Bones: Histology

Summary

This script discusses the composition and structure of bones, highlighting the bone matrix and three key cell types: osteocytes, osteoblasts, and osteoclasts. It differentiates between compact (cortical) bone and cancellous (trabecular) bone, explaining their functions and appearances. The script also describes the Haversian system in compact bones, emphasizing the roles of bone cells in growth, repair, and maintenance.

Takeaways

• 🦴 Bones are primarily made up of a calcified extracellular matrix known as the bone matrix, which is composed mainly of collagen.
• 🌀 There are three main types of bone cells: osteocytes, which are found in lacunae and aid in bone nutrition; osteoblasts, which synthesize and mineralize bone matrix; and osteoclasts, which are responsible for bone resorption.
• 🔍 Morphologically, bones are categorized into two types: compact (cortical) bone, which is dense and closer to the bone surface, and cancellous (trabecular) bone, which is spongy and contains interconnecting cavities.
• 🔬 Under the microscope, bone tissue can be lamellar, where the matrix is arranged in orderly sheets, or woven, with randomly arranged collagen fibers, typically found in growing or healing bones.
• 📖 Compact bone is predominantly found in the outer layers of bones, while trabecular bone is found deeper within, forming a latticework of spicules.
• 💡 The term 'trabecula' is derived from Latin, meaning 'little beam,' which is reflected in the beam-like structures of trabecular bone that support bone marrow.
• 👀 Vertebrae are an example of bones with both compact and trabecular bone, with a thin outer layer of compact bone and a majority of trabecular bone inside.
• 🔎 Osteoclasts, which have multiple nuclei, can be differentiated from other cells like megakaryocytes, which have a single multi-lobed nucleus, by their presence in resorption cavities on the bone matrix.
• 🏗️ Compact bones have structural units known as osteons or Haversian systems, which include concentric lamellae surrounding a central Haversian canal containing blood vessels and nerves.
• 📚 Osteoblasts, which are large and have a basophilic cytoplasm, can be seen lining the inner surface of the Haversian canal, playing a role in bone formation and maintenance.

Q & A

What is the primary composition of bones?

Bones are primarily composed of an extracellular calcified material known as the bone matrix or collagen matrix.

What are the three main types of cells found in bones?

The three main types of cells in bones are osteocytes, osteoblasts, and osteoclasts.

Where are osteocytes located and what is their function?

Osteocytes are found in cavities or lacunae between the layers of the bone matrix and assist with the nutrition of the bone.

What is the role of osteoblasts in bone formation?

Osteoblasts are responsible for synthesizing and mediating the mineralization of the bone matrix.

How do osteoclasts contribute to bone remodeling?

Osteoclasts are large multinucleated cells that remove calcified bone matrix, allowing for constant turnover and remodeling of bones.

What are the two main types of bone based on morphology?

The two main types of bone based on morphology are compact or cortical bone, which is dense and closer to the surface, and cancellous or trabecular bone, which is spongy and found deeper within bones.

What is the difference between lamellar bone and woven bone?

Lamellar bone has a bone matrix arranged in sheets, while woven bone has collagen fibers arranged randomly. Lamellar bone is more mature and makes up 80% of compact bone, whereas woven bone is more immature and is found mostly in developing or healing bones.

What is the Haversian system and what are its components?

The Haversian system, also known as osteons, consists of concentric lamellae with a central canal called the Haversian canal, which contains blood vessels and nerves.

How is bone marrow related to the structure of bones?

Bone marrow is present within the interconnecting spaces formed by trabecular bone spicules and is also found within the spaces of the Haversian system.

What is the difference between a long bone and a flat bone in terms of bone structure?

Long bones typically have denser compact bone along the outer edge, while flat bones have compact bone on both of their outer flat surfaces with a majority of the bone consisting of trabecular bone.

How can osteoclasts be differentiated from other similar-looking cells?

Osteoclasts can be differentiated by identifying their multiple nuclei, as opposed to cells like megakaryocytes, which have a single multi-lobed nucleus.

Outlines

00:00

🦴 Bone Composition and Structure

Bones are made up of a calcified extracellular material known as the bone matrix, which houses three main cell types: osteocytes, osteoblasts, and osteoclasts. Osteocytes are located in lacunae and aid in bone nutrition. Osteoblasts, found on bone surfaces and within the matrix, synthesize and mineralize the bone matrix. Osteoclasts, large cells with multiple nuclei, are responsible for bone matrix resorption, facilitating bone turnover and remodeling. Bones morphologically come in two types: compact (cortical) bone, which is dense and closer to the bone surface, and cancellous (trabecular) bone, which is spongy and contains interconnecting cavities. Under the microscope, bones are seen as either lamellar, with organized sheets of matrix, or woven, with randomly arranged collagen fibers. Lamellar bone is predominantly found in compact bone, while woven bone is typical in developing or healing bones. Long bones have compact bone on the outer edge, whereas flat bones have it on both outer surfaces. Trabecular bone features spicules that form a network with bone marrow in the interconnecting spaces. The vertebrae, with its thin outer layer of compact bone and predominant trabecular bone, exemplifies this structure. Osteoclasts are identifiable within bone marrow, often within resorption cavities on the bone matrix surface, and are distinguished by their multiple nuclei compared to other cells like megakaryocytes.

