1. Neural Tube, Somites and the Placenta
Lecture 3

2. Neurulation Neural tube formation
Notochord stimulates the ectoderm to form the neural tube
Cells between the outer ectodermal layer and the neural tube will develop into the neural crest
Will eventually form parts of the peripheral nervous system, pigment cells (melanocytes), endocrine system and musculoskeletal system

3. In amphibians

9. Neural Tube Defects
anencephaly

10. Spina bifida

11. Somite differentiation
During neural tube formation, the mesoderm transforms into special structures called somites
Landmark to identify the age of the embryo
Develops into the specific organ systems

12. Somite differentiation
Paraxial mesoderm: just lateral to the neural tube = forms the somites
Intermediate mesoderm: gives rise to the urogenital system
Lateral mesoderm
Somatic/Parietal mesoderm and somatopleure
Splanchnic/Visceral mesoderm and splanchnopleure
Coelom = between somatic and splanchnic mesoderm

13. Somite formation in chick embryo

14. Somite differentiation
Somites are committed to produce structures characteristic of their level along the body axis
Somites near the neural tube will develop into sclerotomes  eventually form parts of the vertebrae, ribs and sclapula
Outer cells of the somite become the dermatomyotome  becomes muscle (myotome cells) and dermis (dermatome cells)

15. Somite differentiation
Myotome
Medial myotome = axial muscles
Lateral myotome = will migrate as muscles of the limbs
May help determine age of embryo

16. Somite differentiation
Age of Chick embryo
Number of somites
22 hours
1-2
24 hours
3-4
27 hours 8
30 hours 10
33 hours 12
38 hours 17
55 hours 29
72 hours 36

17. Somite differentiation
Age of human embryo (days)
Number of somites 20
1-4 21
4-7 22
7-10 23
10-13 24
13-17 25
17-20 26
20-23 27
23-26 28
26-29 29
29-34 30
34-35

18. Somite differentiation
Lateral mesoderm separates to 2 layers, with a coelum between them
Somatic/Parietal mesoderm
Forms limb muscles together with the myotomes
Forms lining of the peritoneal, pleural and pericardiac cavities
Splanchnic/Visceral mesoderm and splanchnopleure
Forms wall of the gut tube
Forms outer covering of the internal organs

21. Somite formation in humans

23. Extraembryonic Membranes
Amnion
ectodermally derived
Encloses entire embryo in a fluid filled sac
Yolk sac
Endodermally derived
Encloses the yolk, which provides nutrients for the embryo
Induces formation of blood and blood vessels in the yolk

24. Extraembryonic Membranes
Allantois
Endodermally derived
From the hindgut
Acts as a reservoir for storing or removing wastes and to mediate gas exchange
Chorion or serosa
Outermost extraembryonic membrane
Site for gas exchange for oviparous animals (egg laying)
In mammals, also acts for nutrition, excretion, filtration and synthesis of hormones

25. Extraembryonic membranes in the Chick
Formation of the yolk sac
Splanchnopleure extends into the outer boundary of the yolk, eventually enclosing it
Beginning of primitive gut formation
Foregut
Midgut
Hindgut
Yolk duct and stalk
Formation of the omphalomesenteric arteries and veins  brings nutrients from yolk to the embryo
Initial site of red blood cell production

26. Extraembryonic membranes in the Chick
Formation of the amnion
Derived from the inner layer of the somatopleure
Begins at the head fold, followed next at the tail fold and lateral folds
Head fold, tail fold and lateral folds meet each other to completely enclose the embryo, as they join together at the amniotic raphe

27. Extraembryonic membranes in the Chick
Formation of the chorion
Derived from the outer layer of the somatopleure
Outermost covering
Extends over the yolk sac
Cavity between chorion and amnion = chorioamniotic cavity
Functions to transport Calcium from the egg shell into the embryonic circulation

28. Extraembryonic membranes in the Chick
Formation of the allantois
Arises from the endoderm of the hindgut
Extends and eventually fuses with the chorion
2 Functions
As the respiratory system of the chick embryo to become the chorioallantoic membrane
transports oxygen from outside of the shell to the embryo and releases CO2
As a reservoir for metabolic wastes of the embryo (uric acid)

36. Extraembryonic membranes in humans
Trophoblast cells invade uterus
Formation of the amnion
A cavity forms within the epiblast layer of the inner cell mass
Allantois
Is replaced by the placenta
Yolk sac (previously the blastocoele)
Only for RBC production
Nutrients are supplied by the placenta
Chorion
Forms placenta
Fuses with amniotic cavity and uterus

37. Formation of the placenta
Maternal blood enters the placenta through the intervilous spaces via endovascular invasion by cytotrophoblast cells

38. Functions of the placenta
Exchange of nutrition, metabolic and gaseous products
Transmission of maternal antibodies
Primary respiratory center of the fetus
Production of hormones
Progesterone = to develop uterus
Estrogen = to develop uterus and to promote milk production by the mother
Beta HCG = maintains the corpus luteum
Somatomammotropin = gives fetus priority on blood glucose

39. Amniotic Fluid
Initially produced by amniotic cells and fluid transported from maternal blood
Serves as a protective cushion
Fetus drinks amniotic fluid and urinates back the fluid

44. Cells from yolk sac endoderm transform into splanchnic mesoderm and form cavities within the
Cytotrophoblast + amnion mesoderm = extraembryonic somatic mesoderm  somatopleure (ectoderm + mesoderm)
Yolk sac endoderm + amnion mesoderm = extraembryonic splanchnic mesoderm  splanchnopleure (endoderm + mesoderm)

45. Extraembryonic Somatic mesoderm Extraembryonic Splanchnic mesoderm
Cytotrophobast + syncitiotrophoblast = primary villa (precursor of chorionic villi)

47. Chorionic villi

51. Maternal-Fetal barrier
No direct contact between maternal and fetal blood
Embryo is considered a foreign organism
Prevent invasion by the maternal immune system and protection of embryo from toxic substances ingested by the mother
Diffusion of substances from maternal blood to fetal blood in placenta and vice versa

56. Placenta and twin pregnancy
Dichorionic diamniotic
Monochorionic diamniotic
Monochorionic monoamniotic

59. Twin-Twin Problems