Question

Why are we not inundated with bacterial infections on our skin? and Compare and contrast the two modes of embryonic bone formation. Be sure to highlight what these are and describe the unique developmental cascades that unfold for each.

Answer 1

The different layers and structures of the skin are

• Epidermis
• Basement membrane zone
• Dermis and its structure
• Subcutaneous (adipose) tissue underneath

SKIN

Skin is the largest organ of the body.

It covers approximately 18sq. ft & weighs up to 12 lbs.

It receives approximately 1/3 of circulating blood volume.

It can be nourished via perfusion and topically.

EPIDERMIS

• It is the visible layer of the skin.
• It is divided into 4 - 5 defined stratified layers - containing keratinocytes.
• Outermost layer is called "stratum corneum".
• It is no blood vessels, nourishment originates from small blood vessels in the dermis

STRATUM CORNEUM

• Its main function is to maintain the protective function of the epidermis.
• It consists of 15 - 20 layers or comeocytes ( "dead" skin cells)
• Corneocytes are held together by lipids.
• They are joined together by protein links called desmosomes.
• They create a " brick and mortar" structure of the skin.
• A healthy stratum corneum provides the best line of defense against invasion.

Compare & Contrast the modes of bone formation

Appositional Growth	Interstitial Growth
occurs only on the surface of the tissue Addition to surface Osteoprogenitor cells Bone - Hard tissues Calcified tissue Growth occurs in diameter Occurs at the endosteum It increases thickness	Occurs throughout the tissue Addition within        Chondroblasts - soft tissues Uncalcified tissue Longitudinal Growth Occurs at the epiphyseal plate It increases the length.

Bone Growth in length

The epiphyseal plate is the area of elongation in a long bone includes a layer of hyaline cartilage where ossification to occur in immature bones. The epiphyseal plate is divided in to diaphyseal side and the epiphyseal side. On the epiphyseal side, hyaline cartilage cells are active and are dividing and producing hyaline cartilage are active and are dividing and producing hyaline cartilage matrix. On the diaphyseal side, cartilage calcifies and dies, then is replaced by bone. As cartilage grows, the entire structure grows in length and then is turned into bone. Once cartilage cannot grows further, the structure cannot elongate more.

The epiphyseal plate is composed of five zones of cells and activity. The reserve zone is the region closest to the epiphyseal end of the plate and contains small chondrocytes within the matrix. These chondrocytes do not participate in bone growth but secure the epiphyseal plate to the overlying osseous tissue of the epiphysis.

The proliferative zone is the next layer towards the diaphyses and contains stacks of slightly larger chondrocytes.It makes new chondrocytes to replace those that dies at the diaphyseal end of the plate. Chondrocytes in the next layer, the zone of maturation and hypertrophy, are older and larger than those in the proliferative zone. The more mature cells are situated close to the diaphyseal end of the plate. The longitudinal growth of bone is a result of cellular division in the proliferative zone and the maturation of cells in the zone of maturation and hypertrophy. This growth within a tissue is called interstitial growth.

Bone Growth in diameter

While bones are increasing in length, they are also increasing in diameter. Growth in diameter can continue even after longitudinal growth ceases. This growth by adding to the free surface of bone is called appositional growth. Appositional growth can occur at the endosteum or periosteum where osteoclasts resorb old bones that lines the medullary cavity, while osteoblasts produce new bone tissue. The erosion of old bone along the medullary cavity and the deposition of new bone beneath the periosteum not only increase the diameter of the diaphysis but also increase the diameter of the medullary cavity. This remodeling of bone primarily takes place during a bone's growth.

In adult life, bone undergoes constant remodeling, in which resorption of old or damaged bone takes place that which is resorbed. Injury, exercise and other activities lead to remodeling.