Embryo L3 Flashcards
Organogenesis
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
Neurulation
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
Somites
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
sclerotome
Cells in ventral & medial walls shift position to surround neural tube; form sclerotome
dermatome
Cells between dorsomedial cells & ventrolateral cells form the dermatome
Somite Derivatives
Sclerotome
-Vertebrae & ribs
Dermatome
-Dermis
Myotome
-Muscles
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
Notochord
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.
Anacephaly
No cranial closure of neural tube
Spinabifida
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
Mesoderm
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.
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.
Dermatome
Cells at dorsomedial and ventrolateral edges of upper somite region will form precursors for muscles.
Sclerotome
Closest to neural tube - vertebrae and ribs
Dermatome
Dermis - farthest from neural tube.
Myotome
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.
Endoderm
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.