05:00

🔬 Bone Cell Types and Functions

This paragraph delves into the specific locations and functions of the three main bone cell types. Osteocytes are found in the cavities or lacunae between collagen matrix layers, playing a role in bone nutrition. Osteoblasts are located along the bone surface and line the Haversian canal within the bone, where they are involved in bone synthesis and mineralization. Osteoclasts, characterized by their large size and multiple nuclei, are found on the bone matrix surface and are responsible for bone resorption, a key process in bone remodeling and maintenance.

Keywords

💡Bone matrix

The bone matrix is the extracellular calcified material that forms the primary structural component of bones. It is composed mainly of collagen and minerals, providing both flexibility and strength to the skeletal system. In the video, the bone matrix is described as having osteocytes embedded within it, which assist with the nutrition of the bone, and osteoblasts that synthesize and mediate its mineralization.

💡Osteocytes

Osteocytes are mature bone cells that reside within the cavities, or lacunae, of the bone matrix. They play a crucial role in maintaining the health of bones by sensing mechanical stress and participating in the exchange of nutrients and waste products. The video mentions that osteocytes are found between the layers of the bone matrix, highlighting their importance in bone nutrition.

💡Osteoblasts

Osteoblasts are cells responsible for the formation of new bone tissue. They synthesize the organic components of the bone matrix and are involved in the mineralization process. The video describes osteoblasts as being located mostly along the surface of bones and within the bone matrix, where they contribute to bone growth and repair.

💡Osteoclasts

Osteoclasts are large, multinucleated cells that are essential for bone resorption, the process of breaking down and removing bone tissue. They are particularly important for maintaining bone health by allowing for the constant turnover and remodeling of bones. The video script mentions that osteoclasts are responsible for removing calcified bone matrix.

💡Compact bone

Compact bone, also known as cortical bone, is the dense, hard outer portion of bones that is closer to the surface. It provides strength and protection to the bone. The video script describes compact bone as being found along the outer edge of long bones and on both outer surfaces of flat bones, emphasizing its role in structural support.

💡Cancellous bone

Cancellous bone, or trabecular bone, is the spongy, lattice-like inner portion of bones that is composed of trabeculae or spicules. It is found deeper within bones and is responsible for providing structural support while being lighter and more flexible than compact bone. The video script uses the term 'trabecula' to describe these beam-like structures, which form a network containing bone marrow.

💡Lamellar bone

Lamellar bone is a type of bone tissue characterized by its layered or sheet-like arrangement of the bone matrix. It is more mature and organized compared to woven bone and makes up the majority of bone tissue in the body. The video script notes that 80% of lamellar bone is found within compact bone, indicating its prevalence in the structurally dense areas of bones.

💡Woven bone

Woven bone is an immature form of bone tissue with collagen fibers arranged randomly. It is typically found in developing and growing bones, as well as in bones that have healed after a fracture. The video script mentions that woven bone is more common in areas of the bone that are undergoing rapid growth or repair.

💡Haversian system

The Haversian system, also known as osteons, are the structural units of compact bone. Each osteon consists of concentric layers of bone matrix, or lamellae, surrounding a central canal, the Haversian canal, which contains blood vessels and nerves. The video script describes the Haversian system as a key feature of compact bone, highlighting its importance in bone health and function.

💡Cement line

The cement line is a thin, basophilic line that borders each osteon in the Haversian system. It is a product of bone remodeling and serves as a boundary between old and new bone tissue. The video script mentions the cement line as a distinguishing feature of the osteon, illustrating the dynamic nature of bone tissue.

💡Bone marrow

Bone marrow is the soft tissue found within the cavities of bones, particularly in the trabecular bone. It is responsible for the production of blood cells and plays a role in immune function. The video script describes bone marrow as being present within the spaces formed by trabecular bone, emphasizing its vital role in the bone's internal environment.

Highlights

Bones are primarily composed of a calcified extracellular material known as the bone matrix.

There are three main types of bone cells: osteocytes, osteoblasts, and osteoclasts.

Osteocytes are located in cavities between bone matrix layers and assist with bone nutrition.

Osteoblasts are responsible for synthesizing and mineralizing the bone matrix.

Osteoclasts remove calcified bone matrix, enabling bone turnover and remodeling.

Bones morphologically come in two main types: compact (cortical) and cancellous (trabecular).

Compact bone is dense and found closer to the bone surface, while trabecular bone has interconnecting cavities.

