Bone Tissue and Skeletal System: Comprehensive Notes

Context and scope
- Today’s focus: bone tissue, bone cells, growth of long bones, growth of flat bones.
- Quiz considerations: potential quiz topics; teacher may release a quiz study guide next week outlining exact topics.
- Preview of lab-style activity: practicing with skull anatomy and mock lab tests to simulate exam conditions.
- Plan for a mock skull lab test: typically 15–20 minutes, with a timed format; highlighted as a pivotal weekend study opportunity.
Skull anatomy and labeling practice
- Class exercise involves identifying skull regions from a figure (Figure 1.2 in some edition); regions labeled with numbers (e.g., 4, 6, 8, 1, 9, 5, 2, 7, 3, 10).
- Temporal region discussion: temporal bone located at the sides of the skull; parietal region is superior to it; frontal and nasal bones described as well.
- Specific point: distinguishing the zygomatic bone from its processes
- Zygomatic process of the temporal bone vs temporal process of the zygomatic bone are named connections between two bones.
- Practical study tip: when identifying parts of the temporal bone, expect specificity (e.g., zygomatic process of the temporal bone).
- Lab objective strategy: you may be asked to name multiple structures within a single bone (e.g., a group of structures on the temporal bone). Bracket or group numbers to indicate that they belong to the same bone rather than spelling out each structure’s full bone name.
- When naming conventions matter:
- In some courses you may be asked to append terms like “muscle” after muscle names (e.g., biceps brachii muscle) to satisfy exam expectations.
- For long bones, you might name several structures within a single bone (e.g., humerus one, humerus two) and then indicate they belong to that bone, rather than repeating the bone name for every structure.
- Regional vs systemic anatomy context: the course emphasizes regional anatomy and systemic anatomy (11 body systems).
Regional organization vs lab prep concepts
- The instructor references regional anatomy versus systemic anatomy debates and the practical value of studying by system while recognizing regional structures.
- Appendicular vs axial skeleton: reminder that there is a distinction—appendicular skeleton includes limbs (and girdles), while axial skeleton includes skull, vertebral column, and rib cage.
Endocrine regulation and homeostatic control (negative and positive feedback)
- Endocrine topic preview: thyroid hormone production and regulation (T3 and T4).
- Core pathway (negative feedback loop):
- Hypothalamus releases TRH (thyrotropin-releasing hormone) -> anterior pituitary releases TSH (thyroid-stimulating hormone) -> thyroid gland releases T3 (triiodothyronine) and T4 (thyroxine).
- Elevated levels of T3 and T4 provide negative feedback to suppress further TRH/TSH release, restoring homeostasis.
- Alternate regulatory scenarios: primary vs secondary thyroid disorders
- Primary hyperthyroidism: thyroid gland itself is overactive; high T3/T4 with feedback effects.
- Hypothyroidism due to pituitary or hypothalamic failure: low TSH or TRH leading to low T3/T4.
- Clinical relevance and management
- TSH is a common clinical measure; high TSH with low thyroxine suggests the body is trying to stimulate thyroid production.
- Levothyroxine is a common lifelong treatment to maintain thyroid hormone levels in hypothyroid patients.
- Master gland concept: Pituitary gland is often described as a master endocrine gland; the hypothalamus acts as the main regulatory center for TRH/TSH signaling.
- The speaker notes this topic is part of broader endocrinology covered in more depth in later courses (e.g., MCAT-style relevance).
Bone tissue overview and functional roles
- Primary homeostatic roles of the skeleton:
- Support and movement: bones provide structural support and levers for muscles to produce movement.
- Protection: cranial bones protect the brain; vertebral arch protects the spinal cord; thoracic cage (sternum and ribs) protects the heart and lungs.
- Hematopoiesis: red bone marrow in spongy bone of flat bones and ends of long bones produces red blood cells, white blood cells, and platelets.
- Fat storage: yellow bone marrow stores fat; long bones serve as energy reserves.
- Calcium reservoir: about 99\% of the body's calcium is stored in bone; calcium homeostasis is tightly regulated in the blood.
- Calcium regulation and endocrine interplay
- Calcium is essential for muscle contraction, including smooth muscle contraction, bone mineralization, and bone strength.
- Blood calcium must be buffered to prevent calcification of soft tissues and ensure proper muscle and cardiovascular function.
- Marrow distribution
- Red marrow is prominent in the ends of long bones and in flat bones.
- Yellow marrow predominates in the medullary (central) cavities of long bones.
Bone types and their structural features
- Long bones
- Examples: humerus, radius, ulna; femur; tibia and fibula; metacarpals and digits; metatarsals.
- Key features: a proximal and distal ends (knobs) and a cylindrical shaft (diaphysis); act as levers for movement and bear weight.
- Short bones
- Found in wrists (carpals) and ankles; irregular shapes with varying flat/rounded surfaces; often spongey bone inside with compact bone shell.
- Carpal bones (typical list): Scaphoid, Lunate, Triquetrum, Pisiform, Trapezium, Trapezoid, Capitate, Hamate.
