Muscoskeleton

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Questions and Answers

What are the three subdivisions of a somite?

Mesoderm, Ectoderm, Endoderm

Neural tube, Notochord, Somite

Sclerotome, Dermatome, Myotome (correct)

Paraxial, Intermediate, Lateral plate

During which week of development does the somite period occur?

Fourth week (correct)

First week

Second week

Third week

What does the sclerotome give rise to?

Vertebral column and ribs (correct)

Striated muscle

Dermis

Lateral plate mesoderm

How many pairs of somites does a human typically develop?

40-45 pairs Signup and view all the answers

Which part of the paraxial mesoderm remains unsegmented?

Preotic part Signup and view all the answers

What does the dermatome primarily give rise to?

Some dermis and subcutaneous tissue Signup and view all the answers

What type of muscle does the myotome develop into?

Striated muscle Signup and view all the answers

Which structure does not develop from the paraxial mesoderm?

Lateral plate mesoderm Signup and view all the answers

What structure is formed from mesenchyme near the junction of the transverse process and costal arch?

Costotransverse joint Signup and view all the answers

Which of the following contributes to the development of ribs in the thoracic region?

Rib Signup and view all the answers

Which is the first stage in the development of the sternum?

Formation of sternal bars Signup and view all the answers

When do ossification centers for the manubrium appear during intrauterine life (IUL)?

5th month Signup and view all the answers

At what age is the fusion of the xiphoid process with the body of the sternum typically completed?

40 years Signup and view all the answers

What is a derivative of the notochord?

Body of vertebra Signup and view all the answers

What is the correct sequence of events in sternum development?

Mesenchymal bars, cartilage sternal model, ossification Signup and view all the answers

During which week of intrauterine life do limb buds first appear?

4th week Signup and view all the answers

Which region is characterized by the development of the transverse process as a derivative of the costal element?

Lumbar region Signup and view all the answers

What occurs first in the ossification process of the sternum?

Ossification of the manubrium Signup and view all the answers

What type of muscle proteins are synthesized by myotubes?

Muscle proteins such as actin and myosin Signup and view all the answers

From which myotomes do extraocular muscles develop?

Three preotic myotomes Signup and view all the answers

Which part of the myotome forms flexor and pronator muscles in the upper limb?

Hypaxial part Signup and view all the answers

Which muscle is derived from the first pharyngeal arch?

Masseter Signup and view all the answers

Which of these muscle groups is not derived from myotomes of somites?

Muscles of mastication Signup and view all the answers

What is the initial process that leads to the formation of the body of each vertebra?

Fusion of two adjacent sclerotomes Signup and view all the answers

At what stage does the ossification of vertebrae begin?

During the 6th week of IUL Signup and view all the answers

How many primary ossification centres does each vertebra initially have at birth?

3 Signup and view all the answers

What is the role of the notochordal cells during the formation of the intervertebral disc?

To form the nucleus pulposus Signup and view all the answers

Which of the following structures develops from the dorsal group of sclerotomes?

Vertebral arch Signup and view all the answers

What process describes the fusion of caudal and cranial portions of sclerotomes?

Resegmentation Signup and view all the answers

What are the secondary ossification centres responsible for in vertebrae?

Development of the vertebral body surfaces Signup and view all the answers

Which part of the vertebra is formed by the fusion of primary and secondary ossification centres?

Vertebral body Signup and view all the answers

What do costal processes in the thoracic region elongate to form?

Costal arches Signup and view all the answers

What structure forms the pedicel, laminae, spine, and articular processes of a developing vertebra?

Dorsal group of sclerotomes Signup and view all the answers

What is the initial appearance order of forelimbs and hindlimbs during development?

Forelimbs appear first followed by hindlimbs. Signup and view all the answers

During which week does the apical ectodermal ridge (AER) form?

6th week of intrauterine life. Signup and view all the answers

What role does the apical ectodermal ridge (AER) play in limb development?

