Identify the gross anatomical features of a boneDescribe the histology of bone tconcern, including the feature of bone cells and also matrixCompare and contrast compact and also spongy boneIdentify the frameworks that compose compact and spongy boneDescribe how bones are nourished and innervatedfunction?

Bone tproblem (osseous tissue) differs greatly from various other tproblems in the body. Bone is hard and also many of its features depend on that characteristic hardness. Later discussions in this chapter will certainly present that bone is additionally dynamic in that its form adjusts to accommodate stresses. This section will examine the gross anatomy of bone first and then move on to its histology.

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Gross Anatomy of Bones

A lengthy bone has actually two major regions: the diaphysis and also the epiphysis (Figure 6.3.1). The diaphysis is the hollow, tubular shaft that runs in between the proximal and also distal ends of the bone. Inside the diaphysis is the medullary cavity, which is filled through yellow bone marrow in an adult. The external walls of the diaphysis (cortex, cortical bone) are created of thick and also tough compact bone, a form of osseous tconcern.

Figure 6.3.1 – Anatomy of a Long Bone: A typical long bone showing gross anatomical functions.

The bigger section at each end of the bone is dubbed the epiphysis (plural = epiphyses), which is filled internally through spongy bone, one more kind of osseous tconcern. Red bone marrowhead fills the spaces between the spongy bone in some lengthy bones. Each epiphysis meets the diaphysis at the metaphysis. During growth, the metaphysis consists of the epiphyseal plate, the website of long bone elongation explained later on in the chapter. When the bone stops growing in beforehand adulthood (about 18–21 years), the epiphyseal plate becomes an epiphyseal line watched in the figure.

Lining the inside of the bone surrounding to the medullary cavity is a layer of bone cells called the endosteum (endo- = “inside”; osteo- = “bone”). These bone cells (defined later) reason the bone to thrive, repair, and also reversion throughout life. On the external of bones there is an additional layer of cells that thrive, repair and also remodel bone too. These cells are component of the outer double layered framework called the periosteum (peri– = “around” or “surrounding”). The cellular layer is nearby to the cortical bone and also is spanned by an external fibrous layer of dense ircontinuous connective tproblem (watch Figure 6.3.4a). The periosteum additionally has blood vessels, nerves, and also lymphatic vessels that nourish compact bone. Tendons and also ligaments attach to bones at the periosteum. The periosteum covers the whole external surface other than where the epiphyses accomplish other bones to develop joints (Figure 6.3.2). In this area, the epiphyses are extended with articular cartilage, a thin layer of hyaline cartilage that reduces friction and acts as a shock absorber.

Figure 6.32 – Periosteum and Endosteum: The periosteum forms the outer surchallenge of bone, and also the endosteum lines the medullary cavity.

Flat bones, favor those of the cranium, consist of a layer of diploë (spongy bone), extended on either side by a layer of compact bone (Figure 6.3.3). The two layers of compact bone and also the internal spongy bone work together to protect the internal organs. If the external layer of a cranial bone fractures, the brain is still protected by the intact inner layer.

Figure 6.3.3 – Anatomy of a Flat Bone: This cross-area of a level bone reflects the spongy bone (diploë) covered on either side by a layer of compact bone.Osseous Tissue: Bone Matrix and CellsBone MatrixOsseous tconcern is a connective tissue and also like all connective tconcerns has reasonably few cells and also large quantities of extracellular matrix. By mass, osseous tworry matrix consists of 1/3rd collagen fibers and also 2/3rds calcium phosphate salt. The collagen offers a scaffolding surchallenge for not natural salt crystals to adbelow (view Figure 6.3.4a). These salt crystals form as soon as calcium phosphate and also calcium carbonate integrate to develop hydroxyapatite. Hydroxyapatite also incorpoprices various other not natural salts choose magnesium hydroxide, fluoride, and sulfate as it crystallizes, or calcifies, on the collagen fibers. The hydroxyapatite crystals provide bones their hardness and also toughness, while the collagen fibers offer them a framework for calcification and also provides the bone adaptability so that it deserve to bfinish without being brittle. For instance, if you rerelocated all the organic matrix (collagen) from a bone, it would certainly crumble and shatter readily (view Figure 6.3.4b, top panel). Conversely, if you remove all the inorganic matrix (minerals) from bone and also leave the collagen, the bone becomes overly functional and cannot bear weight (see Figure 6.3.4b, reduced panel).
Figure 6.3.4a Calcified collagen fibers from bone (scanning electron micrograph, 10,000 X, By Sbertazzo – Own work, CC BY-SA 3.0,

