Skeletal development

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STFM: skeletal tissue forming mesencyme
3 Precursors of the skeleton (sources of STFM):: 1. Paraxial mesoderm
2. Lateral plate mesoderm
3. Ectoderm (neural crest)
What specifically does ectoderm form in the skeleton?: structures in the head
What does STFM form at the site of bone/cartilage formation?: Preskeletal condensation
3 Factors influencing STFM differentiation:: -Epithelial-mesenchymal interactions
-Specific transcription factors
-Growth factors
2 specific transcription factors influence STFM differentiation:: Sox-9 (cartilage)

Cbfa-1 (bone)
A growth factor that acts during skeletal development:: Bone morphogenetic factor (BMP)
Specific component of paraxial mesoderm that contributes to STFM:: Scleratome of somites
Specific layer of lateral plate mesoderm that contributes to STFM:: Somatic layer
2 Pathways of bone formation:: -Intramembranous ossification

-Endochondrial ossification
Each pathway starts with:: Preskeletal Mesenchyme Condensation
1st step in forming bones:: STFM cells differentiate into CHONDROBLASTS and form cartilage
Factor that directs differentiation of STFM cells forming bones:: Cbfa-1 = intramembranous oss

Sox-9 = endochondrial oss
Steps 2-4 in forming cartilage:: 2. Chondroblasts -> Chondrocytes
3. Cytes secrete matrix
4. Cartilage forms; surrounding cells are Perichondrium
2 fates of cartilage:: -Remain cartilage for life

-Form bone by intramembranous or endochondrial ossification
How does Intramembranous ossification get started?: During preskeletal condensation, osteoblasts form directly by direction of cbfa-1
What are qualities of bone that forms by intramemb ossification?: Superficial, flat, and grows by adding new bone to edge.
How does Endochondrial ossification get started?: Cartilage forms first to provide a model.
Qualities of bone formed by endochondrial ossification:: -Deep
-Grows in a focused direction
Ossification centers are: sites of collagen synthesis - where ossification occurs.
What info do ossification centers give?: Bone age - indicator of skeletal growth and maturation compared to actual age of patient.
What is bone age?: The amount of epiphyseal cartilage in skeleton
Bone maturation that uses ossification centers:: both - intramemb and endochond.
2 types of ossification centers:: Primary vs. Secondary
Primary ossification centers: -When in devo? -Where?: Prenatal - located at CENTER of bones; radiates from there.
Why does the skeleton have to develop prenatally?: To support respiration
Secondary ossification centers: -When in devo? -Where?: Pre/postnatal, postpuberal; located at ENDS of bones; growth happens from ends->in; disappear in 20's - 30's
Axial skeleton consists of:: -Vertebral column
-Ribs
-Sternum
-Skull
Vertebrae derive from:: Scleratome mesenchyme - beginning at Somite 5
Somitic mesoderm is from:: Paraxial mesoderm
3 components of somites:: -Dermatome
-Myotome
-Scleratome
4 steps in forming vertebrae:: 1. Lateral scleratome moves toward neural tube
2. Some scleratome moves close to notochord = perinotochordal sheath
3. Sclrtm condenses caudally; stay loose cranially
4. Perinotoch sheath arranges too but not in line with other
Loose potion of scleratome is associated with:: spinal nerve and intersegmental artery
Components of a primitive vertebrae:: -Centrum
-Neural arches (pair)
What grows from the neural arches?: Costal and transverse processes
What does the centrum form from?: Loose portion of perinotochordal sheath
What is needed to form a centrum?: 2 adjacent somite pairs
What do the neural arches form from?: Dense portion of scleratome
What does the compact portion of perinotochordal sheath become?: Intervertebral disc
What does the notochord become?: Stays put where it is, as the Nucleus Pulposus
___ pr of somites forms neural arches + intervertebral disc. ___ pr of somites forms centrum: 1 pair

2 pairs
Primitive vertebrae are formed by:: Week 5-6
Regionalization of the spine is achieved by:: differential expression of hox genes along cranial/caudal axis
What type of ossification do vertebrae undergo?: Endochondrial
How many 1' ossification centers develop in each vertebra?: 3-4
What is the neurocentric junction?: Line between centrum and neural arches; cartilage plate remains until 3-6 yrs old.
When do secondary ossification centers form on vertebrae?: Puberty
Primary ossification centers: ____/centrum which ____ ___/neural arch: 2/centrum - usually fuse
1/neural arch
Regionalization signals:: -Hox genes
-Retinoic acid
3 abnormalities of vertebral devo related to abnormal regionalization:: 1. Klippel-feil anomaly
2. Sacralization
3. Lumbarization
Klippel-feil anomaly:: -Fused cervical vertebrae
-Short neck
-Low hair line
Sacralization/Lumbarization:: Change in # of a TYPE but not total number of vertebrae.
Defects of Asymmetry:: -Scoliosis - lateral curvature
-Kyphosis - front curvature
-Lordosis - back
Dysraphism:: General condition caused by failure of neural arches to fuse
2 types of dysraphism:: -Rachischisis (open)

-Spina bifida
What is Rachischisis?: Condition where many vertebrae have unfused processes.
What is Spina Bifida?: Series of conditions where one or a few vertebrae have unfused processes
What is the mildest form of spina bifida?: Spina Bifida Occulta
3 Defects of Formation of vertebrae:: -Hemivertebrae
-Wedge-shaped Vertebrae
-Unsegmented Bars
What can cause hemivetebrae to form?: Failure of primary ossification centers to develop
How do ribs develop?: By lateral expansion of costal process in thoracic spine region
How does the sternum develop?: The ribs come together with 2 Sternal Bands and push them together to form the Sternum.
What is the most common form of accessory ribs? Who is it more common in?: Lumbar ribs - 3x more common in males.
2 divisions of the skull:: -Neurocranium (brain cavity)
-Viscerocranium (face/jaw)
Neurocranium consists of:: -Chondrocranium (floor)
-Calvaria (skull cap)
Viscerocranium consists of 4 basic types of bones: (how do they ossify?): -Ossicles/Styloid process via endochondral ossification