- Flat bones
- Examples: sternum, scapula, cranial bones; provide broad surfaces for muscle attachment and protect internal organs.
- Irregular bones
- Examples: vertebrae, pelvic bones, ethmoid, sphenoid; characterized by odd shapes for diverse attachments and functions.
- Sesamoid bones
- Examples: patella (kneecap); other small sesamoids can occur in tendons or at various joints.
- Function: protect tendon from wear and improve mechanical leverage and tendon gliding.
- Wormian (suture) bones
- Extra bones (or sutural bones) found within sutures, often in the occipital region; sometimes called suture bones.
- Occurrence varies; potential ethnic or developmental associations discussed historically, but not consistently reliable.
The skull: processes, landmarks, and naming conventions
- Bones named for location and connections (e.g., temporal bone, zygomatic bone).
- Processes can be named in relation to another bone rather than the bone they belong to (e.g., zygomatic process of the temporal bone vs temporal process of the zygomatic bone).
- Emphasis on learning both anatomical names and their functional connections to enhance clinical understanding.
Sphenoid and pituitary gland relationship
- The sphenoid bone houses the pituitary gland in its central area (the sella turcica) within the cranial base.
- Understanding tuber cinerum and the greater wings helps situate the pituitary in classic skull anatomy.
Epiphyses, metaphyses, and growth plates in long bones
- Structure in a long bone (example: femur):
- Epiphysis: the ends of the bone; covered with hyaline cartilage (articular cartilage) at joint surfaces.
- Diaphysis: the shaft; compact bone surrounding the medullary cavity.
- Metaphysis: the region between epiphysis and diaphysis; contains the growth plate in children.
- Growth plate (epiphyseal plate): cartilage zone where longitudinal bone growth occurs in children; progressively ossifies as maturation occurs.
- Epiphyseal line: in adults, the growth plate has closed, leaving a bony line where growth occurred.
- Growth mechanism and timing
- Longitudinal growth occurs postnatally from growth centers in the metaphyses; cartilage cells proliferate and are replaced by bone cells.
- As puberty approaches, sex hormones (testosterone and estrogen) accelerate ossification and close growth plates, forming the epiphyseal line.
- Hyaline/articular cartilage
- Ends of epiphyses are covered with hyaline cartilage (articular cartilage) that cushions bone contact in joints and does not ossify under normal conditions.
- Wear or injury to articular cartilage can lead to osteoarthritis due to loss of cushioning.
Periosteum, endosteum, and bone remodeling
- Periosteum: outer covering of bone with two layers
- Fibrous layer: outer, tougher layer that anchors blood vessels and nerves; provides protection and surface for attachment of tendons and ligaments.
- Osteogenic layer: inner layer containing bone-forming cells that contribute to bone growth and healing.
- Endosteum: inner lining of the medullary cavity, also containing bone-forming and bone-resorbing cells.
- Medullary cavity: hollow interior of the diaphysis containing bone marrow (red in active hematopoietic regions as described; yellow marrow within the diaphysis stores fat).
- Osteocytes, osteoblasts, and osteoclasts
- Osteoblasts build bone; osteoclasts resorb bone; osteocytes are mature bone cells embedded in the bone matrix.
- Remodeling process tends to be spatially biased: outer periosteal surfaces favor deposition; inner endosteal surfaces favor resorption, enabling adaptation to mechanical load and repair.
- Practical image cues for study
- The wall of the medullary cavity is lined by the endosteum.
- The outer surface is covered by the periosteum.
- The two membranes (periosteum and endosteum) host different cell populations involved in growth and remodeling.
Radiographic concepts and clinical relevance
- Growth plate visualization on X-ray: presence of blue/gray cartilage in growing children vs. closure in adults highlights a growth plate vs. epiphyseal line distinction.
- In clinical practice, the health of calcium homeostasis and bone remodeling can be inferred from radiographic appearance alongside biochemical markers (e.g., TSH, thyroid hormones discussed in other contexts).
- Osteoarthritis risk associated with articular cartilage wear emphasizes the functional importance of protecting joint surfaces.
Study strategy and connections to broader anatomy
- Emphasis on integrating anatomy across regional and systemic perspectives to understand both structure and function.
- Real-world relevance: bone biology underpins metabolic regulation (calcium homeostasis), endocrinology, hematopoiesis, and musculoskeletal health.
- Interdisciplinary connections: clinical cases (thyroid disorders, calcium regulation) intersect with bone tissue function and homeostasis.
Quick recap of key terms to know for exams
- Epiphysis, Diaphysis, Metaphysis, Epiphyseal plate/line, Articular cartilage, Hyaline cartilage, Periosteum (Fibrous layer, Osteogenic layer), Endosteum, Medullary cavity, Red bone marrow, Yellow bone marrow, Osteoblasts, Osteoclasts, Osteocytes, Wormian (suture) bones, Sesamoid bones, Sphenoid bone, Pituitary gland, TRH, TSH, T3, T4, Hypothalamus, Anterior pituitary, Feedback loops (negative/positive)