It secretes growth factor to stimulate limb mesenchyme outgrowth. Signup and view all the answers

At what stage do the hand and foot plates begin to take shape?

By the 6th week of intrauterine life. Signup and view all the answers

How are digits formed in the developing hand and foot plates?

By programmed cell death in the ectodermal ridges. Signup and view all the answers

What occurs during the rotation of limb buds?

Upper limb rotates 90° laterally, lower limb rotates 90° medially. Signup and view all the answers

What distinguishes the forelimb from the hindlimb developmentally?

Forelimb bud grows faster than the hindlimb bud. Signup and view all the answers

Which structure serves to form the rounded handplate without visible digital rays?

Early limb bud. Signup and view all the answers

Which of the following describes the segmentation of the limb bud?

The limb bud divides into distinctly recognizable parts around the 6th week. Signup and view all the answers

What causes the formation of digital rays in the handplate?

Cell death within the ectodermal ridges. Signup and view all the answers

What primarily develops from the myotome during embryonic development?

Striated muscle Signup and view all the answers

Which part of the somite gives rise to the vertebral column and ribs?

Sclerotome Signup and view all the answers

What characterizes the cross-section of a somite during development?

It is a triangular structure with a cavity. Signup and view all the answers

During which weeks is the somite period of development significant?

3 to 4 weeks Signup and view all the answers

What do the cells of the dermatome give rise to?

Dermis and subcutaneous tissue Signup and view all the answers

Which structure forms from the preotic part of the paraxial mesoderm?

Somitomeres Signup and view all the answers

How many pairs of somites does a human typically develop?

40-45 pairs Signup and view all the answers

What is the role of the paraxial mesoderm in skeletal development?

It generates the axial skeleton and somites. Signup and view all the answers

What is the primary source of development for the extraocular muscles?

3 preotic myotomes Signup and view all the answers

Which muscle is NOT derived from the myotomes of somites?

Palatoglossus muscle Signup and view all the answers

From which branchial arch do the muscles of mastication derive?

1st arch Signup and view all the answers

What structures develop from 4 occipital myotomes?

Muscles of the tongue Signup and view all the answers

Which of the following pairs correctly matches the part of the myotome with its corresponding muscle type?

Epaxial part - Extensor muscles Signup and view all the answers

What is the primary contribution of the dorsal group of sclerotomes in vertebra development?

Formation of vertebral arch and spine Signup and view all the answers

At what age do all secondary ossification centers fuse with the vertebra?

25 years Signup and view all the answers

What does the resegmentation of sclerotomes achieve in vertebra development?

Creates distinct vertebral segments Signup and view all the answers

What forms the nucleus pulposus of the intervertebral disc?

Notochordal cells Signup and view all the answers

Which process involves the elongation of costal processes to form ribs?

Development of costal arches Signup and view all the answers

How many primary ossification centers are present in each vertebra at birth?

3 Signup and view all the answers

What is the result of the chondrification stage during vertebral development?

Formation of vertebral body Signup and view all the answers

What are the distinct parts of each vertebra at birth?

Body and neural arch Signup and view all the answers

What structure forms the pedicel, laminae, spine, and articular processes of a developing vertebra?

Vertebral arch Signup and view all the answers

Which bone is primarily formed by membranous ossification, unlike the others in the upper limb?

Clavicle Signup and view all the answers

What developmental condition is characterized by the complete absence of all four limbs?

Amelia Signup and view all the answers

Thalidomide exposure during pregnancy is primarily associated with which type of limb malformation?

Amelia Signup and view all the answers

Which bone of the lower limb is NOT part of the group that develops from the somatopleuric layer of lateral plate mesoderm?

Scapula Signup and view all the answers

Which of the following malformations results from the failure of webbed fingers or toes to degenerate?

Syndactyly Signup and view all the answers

What is the significance of the clavicle in the ossification timeline of the human skeleton?

It is the first bone to ossify. Signup and view all the answers

Which period of gestation poses the highest risk of limb malformations due to thalidomide exposure?

