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Flashcards in Embryo L3 Deck (67)


Embryonic period
Occurs from week 3 – 8
When primary germ layers give rise to tissues and organs
End of this period, main organ systems established



Process by which the neural plate forms the neural tube
Occurs in the ectoderm via notochordal induction
Completed by end of 4th week

Four main events:
Formation of neural plate
Shaping of neural plate
Invagination of neural plate
Closure of neural groove

Neural folds begin to move together and eventually fuse
This fusion begins in cervical region (5th somite) and progresses both cranially & caudally
This results in the formation of the neural tube


Neural Folds

Laterally elevated edges from invagination. Allow plate to dip further and further into mesoderm (coming closer to one another).


Neural groove

– depressed midregion of neural plate
Neural groove will become the neural canal


Neural PLate

These cells make up neuroectoderm


Neural folds

fuse at midline (progressing both cranially & caudally), forming the neural tube, which separates from the ectoderm


Neural Crest Cell Development

As neural folds fuse, cells at lateral border (crest) of neuroectoderm dissociate
Called neural crest cells
Undergo epithelial-to-mesenchymal transition
Enters the underlying mesoderm


Paraxial mesoderm

Mesoderm near midline proliferate to form thickened paraxial mesoderm

Beginning of week 3, starts to organize into segments, which form cranially to caudally (beginning in occipital region)

Will give rise to most of axial skeleton, skeletal muscles, and dermis of the back


lateral plate mesoderm

More laterally, stays thin, known as lateral plate mesoderm


intermediate mesoderm

In between paraxial and lateral plate mesoderm is the intermediate mesoderm


Intraembryonic cavity

Somatic or parietal layer (lines the amnion)
Splanchnic or visceral layer (lines yolk sac)
The cavity between these two layers is the intraembryonic cavity
Continuous with the extraembryonic cavity on each side of embryo



Division of body
4 occipital (will disappear)
8 cervical
12 thoracic
5 sacral
5 lumbar
8-10 coccygeal (last 5-7 disappear)

Age of embryo can be determined by counting somites


Somite differentiation

Mesoderm cells, undergo epithelization and arrange in donut shape around small lumen
Cells at dorsomedial & ventrolateral edges of upper region of somite form precursors for muscle cells
Cells will become mesenchymal & migrate beneath dermatome and form the myotome



Cells in ventral & medial walls shift position to surround neural tube; form sclerotome



Cells between dorsomedial cells & ventrolateral cells form the dermatome


Somite Derivatives

-Vertebrae & ribs




Intermediate Mesoderm Derivatives

Differentiates into urogenital structures
Urinary system
Reproductive system


Lateral Plate Mesoderm Derivatives

Parietal mesoderm layer along with overlying ectoderm form lateral body wall folds
Will help close the ventral body wall


mesothelial membranes (i.e., serous membranes)

Mesoderm cells of parietal layer surrounding the intraembryonic cavity form -Line the peritoneal, pleural, & pericardial cavities and secrete serous fluid


Visceral mesoderm layer

Visceral mesoderm layer together with overlying endoderm form walls of the gut tube
Mesoderm cells of visceral layer form thin serous membrane around each organ


Blood & Blood Vessel Formation

Blood islands begin to form in mesoderm surrounding wall of yolk sac (week 3)
Then later in the lateral plate mesoderm and other regions


Endoderm Derivatives

Will form the gastrointestinal tract

Lines the ventral surface of the embryo & forms roof of yolk sac


Body Folding

Folding simultaneously occurs in both the sagittal plane (i.e., cranial/caudal folds) as well as the horizontal plane (i.e., lateral folds)

Takes the embryo from a flat disc to a cylinder


Cranial Folding

Oropharyngeal membrane acts as hinge & swings ventrally

Part of yolk sac endoderm is incorporated into embryo as the foregut (proximal portion of GI tract)

Oropharyngeal membrane separates the foregut from the oral cavity (will rupture ~day 26)


