Third week of Embryological development EVENTS: GASTRULATION NEURULATION FOLDING OF THE EMBRYO 7/3/2018 G.LUFUKUJA

Events during 3rd wk appear to occur in threes 3 germ layers of derived from bilaminar embryonic disc 3 new structures appear Primitive streak, notochord & allantois 3 layers appear in chorionic villi Syncytiotrophoblast Intermediate cytotrophoblast Inner mesodermal layer 7/3/2018 G.LUFUKUJA

Other events in third week Formation of neural -plate -tube -crest Formation of somites Formation of intraembryonic coelom 7/3/2018 G.LUFUKUJA

GASTRULATION The main event that occurs during the third week of development is the formation of the trilaminar embryo. This process is called gastrulation. The first sign of gastrulation is the formation in the epiblast of the primitive streak. 7/3/2018 G.LUFUKUJA

Events in Gastrulation 1.    Cell proliferation – cells from the epiblast in the caudal half proliferate at the future median plane causing heaping up of the cells and is the source of a new layer of cells 2.    Cell migration by amoeboid movement – the cells insinuate themselves between the epiblast and hypoblast 3.     Cell determination - the cells arising from the primitive streak are determined to give rise to different tissues 7/3/2018 G.LUFUKUJA

Gastrulation… At the cranial end of the primitive streak, cells proliferate to form the primitive node, which finally undergo apoptosis to form the primitive pit Within the primitive streak there is apoptosis to form the primitive groove 7/3/2018 G.LUFUKUJA

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7/3/2018 G.LUFUKUJA

Gastrulation… At this point a trilaminar disc is formed with the epiblast giving rise to all three germ layers: 1. the epiblast that remains becomes the ectoderm, the cells that replaced the hypoblast become the endoderm, the cells in between becoming the mesoderm (intra-embryonic mesoderm). 7/3/2018 G.LUFUKUJA

Gastrulation The phenomenon under the influence of maternal effect & zygotic genes, thus: Midline mesoderm forms notochord Paraxial mesoderm gives rise to myotomes (skeletal muscle); sclerotomes (vertebra); and dermatomes (dermis). Ectoderm develops into CNS (induced by mesoderm and skin) Endoderm forms the lining of the Alimentary System and RS Mesoderm forms the CVS (induced by endoderm) 7/3/2018 G.LUFUKUJA

Gastrulation (Significance…) Converts the bilaminar to trilaminar disc Establishes the cranio-caudal axis and bilateral symmetry(side by side) of the embryo Induces embryonic anlagen cells to form organ systems (organogenesis) thus gastrulation Signals morphogenesis Establishes 3 germ layers 7/3/2018 G.LUFUKUJA

Mesoderm differentiation 7/3/2018 G.LUFUKUJA

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Formation of the notochord The notochordal process grows out from the primitive node growing as a rod of cells cranially along the midline. Its growth is limited by the buccopharyngeal membrane. The most cranial part of the notochord is at the prochordal plate. 7/3/2018 G.LUFUKUJA

Notochord… Found in all chordate embryos - flexible, rod-shaped body Develops from axial mesoderm Defines the primitive axes of the embryo Replaced by vertebral column in most vertebrates Position: Ventral to the neural tube Persists throughout life as the main axial support of the body in some chordates 7/3/2018 G.LUFUKUJA

Segmentation of the paraxial mesoderm The segments are termed somites. The first somite appears on day 20 at the cranial end close to the prochordal plate. Somites develop in cranio-caudal sequence to form 42 to 44 somites by day 30. The somites are: 4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 3 coccygeal. A few other somites at the caudal end degenerate 7/3/2018 G.LUFUKUJA

The intra-embryonic cavities Intraembryonic coelomic spaces appear in lateral & cardiogenic mesoderm; they coalesce to form horseshe-shaped intraembryonic coelom The intra-embryonic coelom is shaped like an inverted U-tube, and is divided into three parts:pericardial cavity, pleuro-peritoneal canals, intra-embryonic coelom communicates laterally with the extra-embryonic coelom. 7/3/2018 G.LUFUKUJA

Formation of the Neural plate & neural tube The neurectoderm is now termed the neural plate. Like the notochord it is limited craniallly by the buccopharyngeal membrane and caudally by the cloacal membrane. 7/3/2018 G.LUFUKUJA

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Neural crest derivatives 7/3/2018 G.LUFUKUJA

Folding of the Embryo 7/3/2018 G.LUFUKUJA

Changes resulting from folding Note that drastic and important changes occur in the embryonic cavities as a consequence of folding: 1. The amniotic cavity surrounds the embryo completely on all aspects and becomes the predominant cavity. It enlarges progressively. 2. The yolk sac becomes constricted on all sides, and becomes a small sac connected to the midgut by a narrow vitelline duct. It becomes progressively smaller. 3. The extra-embryonic coelom is gradually obliterated by the expanding amnion and eventually disappears completely 7/3/2018 G.LUFUKUJA

Folding of the Embryo... 1. Head fold 2. Tail fold 3. Lateral folds 7/3/2018 G.LUFUKUJA

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7/3/2018 G.LUFUKUJA

Head Fold… As a result of the formation of the head fold: a. The foregut is formed by folding of the endoderm b. The stomodaeum is an invagination of ectoderm, and the buccopharyngeal membrane separates it from the foregut It opens into the amniotic cavity. 7/3/2018 G.LUFUKUJA

Head Fold… c. The pericardial cavity and cardiogenic mesoderm are shifted to the ventral aspect of the embryo and lie ventral to the foregut. d. The part of the transverse mesoderm between the pericardial cavity and the yolk sac is the septum transversum. The liver will develop in it and it forms the diaphragm. e. The amniotic cavity extends ventral to the cranial end of the embryo. 7/3/2018 G.LUFUKUJA

Head Fold… f. The yolk sac is constricted and will later form the primitive foregut. As a result of the formation of the tail fold: The hindgut is formed & The cloaca is an invagination of ectoderm and has the cloacal membrane separating it from the hindgut. 7/3/2018 G.LUFUKUJA

Tail fold… e. The amniotic cavity extends ventral to the caudal end of the embryo. f. The yolk sac is constricted from the caudal end and will form the primitive hindgut 7/3/2018 G.LUFUKUJA

Somites A somite is divided into two parts: The sclerotome is the ventro-medial part of the somite. Cells from the sclerotome migrate medially to surround the notochord and neural tube and form the axial skeleton. 7/3/2018 G.LUFUKUJA

Somites The dermomyotome is the dorso-lateral part of the somite. Cells from the dermomyotome migrate laterally and, as its name implies, gives rise to (i) skeletal muscle and (ii) the dermis of the skin. 7/3/2018 G.LUFUKUJA

Somite… Each anatomical myotome is derived from the embryological dermomyotome that is innervated by a segmental nerve and forms a group of skeletal muscle cells and the dermis of the corresponding segment of ectoderm. 7/3/2018 G.LUFUKUJA

Neural arches The neural tube induces the formation of the neural arches and their fusion across the midline. Defects of closure of the neural tube will also cause failure of fusion of the overlying neural arches. This anomaly has various terms depending on the degree of the defects e.g meningomyelocoele 7/3/2018 G.LUFUKUJA