Session 2 - Lecture 1 - Embryology Flashcards
1 - Title
Development of the peritoneal cavity and the foregut
“Lecture 1 of 2 - q a bit about GI dvlpmnt – essentially trying to understand why the disposition of the viscera is as it is – why we have got omentum, mesentery, why some structures are mobile”
2 - Lecture objectives
Lecture objectives • Primitive gut tube formation • Disposition of the viscera – Mesenteries – Anatomical relationships – Peritoneal vs retroperitoneal • Foregut – Stomach • Rotation of the stomach – Liver (and Pancreas) – Duodenum
“Objectives, translate those into outcomes by adding descriptions – just remind you and take you back to early embryological dvlpmnt and frame us in formation of primitive gut tube. Try to understand how simple linear gut tube comes to be arranged with disposition of the viscera that you become to be more familiar with, add a lil detail to talk about mesenteries, what form, what structures have them what don’t, and the anatomical structures related comprising the GI tract. Peritoneal vs retroperitoneal – introduce it today and again next half of story. And first half of primitive gut tube called foregut – stomach liver, pancreas and most of duodenum – what’s interesting about the foregut – the stomach as it dvlps undergoes a remarkable change in its shape but also its position, and the change in position influences structure, arrangement etc., and influence development, specialisation of the mesenteries – conceptually q challenging, but go through it in step by step way”
3 - Step 1
How do we get an anterior body wall?
How do we get a gut within a cavity?
4 - How do we get an anterior body wall?
How do we get a gut within a cavity?
Folding of the embryo
“embryonic folding”
5 - Embryonic folding (in words)
Embryonic folding (in words) • In the 4th week the embryo folds – Laterally • Creates ventral body wall • Primitive gut becomes tubular – Craniocaudally • Creates cranial & caudal pockets from yolk sac endoderm (beginning primitive gut development)
“In 4th week, trilaminar embryologic disc – has specialised agents in pre-layers, growth of neural tube, specialisation within mesoderm causes trilaminar mesoderm to fold up - essentially the lateral folding of the embryo creates the ventral (posterior) body wall which forms by default as it were, anterior body wall, edges of the disc folding and meeting each other in the midline causing effectively a seal.”
“Reminding you of what we’ve seen before, this video tries to illustrate what’s happening in the longitudinal plane – this is cephalocaudal folding – head and tail fold – neural tube sitting in ectoderm layer, causes disc to push down at cranial end and caudal end – red dot represents position of cardiogenic field, following position of that illustrates what’s happening. LAst year, most interested in fact entire embryo sits in amniotic side, now undergoes dvlpmnt at right place for heart and great vessels – but today we’re oing to focus our attention on primitive gut tube – so here’s our yolk sac down here, and what you’ll see is primitive gut tube formed from pirimitive yolk sac during embryonic folding. Similarly at perpendicular plane – lateral folding, lateral margins of disc, driven by growth of somites, and here’s our yolk sac, watch what’s happening as embryo folds, creates space inside embryonic body with a gut tube suspending within it. This clip/imaginary image is taken at the point of umbilicus – at this stage of dvlpmnt there is an opening between this stage and the umbilicus,”
6 - What is the gut tube?
What is the gut tube? • Endoderm lined tube • Runs the length of the body • Blind pouches at the head and tail ends • Opening at the umbilicus • Splanchnic mesoderm covering
Sagital plane (2:31) - Septum transversum - Foregut Amniotic cavity - Amnion - Hindgut - Allantois - Yolk stalk Umbilical vesicle (yolk sac)
Transverse plane (1:47)
- Amniotic cavity
- Dorsal mesentery
- Somatic mesoderm
- Splanchnic mesoderm
- Embryonic gut tube
- Intraembryonic coelom
https://www.youtube.com/watch?v=LgqDemnumJ4&feature=youtu.be
“So we need to ask ourselves just to add a little bit of detail to what it is – frame shots, pictures from previous videos – so endoderm lined bc that’s the germ layer that lines the primitive gut tube. It runs the length of the body, initially when it forms, it counterintuitively actually doesn’t have openings where you would expect there to be – at head and tail end they’re initially 2 blind ended pouches – where there is an opening is at the umbilicus – particularly important next time we meet when talking about dvlpmnt of small and large intestines. Point of continuity at the umbilicus. So again, you’ll remember your trilaminar embryonic disc, lateral part, mesoderm splits into 2 separate layers – spomatic to do with the body, splanchnic to do with the visceea. Somatic mesoderm lining the body wall and splanchnic mesoderm surrounding the gut tube – think back to derivatives of germ layers, clearly going to give muscular derivaties, splanchnic going to give derivatives of the gut wall.”
7 - Embryonic divisions of the gut
Embryonic divisions of the gut
• Foregut and hindgut begin as blind diverticula
• Midgut has an opening at first and is continuous with the yolk sac
• These embryonic divisions have implications for blood supply and lymphatic drainage in the adult
Head fold - Foregut Midgut - Yolk stalk Yolk sac - Extraembryoni... - Hindgut - Tail fold https://www.slideshare.net/Ramzanken/folding-of-the-embryo
“So now in thinking about how we go from simple linear primitive gut tube we need to then just map out the structures, or the divisions within that primitive gut tube. So 2 blind ended diverticulum – 1 at head end, 1 at tail end. The cranial most part of PGT is called the foregut, bit in mid is midgut which has opening to umbilicus out to yolk sac and caudal most part is hindgut. And what we can do is map all of the organs that comprise the GI tract back to 1 of those divisions of PGT, and essentially that allows us to understanding blood supply, venous/lymphatic drainage etc of all structures, so implications for lymphatic drainage you are studying as a mature individual.”
8 - Derivatives
Derivatives Foregut - Oesophagus Stomach Pancreas, liver & gall bladder Duodenum (proximal to entrance of bile duct)
Midgut - Duodenum (distal to entrance of bile duct) Jejunum Ileum Cecum Ascending colon Proximal 2/3 transverse colon
Hindgut - Distal 1/3 transverse colon Descending colon Sigmoid colon Rectum Upper anal canal Internal lining of bladder & urethra
“Map out the derivatives of each of these zones/regions or divisions of PGT
- foregut will ultimately give us part of GT from oesophagus to duodenum, and v distinct boundary between foregut and midgut – point at which bile duct enters duodenum, amrks boundary. Midgut derivatives are remainder of duodenum, distal to entrance of bile duct, and then all away along to about 2/3 way of transverse colon – less obbious anatomical structure/landmark that marks the boundary, but for understanding, about 2/3 way. And then finally, remainder of hindgut gives everything else – bit often forgotten and overlooked, but we know the hindgut can also holds within it the cloaca which is divided to give us the anorectal canal but also the bladder and urethra, so just don’t overlook that as a consequence.”
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