6th to 8th weeks Signup and view all the answers

What misconception was prevalent regarding the impact of drugs like thalidomide on developing embryos?

The placenta was believed to provide complete protection. Signup and view all the answers

Which of the following conditions involves rudimentary hands and feet attached to the trunk?

Phocomelia Signup and view all the answers

What developmental stage marks the appearance of limb bud primordia?

End of 4th week Signup and view all the answers

How does the growth rate of the forelimb bud compare to the hindlimb bud during development?

Forelimb bud grows faster Signup and view all the answers

What is the primary role of the apical ectodermal ridge (AER) in limb development?

To induce limb growth Signup and view all the answers

At what stage do the digits begin to form in the hand and foot plates?

6th week of development Signup and view all the answers

What effect does the rotation of the limbs have during development?

It shifts the preaxial and postaxial borders Signup and view all the answers

What is indicated by the presence of longitudinal mesodermal condensations in the limb bud?

Initiation of digit formation Signup and view all the answers

During what developmental phase do hand and foot plates take on a flattened shape?

6th week Signup and view all the answers

What happens at the later constriction phase in limb development?

Limb buds become recognizable as segments Signup and view all the answers

Which of the following correctly describes the limb bud's initial shape?

Paddle-shaped Signup and view all the answers

Which set of structures do the digits form from during limb development?

Digital rays and ectodermal ridges Signup and view all the answers

Study Notes

Mesoderm

The intraembryonic mesoderm differentiates into paraxial mesoderm (somites), intermediate mesoderm (nephrotome), and lateral plate mesoderm.
The paraxial mesoderm runs alongside the notochord and neural tube.

Paraxial Mesoderm

The developing otic capsules divide the paraxial mesoderm into preotic and postotic parts.
The preotic part forms the unsegmented head mesoderm, also known as somitomeres.
The postotic part forms 40-45 pairs of segments called somites, which appear in a craniocaudal sequence.

Somites

Somites appear between the 20th and 30th day of development, with the 4th week known as the somite period of development.
Humans have approximately 40-45 pairs of somites.
Each somite is triangular with a cavity.

Somite Subdivision

Somites differentiate into sclerotome, dermatome, and myotome.
The sclerotome is the ventromedial part, migrating medially to surround the neural tube and form the vertebral column and ribs.
The dermatome is the lateral part, migrating to line the ectoderm's deep surface and contribute to some dermis and subcutaneous tissue.
The myotome is the intermediate part, giving rise to striated muscle.

Development of the Vertebral Column

Vertebrae develop from two adjacent sclerotomes that fuse.
The dorsal group of sclerotomal cells form the vertebral arch and spine of vertebrae.
The ventrolateral group of sclerotomal cells form the costal elements.
Chondrification of mesenchymal vertebrae begins during the 6th week of IUL.
Ossification starts during IUL and continues until around 25 years of age.
Each vertebra has three primary ossification centers: one for the centrum and one for each half of the vertebral arch.
At birth, each vertebra has three parts (body and two halves of the vertebral arch) connected by cartilage.
Fusion of these three parts occurs between 3-6 years of age.
Secondary ossification centers develop in five locations: one for the tip of each transverse process, one for the tip of the spinous process, and one for the upper and lower surface of the vertebral body.
Fusion of the secondary ossification centers with the rest of the vertebra occurs by 25 years of age.

Resegmentation of Sclerotomes

Each sclerotome is divided into cranial and caudal portions by an intrasegmental boundary called von Ebner's fissure.
The caudal dense segment of each sclerotome fuses with the cranial loose segment of the sclerotome caudal to it.
This process is called resegmentation of the sclerotomes, resulting in each vertebra being intersegmental in development.

Formation of Intervertebral Disc

Notochordal cells form the gelatinous nucleus pulposus.
Sclerotomal cells give rise to the annulus fibrosus.

