2.1/2.2 bones

Section 2.1 Osseous Tissue Histology & Physiology

Histology

a. Shapes of Bones: Bones come in several basic shapes, & these shapes have both functional & developmental implications. The common bone shape include: long,flat, short, & irregular.

b. Compact vs Spongy Bone: Keep in mind that bones are made up of multiple layers of the superficial solid & dense compact bone & the deep loosely organized sponge-like spongy bone.

Figure 2.1 Bone shapes & some examples

c. The Osteon: The basic organizational & functional unit of osseous tissue is the osteon. Both compact & spongy bones contain osteons, but these are much more numerous & organized in compact bone. Compact bone contains numerous, densely-packed osteons, while the osteons in spongy bone are scattered throughout its trabeculae (flat thin plates) & spicules (rods or spines). A single osteon is comprised of multiple circular & spiral layers of concentric lamellae (singular: lamella). This lamella surrounds a passageway called central canal, which houses blood vessels & nerves. A second type of lamellae are the interstitial lamellae, which are located in spaces between each osteons (inter- means in between, while -stitial means tissue). The final type of lamellae is the circumferential lamellae, which runs parallel to the bone's surface, surrounding the circumference of the bone; it is the most superficial lamellae of the 3 types of lamellae.

In between each lamellae sit the osteocytes (mature bone cells), in small gaps/pits/holes in the bone matrix; these gaps/pits/holes are called lacunae (singular: lacuna). Each lacuna has an osteocyte inside it. Osteocytes can communicate with each other & with the rest of the body through minute (my-noot) channels through the hard bone matrix, known as canaliculi (singular: canaliculus). In a transverse section, the lamellae & central canal give compact bone the appearance of numerous sets of tree rings stacked together. Osteons are apparent in bone tissue due to the “corkscrew” pattern in which immature bones cells called osteoblasts (bone forming/building cells) produce collagen fibers during bone formation. Another type of bone cell is called an osteoclast, which are dubbed as the bone dissolving/crushing cells. Homeostasis between osteoblasts & osteoclasts maintains the osseous tissue.

d. Histological Organization of Long Bones: Long bones are the cylindrical bones of the limbs that are longer than they are wide. The long central portion (shaft) of each long bone is the diaphysis (plural: diaphyses) & houses the medullary cavity. This cavity contains yellow bone marrow (fatty tissue, that can turn back into red bone marrow if needed) in an adult & is lined by a deep thin layer of epithelium, called endosteum (plural: endostea). The deep endosteum also lines the spongy bones & canals within the bone. The complex endpiece of each long bone is called an epiphysis (plural: epiphyses). Each epiphysis contains spongy bone filled with red bone marrow (a tissue that produces blood cells). The diaphysis & epiphyses effectively form as separate bones during development, & on many long bones in an adult you can still see the scar of where these pieces fused together, called the epiphyseal line.

Living bone tissue is encased by a superficial thin layer of dense connective tissue, called periosteum (plural: periostea) & on the articular surfaces of the epiphyses there is also a thin layer of hyaline cartilage—the articular cartilage—that cushions the long bones wherever they meet at the synovial joints. This cartilage also enables the joints to articulate easily with the help of synovial fluid produced in the synovial joint. Just deep to the periosteum is a layer of compact bone, with a layer of spongy bone deep to the compact bone. The compact bone is pierced in several locations by blood vessels & nerves, which enter the bone through nutrient foramina (singular: foramen). Also, a part of the periosteum are the deep perforating fibers which penetrates into the bone matrix to help with its attachment to bone.

In summary, the order of the layers of a long bone from superficial to deep is as follows:

• Superficial layer anywhere around the long bone is covered in periosteum only.

• Superficial layer found on the epiphyses only are covered with articular cartilage.

◦ Intermediate layer of compact bone in between.

• Deep layer if located in the epiphyses contains spongy bone with its spicules & trabeculae are lined with endosteum.

• Deep layer if located in the diaphysis contains the medullary cavity is lined with endosteum as well, from here the endosteum extends to the centeral canals & nutrient foramina.

e. Histological Organization of Flat Bone: Unlike the long bones of the limbs, the flat bones, primarily found in the skull, sternum, scapula, hips, & rib bones lack a medullary cavity. Instead, the flat bones are structured like a sandwich with multiple layers—superficially, flat bones are also covered with a periosteum on each external sides of the bone, followed by a layer of compact bone tissue on either side of the bone, & spongy bone in the center lined with endosteum. This deep layer of spongy bone in a flat bone is referred to as diploë (dip-lo-ee).

In summary, the order of the layers of a flat bone from superficial to deep is as follows:

• Outer superficial layer lined with periosteum

• Intermediate layer of compact bone

• Deep spongy bone (trabeculae) lined with endosteum

• Intermediate layer of compact bone

• Inner superficial layer lined with periosteum

Figure 2.2 A histological view of a bone tissue and its structures. Note: you should be able to understand the structures found here and apply what you've learned to other bone histology images outside of the lab manual.

Figure 2.3 A close-up view of a spongy bone

Figure 2.4 A close-up view of cross-section of a long bone

Figure 2.5 Structures and sections of a long bone

Physiology

Osseous tissue also plays a critical role in the maintenance of calcium homeostasis throughout the body. Bone tissue is as active & dynamic as any tissue in the body, & is continually being broken down & restructured in order to better withstand the stresses that each bone of the skeleton encounters. These stresses are the result of both:

• The weight of various parts of the body

• The strength of the muscles attached to each bone

a. Bone Maintenance & Remodeling: Bone tissue is very adaptable & dynamic, & constantly remodels itself to better suit the stresses placed on it by the rest of the body. According to Wolff’s Law of Bone:

• Bone grows stronger to better withstand the regular forces it encounters

• The architecture of a bone is determined by the mechanical stresses placed on it

b. Calcium Homeostasis: Most of the body’s store of calcium may be found in osseous tissue at any given time. Therefore, bone also plays a central role in calcium homeostasis within the body. The regulation of the availability of free calcium in the bloodstream is primarily under control of 3 hormones:

• Parathyroid Hormone (PTH) & Calcitriol: PTH &, to a lesser extent, calcitriol, encourage bone resorption (breakdown/dissolving of the matrix of osseous tissue, & returning of minerals, including calcium, to the blood- stream). PTH is released from the parathyroid glands, while calcitriol is secreted by the kidneys.

• Calcitonin: promotes bone deposition (mineralization) of the mineral components of bone, lowering blood calcium levels. It is released by the parafollicular cells in the thyroid gland.

• In conclusion: if the blood calcium level is high, it can be lowered by releasing calcitonin in the bloodstream. As opposed to the blood calcium level being low, then it can be increased by releasing PTH or calcitriol in the bloodstream.