Caudal Folding

Cloacal membrane swings ventrally

Portion of yolk sac endoderm is incorporated as the embryonic hindgut (distal portion of GI tract)

Connecting stalk (future umbilical cord) is now attached to ventral aspect of embryo


Lateral Folding

Dorsal part of yolk sac endoderm is incorporated as embryonic midgut
End 4th week, lateral folds fuse, except at region of connecting stalk.
The midgut connection to the yolk sac constricts, leaving a narrower connection: the omphaloenteric duct (vitelline duct)


dorsal mesentery

Parietal & visceral layer of serous membranes continuous at dorsal mesentery
Suspends gut tube from posterior body wall into peritoneal cavity


Ventral mesentery

Only caudal foregut to upper part of duodenum
Results from thinning of septum transversum


The embryonic body cavity is comprised of

Pericardial cavity
2 pericardioperitoneal canals
Peritoneal cavity


Septum Transversum

Mesoderm between the thoracic cavity and yolk sac
Does not separate thoracic and abdominal cavities completely
2 pericardioperitoneal canals


pericardioperitoneal canals

As lung buds grow, they expand into pericardioperitoneal canals


Pleuropericardial folds

Pleuropericardial folds grow in medially from the lateral walls of the pericardial cavity
Dividing it into the definitive pericardial (1) and pleural cavities
Grow in as lung buds develop. As they become a bit more developed, they are in pleural cavities. Lung buds and folds help separate the pleural and cardial cavities.


Formation of the Diaphragm

Pleuroperitoneal folds develop from the dorsal body wall and fuse with the septum transversum, closing off the pericardioperitoneal canals on either side of the foregut

Diaphragm components: 1.) Septum transversum 2.) Pleuroperitoneal membranes 3.) Mesentery of esophagus 4.) Body wall mesoderm



Induces ectoderm to thicken. This thickening becomes the neural plate. Now it is neuroectoderm. As this occurs, invagination (dip down) happens, creating lateral elevations (neural folds).


Neural tube

Neural folds fuse at midline (both cranially and caudally). This separates from the ectoderm. First fusion happens at cervical region at 5th somite (midneck). Continues both cranially and caudally (double zipper). Dips the tube below the surface.



No cranial closure of neural tube



No caudal closure of neural tube.


Notochord during neural tube closure

Acts as a guide to show where to "zip."


End of neuralation

Zipping is done.


Neural crest cells

As folds fuse, some cells get pushed out of neuralectoderm. Extremely important, lead to a bunch of ganglai, melanocytes, other structures. Cells run out laterally and dorsally in embryo. Undergo epithelial to mesenchymal transition (tight together to mobile). Multipotent cell. Can lead to certain connective tissues, ganglia, odontoblasts!!, etc. Muscles



Intraembryonic mesoderm - inside embryo. Once neural tube forms, paraxial mesoderm forms near midline perforation, to intermediate to lateral plate.


Lateral plate mesoderm cavity

Coalesces, separates lateral plate mesoderm into two sections - somatic/parietal layer (lines amnion), visceral/splanchnic lines yolk sac.


Intraembryonic cavity

Forms between somatic and visceral layer - exposed to extraembryonic cavity.


Paraxial Mesoderm

Organizes into segments (somites), form cranially to caudally in pairs (occipital region first). Number of somite pairs is a timeline. Will form most of the axial skeleton, skeletal muscles, dermis of back all come from somites.



"baseballs" close to midline. Cells in paraxial mesoderm start to undergo epithelialization. Will crowd together into donut shape. Small, briefly lived lumen. Helps cells conglomerate with one another. That conglomeration soon shifts to surround the neural tube (forms sclerotome)
Cells between dorsomedial cells and ventrolateral cells form dermatome.



Cells at dorsomedial and ventrolateral edges of upper somite region will form precursors for muscles.



Closest to neural tube - vertebrae and ribs



Dermis - farthest from neural tube.