Development of Ribs

Ribs develop from costal processes in the thoracic region.
Costal processes elongate to form cartilaginous costal arches, which ossify into ribs.
In the cervical, lumbar, and sacral regions, costal processes remain rudimentary, forming the costal element of the transverse process.

Development of Sternum

The sternum develops from mesodermal condensation (lateral plate mesoderm) in the anterior body wall.
The lateral plate mesoderm forms two mesenchymal sternal bars that chondrify into cartilaginous sternal bars.
The two cartilaginous sternal bars fuse to form a cartilaginous sternum model with manubrium, body, and xiphoid process.
Ossification centers for the sternum appear before birth, except for the xiphoid, which ossifies during childhood.
Ossification of the manubrium involves a pair of ossification centers appearing during the 5th month of IUL.
Ossification of the body involves four pairs of ossification centers that appear from the 6th to 9th month of IUL, fusing to form sternebrae.
Fusion of the sternebrae occurs from below upwards, completing by 25 years of age.
The xiphoid process ossifies around the 3rd year of life, fusing with the body by 40 years of age.

Derivative of the Notochord

The notochord forms the nucleus pulposus of the intervertebral disc.

Development of Limbs

Limb buds develop from mesenchyme on the ventrolateral aspects of the body wall, appearing at the end of the 4th week of IUL.
Upper limbs appear before lower limbs.
Limb bud primordia form during the end of the 4th week.
Limb buds develop during the second month, with the upper limb bud growing faster than the lower limbs.
Limb bud primordia enlarge, leading to the formation of limbs.
The ectoderm at the tip of each limb bud thickens, forming the apical ectodermal ridge (AER), which is present until fingers arise.
The AER secretes growth factors that initiate the outgrowth of limb bud mesenchyme.
The terminal region of the limb bud flattens to form the hand and foot plates during the 6th week of IUL.
The hand and foot plates are separated from the rest of the limb bud by a circular constriction, and exhibit five longitudinal mesodermal condensations called digital rays.
Further constriction divides the limb bud into two segments, forming the arm, forearm, and hand in the upper limb; and the thigh, leg, and foot in the lower limb.
Digits are formed within the hand and foot plates as cell death occurs in the ectodermal ridges.
The upper limb rotates laterally by 90°, bringing the preaxial border and thumb to the lateral side.
The lower limb rotates medially by 90°, bringing the preaxial border and big toe to the medial side.

Development of Skeletal Muscle

Myoblasts elongate and fuse to form multinucleated myotubes (syncytium).
Myotubes synthesize muscle proteins (actin, myosin, troponin, etc.) and become muscle fibers.
Muscle protein synthesis pushes nuclei to the periphery.
Adjacent muscle fibers form bundles, fascicles, and eventually complete muscles.

Development of Individual Muscle Groups

Skeletal muscles can be grouped based on development into:
- Muscles of the trunk (body wall)
- Muscles of the branchial arches
- Extraocular muscles
- Muscles of the tongue
- Muscles of the limbs

Development of Extraocular Muscles, Tongue

Extraocular muscles develop from three preotic myotomes, and are innervated by cranial nerves III, IV, and VI.
All tongue muscles (extrinsic and intrinsic), except for the palatoglossus, are derived from four occipital myotomes.

Muscles of the Body Wall

These muscles develop from the myotomes of somites, which have an epaxial (epimere) and a hypaxial (hypomere) part.
The epaxial part forms the extensor muscles of the vertebral column, such as the erector spinae.
The hypaxial part forms the intercostal muscles, muscles of the anterior abdominal wall, muscles of the neck (longus coli, longus capitis, and scalene muscles), and muscles of the ventral midline longitudinal column or strap muscles (rectus abdominis, rectus sternalis and infrahyoid muscles).

