Bone Histology & Physiology Notes (Transcript-Based)

Palatine bone, sutures, and terminology

  • The part of the hard palate that continues with bone is the horizontal plate of the palatine bone; this is the portion you can see if you turn the skull upside down. It is continuous with the palatine process of the maxilla.
  • When talking about anatomy, terms like the occipital condyle, the horizontal plate of the palatine bone, etc., can seem awkward because the same word root appears in multiple terms. Be as complete as you can when naming structures.
  • Always include related terms such as suture and sinus. There were handout errors: the lab handout listed sinuses by bone (ethmoid, sphenoid, maxillary, frontal) but should have explicitly included the word sinus after the listing, and similarly for sutures. The instructor noted this and it will be corrected next term.
  • Some classes required revising grading due to these corrections. In today’s class, we’ll cover the markings and landmarks of bones and address questions as they arise.

Study strategy and class expectations

  • If you’re unsure about a term like the lambdoidal suture, ask questions as you go to avoid missing material.
  • The instructor emphasizes active participation: ask questions if you don’t understand, or you’ll end up with extra free time during the session.
  • Lab practical reminders:
    • Starts at: 02:1002{:}10
    • Arrive by: 02:0002{:}00 to avoid being late.
    • Practical will rotate based on a timer due to a large number of students; you may need to sit in an empty seat at times.
    • Some stations may be unavailable temporarily; patience is a required skill during the lab practical.
  • Spelling warnings: minor spelling mistakes generally won’t be penalized unless they make terms ambiguous. Examples from the instructor’s list:
    • tibula, fibia, sphepoid are unacceptable.
    • Acceptable flexibility includes minor misspellings of terms like maxilla (e.g., one or two letters different).

Bone cells: histology and cellular lineage

  • The four bone-related cell types discussed include:
    • Osteogenic (osteoprogenitor) cells: stem cells that can divide and give rise to osteoblasts.
    • Osteoblasts: bone-forming cells; underlie the periosteum (outer) and endosteum (inner lining).
    • Osteocytes: mature bone cells housed in lacunae within bone tissue; connected by canaliculi that allow nutrient and signal exchange.
    • Osteoclasts: resorptive cells; arise from a different lineage (not from osteoblast lineage).
  • Locations:
    • Under the fibrous membranes surrounding bone: the periosteum (outer fibrous layer) and the endosteum (inner lining).
    • Osteocytes reside inside bone tissue within lacunae; they communicate via canaliculi.
  • Connections: osteogenic cells can become osteoblasts; osteoblasts lay down bone matrix; osteocytes remain embedded in the mineralized matrix.
  • Purpose of osteoclasts: primarily to regulate blood calcium by resorbing bone when calcium is needed in the bloodstream.

Calcium homeostasis and hormonal regulation

  • Calcium is tightly regulated in the blood with a very narrow range to prevent neural, muscular, and cardiac dysfunction. The clinically referenced target range is 9-11 mg/dL9\text{-}11\ \text{mg/dL} for blood calcium.
  • Three major targets/effectors in calcium regulation:
    • The bones (site of calcium storage and mobilization).
    • The kidneys (calcium reabsorption and excretion).
    • The intestines (calcium absorption).
  • Two hormones control blood calcium levels:
    • Parathyroid hormone (PTH): produced by the parathyroid glands. The primary regulator of increasing blood calcium when it is low.
    • Calcitonin: produced by the thyroid gland. Acts to oppose PTH, generally lowering blood calcium when it is high.
  • Mechanisms and negative feedback:
    • If blood calcium is too low: the parathyroid glands secrete PTH; PTH stimulates osteoclasts to resorb bone, releasing Ca^{2+} into the blood, thereby increasing blood calcium.
    • If blood calcium is too high: the thyroid produces calcitonin; calcitonin inhibits osteoclast activity and can promote osteoblast activity, leading to calcium deposition in bone and a decrease in blood calcium.
    • This is a classic negative feedback loop: low Ca^{2+} triggers responses that raise it; high Ca^{2+} triggers responses that lower it.
  • Osteoclasts and PTH create a direct link between bone remodeling and blood calcium levels.
  • Practical implication: elderly individuals often experience bone density loss because osteoclast activity outpaces osteoblast activity with aging.

