Skip to main content
Biology LibreTexts

7.2: Osseous Tissue (Bone Tissue)

  1. Last updated
  2. Save as PDF
  • Page ID
    53598

    • \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\)

    Osseous Tissue (Bone Tissue)

    Bone tissue (osseous tissue) is a hard and mineralized connective tissue. Bone tissue is made up of different types of bone cells. Osteoblasts and osteocytes are involved in the formation and mineralization of bone; osteoclasts are involved in the resorption of bone tissue. Modified (flattened) osteoblasts become the lining cells that form a protective layer on the bone surface. The mineralized matrix of bone tissue has an organic component of mainly collagen called ossein and an inorganic component of bone mineral made up of various salts. Bone tissue is a mineralized tissue of two types: compact bone (cortical) and spongy bone (trabecular or cancellous).

    This illustration shows a cross section of a cranial bone, constructed somewhat like a sandwich. The topmost and bottommost layers are the thin, translucent, periosteum. The upper and lower periosteum cover an upper and lower layer of compact bone, respectively. The compact bone is solid, with each layer occupying about one tenth of the thickness of the cranial bone. The majority of the cross section is occupied by the spongy bone, or diploe, sandwiched between the upper and lower compact bone. The spongy bone contains many crisscrossing threads of bone. Dark air spaces occur between the threads, giving the bone a porous appearance, much like that of a sponge or Swiss cheese.

    Above: A cross section of bone tissue shows the outer layers are composed of compact bone and the inner layers are composed of spongy bone. A membrane composed of dense connective tissue called the periosteum forms the outermost layer of bone.

    Illustrated diagram shows the structure of bone on a macroscopic and microscopic level.

    Illustrated diagram shows the structure of bone on a macroscopic and microscopic level.

    Above: These illustrations of the structure of bone shows compact bone making up outer layers of bone and spongy bone making up the inner layers of bone. Compact bone is organized into osteons, circular structures surrounding a central canal where blood vessels (red and blue), nerves (yellow), and lymphatic vessels (green) can pass through the bone tissue. Circular rings called lamellae surround the central canals in each osteon. Spongy bone consists of slender bony projections called trabeculae. The trabeculae are surrounded by bone marrow tissues. The outermost layer of compact bone is surrounded by dense connective tissue called the periosteum. Between the periosteum and the osteons of compact bone are layers of circumferential lamellae, tissue that surround the entire bone in rings, similar to how the lamellae of an osteon surrounds the central canal.

    Bone is not uniformly solid, it consists of a flexible matrix and bound minerals, which are intricately woven and endlessly remodeled by a group of specialized bone cells. Their unique composition and design allow bones to be relatively hard and strong, while remaining lightweight. Bone matrix is 90 to 95% composed of elastic collagen fibers, also known as ossein and the remainder is ground substance. The elasticity of collagen improves fracture resistance. The matrix is hardened by the binding of inorganic mineral salt calcium phosphate in a chemical arrangement known as hydroxyapatite. It is the bone mineralization that give bones rigidity.

    Bone is actively constructed and remodeled throughout life by special bone cells known as osteoblasts and osteoclasts. Within any single bone, the tissue is woven into two main patterns, known as compact bone and spongy bone, and each with different appearances and characteristics.

    Compact Bone

    The hard, outer layer of bone is composed of compact bone also called cortical bone being much denser than spongy bone. The cortical bone gives bone its smooth, white, and solid appearance, and accounts for 80% of the total bone mass of an adult human skeleton. It facilitates bone's main functions: to support the whole body, protect organs, provide levers for movement, and store and release minerals, mainly calcium and phosphorous. It consists of multiple microscopic functional units, called an osteon. The osteons (tissue organizational structure known as the Harvesian system) are arrange as multiple layers of osteoblasts and osteocytes around a central canal or Haversian canal where blood vessels, nerves, and lymphatic vessels pass through compact bone. The blood vessels, nerves, and lymphatic vessels passing through the central canals connect with other central canals of other osteons and with the periosteum through channels oriented at right angles known as Volkmann's canals.