In an EM image of the tibia, compact bone appears blue and trabecular bone yellow.

Microscopically, bones are organized as layered (lamellar) or woven bone.

Lamellar bone makes up 80% of compact bone and 20% of trabecular bone.

Woven bone has randomly arranged collagen fibers and is found in growing and healing bones.

Long bones have compact bone on the outer edge, while flat bones have it on both outer surfaces.

Trabecular bone's spicules form a network with spaces containing bone marrow.

Vertebrae have a thin outer layer of compact bone with the majority being trabecular bone.

Osteoclasts can be found within the bone marrow, sitting in resorption cavities.

Osteoblasts line the inner surface of the Haversian canal and have a basophilic cytoplasm.

Compact bones have structural units called the Haversian system or osteons.

An osteon consists of concentric lamellae with a central Haversian canal containing blood vessels and nerves.

The cement line is a thin line along the border of each osteon.

Lacunae within osteons contain remnants of osteocytes.

Transcripts

Bones are composed primarily of an extracellular calcified material called the bone matrix

or collagen matrix.

It also has three main types of cells: osteocytes, osteoblasts, and osteoclasts.

Osteocytes are found in cavities or lacunae between the layers of the bone matrix and

assist with the nutrition of the bone.

Osteoblasts are found mostly along the surface of bones, but also within the bone matrix

itself.

They’re responsible for both synthesizing and mediating the mineralization of the bone

matrix.

Osteoclasts are large multinucleated cells that are responsible for removing calcified

bone matrix and allow for the constant turnover and remodelling of bones.

Morphologically, there are two main types of bone.

The compact or cortical bone is the dense portion that’s found closer to the surface

of bones.

The deeper portion of bones have interconnecting cavities and is called the cancellous or trabecular

bone.

In this electron microscopy or EM image of the tibia, the compact bone is also blue,

and the more central trabecular bone is yellow.

Microscopically, both compact and trabecular bones will be organized or arranged in two

forms.

Layered or lamellar bone has a bone matrix that’s arranged in sheets.

80% of lamellar bone is found within the compact bone and only 20% is found within the trabecular

bone.

The more immature woven bone, has collagen fibers that are arranged randomly.

This form of bone is found mostly within developing and growing bones, as well as bones that have

healed after being fractured.

Alright, let’s compare sections of a long bone and a flat bone.

This long bone is a partial cross-section of the head of a long bone, and the flat bone

is a cross-section from the skull.

Both sections have been decalcified in order to make it easier to cut the bone into thin

sections, but the collagen is still present in the slides.

Since the denser compact bone is typically found closer to the surface, it’s seen in

the long bone along the outer edge, whereas the flat bone has compact bone on both of

its outer flat surfaces.

“Trabecula” means “little beam” in latin, which is why the trabecular bone has

small beam-like structures that are also called spicules.

The spicules connect with each other to form a network of interconnecting spaces that contain

bone marrow.

The vertebrae is another example of a bone that has both types of morphology, but its

compact bone is only a very thin outer layer, with the majority of the bone consisting of

trabecular bone.

In this image, we can also see that the bone marrow is still present within the spaces

formed by the spicules.

The osteoclasts can be found within the bone marrow.

In this image, the osteoclast is sitting within a resorption cavity it formed on the surface

of the bone matrix.

Identifying the multiple nuclei within the osteoclast can help differentiate it from

other similar-looking cells such as megakaryocytes, which only have a single multi-lobed nucleus

instead.

Okay, let’s take a closer look at compact bones, which have structural units called

the Haversian system or osteons.

An osteon consists of concentric lamellae or layers, with a diameter of about 100 to

250 µm and a central canal called the Haversian canal.

This canal contains small blood vessels and nerves, as we can see in this image.

we can see a small blood vessel and remnants of nerve fibers as well.

The concentric layers of mineralized bone matrix radiate out from the canal and are

called Haversian Lamellae.

Along the border of each osteon is a thin purple or basophilic line that’s called

the cement line.

The white spaces throughout the osteon are the lacunae, which contain remnants of osteocytes.

In this image of a haversian canal, the osteoblasts can be seen lining the inner surface of the

Haversian canal.

Osteoblasts are large spindle-shaped or cuboidal cells that have a basophilic cytoplasm.

Alright, as a quick recap…

Morphologically, bones have two main types.

The denser compact or cortical bone, which is found closer to the surface, and the cancellous

or trabecular bone, which is the spongy bone that consists of trabecula or spicules that

form interconnecting cavities.

Microscopically, the lamellar bone is arranged in sheets of collagen matrix and woven bone

has collagen fibers that are arranged randomly.

There are also three main cell types that are part of the bone.

Osteocytes are found in cavities or lacunae between the layers of collagen matrix.

Osteoblasts are found along the surface of bones and also lining the Haversian canal

within the bone.

And osteoclasts are large multinucleated cells that can be found on the surface of the bone

matrix.