Early Embryology and the Gut Tube Flashcards
By what week (post-fertilisation) of development have many of the major organ systems formed?
Week 8.
What is the approximate length of an 8 week old embryo?
30mm.
As the fertilised ovum moves down the uterine tube and undergoing mitotic division, what is surrounding the ovum?
Zona pellucida.
What is the Zona pellucida?
A thick transparent membrane surrounding the ovum.
List 2 functions of the Zona pellucida.
- To prevent polyspermy.
2. Protect the ovum.
What is a morula?
A solid ball of cells resulting from division of a fertilised ovum, and from which a blastula is formed.
How many cell types form a blastocyst? What are they and how are they arranged?
Trophoblast and embryoblast. The outer layer is the trophoblast and the inner layer is the embryoblast.
What does a morula develop into?
Blastocyst.
Around what day of fertilisation is the Blastocyst formed?
Day 5.
What is the fluid within a Blastocyst called?
Blastocoele
Around which day does the Zona pellucida begin to degrade and disappear?
Day 6
List 2 reasons why it’s vital that the Zona pellucida disappears.
- To enable to Blastocyst to grow rapidly.
2. To enable to Blastocyst to stick to the epithelial covering of the uterine lining.
What new cell layer is formed when the Blastocyst moves into the endometrium?
Syncytiotrophoblast.
On what day is the bilaminar embryonic disc created?
Day 8
What are the 2 layers of the bilaminar embryonic disc?
Epiblast and hypoblast.
On day 9 of fertilisation, what fluid filled sac begins to develop below the hypoblast?
Primary yolk sac.
What is a syncytium?
A single cell or cytoplasmic mass containing several nuclei, formed by fusion of cells or by division of nuclei.
On day 12 of fertilisation, what now surrounds the developing embryo?
Extra-embryonic mesoderm.
What can be found within the extra-embryonic mesoderm surrounding the developing embryo? What will they eventually do?
Fluid filled cavities called extra-embryonic coelom, which will join together around the embryo to form a fluid filled sac.
In what week does gastrulation occur?
Week 3
What is gastrulation?
The rapid division and downwards migration of epiblast cells to create a three layered (trilaminar) embryonic disc.
Which 2 key structures can be found towards the tail end of the developing embryo?
Primitive node and primitive streak.
In the developing embryo, what is the primitive streak?
A raised area of epiblast extending from the cloaca to the primitive node.
In the developing embryo, what is the primitive groove? What is happening to the cells there?
An indentation in the centre of the primitive streak - it marks the region where cells are rapidly migrating downwards to form the three layered disc.
Cells from the primitive streak migrate downwards to create which new cell layer?
Intraembryonic mesoderm.
Cells from the primitive node and pit form which structure in the embryo?
The notochord
In which layer of the trilaminar disc does the notochord sit?
The mesoderm
What should eventually happen to the primitive node and streak? What will form if this does not happen?
They should disappear - if not can lead to the formation of a teratoma at the distal end of the vertebral column.
What does the ectoderm form?
Skin and neural tissue.
What does the mesoderm form?
Serous membranes, somites, renal, and gonad
What does the endoderm form?
Lining of the gut tube, respiratory system, and urinary system.
How many degrees, in what direction, and around what axis does the gut tube rotate during development?
270 degrees anticlockwise around the SMA.
What is the condition in which the gut tube does not fully return to the abdomen? Is the gut tube in direct contact with the outside world?
Omphalocele - not in direct contact, surrounded by amniotic membrane.
What is the condition at birth in which the gut tube herniates through the abdominal wall?
Gastroschisis.
Which structure separates the cloaca into the bladder and rectum?
Urorectal septum
