1. Introduction to Embryology 3rd and 4th week of development
Prof. Abdulameer Al-Nuaimi

2. Developing Embryo Blastocyst (Embryoblast) Trophoblast
Blastocyst cavity
Primary yolk
sac
Exocoelomic membrane
Developing Embryo
Blastocyst

3. Primary
Yolk sac
Prim yolk sac
Seco y sac

4. chorion Further development of the trophoblast
connecting stalk appears, it contains capillaries which are connected to the chorionic plate (Extraembryonic Somatopleuric mesoderm) and the embryo.
chorion 
Is a membrane that exist during pregnancy between the developing fetus and mother. It is formed by extraembryonic mesoderm and the two layers of trophoblast, it surrounds the embryo and other membranes.
Chorionic villi emerge from the chorion, invade the endometrium, and allow transfer of nutrients from maternal blood to fetal blood. This commences the formation of placenta. In this way, the embryo starts getting its nutrients and oxygen through this stalk.

5. chorion (Langman’s Medical Embryology) Somatopleuric meso
Buccopharyngeal membrane
(Langman’s Medical Embryology)
Somatopleuric meso
Chorio. villi
chorion 
Endoderm
Extraembryonic
Splanchnopleuric
Mesoderm
(visceral)
Cytotrophoblast
Syncytiotrophoblast
(primitive yolk sac)

6. Gastrulation
 Gastrulation is an early phase in the embryonic development of most animals
During this phase, the embryoblast develops into
a trilaminar ("three-layered") structure known as the gastrula.
The three germ layers are known as
1- Ectoderm
2- Mesoderm 
3- Endoderm

7. Gastrulation:
occurs during the third week.
It begins with the appearance of the Primitive streak (an elongating primitive groove with a node anteriorly). Primitive streak appears on the dorsal mid sagittal surface of the epiblast, along the anterior-posterior axis of the embryo.
Cells of the Epiblast migrate toward the Primitive streak, they are detached from the Epiblast and slip beneath it (invagination). This process is controlled by fibroblast growth factor 8 (FGF8) which is synthesized by Streak cells.
Invaginating cells give rise to Mesoderm, cells gradually migrate beyond the margin of the embryonic disc and establish contact with the extraembryonic mesoderm.

8. Other invaginating cells displace the Hypoblast, creating the embryonic Endoderm.
Epiblast remains as Ectoderm.
Prenotochordal cells invaginating in the node of primitive streak forming the Notochord.
The Notochord forms a midline axis, which serves as the basis of the axial skeleton.

9. Gastrulation (3rd week embryo)
(Langman’s Medical Embryology)
Dorsal surface
Of the embryo
Notochrd
Primitive
streak
Longitudinal section
In the embryo
Transverse section
In the embryo
Gastrulation (3rd week embryo)

10. Gastrulation www.google.co.uk/search? A,B,C Cross section
D Longitudinal section
Parietal Mesoderm
Endoderm
TV. Sec
Visceral Mesoderm A
TV Sec B C
TV Sec D
L. Sec

11. Differentiation of the germ layers
During the embryonic period the three germ layers (Ectoderm, Mesoderm and Endoderm) differentiates into their own tissues and organ systems.
The Ectodermal layer:
Gives rise to the following structures that are in contact with the outside of the body
1- Central nervous system.
2- Peripheral nervous system.
3- Sensory epithelium of nose, ear and eye.
4- Epidermis of skin, hair and nails.
5- Pituitary, mammary and sweat gland.
6- Enamel of the teeth.
Ectoderm
Parietal Mesoderm
Endoderm
Visceral Mesoderm

12. Mesoderm
Is described as three parts
1- Paraxial plate mesoderm: it gives rise to Somites
Somites give rise to supporting tissues of the body
a- Myotome (muscle tissue)
b- Sclerotome (cartilage and bone)
c- Dermatome (Dermis of the skin)
2- Intermediate plate mesoderm: generates the urogenital system—the kidneys, the gonads, and their respective duct systems.
3- Lateral plate mesoderm:  is found at the periphery of the embryo. It splits into two layers, the somatic (parietal) layer mesoderm and the splanchnic (visceral) layer mesoderm.
 a- Somatic (Parietal) layer: forms
1- the future body wall.

