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The microscopic structural unit of compact bone is called an osteon    , or Haversian system. Each osteon is composed of concentric rings of calcified matrix called lamellae (singular = lamella). Running down the center of each osteon is the central canal    , or Haversian canal, which contains blood vessels, nerves, and lymphatic vessels. These vessels and nerves branch off at right angles through a perforating canal    , also known as Volkmann’s canals, to extend to the periosteum and endosteum.

The osteocytes are located inside spaces called lacunae (singular = lacuna), found at the borders of adjacent lamellae. As described earlier, canaliculi connect with the canaliculi of other lacunae and eventually with the central canal. This system allows nutrients to be transported to the osteocytes and wastes to be removed from them.

Spongy (cancellous) bone

Like compact bone, spongy bone    , also known as cancellous bone, contains osteocytes housed in lacunae, but they are not arranged in concentric circles. Instead, the lacunae and osteocytes are found in a lattice-like network of matrix spikes called trabeculae    (singular = trabecula) ( [link] ). The trabeculae may appear to be a random network, but each trabecula forms along lines of stress to provide strength to the bone. The spaces of the trabeculated network provide balance to the dense and heavy compact bone by making bones lighter so that muscles can move them more easily. In addition, the spaces in some spongy bones contain red marrow, protected by the trabeculae, where hematopoiesis occurs.

Diagram of spongy bone

This illustration shows the spongy bone within the proximal epiphysis of the femur in two successively magnified images. The lower-magnification image shows two layers of crisscrossing trabeculae. The surface of each is dotted with small black holes which are the openings of the canaliculi. One of the trabeculae is in a cross section to show its internal layers. The outermost covering of the lamellae is called the endosteum. This endosteum surrounds several layers of concentric lamellae. The higher-magnification image shows the cross section of the trabeculae more clearly. Three concentric lamellae are shown in this view, each possessing perpendicular black lines. These lines are the canaliculi and are oriented on the round lamellae similar to the spokes of a wheel. In between the lamellae are small cavities called lacunae which house cells called osteocytes. In addition, two large osteoclasts are seated on the outer edge of the outermost lamellae. The outermost lamellae are also surrounded by groups of small, white, osteoblasts.
Spongy bone is composed of trabeculae that contain the osteocytes. Red marrow fills the spaces in some bones.

Aging and the…

Skeletal system: paget’s disease

Paget’s disease usually occurs in adults over age 40. It is a disorder of the bone remodeling process that begins with overactive osteoclasts. This means more bone is resorbed than is laid down. The osteoblasts try to compensate but the new bone they lay down is weak and brittle and therefore prone to fracture.

While some people with Paget’s disease have no symptoms, others experience pain, bone fractures, and bone deformities ( [link] ). Bones of the pelvis, skull, spine, and legs are the most commonly affected. When occurring in the skull, Paget’s disease can cause headaches and hearing loss.

Paget's disease

This illustration shows the normal skeletal structure of the legs from an anterior view. The flesh of the legs and feet are outlined around the skeleton for reference. A second illustration shows the legs of someone with Paget’s disease. The affected person’s left femur is curved outward, causing the left leg to be bowed and shorter than the right leg.
Normal leg bones are relatively straight, but those affected by Paget’s disease are porous and curved.

What causes the osteoclasts to become overactive? The answer is still unknown, but hereditary factors seem to play a role. Some scientists believe Paget’s disease is due to an as-yet-unidentified virus.

Paget’s disease is diagnosed via imaging studies and lab tests. X-rays may show bone deformities or areas of bone resorption. Bone scans are also useful. In these studies, a dye containing a radioactive ion is injected into the body. Areas of bone resorption have an affinity for the ion, so they will light up on the scan if the ions are absorbed. In addition, blood levels of an enzyme called alkaline phosphatase are typically elevated in people with Paget’s disease.

Bisphosphonates, drugs that decrease the activity of osteoclasts, are often used in the treatment of Paget’s disease. However, in a small percentage of cases, bisphosphonates themselves have been linked to an increased risk of fractures because the old bone that is left after bisphosphonates are administered becomes worn out and brittle. Still, most doctors feel that the benefits of bisphosphonates more than outweigh the risk; the medical professional has to weigh the benefits and risks on a case-by-case basis. Bisphosphonate treatment can reduce the overall risk of deformities or fractures, which in turn reduces the risk of surgical repair and its associated risks and complications.

Blood and nerve supply

The spongy bone and medullary cavity receive nourishment from arteries that pass through the compact bone. The arteries enter through the nutrient foramen    (plural = foramina), small openings in the diaphysis ( [link] ). The osteocytes in spongy bone are nourished by blood vessels of the periosteum that penetrate spongy bone and blood that circulates in the marrow cavities. As the blood passes through the marrow cavities, it is collected by veins, which then pass out of the bone through the foramina.

In addition to the blood vessels, nerves follow the same paths into the bone where they tend to concentrate in the more metabolically active regions of the bone. The nerves sense pain, and it appears the nerves also play roles in regulating blood supplies and in bone growth, hence their concentrations in metabolically active sites of the bone.

