104 - Development 1 Flashcards
Frequency of birth defects
~1:33 live births
Most-common structural defects
Heart defects (~1:200)
Rate of initial division of fertilised egg
Division every 24 hours
Stage of development at 3 days
Morula (16 cells)
Stage of development at 4 days
Blastocyst undergoes cavitation to form a blastocoele
Where is the egg fertilised?
Fallopian tube
Morula
16-cell stage, at around 3 days
Arrangement of cells in a blastocyst
Two cell types: trophoblast (outer epithelial layer), inner cell mass (that becomes the embryo). Trophoblast forms a ring around the inner cell mass and the blastocoele
What does the trophoblast do?
Forms some extraembryonic structures like the placenta.
When does the blastocyst implant into the uterine wall?
5 - 10 days
Formation of bilaminar disc 1 2 3
1) Inner cell mass splits into the epiblast and hypoblast. 2)Part of the epiblast becomes the embryo. 3) Hypoblast expands to line interior of trophoblast to form the yolk sac.
Role of the yolk sac in humans
Not much. Doesn’t fill with yolk, embryo relies on maternal supply of nutrients.
Gastrulation
Formation of endoderm, mesoderm and ectoderm from bilaminar disc (epiblast layer).
Broad derivatives of ectoderm
Nervous system and epidermis
Broad derivatives of mesoderm
Blood, heart, kidneys, gonads, most bones, connective tissues
Broad definition of endoderm
Epithelium of gut and associated organs
Neural tube formation 1 2 3 4
1) Notochord induces overlying ectoderm to form neural plate 2) Neural plate forms neural groove 3) Neural plate folds around groove to form neural tube 4) Neural tube is formed, with neural crest sandwiched between neural tube and epidermis
Notochord
Formed from mesoderm. First structure to form in an embryo, runs down midline of developing embryo.
Role of notochord
Organisation of embryo
*Neural tube formation
Neural crest derivatives
Largely PNS. • Dorsal root ganglia • Sympathetic and parasympathetic ganglia • Enteric ganglia • Schwann cells • Melanocytes • Dentine • Muscle, cartilage and bone of skull, jaws, face and pharynx
Prevalence of cleft palate
1:1000 live births
How does cleft palate come about?
Issue with migration of neural crest cells.
Mesodermal derivatives
• Dermis • Muscles • Skeleton • Urogenital tract • Heart and blood vessels • Wall of gut and respiratory tract • Haemopoietic tissue • Pleura, peritoneum, pericardium
Difference between mesoderm and mesenchyme
Mesoderm is one of the three germ layers. Mesenchyme refers to the shape and behaviour of cells. Used in contrast to epithelial cells (epithelial cells crowd together, form sheets, mesenchymal cells lose contact with one another, move independently, are motile).
Divisions of mesoderm 1 2 3
1) Paraxial mesoderm (closest to midline) 2) Intermediate mesoderm 3) Lateral mesoderm (further from midline)
Fate of paraxial mesoderm
• Dermis of skin • Axial skeleton • Axial and limb muscles
Fate of intermediate mesoderm
Urogenital system
Fate of lateral mesoderm
• Ventrolateral body wall • Limb skeleton • Visceral pleura, peritoneum • Blood vessels and blood forming tissue • Heart • Wall of gut and respiratory tissues
Organisation of paraxial mesoderm
• Swellings (somitomeres) appear progressively down length of paraxial mesoderm • At the 20 somitomere stage, the eighth pair of somitomeres becomes an independent pair of somites • Somitomeres are replaced by somites from the 8th somite caudally.
Somites
Somitomeres become independent from paraxial mesoderm from the 8th somitomeres caudally, become somites.
Where is the 8th somitomere?
Equivalent of the neck. Above this becomes the head, below becomes the rest of the body
*Development of somites 1 2
1) Somites split into sclerotome and dermomyotome.
2) Dermomyotome develops into dermatome and myotome.
What do the divisions of somites become? 1 2 3
1) Sclerotome becomes axial skeleton 2) Dermatome becomes dermis of skin 3) Myotome becomes axial muscles, appendicular muscles
What forms the axial skeleton and appendicular skeleton?
Sclerotome (from paraxial mesoderm) becomes axial skeleton. Lateral mesoderm becomes appendicular skeleton
When does limb development begin?
End of fourth week of development
What initiates limb development?
Mesoderm
How do limb dermatomes come about?
When limb is budding, it pulls overlying ectoderm with it.
How does the mesoderm form a limb? 1 2 3
1) Mesoderm forms a free limb patch, which will become a limb. 2) If free limb patch is removed, surrounding cells will compensate. 3) If whole patch is removed, no limb will develop
Limb development 1 2 3
1) Mesoderm forms free limb patch, which begins limb development. 2) Overlying ectoderm forms apical ectodermal ridge, takes over limb development. 3) Damage to ectoderm can result in issues in limb development (EG: diplopodia). Ectoderm is fragile, relatively easy to mechanically injure.
Name for developmental fusion of fingers
Syndactyly
