01: Bone Biology Basics

Question 1

What is the endosteum?

Answer 1

The endosteum (plural endostea) is a thin vascular membrane of connective tissue that lines the inner surface of the bony tissue that forms the medullary cavity of long bones.

Question 2

What is the periosteum?

Answer 2

The periosteum is a membrane that covers the outer surface of all bones, except at the articular surfaces (i.e. the parts within a joint space) of long bones.

It is a composite biomaterial (tissue) composed of two main layers, including an outer fibrous layer and inner cambial layer.

Question 3

What type of tissue and cells does the periosteum consist of?

Answer 3

The periosteum consists of dense irregular connective tissue. It is divided into an outer “fibrous layer” and inner “cambium layer” (or “osteogenic layer”).

The fibrous layer contains fibroblasts, while the cambium layer contains progenitor cells that develop into osteoblasts.

Question 4

What is the medullary cavity?

Answer 4

The medullary cavity (medulla, innermost part) is the central cavity of bone shafts where red bone marrow and/or yellow bone marrow (adipose tissue) is stored; hence, the medullary cavity is also known as the marrow cavity.

The medullary cavity is the area inside any bone (long, flat, etc.) that holds the bone marrow.

Question 5

Where is the medullary cavity located? What are its walls composed off? What is it lined with?

Answer 5

Located in the main shaft of a long bone (diaphysis) (consisting mostly of compact bone), the medullary cavity has walls composed of spongy bone (cancellous bone) and is lined with a thin, vascular membrane (endosteum).

Question 6

What is hematopoiesis?

Answer 6

The formation of blood cellular components. All cellular blood components are derived from haematopoietic stem cells. In a healthy adult person, approximately 10¹¹–10¹² new blood cells are produced daily in order to maintain steady state levels in the peripheral circulation.

Question 7

What is “osteocalcin”? What does it do?

Answer 7

Osteocalcin is a small (49-amino-acid) noncollagenous protein hormone found in bone and dentin.

Osteocalcin is secreted solely by osteoblasts and thought to play a role in the body’s metabolic regulation. In its carboxylated form it binds calcium directly and thus concentrates in bone, but genetic evidence has revealed that it does not play an important role in bone mineralization.

Question 8

Why do people look at osteocalcin levels?

Answer 8

As osteocalcin is produced by osteoblasts, it is often used as a marker for the bone formation process. It has been observed that higher serum osteocalcin levels are relatively well correlated with increases in bone mineral density during treatment with anabolic bone formation drugs for osteoporosis, such as teriparatide. In many studies, osteocalcin is used as a preliminary biomarker on the effectiveness of a given drug on bone formation

Question 9

How are bones usually divided for classification?

Answer 9

Bones are divided by location into axial or appendicular groups.

Question 10

What is the axial group in bone nomenclature?

Answer 10

Set of bones found along the body’s vertical axis.

They are the foundation that protects internal organs and provided skeletal support.

Includes 80 bones divided into the skull, vertebral column, and thoracic cage regions.

Question 11

What is the appendicular group in bone nomenclature?

Answer 11

Appendicular bones are those that help us move around.

Everything that is not considered part of the axial group.

Question 12

List the groups of bone classification by shape and an example.

Answer 12

Long bones (longer than they are wide)-ex: tibia, fibula, metacarpals.

Short bones-ex: talus, scaphoid, lacunae.

Flat bones (thinner)-ex: sternum, scapula, those that make up brain case.

Irregular bones ex: vertibrae, pelvis.

Question 13

Describe in a basic way the fundamental internal structure of bone

Answer 13

Although different shapes, all bones have a similar interanl structure:

• dense, smooth looking external layer of compact, or cortical bone.
• the cortica bone surrounds a porous internal layer of spongy bone tissue made up of tiny cross hatching supports called trabeculae.
• the trabeculae is where you find red and yellow bone marrow.

Question 14

Explain red bone marrow vs yellow bone marrow?

Question 15

Describe the arrangement of bone tissues based on groups of bone classification

Answer 15

Flat, short, and irregular bones- tissues have spongy bone in the middle and are surrounded by compact bone. Think of a spongy bone sandwhich on compact-bone bread.

Long bones-Spongy bone and its red marrow are concentrated at the tips. These flared ends, or epiphyses bookend the bone’s shaft, or diaphysis, surround a hollow medullary cavity that’s full of yellow marrow.

Question 16

What are osteons?

Answer 16

Osteons are the fundamental functional unit of much compact bone. Osteons are roughly cylindrical structures that are typically between 0.25 mm and 0.35 mm in diameter cylindrical, and are weight-bearing structures that run parallel to bone’s axis.

Question 17

Describe the structure of osteons

Answer 17

Each osteon consists of concentric layers, or lamellae, of compact bone tissue that surround a central canal, the haversian canal.

The haversian canal contains the bone’s blood supplies. The boundary of an osteon is the cement line.

Question 18

What are the contents of an osteon’s lamellae?

Answer 18

Lamellae is filled with collagen fiberes that run in the same direction.

Structures of a neighboring lamellae, either on the inside or outside, run in a different direction creating an alternating pattern that reinforces the structure against torsion stress.

Question 19

What are lacunae?

Answer 19

In histology, a lacuna is a small space containing an osteocyte in bone or chondrocyte in cartilage.

The lacunae are situated between the lamellae, and consist of a number of oblong spaces. Each lacuna is occupied during life by a branched cell, termed an osteocyte (bone-cell or bone-corpuscle). A lacuna never contains more than one osteocyte.

Lacunae are connected to one another by small canals called canaliculi.

Sinuses are an example of lacuna.

Question 20

What are osteocytes?

Answer 20

An osteocyte, a star-shaped type of bone cell, is the most commonly found cell in mature bone tissue, and can live as long as the organism itself. Osteocytes do not divide and have an average half life of 25 years.

Osteocytes are simply osteoblasts trapped in the matrix that they secrete. During bone formation, an osteoblast is left behind and buried in the bone matrix as an “osteoid osteocyte”, which maintains contact with other osteoblasts through extended cellular processes.

Question 21

What do osteocytes do?

Answer 21

Although osteocytes are relatively inert cells, they are capable of molecular synthesis and modification, as well as transmission of signals over long distances, in a way similar to the nervous system.

Osteocytes are thought to be mechanosensor cells that control the activity of osteoblasts and osteoclasts within a basic multicellular unit (BMU), a temporary anatomic structure where bone remodeling occurs.

Question 22

What are osteoblasts?

Answer 22

They are bone-building cells, and they’re actually what constructs your bone in the first place. The process of bone formation is carried out by a group of organized groups of connected osteoblasts.

Osteoblasts are specialized, terminally differentiated products of mesenchymal stem cells. They synthesize dense, crosslinked collagen and specialized proteins in much smaller quantities, including osteocalcin and osteopontin, which compose the organic matrix of bone.

A group of organized osteoblasts together with the bone made by a unit of cells is usually called the osteon.

Question 23

What are osteoclasts?

Answer 23

A large multinucleated type (human osteoclasts on bone typically have five nuclei and are 150–200 µm in diameter) of bone cell that breaks down bone tissue. This function is critical in the maintenance, repair, and remodelling of bones of the vertebral skeleton.

The osteoclast disassembles and digests the composite of hydrated protein and mineral at a molecular level by secreting acid and a collagenase, a process known as bone resorption. This process also helps regulate the level of blood calcium.