this lecture covered some basic ideas about the formation of limbs in an embryo. it covered: the structural development of limbs from somites, the formation of bones, formation of blood vessels, and some structural abnormalities.
somites are described as "paraxial mesodermal segments" that ultimately differentiate into three parts: sclerotome, which forms the spine and ribs, dermatome, which forms the dermal layer of the skin, and myotome, which forms the muscles of the back and limbs. the limb "bud" starts growing off of the lateral plate mesoderm, with migrating somatic cells contributing to its development as well. it grows outward, developing along the proximal-distal axis via FGF, spearheaded by the apical ectodermal ridge. the "zone of polarization activation" is what develops patterns in the caudal-cranial axis (such as the different sizes and shapes of the 5 fingers) via Shh. these two mechanisms feed off of each other in a positive feedback loop, where Shh from the ZPA promotes the FGF in the AER and vice versa, through intermediates called formin, gremlin, and bone morphogenetic protein.
the process of endochondrial ossification is then described briefly: mesenchymal cells condense and form "pre-cartilaginous aggregates". cartilage producing cells (chondroblasts) secrete BMP to promote growth and Ihh to promote BMP, another positive feedback mechanism. chondroblasts then secrete hyaline cartilage, and become chondroblasts that reside in empty pools called lacunae. hydroxyapetite, an inorganic calcium phosphate crystal, is then secreted and eventually kills off the chondrocytes via apoptosis. osteoblasts then secrete bone matrix and bone is formed. bone remodeling occurs through the balance of osteoblast (bone forming cells, formed from mesenchymally derived osteoprogenitor cells) and osteoclast (bone destroying cells, derived from fused mononuclear hemopoetic progenitor cells) activity on opposite ends of a surface of bone.
some facts about blood vessel development: angioblasts form vascular plexuses which form the aorta and cardinal veins. central artery distributes blood to marginal sinus, which becomes the basilic and cephalic veins. central artery becomes brachial and interossei arteries.
finally, some abnormalities of limb development: amelia is absence of limbs. meromelia is absence of limb segments. phocomelia is absence of proximal limb segments but normal distal segments. polydactylyl is extra digits. syndactylyl is webbed digits.
questions
1. what are somites made from and what induces them?
2. somites divide into...
3. what does the sclerotome become?
4. what does the dermatome become?
5. what does the myotome become?
6. describe the production of cartilage producing cells.
7. what do Wnt genes do?
8. where do the limb buds originate?
9. limb develops from both...
10. what is skeletal muscle formed from?
11. what is dermis formed from?
12. what is the epidermis formed from?
13. what are sensory neurons formed from?
14. what is the AER and what does it do?
15. what is the ZPA and what does it do?
16. what does the myotome form in the limbs?
17. describe the interaction between the AER and ZPA.
18. what are the signaling factors between the AER and ZPA?
19. describe the process of endochondral ossification.
20. what do cartilaginous cells secrete?
21. what is mineralization triggered by?
22. what are osteoblasts derived from?
23. epiphyseal plates expand...
24. achondroplasia is...
25. bone remodeled by...
26. osteoblasts are derived from...
27. osteoclasts are derived from...
28. osteoclasts are activated by...
29. amelia...
30. meromelia...
31. phocomelia...
32. polydactyly...
33. syndactyly...
34. angioblasts form...
35. central artery distributes...
36. marginal sinus becomes...
37. central artery becomes...
answers
1. made from paraxial mesoderm segments, induced by hox genes.
2. sclerotome, dermatome, myotome.
3. vertebrae and ribs
4. dermis
5. muscles of back and limbs
6. Shh from notochord and neural tube induce Pax (1,9) genes, which converted somitic cells into chondroblasts.
7. influence the conversion of dorsal somitic cells into dermatome and myotome.
8. lateral plate mesoderm.
9. local limb bud tissue, as well as migrating tissue from somites.
10. migrating mesoderm cells from myotome differentiate into muscle (following tendons during migration)
11. dermatomal cells
12. ectoderm
13. neural crest cells.
14. apical ectodermal ridge, forms bones along the proximal-distal axis via FGF.
15. forms bones along the cranial-caudal axis via Shh.
16. dorsal-ventral compartment muscles via Wnt.
17. the two centers have a positive feedback system where FGF from the AER maintains Shh expression at the ZPA and Shh in turn activates FGF in AER.
18. formin, gremlin, bone morphogenetic protein (BMP)
19. mesenchymal cells in center of limb condense into "pre-cartilaginous aggregates". chondroblasts form hyaline cartilage matrix. chondroblasts enlarge and form lacunae. hydroxyapetite deposits on partitions between lacunae. cartilage cells die by apoptosis. capillaries vascularize calcified cartilage. osteoblasts deposit bone matrix which replaces cartilage.
20. BMP to promote growth and Ihh to promote production of BMP
21. local secretion of alkaline phosphatase.
22. mesenchymally derived osteoprogenitor cells.
23. with growth hormone
24. premature ossification of epiphyseal plates
25. selective deposition and resorption of bone from opposing surfaces, depending on relative activity of osteoblasts and osteoclasts.
26. osteoprogenitor cells
27. "fused mononuclear hemopoietic progenitor cells"
28. cytokine signaling.
29. absence of limbs
30. absence of limb segments
31. absence of proximal end of limb with normal distal end
32. extra digits
33. webbed digits
34. vascular plexus with aorta and cardinal veins
35. blood through capillaries into marginal sinus
36. cephalic and basilic veins
37. brachial and interosseus arteries
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