Skeletal System

6 Functions• Support • Protection • Movement • Mineral Storage • Site of blood cell production • Storage of energy

Functions 5 & 6• Site of blood cell production • Red bone marrow produces red

blood cells from a process called hematopoiesis.

• Yellow bone marrow stores lipids (fats) for energy

Parts of a Long Bone• Diaphysis: the shaft or long,

main cylindrical portion of the bone

• Epiphyses: The extremities or ends of the bone

• Metaphysis: In mature bone, the region where the diaphysis joins the epiphysis.

• Articular Cartilage: A thin layer of hyaline cartilage covering the epiphysis where the bone forms an joint with another bone

• Periosteum: A membrane around the surface of the bone not covered by articular cartilage.

• Medullary (marrow cavity): The space within the diaphysis that contains the fatty yellow bone marrow.

• Endosteum: The lining of the medullary cavity that consists of osteoprogenitor cells.

Histology• The skeletal system consists of four types of connective

tissue • Cartilage • Bone • Bone Marrow • Periosteum

Osseous (Bone) Tissue• Contains lots of extracellular matrix surrounding widely

separated cells.

• The matrix consists of inorganic components • Mineral salts

• And organic components • Mostly collagen fibers

4 Types of Cells in Bone1. Osteoprogenitor Cells: cells that undergo mitosis to

become osteoblasts.

2. Osteoblasts: cells that form bone, but do not have the ability to divide by mitosis.

3. Osteocytes: A mature bone cell, and maintain the daily cellular activities of bone tissue.

4. Osteoclasts: function in bone reabsorption, which is important in the development, growth, maintenance, and repair of bone.

Unique to Bone• Unlike other connective tissue the matrix of bone

contains an abundance of mineral salts.

• These salts are deposited by osteoblasts in the internal framework formed by collagen fibers.

• The tissue then hardens through a process called calcification

2 Types of Osseous Tissue• Compact (Dense) Bone Tissue • Contains very few spaces • Forms the external portion of the bones • Provide protection and support

• Spongy (Cancellous) Bone Tissue • Usually does not contain true osteons (tree ring structures) • Consists of irregular latticework of thin plates called

trabeculae • These spaces are filled with red bone marrow

How does bone form?

• When a human fetus is developing it is composed of mainly fibrous connective tissue and hyaline cartilage.

• Ossification (the process by which bones form) begins typically around the sixth or seventh week of embryonic life and continues throughout adulthood.

Endochondral Ossification• The replacement of cartilage by bone.

• Most bones are formed this way by the process is best observed in long bones.

1. In the beginning the stem cells come together to shape the future bone.

2. These cells develop into chondroblasts which produce the cartilage matrix (specifically hyaline cartilage).

3. Formation of the bone collar around the hyaline cartilage model.

4. The osteoblasts within the cartilage produce a thin shell of compact bone in the center of the model.

5. Invasion of the internal cavity by blood vessels and spongy bone begins to form.

6. The medullary cavity begins to form as well as secondary ossification centers at the epiphyses.

7. Ossification of the epiphyses which when completed hyaline cartilage remains only in the epiphyseal plates and articular cartilage.

Growth and Maintenance• Growth occurs when

cartilage cells are produced on the espiphyseal side of the plate while the cartilage cells on the diaphyseal are being destroyed and turned into bone.

• The thickness of the plate will remain relatively constant while the bone of the diaphyseal side of the plate increases in length.

• This process is controlled by the hormone, Human Growth Hormone (HGH)

Growth and Maintenance• The regulatory signals that

send out HGH slow down as we get older.

• This causes the cartilage on the epiphyseal side to the plate to stop dividing and eventually the diaphyseal side catches up.

• When the epiphysis and diaphysis fuse together then no more bone growth can occur.

• This is accelerated by sex hormones at puberty

Still Growing?

Remodeling• Like your skin, bone is

constantly being replaced throughout your life.

• This process goes from the inside out.

• Osteoclasts are responsible for the reabsorption of bone tissue.

Minerals and Vitamins• Sufficient amounts of

calcium & phosphorus are what make the bone matrix hard and strong.

• Vitamin D is required for the absorption of calcium from the digestive tract.

• Vitamin C, A and B12 also play a role in bone growth and remodeling.

Mineral Storage• Your bones are not the only organs that need calcium

and therefore act as a storage site for this mineral.

• Your bones also store phosphorous which are required to produce nucleic acids (DNA and RNA).

Calcitonin (CT) and Parathyroid (PTH)

• Calcitonin (CT) is a hormone released from the thyroid gland in response to high levels of calcium in the blood.

• It inhibits osteoclastic activity and promotes bone growth and calcium storage, which brings the levels in the blood down.

• In response to low levels of calcium in the blood, your parathyroid glands secrete a hormone called parathyroid hormone (PTH).

• This promotes the activity of osteoclasts which will release calcium into the blood.

Exercise and Bone• The bones in your body will

alter their strength in response to mechanical stress.

• Bones become stronger under this stress because they increase the deposition of mineral salts.

• However, if mechanical stress is not applied then reabsorption does not happen and your bones become weaker through a process called demineralization.

Demineralization• The main mechanical stress

on bones results from weight bearing exercise, and the force of gravity.

• If a person is bedridden or is in a cast the strength of the unstressed bone diminished because the reabsorption of minerals has stopped.

• This is why when astronauts experience weightlessness they lose bone strength (mass) quickly because there is nothing acting on them.

Aging2 principal events happen to our bones as we age.

1. A loss of calcium from bones • Starts at age 30 for females and accelerates quickly around

age 40-45 because estrogen levels start to decrease. • In males loss does not typically begin until age 60 • The loss of calcium from bones causes osteoporosis.

2. A decreased rate in protein formation which results in a decreased ability to produce organic bone matrix. This causes more accumulation of inorganic matrix making the bones brittle.

Osteoporosis• An age related disordered characterized as a decrease in

bone mass resulting in an increased susceptibility to fractures.

• The sex hormone estrogen normally promote bone formation by stimulating osteoblasts.

• But as we age (especially females) these estrogen levels decrease and bone formation slows down.

• Women are more susceptible to this disordered because estrogen promotes bone formation without the need for an increased amount of calcium deposition.

Symptoms• Causes shrinkage of back bone

and height loss

• Affects entire skeleton even teeth!

• Increased susceptibility to fractures.

Other Factors that cause Osteoporosis

1. Body build (shorter people have greater risk because they have less total mass)

2. Weight (since adipose is a good source of estrogen that slows bone loss)

3. Smoking 4. Calcium deficiencies and malabsorption 5. Vitamin D deficiency 6. Lack of Exercise 7. Certain drugs (alcohol, diuretics, cortisone, etc.) 8. Premature menopause

Rickets• A vitamin D deficiency in children in which the body

does not absorb calcium and phosphorus.

• This results in the bones in the leg bowing because the weight of the upper body is too much for them to support.

Herniated (Slipped) Disc• A protrusion (sticking out) of the inner, soft elastic

material of an intervertebral disc.

Fractures• A fracture is any break in a bone.

• We will look at 6 different kinds • Partial (incomplete) • Complete • Closed (simple) • Open (compound) • Comminuted (multifragmentary) • Compressed

Partial (Incomplete)• The break across the bone is incomplete

Complete• The break across the bone is complete

Closed (Simple)• The bone does not break through the skin

Open (Compound)• The broken ends of the bone protrude through the skin

Comminuted (multifragmentary)

• When the bone is broken into two or more pieces.

Compressed• When the bone is crushed