-Facial bones/Jaws via Intramembranous ossification
How many bones are in the skull?: 28
Calvaria derives from:: Neural crest
Chondrocranium (floor) derives from:: Paraxial mesoderm
Face/jaws derive from:: notochord
How does each ossify: -Calvaria -Chondrocranium: Calvaria: intramembranous
Chondrochranium: endochondral
Chondrocranium: -derived from? -How does it form?: -Mesoderm (scleratom)
-Forms by fusion of several chondrogenic centers
What allows the chondrocranium to grow longitudinally?: Spheno-ethmoidal and Spheno-occipital cartilage plates
What is the Calvaria primarily derived from?: Neural Crest Ectoderm
What are Fontanelles?: Intersections between cranial sutures that run perpendicular
What are the 4 fontanelles, and what is each one's reference pt?: Anterior - Bregma
Posterior - Lambda
2 Anterolateral - Pterion
2 Posterolateral - Asterion
What is the last fontanelle to close after birth?: Anterior - between 1.5 - 2 yrs
Face/Jaws -Derived from? -How do they form?: -Neural crest ectoderm
-Intramembranous ossification
What part of the viscerocranium forms by endochondrial ossification? Which arch?: -Malleus/incus (Arch 1)
-Stapes/Styloid process (Arch 2)
What are the calvaria bones?: -Interparietal part of occipital
-Parietal
-Frontal
What are the first fontanelles to close?: Posterior and Anteriolateral
-2 to 3 months
What are the next fontanelles to close?: Posteriorlateral - 1yr
And last?: Anterior - 1.5 to 2 yrs
What is a Dysraphic malformation of the skull referring to?: Absence of calvaria bones, or an opening between them.
What is a severe form of Dysraphic skull?: Acrania - absence of the calvaria.
What are 3 defects related to sutures?: 1. Craniosynostosis
2. Craniofacial Dysostosis
3. Cloverleaf skull
What is Craniosynostosis?: Premature closure of the sutures
What sutures do most Craniosynostoses involve? -What sex is it more common in?: 50% - sagittal
30% - coronal
More often in males
What defect is associated with Craniosynostosis?: Defective FGF Receptor 1/2 genes
What is the result of craniosynostosis?: Assymetric skull
What is Cloverleaf skull?: Severe form of craniosynostosis; Skull grows in 3 directions
What is a Dysraphic skull defect always associated with?: Meroanencephaly
What is microcephaly?: Normal face, but small calvarium
-Assoc w/ defective cerebral hemisphere growth
What is an example of a Craniovertebral Junction defect?: Occipitalization of the Atlas
What are common features associated with craniovertebral junction defects?: -Torticollis
-Short neck
-Low nuchal hairline
-Restricted movement of head on the spine
What could cause Craniovertebral Junction Defects?: Defective Hox signalling and regional patterning
What is Macrocephaly?: Enlarged skull due to excess bone thickness
Appendicular Skeleton consists of:: -Limb girdles
-Limb bones
Appendicular skeleton is derived from:: Somatic mesoderm
2 Anomalies of Pectoral girdle formation:: 1. Cleidocranial Dysplasia
2. Sprengel Anomaly
What is Cleidocranial dysplasia?: Hypo or Aplastic Clavicles
What is Sprengel Anomaly?: High set scapula - hypoplastic
How do joints form?: 1. Cartilages approach
2. Perichondrium forms and surrounds future joint
3. Interzonal mesenchyme differentiates
What 3 types of joints can form from Interzone Mesenchyme differentiation?: 1. Fibrous
2. Cartilaginous (hyaline)
3. Synovial
What are the 2 components of Interzone mesenchyme in Synovial joints, what do they become?: 1. Peripheral - becomes ligaments
2. Central - becomes synovial cavity.
What do skeletal tissue dysplasias affect? What problem causes these?: -Growth in all/part of skeleton
-Abnormal extracellular matrix production
What does the term Chondrodystrophia describe?: A group of disorders characterized by disproportionate growth.
What is the most common chondrodystrophia?: Achondroplasia - the most common type of dwarfism.
What type of stature does a person with Achondroplasia have?: -Normal trunk
-Short limbs
-Large cranium, small face
What mutation is associated with Achondroplasia?: FGF Receptor 3 gene
Marfan Syndrome: -spidery long fingers
-aortic aneurisms
-joint hypermobility
-defect of Fibrillin prodctn
Mucopolysaccharidoses: -Lysosomal storage disorder
-Chronic, progressive, multisystem deterioration
Osteogenesis Imperfecta: Collagen type I defect - brittle bones; respiratory problems
Which chondrodystrophia is not dominant inheritance?: Mucopolysaccharidoses
2 Endocrine disorders that affect skeletal devo:: -Hyperpituitarism (too much GH)

-Hypothyroidism (Cretinism)
What is Gigantism?: -Rare form of hyperpituitarism
-If GH overproduction is prior to epiphyseal closure
What is the disease called if GH overproduction is AFTER epiphyseal closure?: Acromegaly
What features are associated with Acromegaly?: Enlarged soft tissues and forehead
What age determines whether Acromegaly or Gigantism results?: -Prepuberty = Gigantism

-Postpuberty = Acromegaly
What are the common names for Hypopituitarism?: -Cretinism
-Pituitary dwarfism
What symptoms result from cretinism?: -Mental retardation
-Skeletal/ear anomalies
-Younger bone age than actual age

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