C. A.Introduction
The earth presents animals with two problems:
- It is quite different from place to place. Some areas are suitable for living and other places are not.
- The earth has gravity. Animals with a stiff skeletal system can resist the downward pull. Structures are often movable and serve as places of attachment for muscles that help animals move from place to place.
D. B. Types of Skeleton systems
- Hydrostatic skeleton
Fluid is held under pressure in a closed body compartment seen in coelenterates, flatworms, nematodes and annelids. The animals control movement by changing the shape of the body compartment with muscles.
- Exoskeleton
Hard encasement deposited on the surface of an animal. Mollusks have a shell secreted by the mantle. Arthropods have a jointed exoskeleton secreted by the epidermis. About 30-50% of the cuticle is made of chitin. Some arthropods add calcium salts to the chitin.
- Endoskeleton
Hard support elements, the bones, are buried within the soft tissue of an animal, ie. Echinoderms have a hard plate beneath the skin while vertebrates have an articulated endoskeleton.
C. Vertebrate Endoskeleton
Most vertebrates are made up of bones, except for some cartilaginous fishes, and have a bony endoskeleton with some cartilaginous structures.
1. Functions of vertebral skeletons
a. Support
b. Allows for movement via joints
c. Forms blood cells through bone marrow
d. Protect organs
e. Detoxifies certain poisons which are removed from parts of the body and brought to the bones
f. Allows for certain places for muscle attachment
g. stores minerals
We will use the human skeleton as a representative vertebrate endoskeleton. The skeleton is divided into two components.
Axial: skull, vertebral column, and bones of thorax
Appendicular: limbs, pectoral, and pelvic girdles.
2. Joints
D. Structure of a Bone
1. Bone parts
a. Epiphysis
The end of the bone. Often it is covered with smooth hyaline cartilage
b. Diaphysis
The shaft of the bone that is covered with the perichondrium
c. Perichondrium
The vascularized, fibrous outer covering of the bone which provides the bone with blood vessels
d. Endosteum
The compact boe of the diaphysis
e. Medullary canal
The inner canal of the bone. This canal contains the bone marrow and the yellow marrow. The yellow marrow is the storage area for fat
2. Bone development
In vertebrates, the hyaline cartilage is the forerunner of the long bones.
a. early in fetal development, the membranous perichondrium grows around the cartilage,
b. The cartilage forming cells, deep in the center of the dense cartilage bed, increase in size After the cell enlargement they die and disintegrate which leaves behing the cavities of that the cells have created. Bone will form in the cavities.
c. The first bone will start to form. This is called the primary ossification.
d. The blood vessels from the perichondrium invade the cartilage bed. The blood first carries calcium salts and osteoblasts, the bone building cells.
e. Osteoblasts use the calcium to form a thin bony wall surrounding open pockets of spongy bone. This lies in the center of the cartilage bed.
f. The walls between the spaces in the center of the bed suddenly break down. This leaves a large space for the bone marrow.
g. On the outside of the cartilage. Cells from the perichondrium lay down a collar of bone around the model. This bone is not thin, but dense and compact. As the bone accumulates, the bone gets wider. Bones lengthen at the two newborn forming centers at either end- the epiphysial plates.
h. These plates yield to bone forming cells on either side of each plate they lay down material to make the bone gow longer, a process which stops at 18-24 years of age.
3. Ossification continues
The bone cells become entombed in the small hardened cavities and enter a quiet like state. They can communicate with each other through a series of capillary canals. There are web like structures called the Haversian system. This is the unit of bone. Each consists of a Haversian Canal, which contains blood vessels, surrounded by a concentric ring called the Lamellae of calcified bone. Within each ray are obsteoblasts eac cell had a minute cavity called a lacuna. Between bone cells are little canals called Canaluculi which connect with the Haversian Canals. Bone cells exchange materials by way of circulating the materials through Canaliculi. If the bone is broken, osteoblasts in the periosteum produce bone. The calcium for bone formation comes from the blood. Calium is also necessary for muscle contraction.