COMPONENTES DE LA LECHE MATERNA

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thereby entering the Volkmann’s canals to supply the outer one third (1/3) portion of the cortex.

Remember that the inner 2/3 of the cortex was supplied by the nutrient artery discussed above.

Epiphyseal arteries:

These are the arteries of epiphyses and are derived from the peri-articular vascular arcades found on the non-articular bony surfaces. This area also has numerous foramina out of which only few are the entrance points of these arteries while the remaining are the venous exits.

Metaphyseal arteries:

These arteries are derived from the neighboring systemic vessels. These arteries directly go into the metaphyses and reinforce the metaphyseal branches of the primary nutrient artery.

Nerve supply of bones

As with all other living tissues, the bone is innervated by peripheral nerves so that it can coordinate with the central nervous system. The important part of this coordination is formed by the sensory signals coming from the bones. The brain reads these signals and makes necessary changes to avoid any damage to bones and other body tissues.

Hilton’s Law:

Hilton’s law explains the pattern of innervation of bones by the peripheral nerves. According to this law, the nerve supplying a muscle will also supply the underlying bone. Thus if a group of muscles over a bone receive nerve supply from a specific nerve, the bone, over which the muscles lie will also be innervated by the same nerve.

Features of innervation of bones:

The innervation of nerves shows the following common features:

 Nerves accompany the blood vessels so if you have to find the specific nerve supplying a specific bone, you will have to look for the nerves which accompany the arteries and veins of a bone.

 Most of the nerves coming to a bone are sympathetic and vasomotor in function.

 Some of the nerves are sensory and such nerves are distributed to the articular ends and periosteum of the bones.

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place during the second month of intrauterine life. This indicates that the bones which start ossifying before second month are membrane bones and the bones which start ossifying after 2nd month of intrauterine life are cartilaginous bones. There is another type of ossification in which a bone partly ossifies from membrane and partly from cartilage. These bones are known as membro-cartilaginous bones.

Process of ossification:

With the explanation of different types of ossification in bones it is also important to explain what ossification is. Ossification is the process by which bone is formed. It is started at certain sites known as centers of ossification each of which is a point where lying down of lamellae (bone formation) is started by the activity of osteoblasts. Osteoblasts are bone forming cells and secrete collagen and other substances that form the ground substance of bone. The centers of ossification may be primary or secondary. The primary centers of ossification appear before birth and are the first to start the process of ossification. The secondary centers of ossification mostly appear after birth but there are few exceptions to this that is some secondary centers do appear before birth. The secondary centers are sites where process of ossification starts after it has started in primary centers.

Growth of long bones

All long bones of the body (except clavicle) are formed from a cartilaginous model by the process of ossification. Once a young bone is formed, the growth takes place in three steps.

Growth in length

A long bone grows in length my multiplication of cells in the epiphyseal plate of cartilage. The cartilage cells divide and increase in number. The zone of active division in the epiphyseal plate of cartilage lies towards the epiphysis (end of the bone). This means that newly formed cartilage cells will push the older, larger cells towards the diaphysis (shaft of the bone).

Eventually these cartilage cells are replaced by osteocytes (bone cells), thus increasing the length of the bone. It should be kept in mind that after puberty, when the epiphyseal plate of cartilage no more exists, the growth in length of a bone stops completely.

103 Growth in thickness

A long bone grows in thickness by multiplication of cells in the deeper layer of periosteum.

The cells lying in the deeper layer of periosteum are known as osteoblasts (bone forming cells).

These cells divide continuously and form the osteocytes, thus increase the thickness of bone.

Remodeling of bone:

The growth takes place by deposition of new bony tissue on the surface and at the ends. This is process of bone deposition by osteoblasts is called appositional growth or surface accretion.

The appositional growth can result in shape alterations, however, in order to maintain the shape, the unwanted bone must be removed. This job is done by another class of cells known as osteoclasts. The process of reshaping the bone is called remodeling and it is one of the major factors responsible for increasing size of marrow cavity.

Types of cells in bones

Bone is formed by three primary cell types: Osteoblasts, Osteocytes and Osteoclasts.

Osteoblasts:

Osteoblasts are bone-forming cells that descend from osteoprogenitor cells. They form a protein mixture known as osteoid, which mineralizes to become bone. Osteoid is primarily composed of Type I collagen. Osteoblasts also manufacture hormones, such as prostaglandins, to act on the bone itself. They robustly produce alkaline phosphatase, an enzyme that has a role in the mineralization of bone, as well as many matrix proteins. Osteoblasts are the immature bone cells, and eventually become entrapped in the bone matrix to become osteocytes, which are the mature bone cells. All bone lining cells are osteoblasts.

Osteocytes:

Osteocytes are mature bone cells that originate from osteoblasts, which have migrated into and become trapped and surrounded by bone matrix, produced by them. The spaces they occupy are known as lacunae. Osteocytes have many processes that reach out to meet osteoblasts and other osteocytes probably for the purposes of communication. Their functions include formation of bone, maintenance of matrix and homeostasis of Calcium.

104 The spongy bone

Osteoclasts:

Osteoclasts are the cells responsible for bone resorption and remodelling. They are large, multinucleated cells located on bone surfaces in what are called Howship’s lacunae or resorption pits. These lacunae, or resorption pits, are left behind after the breakdown of the bone surface. Because the osteoclasts are derived from a monocyte stem-cell lineage, they are equipped with phagocytic-like mechanisms similar to circulating macrophages.