Development of the Skeletal System

Questions and Answers

What are the two layers of the lateral plate mesoderm called?

Somatic and parietal mesoderm layers

Splanchnic and visceral mesoderm layers

Splanchnic and parietal mesoderm layers (correct)

Somatic and visceral mesoderm layers (correct)

How does somitomere formation progress in the embryo?

Randomly throughout the embryo

In a cephalocaudal manner (correct)

From the caudal to cephalic region

Uniformly throughout the mesoderm

Which of the following correctly represents the number of segments in somites from the occipital region caudally?

5 occipital, 10 cervical, 10 thoracic, 8 lumbar

6 occipital, 8 cervical, 12 thoracic, 5 lumbar

4 occipital, 6 cervical, 10 thoracic, 8 lumbar

4 occipital, 8 cervical, 12 thoracic, 5 lumbar (correct)

What signifies the development stage of an embryo in terms of somite count?

The number of somites present Signup and view all the answers

Which statement about the fate of somites is true?

Some somites disappear during development Signup and view all the answers

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

It causes the mesenchyme to remain as undifferentiated cells. Signup and view all the answers

During the formation of digits, what process leads to the separation of the apical ectodermal ridge into five parts?

Cell death occurring in the AER. Signup and view all the answers

What do the rotations of the upper and lower limbs during development signify?

Adaptation for bipedalism and functional positioning. Signup and view all the answers

At what stage of development do the first hyaline cartilage models of the extremities begin to form?

6 weeks of gestation. Signup and view all the answers

What process leads to the formation of joints during limb development?

Chondrogenesis being arrested and the development of a joint interzone. Signup and view all the answers

Study Notes

Organization of the Mesoderm

Mesoderm is classified into three regions: paraxial, intermediate, and lateral plate mesoderm.
Lateral plate mesoderm divides into somatic (parietal) and splanchnic (visceral) layers, lining the intraembryonic cavity.
Intermediate mesoderm links paraxial and lateral mesoderm and plays a vital role in development.

Somitomeres and Somites

In the third week, paraxial mesoderm organizes into somitomeres, appearing first in the cephalic region.
Somitomeres consist of concentric mesodermal cell whorls and contribute to head mesenchyme.
Somitomeres caudally organize into somites: 4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 8-10 coccygeal pairs.
Somites form the axial skeleton and indicate the age of the embryo.

Development of the Skull

The skull develops from mesodermal and neural crest cells, forming the cranial vault and base.
Newly formed skull in newborns shows fontanelles and sutures, crucial for brain growth and passage through birth canal.

Development of the Limbs

Apical ectodermal ridge (AER) forms, influencing mesenchymal cells in limb buds to remain undifferentiated.
Limb development occurs proximodistally; fingers and toes form via programmed cell death in AER.
Upper limbs develop faster than lower limbs, rotating 90° laterally; lower limbs rotate medially.

Formation of Joints

Joints arise from interzones in cartilaginous condensations where chondrogenesis stops, leading to joint cavity formation through cell death.
Joint capsules differentiate from surrounding cells to create functional joints.

Clinical Correlates: Bone Age

Radiologists assess ossification centers via X-rays to determine children's bone maturation.
Ultrasonography can provide insights on fetal growth and gestational age.

Limb Abnormalities

Abnormalities include meromelia (partial absence), amelia (complete absence), phocomelia (rudimentary limbs), and micromelia (short limbs).
Additional anomalies: polydactyly (extra digits), ectrodactyly (absence of digits), syndactyly (fused digits).

Vertebral Column, Ribs, and Sternum

Sclerotome cells envelop spinal cord and notochord, forming vertebral column structures.
Notochord degenerates, forming nucleus pulposus; annulus fibrosus arises from surrounding tissue.
Ribs originate from thoracic vertebrae's costal processes, while the sternum develops from somatic mesoderm and forms through fusion of sternal bands.

Clinical Correlates: Vertebral Defects

Asymmetrical fusion or absence of vertebrae can lead to scoliosis.
Spina bifida results from imperfect vertebral arch fusion; outcomes range from spina bifida occulta (no neurological deficits) to spina bifida cystica (exposed neural tissue).

Molecular Regulations of Bone Development

Hox genes determine limb positioning along the craniocaudal axis and influence bone shape.
Fibroblast growth factors (FGFs) are critical for regulating growth along three body axes.
Bone morphogenetic proteins (BMPs) induce AER formation, guiding limb development.

Muscular System Development

Muscular system (skeletal, smooth, cardiac muscle) primarily derives from mesodermal layer.
Skeletal muscle originates from paraxial mesoderm; myogenic precursors arise from somatic and splanchnic mesoderm.
Myogenesis is marked by the elongation of myoblasts and their fusion into myotubes, leading to skeletal muscle formation.

Molecular Regulations of Muscle Development

BMP4 and FGFs from lateral plate mesoderm signal gene expression for muscle formation.
MYO-D and MYF5, key muscle-specific genes, activate pathways for myoblast development into myofibers, defining skeletal muscle structure.

Description

This quiz covers the intricate development of the skeletal system, focusing on the organization of mesoderm, development of the skull and limbs, as well as clinical correlates. It includes important details on somitomeres, somites, and molecular regulations of bone development, making it essential for understanding human anatomy.