

• Knee

Review Questions (Selected)

Sample Questions for Study

• What is the only movable joint in the skull? Answer: Temporomandibular joint (TMJ).

• Which bone contains the foramen magnum? Answer: Occipital bone.

• What structure passes through the foramen magnum? Answer: Spinal cord.

• Name the six bones that form the orbit of the eye. Answer: Frontal, lacrimal, ethmoid, zygomatic, maxillary, sphenoid (and palatine).

• Which ribs are called "true ribs"? Answer: Ribs 1-7, attached directly to sternum.

• Which ribs are called "false ribs"? Answer: Ribs 8-12, not directly attached to sternum.

• Which ribs are called "floating ribs" and why? Answer: Ribs 11-12, not attached to sternum at all.

• What part of the humerus is a common site of fractures? Answer: Surgical neck.

• The "shin" is the common name for which bone? Answer: Tibia.

Additional info:

• Some anatomical terms and relationships were inferred from context and standard anatomical knowledge.

• Table entries for bone composition experiments were completed based on typical laboratory outcomes.