Muscles of Pharyngeal Arches

Muscles of the pharyngeal arches develop from the mesoderm of the pharyngeal arches.
The 1st arch forms the muscles of mastication (temporalis, masseter, lateral and medial pterygoid), tensor tympani, tensor veli palatini, anterior belly of the digastric, and mylohyoid.
The 2nd arch forms muscles of facial expression, the posterior belly of the digastric, the stapedius, and the stylohyoid.
The 3rd arch forms the stylopharyngeus.
The 4th arch forms the cricothyroid, constrictors of the pharynx, and muscles of the palate (excluding tensor veli palatini).
The 6th arch forms the intrinsic muscles of the larynx (excluding cricothyroid).
The 5th arch degenerates.

Development of Muscles of the Limbs

In the 5th week, myotomes of the limb bud form anterior and posterior condensations.
The anterior mesenchymal condensation forms the flexor and pronator muscles in the upper limb, and the extensor and adductor muscles in the lower limb.

### Development of Musculoskeletal System

Somite formation: Begins around the 20th day of development and continues into the 4th week. Each somite gives rise to different tissues.
Somite subdivisions: Somites differentiate into three parts:
- Sclerotome: Forms the vertebral column and ribs.
- Dermatome: Forms some dermis and subcutaneous tissue.
- Myotome: Forms striated muscle.
Vertebral Column Formation:
- Chondrification: Begins in the 6th week of intrauterine life (IUL) with the formation of cartilaginous vertebrae.
- Ossification: Starts during IUL and continues until 25 years of age.
  - Primary ossification centers form the body and vertebral arches.
  - Secondary ossification centers form the transverse and spinous processes.
Resegmentation of sclerotomes:
- Each sclerotome divides into cranial and caudal segments.
- The caudal segment of one sclerotome fuses with the cranial segment of the next, resulting in intersegmental vertebrae.
Intervertebral disc formation:
- The notochord forms the nucleus pulposus.
- Sclerotomal cells surround the nucleus pulposus to form the annulus fibrosus.
Rib formation:
- Develop from costal processes in the thoracic region.
- Costal processes elongate to form cartilaginous costal arches, which ossify into ribs.
Limb Bud Development:
- Formation: Begins in the 4th week of development.
- Apical Ectodermal Ridge (AER): Thickened ectoderm at the tip of the limb bud.
  - It secretes growth factors that stimulate limb outgrowth.
- Hand and Foot Plates: Form in the 6th week as the limb bud flattens.
- Digit Formation: Ectodermal ridges separate and form digits.
Limb Rotation:
- Upper limbs rotate 90° laterally, positioning the thumb laterally.
- Lower limbs rotate 90° medially, positioning the great toe medially.
Limb Bone Development:
- Bones of the upper and lower limbs develop from the somatopleuric layer of the lateral plate mesoderm.
- Ossification occurs through endochondral ossification, except for the clavicle, which also has membranous ossification.
Musculoskeletal Development:
- Myoblast Fusion: Myoblasts fuse to form multinucleated myotubes.
- Muscle Fiber Formation: Myotubes synthesize muscle proteins and develop into muscle fibers.
Muscle Groups:
- Muscles of the Trunk (Body Wall): Develop from myotomes of somites.
  - Epaxial part forms extensor muscles of the vertebral column.
  - Hypaxial part forms intercostal, abdominal wall, neck, and strap muscles.
- Muscles of Branchial Arches: Develop from mesoderm in the pharyngeal arches.
- Extraocular Muscles: Develop from preotic myotomes.
- Muscles of the Tongue: Develop from occipital myotomes.
- Muscles of the Limbs: Develop from myotomes of the limb buds.
Clinical Correlation:
- Thalidomide Babies: Congenital malformations (amelia, phocomelia, meromelia) due to thalidomide use during early pregnancy.
- Syndactyly: Webbed fingers or toes from failure of web degeneration between digits.
- Amelia: Complete absence of all four limbs.
- Phocomelia: Rudimentary hands and feet directly attached to the trunk.
- Meromelia: Short limbs with three segments.

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Description

Explore the complexities of embryonic development with a focus on the mesoderm and its subdivisions, including paraxial mesoderm and somite formation. Understand the significance of somite differentiation into sclerotome, dermatome, and myotome during the critical weeks of human development.