Diagram of blood and nerve supply to bone

This illustration shows an anterior view if the right femur. The femur is split in half lengthwise to show its internal anatomy. The outer covering of the femur is labeled the periosteum. Within it is a thin layer of compact bone that surrounds a central cavity called the medullary or marrow cavity. This cavity is filled with spongy bone at both epiphyses. A nutrient artery and vein travels through the periosteum and compact bone at the center of the diaphysis. After entering the bone, the nutrient arteries and veins spread throughout the marrow cavity in both directions. Some of the arteries and veins in the marrow cavity also spread into the spongy bone within the distal and proximal epiphyses. However, additional blood vessels called the metaphyseal arteries and the metaphyseal veins enter into the metaphysis from outside of the bone.
Blood vessels and nerves enter the bone through the nutrient foramen.

Watch this video to see the microscopic features of a bone.

Chapter review

A hollow medullary cavity filled with yellow marrow runs the length of the diaphysis of a long bone. The walls of the diaphysis are compact bone. The epiphyses, which are wider sections at each end of a long bone, are filled with spongy bone and red marrow. The epiphyseal plate, a layer of hyaline cartilage, is replaced by osseous tissue as the organ grows in length. The medullary cavity has a delicate membranous lining called the endosteum. The outer surface of bone, except in regions covered with articular cartilage, is covered with a fibrous membrane called the periosteum. Flat bones consist of two layers of compact bone surrounding a layer of spongy bone. Bone markings depend on the function and location of bones. Articulations are places where two bones meet. Projections stick out from the surface of the bone and provide attachment points for tendons and ligaments. Holes are openings or depressions in the bones.

Bone matrix consists of collagen fibers and organic ground substance, primarily hydroxyapatite formed from calcium salts. Osteogenic cells develop into osteoblasts. Osteoblasts are cells that make new bone. They become osteocytes, the cells of mature bone, when they get trapped in the matrix. Osteoclasts engage in bone resorption. Compact bone is dense and composed of osteons, while spongy bone is less dense and made up of trabeculae. Blood vessels and nerves enter the bone through the nutrient foramina to nourish and innervate bones.

Questions & Answers

Minimum blood pressure
Hasnain Reply
120/80
AmirHameed
90/60
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120/80
Sri
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IDDRISU
which of the following is the epithelial tissue that lines the interior of blood vessels?
Firomsa Reply
endothelial tissues
Osaki
Endothelial tissue
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what is the function of the skeletal system?
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riad Reply
the long bone with two primary center's of ossification for shaft is ________
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medulla
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There are only six days during any cycle when a woman can get pregnant - the five days leading up to ovulation and the 24 hours after ovulation. This is because sperm can live for up to 5 days in a woman's body, and the ovum lives for only 12-24 hours.
THE
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It's the men version of menopause!
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If our blood sugar drops below 70 mg/dL, we may have symptoms, such as feeling tired, weak, or shaky, become confused or drowsy or even lose consciousness and possibly die.
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symptoms of hypoglycemia include: shakiness, dizziness, headache, confusion, rapid pulse rate, sweating, Hunger, etc.
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what is homeostasis
Nikky Reply
what is respiratory
Nikky
Respiratory is breathing
Atone
respiration is an exchange of gases (O2 & CO2) between the body and outside environment.
joey
homeostasis is internal balance.
Ali
Respiratory is system where the body takes in oxygen n releases carbondioxide
Hassan
respiratory system is a system where exhalation and inhalation takes place
Kom
1)inhalation: active diaphragm & external intercoastal muscles 2)exhalation: passive (allow muscle group to relax)
Shoukat
mantaince of body is called homeostasis
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Sakshi
Face ki he ethmoid bone
Rakesh
ethmoid bone. Irregularly shaped bone anterior to the solenoid. Forms the roof of the nasal cavity, upper nasal septum, and part of the medial orbit walls.
LaKecia
Its unpaired bone of skull that saprate the nasal cavity from the brain it's located at the roof of the nose between the two orbits it's bone is one of the bones that make up the orbit of eye.
SARFARAJ
sphenoid I meant
LaKecia
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Rakesh
till me fast my ans
Rakesh
Also included in the ethmoid bone is the crista galli and the cribriform plates. The plates help form the roof of the nasal cavity and the floor of the anterior cranial fossa.
LaKecia
the outer most covering of the brain ( the dura mater) attached to the crista galli and helps secure the brain in the cranial cavity
LaKecia
Sakshi Mishra. I hope that helped!
LaKecia
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Khubaib
synovial joint will have present in wrist
sk
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sk
respiration is the process of inspiration an expiration
Bel
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sk
The protection of internal envirnment from the harm of external envirnmen is called as homeostasis.
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skin
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Source:  OpenStax, Anatomy & Physiology. OpenStax CNX. Feb 04, 2016 Download for free at http://legacy.cnx.org/content/col11496/1.8
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