Embryonic Disc Formation Flashcards
By what week post-fertilisation have many of the major organ systems formed?
8 weeks
Describe the formation of a Morula
Day 1 - 4
Morula forms by repeated mitotic divisions
Describe the formation of a Blastocyst
Day 5
Blastocyst forms: Trophoblast and Embryoblast (Inner cell mass)
What happens on day 6?
Implantation
What needs to disintegrate before implantation?
Zona Pellucida
What happens on day 8?
Amniotic Cavity opens up
Bilaminar disc: Epiblast (Top)
Hypoblast (Bottom)
What happens on day 9?
Primary Yolk Sac forms below hypoblast
What is a syncytium?
A single cytoplasmic mass containing multiple nuclei
What cell layers will eventually form the placenta?
Extraembryonic mesoderm, Syncytium and Cytotrophoblasts
What happens on day 12?
Extraembryonic mesoderm develops and forms cavities within (Will eventually form Chorionic Cavity)
What happens on day 13?
Connecting stalk forms and chorionic cavity fills with fluid.
How is the trilaminar disc formed?
Week 3 - Gastrulation
Rapid epiblast cell division at the primitive groove and downwards migration forms 3 layers: Ectoderm, Mesoderm and Endoderm.
What is the Primitive node and pit?
Week 3 - Neural Tube Formation
Raised area of epiblast. Cells migrate from the node to form the Notochord.
Tip: Node = ‘Nodochord’
What are the derivatives of the Endoderm?
Lining of gut tube, respiratory system and urinary system
What are the derivatives of the Ectoderm?
Skin & neural tissue
What are the derivatives of the Mesoderm?
Organs and muscles
Define Somites
Somites form parts of the axial skeleton (skull, vertebrae, ribs), associated musculature and the dermis of the skin
Describe the 2 directions of embryonic folding
Longitudinal - Moves heart, brain, mouth & anus into an adult position (process called Reversal)
Lateral - draws amniotic cavity around the embryonic disc
Briefly describe gut tube folding
The gut tube grows rapidly and herniates out of the umbilical region. The gut tube rotates 270º anticlockwise along the axis of the SMA
Name 2 pathologies of gut tube development
Omphalocele - The gut tube may not fully return to the abdomen
Gastroschisis - Gut tube may herniate through the abdominal wall
What is a volvulus
Twisting of a loop of intestine and supporting mesentery resulting in bowel obstruction.
Name the remnants of the vitellointestinal duct and their clinical significance.
Cyst, band, fistula, sinus or diverticulum
Clinical:
Cyst - protrusion from umbilicus
Fistula - faeces leaking from umbilicus
Meckel’s Diverticulum - can inflame and cause tenderness
What is the cloaca?
The cloaca is separated by the urorectal septum into the urogenital sinus (developing bladder) and rectum
What problem(s) can occur if the urorectal septum does not form properly?
Imperforate Anus
What are pharyngeal arches and what to they form?
4 Interconnecting loops of ectoderm, mesoderm and endoderm (lower face and neck)
What layer does the notochord form within?
Mesoderm - creates a longitudinal axis in the embryo
What does the 1st pharyngeal arch form?
Nerve supply: CNV (Trigeminal nerve)
Muscles: e.g. Mastication and tensor tympani
Bones: e.g. malleus, incus, maxilla, zygoma & mandible
