Bone Marrow, Bone Development, and Mineral Homeostasis

Vocabulary practice flashcards covering bone marrow anatomy, intramembranous and endochondral ossification, bone deposition and remodeling, skeletal pathologies, and hormonal regulation of calcium homeostasis.

Red Marrow

Hemopoietic tissue resembling thick blood that produces blood cells; found in the medullary cavity and spongy bone trabeculae of the vertebrae, ribs, sternum, pelvic girdle, and proximal heads of the femur and humerus in adults.

Yellow Marrow

Fatty marrow located within the medullary cavities of long bones in adults.

Ossification

The process of bone formation that begins during the 6th or 7th week of embryonic life.

Intramembranous Ossification

Bone formation that occurs through the condensation of mesenchyme into trabeculae, producing the flat bones of the skull, most cranial and facial bones, the mandible, and the clavicle.

Osteoid Tissue

Uncalcified bone matrix laid down by osteoblasts on mesenchymal trabeculae during intramembranous ossification.

Endochondral Ossification

A method of bone formation where bone is preformed in hyaline cartilage; forms long bones and all bones not formed via intramembranous ossification.

Mineralization

A crystallization process in which osteoblasts produce spiraled collagen fibers along the osteon that become encrusted and hardened by calcium and phosphate ions from blood plasma when ion concentrations reach the solubility product.

Ectopic Calcification

Abnormal calcification that occurs in soft tissues such as the lungs, brain, eyes, muscles, tendons, or arteries (arteriosclerosis).

Ruffled Border

The specialized surface of osteoclasts that secretes hydrochloric acid ($\text{pH } 4$) to dissolve bone minerals during mineral resorption.

Acid Phosphatase

An enzyme secreted by osteoclasts during mineral resorption to digest collagen fibers in the bone matrix.

Interstitial Growth

The process by which bones increase in length via growth within the epiphyseal plate.

Appositional Growth

The process by which bones increase in width through osteoblasts depositing matrix in layers onto the outer bone surface.

Wolff's Law of Bone

The principle stating that the architecture of a bone is determined by mechanical stresses placed upon it, mediated through the collaborative action of osteoblasts and osteoclasts.

Achondroplastic Dwarfism

A form of dwarfism resulting from a spontaneous mutation during DNA replication that causes failure of cartilage growth, leading to long bones ceasing growth in childhood, a normal torso, and short limbs.

Pituitary Dwarfism

A condition caused by a lack of growth hormone secretion, resulting in short stature with normal proportional body development.

Phosphate (Skeletal Storage)

A major mineral component of DNA, RNA, ATP, pH buffers, and phospholipids, with approximately $750\,\text{g}$ stored in the adult skeleton.

Calcium (Skeletal Storage and Normal Plasma Level)

An essential ion required for neuron function, muscle contraction, blood clotting, and exocytosis; represented by $1100\,\text{g}$ in the adult skeleton with a normal blood plasma concentration of $10\,\text{mg/dL}$.

Hypocalcemia

A deficiency of blood calcium that causes nervous system excitability by allowing sodium channels to open more easily; leads to muscle spasms, tremors, or tetany at $6\,\text{mg/dL}$, and laryngospasm and suffocation at $4\,\text{mg/dL}$.

Hypercalcemia

An excess of blood calcium that binds to cell surfaces, making sodium channels less likely to open and depressing the nervous system; causes muscle weakness, sluggish reflexes, and cardiac arrest at $12\,\text{mg/dL}$.

Calcitriol

The active hormone form of vitamin D synthesized sequentially by epidermal keratinocytes, the liver, and kidneys; raises blood calcium by stimulating intestinal and skeletal calcium absorption, osteoclast stem cell differentiation, and renal calcium reabsorption.

Cholecalciferol-D3

The compound produced when ultraviolet (UV) radiation and epidermal keratinocytes convert a steroid derivative during the initial step of calcitriol synthesis.

Calcidiol

An intermediate product in calcitriol synthesis formed in the liver by converting cholecalciferol-D3.

Rickets and Osteomalacia

Disorders characterized by abnormal bone softness due to vitamin D deficiency, termed rickets in children and osteomalacia in adults.

Calcitonin

A hormone secreted by C cells of the thyroid gland when calcium levels rise too high; corrects hypercalcemia by reducing osteoclast activity by up to $70\%$ and increasing osteoblast number and activity.

Parathyroid Hormone (PTH)

A hormone released by glands on the posterior surface of the thyroid in response to low blood calcium; corrects hypocalcemia by raising blood calcium, promoting renal calcium reabsorption, stimulating calcitriol synthesis, and releasing RANKL.

RANKL

Osteoclast-stimulating factor released by osteoblasts under the direction of parathyroid hormone (PTH) to increase the osteoclast population.

Premature Closure of Growth Plates

The early cessation of bone elongation caused by the teenage use of anabolic steroids, resulting in short adult stature.