- Loose CT: Such as areolar and adipose (fat) tissue.
- Dense CT: Such as tendons and ligaments.
- Fluid CT: Blood and lymph.
- Supporting CT: Cartilage and bone.
Muscle and Nervous Tissue
Muscle tissue is specialized for contraction to produce movement. It exists in three forms: skeletal (voluntary and striated), cardiac (involuntary, striated, and featuring intercalated discs), and smooth (involuntary and non-striated).
Nervous tissue is responsible for communication via electrical impulses. It consists of neurons, which transmit signals, and neuroglia, which provide essential support and protection for the neurons.
Exam Tip: Regeneration Capacity
Not all tissues heal equally. Epithelial tissue has excellent regeneration, while cartilage has poor regeneration due to its avascular nature. Cardiac muscle has very poor regeneration capacity compared to skeletal muscle.
The Skeletal System: Structure and Function
The skeletal system is much more than a static frame; it is a dynamic organ system providing support, protection of vital organs, leverage for movement, mineral storage, and hematopoiesis (the production of blood cells within the red bone marrow).
Bone Classification and Anatomy
Bones are classified by their shape into long, short, flat, irregular, and sesamoid bones.
A typical long bone consists of several key regions:
- Epiphysis: The expanded ends of the bone, which contain spongy bone and articular cartilage.
- Diaphysis: The long, central shaft.
- Medullary Cavity: The hollow space within the diaphysis, which houses yellow bone marrow (fat storage) in adults.
- Periosteum: The fibrous outer covering essential for fracture repair.
Microscopic Composition: Compact vs. Spongy Bone
Bones are composed of two distinct structural types:
- Compact Bone: A dense, heavy outer shell organized into functional units called osteons. Each osteon consists of concentric rings of matrix called lamellae, with small spaces called lacunae that house osteocytes. These cells communicate through tiny channels called canaliculi.
- Spongy (Trabecular) Bone: A lightweight, honeycomb-like network of trabeculae that houses red bone marrow.
The bone matrix itself is a composite material that balances strength and flexibility:
- Organic Component: Primarily collagen fibers, which provide tensile strength and flexibility.
- Inorganic Component: Hydroxyapatite crystals, represented by the formula:
Ca10(PO4)6(OH)2
These crystals provide the hardness and compressive strength required to support weight.
Bone Biology: Cells, Remodeling, and Growth
The Four Bone Cell Types
Bone is a living tissue maintained by a constant cycle of remodeling driven by four specialized cells:
- Osteogenic Cells: Stem cells that divide to produce osteoblasts.
- Osteoblasts: The "builders" that secrete the bone matrix (osteoid).
- Osteocytes: Mature bone cells that maintain the matrix and communicate via canaliculi.
- Osteoclasts: Large cells derived from the blood lineage that break down and resorb bone using enzymes and acid.
Bone Remodeling and Wolff's Law
Bone remodeling is the continuous process where osteoclasts remove old bone and osteoblasts deposit new bone. This process allows the skeleton to respond to mechanical stress, a principle known as Wolff’s Law. Increased physical load (like weight-bearing exercise) stimulates osteoblast activity, making bones thicker and stronger, whereas inactivity can lead to bone loss.
Ossification (Bone Formation)
Bone develops through two primary processes:
- Intramembranous Ossification: The formation of flat bones, such as those in the skull.
- Endochondral Ossification: The process used for most bones in the body, where a hyaline cartilage model is gradually replaced by bone.
In endochondral ossification, growth occurs at the epiphyseal growth plate—a region of active cartilage between the epiphysis and diaphysis. Once a person reaches maturity, hormonal signals cause the growth plate to close (epiphyseal closure), leaving behind an epiphyseal line and ending longitudinal growth.
Clinical Note: Hormonal Influence
During puberty, testosterone in males promotes faster chondrocyte division, leading to rapid lengthening, while estrogen in females stimulates a growth spurt but ultimately triggers earlier growth plate closure.
