Skeletal Unit:

Skeletal System

Ossification- hardening

osteogenesis - Bone formation

The body is composed of 206 bones along with cartilage, tendons, and ligaments

Bone formation:

In the early stages of development, the embryo’s skeleton consists of fibrous membranes and hyaline cartilage
By the sixth or 7th week of embryonic development, the actual process of bone development (Ossification) (Ostengenesis) begins.

The process of Osteogenesis:

Begins with progenitor cells
Progenitor cells are descendants of stem cells that further differentiate to create specialized cell types
Each progenitor cell is only capable of differentiating into cells that belong to the same tissue or organ. They are similar to stem cells but they already have a “target” tissue or organ

Bone continues to develop:

The framework is a flexible and semi-solid matrix
A matrix is a large network of proteins and other molecules that surround, support, and give structure to cells and tissues in the body
It is produced by chondroblasts (Immature cartilage cells), water, and other material
The matrix surrounds and isolates chondrocytes (Mature chondroblasts). The connective tissues cartilage is avascular therefore; nutrients are brought in by diffusion through the matrix
Remember this is why damaged cartilage doesn’t repair as readily as most tissues do

Bone tissue:

Most bones develop using hyaline cartilage structures as their “Models”
Ossification or osteogenesis involves 2 steps
During fetal development, a framework is laid down that determines where bones will form

Final formation:

The hyaline cartilage dissolves as the bone completes its formation
There are 2 major places where the hyaline cartilage remains. One is the end of the bone and in the area of the bone where growth takes place which is the epiphyseal plate

Remodeling:

Bones are not inactive structures that form and never change. A bone is a dynamic and active tissue. As a person grows, so do their bones, as people age bones also change. The process of skeletal change is called remodeling. Remodeling is your bone’s way of replacing old parts with new parts
Bones are remodeled continually in response to 2 factors.

The Changing Skeleton:

When the blood calcium levels drop below the optimal level, then the parathyroid gland (glands in your throat) are stimulated to release a hormone to the blood. (you don't want this to happen.)
This hormone activates cells called osteoclasts (multinucleated cells containing lots of mitochondria and lysosomes) these are bone-destroying cells in bones, that break down the bone matrix__ to release calcium from the matrix into the blood. If the levels of calcium are too high, calcium is deposited in the bone matrix.
The bone matrix acts as a storage area for calcium. The process osteoclasts go through is referred to as resorption.
Resorption is when the parathyroid gland releases osteoclasts into the bones to break them down into calcium, and release that calcium into the blood. And vise versa if the bone matrix levels of calcium are low.

Stress, muscle pull, and gravity

The stress of muscle pull and gravity acting on the skeleton determines where the bone matrix is to be broken down by the osteoclasts or formed so that the skeleton remains strong.
(Older women often have what we call weak and fragile bones. This is most often due to a deficiency of estrogen. This deficiency leads to increased bone remodeling, where bone resorption outpaces bone formation and decreases bone mass.)

Axial skeleton:

The skeleton can be divided into two main parts. The axial skeleton consists of 80 bones. The primary bones of the axial skeleton are the skull, spine, ribs, and sternum (thorax).

Appendicular skeleton:

The appendicular skeleton consists of 126 bones. The primary bones of this skeleton are the shoulder or pectoral girdle, arms, hands, pelvic girdle, legs, and feet.

Functions of bones:

Bones are composed of about 50% water and 50% a solid, calcified, rigid substance known as osseous (AH see us) tissue.