Figure 6.3.4b Contributions of the organic and inorganic matrices of bone. Image from Ammermale figure 6-5, Pearson

Bone Cells

Although bone cells write much less than 2% of the bone mass, they are crucial to the attribute of bones. Four forms of cells are found within bone tissue: osteoblasts, osteocytes, osteogenic cells, and also osteoclasts (Figure 6.3.5).

Figure 6.3.5 – Bone Cells: Four kinds of cells are discovered within bone tissue. Osteogenic cells are undifferentiated and also build right into osteoblasts. Osteoblasts deposit bone matrix. When osteoblasts acquire trapped within the calcified matrix, they become osteocytes. Osteoclasts build from a various cell family tree and also act to resorb bone.

The osteoblast is the bone cell responsible for creating brand-new bone and also is uncovered in the growing sections of bone, consisting of the endosteum and the cellular layer of the periosteum. Osteoblasts, which execute not divide, synthedimension and also secrete the collagen matrix and also other proteins. As the secreted matrix surrounding the osteoblast calcifies, the osteoblast come to be trapped within it; as a result, it transforms in framework and becomes an osteocyte, the main cell of mature bone and the the majority of widespread form of bone cell. Each osteocyte is situated in a little cavity in the bone tworry dubbed a lacuna (lacunae for plural). Osteocytes maintain the mineral concentration of the matrix using the secretion of enzymes. Like osteoblasts, osteocytes absence mitotic task. They can interact via each other and also receive nutrients by means of long cytoplasmic processes that extfinish via canaliculi (singular = canaliculus), networks within the bone matrix. Osteocytes are linked to one another within the canaliculi using gap junctions.

If osteoblasts and osteocytes are inqualified of mitosis, then just how are they repleniburned once old ones die? The answer lies in the properties of a third category of bone cells—the osteogenic (osteoprogenitor) cell. These osteogenic cells are unidentified via high mitotic activity and also they are the only bone cells that divide. Immature osteogenic cells are discovered in the cellular layer of the periosteum and also the endosteum. They identify and also construct into osteoblasts.

The dynamic nature of bone suggests that new tconcern is constantly developed, and also old, injured, or unvital bone is liquified for repair or for calcium release. The cells responsible for bone resorption, or breakdown, are the osteoclasts. These multinucleated cells originate from monocytes and macrophperiods, two types of white blood cells, not from osteogenic cells. Osteoclasts are continually breaking dvery own old bone while osteoblasts are continually forming brand-new bone. The recurring balance between osteoblasts and also osteoclasts is responsible for the constant but subtle reshaping of bone. Table 6.3 reviews the bone cells, their functions, and also areas.

Bone Cells (Table 6.3)Cell typeFunctionLocation
Osteogenic cellsDevelop right into osteoblastsEndosteum, cellular layer of the periosteum
OsteoblastsBone formationEndosteum, cellular layer of the periosteum, growing sections of bone
OsteocytesMaintain mineral concentration of matrixEntrapped in matrix
OsteoclastsBone resorptionEndosteum, cellular layer of the periosteum, at sites of old, injured, or unneeded bone
Compact and Spongy Bone

Most bones contain compact and spongy osseous tissue, however their circulation and also concentration differ based upon the bone’s all at once function. Although compact and spongy bone are made of the exact same matrix materials and also cells, they are various in how they are organized. Compact bone is dense so that it can withstand compressive forces, while spongy bone (additionally called cancellous bone) has actually up spaces and is supportive, yet likewise lightweight and can be conveniently remodebrought about accommodate transforming body demands.

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Compact Bone

Compact bone is the denser, more powerful of the two kinds of osseous tproblem (Figure 6.3.6). It provides up the external cortex of all bones and is in prompt call with the periosteum. In lengthy bones, as you move from the external cortical compact bone to the inner medullary cavity, the bone transitions to spongy bone.