Bone formation Flashcards
Types of the bone
There are two main types in the
skeletal system related to both
their location and function.
- The axial skeleton: is found
along the longitudinal (vertical)
axis of the body and the skull
and vertebrae.
- The appendicular skeleton:
includes the bones of the limbs
and the girdles to support the
limbs.
Bone classification
Bones are classified as:
- Short: small bones of hands and
knees,
- Long: femur, tibia
- Flat skull
- Irregular: spinal, maxilla,
mandible
- Long bones and flat bones contain
bone marrow
Functions of the bone
Structural support: forms the shape and frame
- Physical protection of vital organs: brain, and other internal organs
- Movement: Muscle attachment; levers
- Blood cell formation: harbor Hematopoietic stem cells
- Mineral storage and regulation: Plasma Bone
- Remodeling: both physiological and pathological, such as growth and ageing
Structure of the bone
Compact bone: dense bone surrounds the
marrow cavity
- Trabecular bone (also called cancellous or
spongy bone) consists of delicate beams of bone,
trabeculae, space and sponge like network. The
ends of long bones
Cells of the bone
Progenitor cells: mesenchymal
stem cells or osteogenic cells
› Osteoblast: cells that build the
bone
› Osteocytes: cells embedded in
the bone tissue, inactive
› Osteoclast: cells that break
down the bone
Osteocyte Differentiation
MSC (Mesenchymal Stem Cell) → Becomes an osteoprogenitor cell.
Osteoprogenitor Cell → Differentiates into a pre-osteoblast (proliferation and matrix synthesis).
Pre-osteoblast → Matures into an osteoblast, which produces bone matrix.
Mature Osteoblast → Can either become a:
Lining cell (if inactive).
Osteocyte (embedded in the bone matrix, functioning in bone maintenance).
Origin of Mesenchymal Stem Cells
Mesoderm : Mesenchymal stem cells arise from the mesoderm, forming the majority of bones.
Ectoderm : Neural crest cells (from ectoderm) form the anterior skull and facial bones.
Endoderm: does not contribute to bone formation.
Intramembranous Ossification (Flat Bone Formation)
Occurs mainly in skull and facial bones, following these steps:
Mesenchymal cells (neural crest origin) differentiate into osteoblasts, forming ossification centers.
Osteoblasts secrete collagen and become trapped, turning into osteocytes.
Trabecular bone and periosteum begin forming.
Cortical bone develops on the surface of trabecular bone.
Blood vessels invade, creating red bone marrow.
Periosteum mediates new bone growth.
Endochondral Ossification (Long Bone Formation)
Occurs in long bones and follows these steps:
Mesoderm-derived mesenchymal stem cells differentiate into chondrocytes.
Chondrocytes secrete collagen type II, forming a hyaline cartilage model.
Osteoblasts migrate into the cartilage model.
Blood vessels invade, forming the medullary cavity.
New bone formation occurs at the epiphyseal plate.
Origin of Craniofacial Bones
Bones of the skull and face come from two primary sources:
Mesoderm (Paraxial) → Forms the posterior skull.
Neural crest cells (from Ectoderm) → Form the anterior skull and facial bones.
The posterior part of the skull (parietal and occipital bones) originates from mesenchymal stem cells.
The frontal and facial bones (maxilla, mandible, nasal, zygomatic, etc.) originate from neural crest cells.
📌 Key point: The neural crest cells play a critical role in forming the face and anterior skull, whereas the mesoderm forms the back of the skull.
Cranial Development in Infants
At birth, the skull is not fully fused to allow flexibility during childbirth and postnatal brain growth.
Fontanelles (soft spots) are present, which gradually close over time:
- Anterior fontanelle closes around 18 months.
- Cranial sutures fully close by 24 years of age.
Growth of the maxilla continues until ~15 years, while the mandible develops until ~20 years.
📌 Key point: The skull remains flexible in infancy to accommodate brain growth, and full ossification takes many years.
Growth of the Cranium and Face
Compared to adult size:
- 87% of cranial growth is completed by age 2.
- 98% is completed by age 18.