Muscle - midway between dermatome and sclerotome. As this development progresses, notochord will eventually degenerate, except for the inner portion (nucleus pulposis) will become the inner spinal cord.


Intermediate mesoderm

Urinary system, repro system - differentiates into urogenital system.


Parietal mesoderm layer

Along with overlying ectoderm form lateral body wall folds. Inside the body cavity (intraembryonic body cavity). Helps close body cavity by folding. This surrounds intraembryonic cavity. Thus lines different body cavities - forms serous membranes (mesothelial).


Visceral mesoderm layer

Directly surrounds yolk sac. With overlying endoderm forms walls of the gut tube. Surrounds internal organs.


Blood islands

Begin in (extra-embryonic) mesoderm surrounding yolk sac wall. Occurs later in lateral plate mesoderm and other regions. These migrate to surrounding tissues.



Forms gastrointestinal tract. Lines ventral surface of the embryo and forms roof of yolk sac.


Body folding

Just a flat shape. Must become a 3D object. Lateral and sagittal (cranial/caudal) begins to fold inward.


Cranial/caudal folding

Early week 4/mid week 3, you have neural tube occuring. As this occurs, cranial fold hinges ventrally. you also see growth occur in the caudal end, so this begins to fold ventrally. This swinging/folding brings with you the endoderm in that layer. In head region, endoderm forms part of gut region. In tail region you have endoderm that forms a tube, which will become the gut.


Septum transversum

Part of diphragm. Mesodermal tissue that was a little inferior to oropharyngial membrane. Will be brought ventrally and form the seperation into distinct cavities. These will form pericardial cavity, pericardioperitoneal canals (connect above septum transversum to peritoneal cavity of embryo), peritoneal cavity.


Oropharyngeal membrane

On cranial end, lined by endoderm and mesoderm. This acts as the actual hinge during folding. Heavy growth of tube leads to covering of this structure. As you fold, you bring underlying endoderm with it. How it forms forgut and proximal portion of GI tract. Oral cavity needs to be opened up, oropharyngeal cavity will deteriorate and rupture into developing gut tube (forgut = endoderm - yolk sac endoderm). **Notochord is not folding, does not get pulled.


Caudal folding

Ventral swing. The hinge is formed much quicker, folds under on the cloacal membrane (hinge). Endoderm and mesoderm fused together (like oropharyngeal membrane). Endoderm is still coming with it, now it will line hindgut. Cloacal membrane will also rupture (anus). As this swinging occurs, the connecting stalk is brought along and is swung to ventral surface, where the umbilical cord will form.


Lateral folding

Lateral edges come down to ventral surface. As they start to come ventrally to close off ventral surface. You see that they will fuse with one another. At umbilicus (connecting stalk) there is no complete closure. There is a midgut connection with the yolksac.


Omphaloenteric duct (vitelline duct)

At umbilicus (connecting stalk) there is no complete closure. There is a midgut connection with the yolksac.


Embryonic cavity

Forms parietal, pleural cavities. Now, you see parietal layer (inside body cavity), visceral layer.


Parietal and visceral layer of serous membranes fuse at

the dorsal mesentery (double layer). Suspends gut tube from posterior body wall into peritoneal cavity.


Ventral mesentery

Caudal foregut to upper part of duodenum. Results from thinning of septum transversum.


Septum transversum

Mesoderm between the thoracic cavity and yolk sac
Does not separate thoracic and abdominal cavities completely
2 pericardioperitoneal canals - strips of white going down.


Lung buds

As lung buds grow, they expand into pericardioperitoneal canals


Diaphragm formation

Pleuroperitoneal folds (mesodermal tissue) come in closer towards septum transversum (ventral). As they fuse, they help close off thoracic cavity above to abdominal cavity below. Other piece to diaphragm - parietal portions of mesoderm are incorporated with some mesentary of esophagus are incorporated into diaphragm.