UNSW Embryology

Musculoskeletal Development - Stage 13/14

© Dr Mark Hill (2009)

Acknowledgements

Introduction

This page looks at musculoskeletal events that are occuring at early embryonic Carnegie stage 13/14. The features to observe are mainly the rostrocaudal development of the somites and the early limb buds.

Somites near the head and through the trunk have begun dispersing into the separate dermal, myotome and sclerotomal components. Towards the tail, newly forming somites with central somitoceol (cavity) are visible. Mesenchyme in the sclerotomal regions will begin to differentiate initially as cartilage, to be replaced later by endochondrial ossification.

The upper limb bud is the most obvious lateral expansion on the trunk, with only small lower limbbud bulges towards the tail. Within the upper limb bud undifferentiated mesenchyme (embryonic connective tissue) and early forming blood vessels are visible, and at tip surface a clearly visible apical ectodermal ridge.

Embryo Stage 13/14

Human Stage 13 embryo- Somites clearly visible beneath surface.	Human Stage 14 embryo- Somites clearly visible beneath surface. Note the further development of forelimb and hindlimb buds.

Stage 13/14 Embryo Somites

G5: Tangential section through the lower curvature of the embryo (neural tube cut twice). Newly formed somites containing somitocoeles and consisting of dark, lateral dermomyotomes and more diffuse, medial sclerotomes. Notochord. Amnion and amniotic cavity.

Due to the development of somites in a rostrocaudal sequence, these somites are relatively and observably more immature (formed later) than those seen in D1-D3 below.

D1: The somite development overview (on page 1) shows this organization rotated 90^o

D2:

D3:

Surface bulge of a somite, either side of neural tube. Somites at this level have spread from their original ball shape.

• Somite components
  • dermatome- dark region lying directly under surface ectoderm.
  • myotome- slightly paler colour beneath dermatome.
  • sclerotome- very pale region surrounding notocord and between neural tube and dorsal aorta.
• Notochord- dark spot ventral to neural tube.
• Paired dorsal aortae, giving segmental branches.
• Inferior cardinal veins (lateral, empty). Dorsal root ganglion- dark patches either side of neural tube.
• Spinal nerve passing between sclerotome and myotome.
• Neural tube- note the alar and basal laminae of cells, sulcus limitans.

(sections from level of lung buds to stomach)

Axial Skeleton - Vertebrae

(See also below notes on vertebral development )

G7: Cervical region: dark masses of dorsal root ganglia. Lumbar region: dorsal aorta with its dorsal segmental arterial branches. Between each dorsal segmental artery is a darker-staining mass of mesenchyme (the dark part of a sclerotome) which is the anlage of the intervertebral disc. The dorsal segmenta lartery itself marks the location of the centre of the light-staining part of the sclerotome, which is the future vertebral body. The dark band dorsal to the sclerotomes is the basal lamina of the wall of the neural tube.

G6: Cervical region (neural tube cut very obliquely): note the wavy notochord and thin roof and floor plates. Thoracic region: alternating light and dark parts of the sclerotomes (vertebral bodies and I.V. Discs, respectively). Lumbar region: tiny, dorsal segmental branches of dorsal aorta Each little branch is aligned almost parallel to the cranial border of the next caudal dorsal root ganglion (segmented dark masses). Sacral region: oblique section of neural tube, notochord.

Formation of Vertebra

The vertebra of the spinal cord go through a lengthy period of maturation which I have briefly summarized below. A key aspect of development for vertebra (spinal cord) and skull (brain) is to create a flexible enclosure to allow continued growth of the neural components (that are relatively immature).

• Precartilaginous Stage (Human wk4 - wk6)
  • shown in stage 13/14 embryo
  • generated by the sclerotomal mesenchyme surrounding the notochord
  • vertebra form out of register with the original somites
  • each sclerotome has 2 components
    • cranial loose cells
      • fuses with next cranial segment
    • caudal dense cells
      • upper part of which forms annulus fibrosus of intervertebral disc (IVD)
      • lower part fuses with next caudal segment (cranial+caudal=centrum)
  • notochord forms nucleus pulposus (surrounded by sclerotomal annulus fibrosus) of IVD
• Cartilaginous Stage (Human wk6 - )
  • chondrification centers form in mesenchyme of centrum
  • replaces mesenchyme with cartilage
• Bony Stage (Human wk7 - 25 years)
  • shown in stage 22 embryo
  • primary ossification centers form in cartilage
    • 2 dorsal- each vertebral arch
    • 1 ventral- centrum
  • at birth vertebra is these 3 boney parts connected by cartilage
  • postnatally 3-5 years- arch fuses
  • puberty- secondary ossification centers form on body and facets

Limbs

Upper Limb

D3: Cranial edge of forelimb fold (future shoulder region). Note the superolateral extent of the coelomic serosa with respect to the limb fold.

D4,D5: Note axillary fossa appearing on each side and local thickening of the ectoderm in the region of the fossa. Note thinnest ectoderm stretched over heart and neural tube.

D6-D7: Limb folds and fossae. Greatest thickening of the ectoderm is at apex of the limb fold: the apical ectodermal ridge. The mesenchyme of the limb is densest under the ectoderm. Note the narrow distance between the ectoderm of the axillary fossa and the serosa of the body cavity. The limb fold has distinct flexor and extensor surfaces: the flexor surface is narrower and has thicker ectoderm. Note blood vessels close to flexor surface

Lower Limb

E1-E3: Note blood vessels in limb fold mesenchyme, and pale-staining nerves at root of limb (E1).

E4: Spinal nerves (part of future brachial plexus) directed towards the flexor side of the limb fold. Note also the section of the lumbosacral region with neural tube and hindlimb bulge.

E5-E7: Hindlimb fold. Forelimb fold.

F1,F2: Caudal limit of forelimb. Broad extensor surface. Dense mesenchyme of hindlimb. Note lumbar somites, somitocoeles.

Stage 13/14 Embryo (Pig)

A1	A2	A3	A4	A5	A6	A7
B1	B2	B3	B4	B5	B6	B7
C1	C2	C3	C4	C5	C6	C7
D1	D2	D3	D4	D5	D6	D7
E1	E2	E3	E4	E5	E6	E7
F1	F2	F3	F4	F5	F6	F7
G1	G2	G3	G4	G5	G6	G7

Glossary of Terms

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

Quick Links

Development Notes ----------------- Menstrual Cycle Week 1 Week 2 Week 3 Week 4 Placenta Serial Images Genes/DNA Molecular Dev Abnormal Dev Postnatal Dev Notes Index

System Notes ----------------- Heart/Cardiovascular Respiratory Coelomic cavity Gastrointestinal Tract Head and Neck Neural Neural Crest Eye Ear Nose Endocrine Urogenital Musculoskeletal Integumentary

Movies ----------------- Embryo Growth Ultrasound Week 1 Week 2 Week 3 Week 4 Stage 13/14 Stage 22 Neural Head and Neck Heart Vasculature Gastrointestinal Urogenital Other Embryos Larsen Internet

Other Embryos ----------------- Chicken Fly Frog Mouse Rabbit Rat Worm Zebrafish Guinea Pig

UNSW Embryology ISBN: 978 0 7334 2609 4

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