The growth of the face follows a different pattern than the rest of the body.
Puberty does not significantly affect cranial growth, unlike other bones.
📌 Key point: The skull grows rapidly in early childhood, and most of its development is complete before adulthood.
Ossification of the Maxilla (Intramembranous Ossification)
The maxilla forms through intramembranous ossification.
Starts in the 7th week of fetal development from neural crest mesenchyme.
==> Centers of ossification appear in the maxillary processes of the 1st pharyngeal arch.
Like the cranial bones, the maxilla arises from neural crest cells (from ectoderm), not mesoderm.
Growth continues postnatally until 14-15 years, driven by periosteal osteoblast activity.
📌 Key point: The maxilla forms directly from mesenchymal tissue, without a cartilage model.
Since it forms without cartilage, it grows differently from long bones that undergo endochondral ossification.
Mandible Formation (Both Ossification Processes)
The mandible is unique because it forms through both intramembranous and endochondral ossification:
Mandibular processes of the 1st
pharyngeal arches
– Main body: Intramembranous
ossification
Condyle = Endochondral ossification (cartilage precursor).
Forms from the 1st pharyngeal arch.
📌 Key point: The mandible is the only bone that uses both types of ossification
Inherited Mandibular Prognathism (Habsburg Jaw)
Habsburg jaw is a condition characterized by an overdeveloped mandible (prognathism).
Found in European royal families due to inbreeding.
Genetic mutations affecting jaw growth lead to mandibular overgrowth.
📌 Key point: The Habsburg jaw is an example of genetic influences on craniofacial bone growth.
Middle Ear Ossicles and Development
The middle ear contains three ossicles (small bones) that play a role in hearing:
Malleus (hammer)
Incus (anvil)
Stapes (stirrup)
These bones originate from the pharyngeal arches:
1st pharyngeal arch →Malleus and Incus
2nd pharyngeal arch →Stapes
Development of the Middle Ear
6 weeks of fetal development: The auditory ossicles begin forming from neural crest cells via endochondral ossification, meaning they develop from cartilage precursors
5 months of fetal development: The auditory canal enlarges, freeing the ossicles, which are now functional.
📌 Key point: The middle ear bones develop through endochondral ossification and are derived from neural crest cells.
Pharyngeal Arch Disorders
Developmental disorders affecting the 1st and 2nd pharyngeal arches can lead to craniofacial abnormalities:
- Treacher Collins Syndrome
1 st and 2 nd pharyngeal arches involved;
Caused by a TCOF1 gene mutation.
Mandibular ramus height was significantly shorter - Pierre Robin Sequence
Sox 9 mutation affects cartilage formation.
Causes Mandibular body length is significantly smaller retracted tongue, leading to breathing and feeding difficulties. - Hemifacial Microsomia
Second most common craniofacial disorder after cleft lip.
Leads to underdeveloped facial bones and muscles.
Often associated with ear abnormalities and hearing loss
Osteogenesis Imperfecta (Brittle Bone Disease)
📌 Key point: Defective collagen synthesis leads to fragile bones that fracture easily.
Caused by mutations in collagen genes (COL1A1, COL1A2).
80% autosomal dominant, 20% autosomal recessive.
Bones break easily with minimal injury.
Features:
Brittle bones
Blue sclera (whites of the eyes appear blue)
Dental abnormalities
Example: Mr. Glass in Unbreakable (a pop culture reference).
Maternal Health and Bone Development
Vitamin D and Calcium Deficiency
– Can lead to bone growth defects in the fetus.
Fetal Alcohol Syndrome
– Causes craniofacial deformities.
📌 Key point: Proper maternal nutrition is critical for fetal bone and skull development.
Bone Remodeling and Repair (osteoporosis)
Bone is constantly broken down and rebuilt by two types of cells:
Osteoblasts (bone-building cells) → Active in youth, under stress, and with proper hormones.
Osteoclasts (bone-resorbing cells) → Increased activity with aging, hormonal, poor nutrition (Vit D or Ca deficiency), lack of stress (exercise + weights).
📌 Key point: Balanced bone remodeling is essential to maintain bone mass and strength.