13. b- Splanchnic layer (visceral layer): forms 1-the circulatory system.
2- Connective tissue for the glands.
3-Muscle, connective tissue and peritoneal components, of the wall of the gut.
Somatic
(parietal)
mesod
Parietal Mesoderm
Transverse Section
Splanch nopleur
Meso
(visceral)
Somatic
(parietal)
Mesoder
Second. Y. S
Endodearm
Splanchnopleuric
Mesoderm (visceral)

14. 3- Epithelial lining of the tympanic cavity and auditory tube.
Endoderm:
Gives rise to the:
1- Epithelial lining of the gastrointestinal tract, respiratory tract, and urinary bladder.
2- Parenchyma of the thyroid gland, parathyroid gland, liver and pancreas.
3- Epithelial lining of the tympanic cavity and auditory tube.

15. Differentiation of Paraxial mesoderm and Somites formation
During development, the paraxial mesoderm is divided into paired segments of tissue called Somites on each side of the Notochord, they develop in a cephalocaudal direction.
The first pair of Somites appears at day 20 and then at a rate of about three pairs per day until 42–44 pairs are formed.
At the beginning of the fourth week, somites differentiate into 1-Myotomes (form skeletal muscles)  
2-Sclerotomes (form bones and cartilages).
Some cells from the sclerotomes surround the notochord and spinal cord, and give rise to the vertebral column.
3-Dermatomes (form the dermis of the skin)

16. Somites Formation

17. Oropharyngeal membrane
Located at the cranial end of the embryonic disc, it consists of a small region of tightly adherent ectoderm and endoderm cells, there is no intervening mesoderm. It represents the future opening of the oral cavity
Cloacal membrane
Formed at the caudal end of the embryonic disc
It consists of tightly adherent ectoderm and endoderm cells, there is no intervening mesoderm. When this membrane appears, the posterior wall of the yolk sac forms a small diverticulum, the Allantois which extends into the connecting stalk
Sagittal Section
Langman’s Medical Embryology)
Endoderm

18. Folding of the embryo
During the fourth week (days 22-28). , folding of the flat trilaminar embryonic disk into a cylindrical embryo takes place.
The embryo passes into
1- Longitudinal Folding in the Median Plane (cephalocaudal)
2- lateral folding: Transverse Folding in Horizontal Plane, i.e. right and left lateral to medial folding
Cephalocaudal and lateral flexion commence at the same time. These flexions take place due to the vigorous growth of the Amnion and Embryonic disc in contrast to the yolk sac which does not grow, It is also stimulated through gene expression.

19. The Cranial area of the embryo contains the buccopharyngeal
(Oropharyngeal)Membrane, the cardiogenic area (the heart) and Septum transversum,
Cranial Flexion brings the Buccopharyngeal membrane, cardiogenic area and septum transversum ventraly, forming the ventral surface of the future face, neck and chest. It brings the heart into its thoracic position and septum transversum to the diaphragm.
Longitudinal section in the
embryo

20. caudal flexion
Brings the cloacal membrane onto the ventral surface of the embryo.
lateral folding
Results in the incorporation of a portion of the yolk sac (which is lined with endoderm) into the embryo to form the primitive gut.
It also leads to the formation of the body cavities
The remaining part of the yolk sac and Allantois remain outside the embryo.

21. (Langman’s Medical Embryology)
Parietal mesoderm
Endoderm
Visceral mesoderm

22. Formation of body wall Parietal Mesoderm Visceral mesoderm
Lat. Fold
Formation of body wall
Langman’s Medical Embryology)

23. Longitudinal section in Human embryo

24. The Primitive gut, is a blind-ended tube and is described as
Foregut, Midgut and Hindgut.
The Midgut, remains connected temporarily to the yolk sac by means of Vitelline duct (yolk stalk)
Foregut extends from Oropharyngeal membrane Liver bud.
Midgut: from Liver bud end of Rt. 2/3 of Transverse Colon.
Hindgut: from beginning of Lt. 1/3 of Transverse Colon Cloacal membrane
Yolk sac
Vitelline duct

25. Congenital Anomalies

26. Ectopic Pregnancy

27. Neural tube defects (Ectodermal defects)

28. Capillary Hemangioma
(Mesodermal defect)

29. Endodermal defects

30. Summary
Appearance of Connecting stalk and Embryonic villi commences the Placental formation.
Gastrulation takes place during the third week: The embryoblast develops into a trilaminar structure known as gastrula.
Cephalocaudal and Lateral folding change the Embryo into cylindrical structure.

31. Thank you