Early development 2: segmentation and folding

Embryonic folding

Process by which a flattened 2D germ disc becomes 3D.

Growth in length and stagnation of growth of yolk sac = cranio-caudal flexion and lateral folds

coelom pinched off and umbilicus forms

Partitioning

ingrowth of pleuropericardial folds - subdivides primitive coelom into pericardial, pleural and peritoneal cavities.

When does somitigenesis start?

during neural tube closure prior to embryonic folding

Somitogenesis steps

Paraxial mesoderm flanks notochord on either side

becomes segmented

each segment = a somite (patterned as an independent unit)

What do somites form?

Segmented structure of axial skeleton (e.g. vertebrae, ribs)

associated muscles

overlying dermis

Segmentation

1st signs occur in cervical regions and spreads caudally

loose whorls of cells bud off from paraxial mesoderm

each whorl condenses into a block of columnar cells that surround a small cavity

Somite differentiation closest to the neural tube

sclerotome

Lose compact structure; migrate around neural tube & notochord

will form bones of axial skeleton

Somite differentiation further from the neural tube

Remain epithelial

Subsequently differentiate into medial and peripheral layers

Medial layer = Myotome

Peripheral layer = dermatome

Myotome

gives rise to muscles of the main body (e.g. intercostal and abdominal)

some cells migrate to limbs to start limb musculature development

dermatome

forms dermis

production of vertebra

each vertebra produced by cells from 2 adjacent sclerotomes

allows route for spinal nerves to pass between vertebra to muscles

What does somite development depend on?

Cranial-caudal position of the somites

Transplanting posterior somite in an anterior position = structures according to position of origin

2 main aspects to embryo folding

rostral growth

formation of gut tube through ventral closure

Formation of the septum transversum

During rostral growth

Thickening of mesoderm at margin of cephalic part of the disc

comes to lie between cardiac region and yolk sac during folding

will separate thoracic and abdominal cavities

gives rise to the diaphragm

Creation of the gut tube

occurs during ventral closure

lateral margins of disc flex and grow

form a cylinder - now ectoderm on the outside and endoderm on inside

formation of allantois due to caudal growth displacing the connecting stalk to next to the yolk sac

neck of yolk sac becomes restricted to narrow stalk, called vitelline duct

central part of each block of lateral plate mesoderm vacuolates and becomes fused, creating central cavity in the embryo - intra-embryonic coelom

What forms from the rostral and caudal ends of the gut?

rostral - buccopharyngeal membranes

caudal - cloacal membranes

both later rupture to form orifices

Allantois

blind ended sac at the end of the gut tube

important in birds as a way of disposing waste materials from developing embryo

What can form from allantois that has failed to close properly?

urachal fistula

Development of the serous membranes

After ventral folding

One layer of lateral plate mesoderm surrounds gut = splanchnopleuric mesoderm

Other layer lines inside of body = somatopleuric mesoderm

both form serous membranes of body cavity

formation of organs relative to these 2 dictates retroperitoneal or intraperitoneal

Foregut, midgut, hindgut

Blind ending tubes at cranial, middle and caudal ends of the gut tube

Midgut still connects to the yolk sac

Foregut terminates with buccopharyngeal membrane

Hindgut terminates with cloacal membrane

Derivatives of endodermal gut tube

Gut formed from inner of 3 concentric tubes formed during craniocaudal and lateral folding

Outgrowths from endodermal tube - lungs and digestive glands

gut tube is reorganised to give stomach and small intestines depending on the species.

Further folds that grow out of body wall in thoracic region to subdivide the embryo

pleuropericardial fold - see next flashcard

2 others - caudally to the lungs, close off gap between septum transversum and posterior body wall. Fuse with septum transversum and together form diaphragm

Pleuropericardial fold

Grow from lateral body wall

separate heart from the lungs

ultimately forms fibrous pericardium as lungs grow and expand

contain phrenic nerves