    Microscopic image of compact bone tissue with osteons labeled.

    Above: Compact bone. Tissue is magnified by 40x.

    Osteons are metabolically active, and as bone is reabsorbed and created the nature and locations of the cells within the bone change. Compact bone is covered by a periosteum on its outer surface, and an endosteum on its inner surface. The endosteum is the boundary between the compact bone and the spongy bone. The periosteum is formed of two layers the outer fibrous layer made of dense irregular connective tissue. The outer layer of periosteum contains blood vessels and nerves and the inner layer of periosteum contains osteogenic cells that become osteoblasts (build the matrix) and osteoclasts (reabsorb the matrix). The fibers of the periosteum extend to form Sharpey’s fibers that anchor the periosteum to the bone.

    Microscopic image of compact bone tissue with osteocyte in lacuna, osteon, central canal, and lamella labeled.

    Above: Compact bone. Tissue magnified by 200x.

    Spongy Bone

    Spongy bone, also called trabecular bone or cancellous bone, is the internal tissue of bone and is an open cell porous network. Spongy bone has a higher surface-area-to-volume ratio than compact bone because it is less dense. This makes it weaker and more flexible. The greater surface area also makes it suitable for metabolic activities such as the exchange of calcium ions. Spongy bone is typically found at the ends of long bones, near joints and in the interior of vertebrae. Spongy bone is highly vascular and often contains red bone marrow where hematopoiesis, the production of blood cells, occurs. The primary anatomical and functional unit of spongy bone is the trabecula. The trabeculae are aligned towards the mechanical load distribution that a bone experiences within long bones such as the femur.

    Thin formations of osteoblasts, covered in endosteum, create an irregular network of spaces, known as trabeculae. The words spongy bone refers to a tiny lattice of boney extensions (trabeculae) that form the tissue. Within these spaces are bone marrow and hematopoietic stem cells that give rise to platelets, red blood cells (erythrocytes) and white blood cells (leukocytes). Trabecular marrow is composed of a network of rod- and plate-like elements that make the overall organ lighter and allows room for blood vessels and marrow. Trabecular bone accounts for the remaining 20% of total bone mass but has nearly ten times the surface area of compact bone.

    Microscopic image of spongy bone and bone marrow.

    Above: Spongy bone tissue (trabeculae) with pockets of bone marrow surrounding it. Tissue is magnified by 400x.

    Bone Marrow

    Bone marrow, also known as myeloid tissue in red bone marrow, can be found in almost any bone that contains spongy tissue. In newborns, all such bones are filled exclusively with red marrow, but as the child ages the hematopoietic fraction decreases in quantity and the fatty fraction called marrow adipose tissue (MAT) or yellow bone marrow increases in quantity. In adults, red marrow is mostly found in the bone marrow of the femur, the ribs, the vertebrae and pelvic bones.

    Bone Cells

    Bone is a metabolically active tissue composed of several types of cells. These cells include osteoblasts, which are involved in the creation and mineralization of bone tissue, osteocytes, and osteoclasts, which are involved in the reabsorption of bone tissue.

    The top of this diagram shows the cross section of a generic bone with three zoom in boxes. The first box is on the periosteum. The second box is on the middle of the compact bone layer. The third box is on the inner edge of the compact bone where it transitions into the spongy bone. The callout in the periosteum points to two images. In the first image, four osteoblast cells are sitting end to end on the periosteum. The osteoblasts are roughly square shaped, except for one of the cells which is developing small, finger like projections. The caption says, “Osteoblasts form the matrix of the bone.” The second image called out from the periosteum shows a large, amorphous osteogenic cell sitting on the periosteum. The osteogenic cell is surrounded on both sides by a row of much smaller osteoblasts. The cell is shaped like a mushroom cap and also has finger like projections. The cell is a stem cell that develops into other bone cells. The box in the middle of the compact bone layer is pointing to an osteocyte. The osteocyte is a thin cell, roughly diamond shaped, with many branching, finger-like projections. The osteoctyes maintain bone tissue. The box at the inner edge of the compact bone is pointing to an osteoclast. The osteoclast is a large, round cell with multiple nuclei. It also has rows of fine finger like projections on its lower surface where it is sitting on the compact bone. The osteoclast reabsorbs bone.