Factors influencing bone density and osteoporosis risk

  • Postmenopausal women are at high risk for osteoporosis due to decreased sex hormones, which help maintain bone density.
  • Hormone replacement therapy (HRT) can have benefits for osteoporosis in some cases, though medical decisions should be individualized.
  • Two major lifestyle and physiological factors that influence bone density:
    • Weight-bearing exercise (moderate activity helps maintain bone density).
    • Hormonal status and adipose-derived estrogen: adipose tissue serves as a reservoir for steroid hormones; some body fat helps maintain hormone levels that protect bone.
  • Observations from a clinical discussion:
    • Individuals with very low body weight and low body fat may experience disrupted menstrual cycles due to reduced estrogen production, which can affect bone density.
    • In a patient named Susan, recommendations included hormonal regulation and moderate weight gain through exercise to optimize bone density.
  • Other risk factors discussed include genetics (family history) and lack of exercise; multiple factors can contribute to osteoporosis.
  • Important takeaways:
    • The two main protective strategies are maintaining sex hormone balance (when medically appropriate) and engaging in weight-bearing exercise.
    • Osteoporosis is influenced by hormones, body composition, genetics, and activity level.

Bone marrow: red vs. yellow and skeletal anatomy context

  • Red bone marrow (myeloid tissue) produces formed elements of blood (red blood cells, white blood cells, platelets).
    • In infants, most medullary cavities and trabecular (spongy) bone produce red marrow.
    • In adults, red marrow is mostly in the proximal epiphyses of long bones and in certain skull bones; much of the distal appendicular skeleton becomes yellow marrow with age.
    • Most short, flat, irregular, and some sesamoid bones retain red marrow regions centered near the trunk.
  • Yellow bone marrow consists primarily of adipose tissue and serves as an energy storage site; it does not contribute to hematopoiesis and has limited cushioning/insulation roles.
  • Terminology overlap:
    • Spongy bone = cancellous bone = trabecular bone = diploë (in skull bones, “dipole” is used). In skull bones, some use “diploë”; commonly referred to as spongy/trabecular tissue.
  • The distinction between red and yellow marrow reflects functional shifts with age and site-specific marrow requirements.

Bone structure and bone formation during development

  • Two primary modes of bone formation in the fetus:
    1) Intramembranous ossification: bone forms directly within a connective tissue membrane, without a cartilage template.
    2) Endochondral ossification: bone forms by replacing a cartilage template with bone.
  • Both processes contribute to long bone growth and fracture repair; cartilage serves as a temporary scaffold that is later replaced by bone.
  • Embryonic connective tissue origin:
    • The primitive connective tissue is called mesenchyme. Within this tissue, osteogenic cells arise and differentiate into osteoblasts.
  • Ossification centers:
    • In intramembranous ossification, ossification centers form within the mesenchymal membrane where osteoblasts begin to lay down bone matrix.
    • These centers are scattered throughout the membrane (e.g., at various sites), and the surrounding mesenchyme condenses to form the periosteum.
  • Process summary:
    • Mesenchymal cells differentiate into osteogenic cells, which then become osteoblasts.
    • Ossification centers secrete ground substance that becomes calcified, forming bone.
    • The membrane surrounding the forming bone differentiates into the periosteum; bone formation proceeds from multiple centers outward.

Miscellaneous notable points and anecdotes

  • A discussion point about a recent article on genetics and bone marrow: researchers found that stem cells in the lungs can contribute to bone marrow reconstitution under certain conditions, illustrating surprising interrelationships between organ systems and stem cell reservoirs.
  • The instructor emphasized connecting lectures with real-world relevance and encouraged students to ask questions throughout to reinforce understanding of landmarks, sutures, and sinus terminology.

Quick glossary and key terms

  • Palatine bone, horizontal plate, palatine process
  • Suture, lambdoid suture, sinus
  • Osteogenic (osteoprogenitor) cell
  • Osteoblast
  • Osteocytes, lacunae, canaliculi
  • Osteoclast
  • Periosteum, endosteum
  • Intramembranous ossification
  • Endochondral ossification
  • Mesenchyme / mesenchymal membrane
  • Ossification centers
  • Red bone marrow vs. Yellow bone marrow
  • Diploë (skull bones), cancellous/trabecular/spongy bone
  • Parathyroid hormone (PTH)
  • Calcitonin
  • Calcium homeostasis: $[\mathrm{Ca^{2+}}]$ range approximately 9–11 mg/dL9\text{--}11\ \text{mg/dL}
  • Negative feedback regulation of blood calcium
  • Osteolysis (bone resorption) and osteogenesis (bone formation)
  • Hormone replacement therapy (HRT) context for osteoporosis