Skeletal System

The skeletal system is a complex framework that supports the body.
It consists of:
- Cartilage: Precursor for most bones, covers many joint surfaces.
- Bones: The solid structures of the skeleton.
- Ligaments: Connective tissues that hold bones together at joints.
- Bone marrow: Enclosed within bones.

Support: Provides a framework for the body and soft organs.
Protection: Safeguards vital organs such as the brain, spinal cord, and other essential organs.
Anchorage: Acts as levers for muscles to create movement.
Mineral Storage: Stores essential minerals such as calcium and phosphorus.
Blood Formation: Hematopoiesis occurs in red bone marrow found in certain bones.
Storage of Fat: Triglycerides are stored in bone cavities, providing an energy source.
Hormone Production: Osteocalcin plays a role in regulating insulin secretion, glucose homeostasis, and energy expenditure.

Knowledge of bone anatomy and the remodeling process is crucial for addressing issues in patients with bone diseases like osteoporosis.

Definition: A type of connective tissue that has a hard, calcified matrix.
Forms:
- Compact Bone: Dense and solid in structure.
- Spongy Bone: Porous with spaces, providing structural integrity and reducing weight.
Contains various types of tissues including blood vessels, nervous tissue, cartilage, adipose tissue, and fibrous connective tissue.

Osteogenic Cells:
- Mitotically active stem cells located in the periosteum and endosteum.
- Differentiate into osteoblasts or bone-lining cells when stimulated.
- Some remain as stem cells for future growth.
Osteoblasts:
- Bone-forming cells that synthesize the organic component of bone and promote mineralization.
- Typically found in a single layer on the bone surface and are mitotically active.
Osteocytes:
- The most abundant bone cells, they mature from osteoblasts and are found in lacunae.
- Maintain bone matrix and act as stress sensors, responding to mechanical stimuli.
- Communicate with osteoblasts and osteoclasts for bone remodeling.
Bone-Lining Cells:
- Flattened cells on bone surfaces that help maintain the matrix alongside osteocytes.
- Known as periosteal cells on external surfaces and endosteal cells on internal surfaces.
Osteoclasts:
- Large, multinucleated cells responsible for bone resorption.
- Secrete hydrochloric acid and enzymes to dissolve bone tissue and are derived from a different stem cell lineage compared to osteoblasts.

Appears disorganized, but is structured along lines of stress for resistance.
Consists of trabeculae, which confer strength, resembling cables on a suspension bridge.
Does not contain osteons but includes irregularly arranged lamellae and osteocytes interconnected by canaliculi.
Capillaries in the endosteum supply necessary nutrients.

Composed of two main components:
- Organic Matter (1/3): Mainly collagen and various proteins, providing flexibility and tensile strength.
- Inorganic Matter (2/3): Primarily calcium phosphate and other minerals, essential for structural rigidity and preventing bending under load.

Definition: The process of forming the bony skeleton, beginning in the second month of embryonic development.
Methods of Ossification:
- Endochondral:
- Bone forms by replacing hyaline cartilage; responsible for forming most of the skeleton.
- Begins at primary ossification centers during fetal development.
- Intramembranous:
- Bone develops from fibrous membranes; primarily forms flat bones of the skull.
Lifelong Bone Remodeling: Occurs throughout an individual's lifetime, impacting size and shape in response to applied forces.

Formation of Bone Collar: Around diaphysis of the hyaline cartilage model.
Calcification of Cartilage Matrix: Cartilage in the center of the diaphysis calcifies and develops cavities.
Spongy Bone Formation: The periosteal bud invades internal cavities to form spongy bone.
Medullary Cavity Formation: Diaphysis elongates, forming a medullary cavity and secondary ossification centers in epiphyses.
Epiphyseal Ossification: The epiphyses ossify; post-development, cartilage remains only in the epiphyseal plates and articular surfaces.

Begins with a sheet of embryonic connective tissue.
Embryonic cells secrete collagen and mineral salts to harden the tissue, forming thin trabeculae.
Creates compact bone sandwiched around spongy bone.

Bones continuously remodel throughout life, adjusting to structural and stress changes.
Weight-bearing exercises stimulate osteoblast activity, increasing bone mass.
During late adolescence, the epiphyseal plate thins and is replaced by bone, ceasing length growth:
- Completion occurs around age 18 for females and 21 for males.

Bone functions as a metabolically active organ, regulating minerals in the body.
Primary reservoir for calcium and phosphate; vital for numerous physiological processes.
Regulation Mechanisms:
- Mineral Deposition: Osteoblasts extract calcium and phosphate from the blood, depositing them in the bone.
- Mineral Resorption: Osteoclasts dissolve bone, releasing minerals back into the bloodstream for various uses.