    Above: Illustration showing the different locations and appearances of the different types of bone cells.

    Osteoblast

    Osteoblasts are mononucleate bone-forming cells. They are located on the surface of osteon seams and make a protein mixture known as osteoid, which mineralizes to become bone. The osteoid seam is a narrow region of newly formed organic matrix, not yet mineralized, located on the surface of a bone. Osteoid is primarily composed of type I collagen. Osteoblasts also manufacture hormones, such as prostaglandins, to act on the bone itself. The osteoblast creates and repairs new bone by actually building around itself. First, the osteoblast puts up collagen fibers. These collagen fibers are used as a framework for the osteoblasts' work. The osteoblast then deposits calcium phosphates which are hardened by hydroxide and bicarbonate ions. The brand-new bone created by the osteoblast is called osteoid. Once the osteoblast is finished working it is actually trapped inside the bone once it hardens. When the osteoblast becomes trapped inside a space (lacuna), it becomes known as an osteocyte. Other osteoblasts remain on the top of the new bone and are used to protect the underlying bone, these become known as lining cells.

    Osteocyte

    Osteocytes are mostly inactive osteoblasts. Osteocytes originate from osteoblasts that have migrated into and become trapped and surrounded by bone matrix that they themselves produced. The spaces they occupy are known as lacunae. Osteocytes connect to osteoblasts and other osteocytes for the purposes of communication. Osteocytes remain in contact with other cells in the bone through gap junctions—cell-to-cell attachments specialized for cell-to-cell communication—which pass through small channels in the bone matrix called the canaliculi.

    Osteoblast

    Osteoclasts are very large multinucleate cells that are responsible for the breakdown of bones by the process of bone resorption. New bone is then formed by the osteoblasts. Bone is constantly remodeled by the resorption of osteoclasts and created by osteoblasts. Because the osteoclasts are derived from a monocyte stem-cell lineage, they are equipped with phagocytic-like mechanisms similar to circulating macrophages. Osteoclasts mature and/or migrate to discrete bone surfaces. Upon arrival, active enzymes, such as tartrate resistant acid phosphatase, are secreted against the mineral substrate. The reabsorption of bone by osteoclasts also plays a role in calcium homeostasis.

    Extracellular Matrix

    Bones consist of living cells embedded in a mineralized organic matrix. This matrix consists of organic components, mainly type I collagen – "organic" referring to materials produced as a result of the human body – and inorganic components, primarily hydroxyapatite and other salts of calcium and phosphate. Above 30% of the acellular part of bone consists of the organic components, and 70% of salts. The collagen fibers give bone its tensile strength, and the interspersed crystals of hydroxyapatite give bone its compressive strength. These effects are synergistic.

    The inorganic composition of bone (bone mineral) is primarily formed from salts of calcium and phosphate, the major salt being hydroxyapatite (Ca10(PO4)6(OH)2). The exact composition of the matrix may be subject to change over time due to nutrition and biomineralization, with the ratio of calcium to phosphate varying between 1.3 and 2.0 (per weight), and trace minerals such as magnesium, sodium, potassium and carbonate also being found.

    Type I collagen composes 90–95% of the organic matrix, with remainder of the matrix being a homogenous liquid called ground substance consisting of proteoglycans such as hyaluronic acid and chondroitin sulfate, as well as non-collagenous proteins such as osteocalcin, osteopontin or bone sialoprotein. Collagen consists of strands of repeating units, which give bone tensile strength, and are arranged in an overlapping fashion that prevents shear stress.

    This page titled 7.2: Osseous Tissue (Bone Tissue) is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Rosanna Hartline.

    1. Back to top
    • Was this article helpful?

    Recommended articles

    1. Article type
      Section or Page
      Author
      Rosanna Hartline
      License
      CC BY-NC-SA
      License Version
      4.0
    2. Tags
      This page has no tags.