Embryology Flashcards
What is cleavage?
The first mitotic division of the zygote, occurs 30 hours after fertilisation
Where does fertilisation occur?
Ampulla of Fallopian tube
What is the structure and function of the zone pellucid a
Prevents polyspermy. It is a glycoprotein shell that breaks down when it is a blastocyst
What is the process that leads to the formation of the blastocyst
Compaction
What is the optimum site for implantation
High on the posterior uterine wall
State two complications arising from implantation not occurring in the correct place.
Ectopic pregnancy - implantation in Fallopian tube or less commonly in the abdomen.
Placenta praevia - implantation low in the uterus, placenta can haemorrhage during delivery
When are the pre embryonic, embryonic and fetal stages?
Pre embryonic is weeks one and two. Embryonic is weeks three to eight. Fetal is nine to thirty eight.
What are the two cells that are present following cleavage called?
Blastomeres
When are the cells totipotent
When in the morula
At what stage do you introduce the fertilised ovum in IVF?
The four or eight cell stage, to increase likelihood of implantation
Give the names of the two distinct cell masses and the cavity in a blastocyst
Embryoblast (inner cell mass), trophoblast(outer cell mass), blastocyst cavity.
After compaction, are the cells of the blastocyst totipotent?
No, they are pluripotent. The inner cell mass will go on to form the tissues of the embryo, and the outer cell mass will go on to form the support structures.
What are the two layers of the bilaminar disk?
What are the corresponding cavities?
What cavity do they sit in?
What layers does the outer cell mass differentiate into?
Epiblast and hypoblast.
Amniotic cavity and yolk sac
Chorionic cavity
Cytotrophoblast and syncytiotrophoblast.
Why can PV bleeding occur at implantation, and why does this make it difficult to date a pregnancy?
The blastocyst releases enzymes to break down the endometrium. The endometrium is highly vascular, so bleeding can occur. This can be confused with a menstrual period, so the pregnancy might not be accurately dated
Where does the blastocyst implant
Uterine stroma
How does the support system of the blastocyst change following implantation?
Histiotrophic (simple diffusion) to haemotrophic (blood flow established). Lacunae in syncytiotrophoblast fuse with maternal capillaries.
What pushes the cytotrophoblast membrane away from the primitive yolk sac membrane?
What does this go on to form?
Acellular extraembryonic reticulum.
Extraembryonic mesoderm, then as spaces around it combine to form chorionic cavity, it forms connecting stalk which is the future umbilical cord
How does the secondary yolk sac form?
The hypoblast produces cells that migrate and form a smaller cavity within primitive yolk sac (exocoelomic cavity)
During its formation large parts of the primitive yolk sac pinch off to form exocoelomic cysts
What lines the chorionic cavity?
Chorionic plate -extraembryonic mesoderm.
What proportion of zygotes are lost in the first 2-3 weeks?
50% Pregnancy often undiagnosed
How many diagnosed pregnancies miscarry?
What proportion of women suffer recurrent miscarriages?
15%
1%
What effect does the notochord have on overlying ectoderm?
Directs conversion to neuro ectoderm. The effect is limited by how far molecules diffuse.
What is the next structure to form once the ectoderm has been induced to form neuroEctoderm?
The edges of the neural plate curl out of the plane and form the neural tube.
From which part of the mesoderm do the somites form? In which way does their formation proceed?
Paraxial mesoderm (cells close to the midline and neural tube) Cephalocaudally.
What is a somite?
How can they be used to age an embryo?
How many form, and how many are left once some have regressed?
Initially exist as a ball of mesoderm, then undergo epithelization to form donut shape around a small lumen.
Appear regularly at 3 per day so can accurately age an embryo by the number of somites.
42-44, regresses to 31.
What are the four different parts of mesoderm present after neurulation?
Paraxial
Intermediate
Splanchnic (viscera) continuous with mesoderm covering the yolk sac
Somatic continuous with mesoderm covering the amnion
What is the space between the splanchnic and somatic mesoderms?
Intraembryonic coelom
What does the intermediate mesoderm develop into?
Urogenital tract
What part of the somite breaks down in organised degeneration?
Ventral (back) wall - goes to form sclerotome
What does the sclerotome give rise to?
Vertebrae and ribs
What does the dorsal portion of the somite form?
Dermomyotome
What does the dermomyotome give rise to, and by what process?
Myotome proliferates and migrates. Goes to form muscles in the back, intercostal muscles, and some limb muscles.
Dermotome disperses, goes to form dermis.
What is the implication of segmentation?
Divides the body into repeating structures. Eg vertebrae, ribs, intercostal muscles, spinal cord segments.
Guides innervation.
What does the hypomere give rise to?
What does the epimere give rise to?
What innervates them?
Hypaxial muscles which are ventral to the horizontal septum of the vertebrae. (Eg muscles of the abdomen, diaphragm)
Epaxial muscles which are dorsal to horizontal septum of the vertebrae
Epimere innervated by dorsal branch, hypomere innervated by ventral branch, of same nerve.
How many pairs of spinal nerves are there?
31 pairs
What does splanchnic or visceral mesoderm (lateral plate mesoderm) give rise to?
The smooth muscle, connective tissue, and vasculature of the gut.
What does the somatic or parietal mesoderm (lateral plate mesoderm) give rise to?
Connective tissue of limbs. Dermis of body wall and limbs. Sternum
What folding occurs, why, and what results from it?
Cephalocaudal folding and lateral folding.
Driven by bulging of brain vesicles into amniotic cavity and lengthening of the neural tube.
Gives rise to embryo with ectoderm on the outside, with umbilical region where connecting stalk on yolk sac duct remain attached
What does folding achieve?
New cavity inside the embryo -primitive gut.
Ventral body wall
Pulls amniotic membrane around embryo - suspended in amniotic sac
Pulls connecting stalk ventrally
Puts angiogenic tissue in the right place
Creates new body cavity - embryonic body cavity.
What are the three germ layers of the embryonic disc? By what process do they form?
Ectoderm, mesoderm, endoderm. Gastrulation.
What structure marks the beginning of gastrulation?
Primitive streak, with a primitive pit in a primitive node.
What is the legal significance of the primitive streak?
Cannot store an embryo once it has appeared.
At which end of the primitive streak in the primitive node located?
Cranial end
In what direction does development proceed?
Cranial to caudal
From which cells of the bilaminar disk does the trilaminar disk derive?
Epiblast - hypoblast regresses.
How does the trilaminar disk form?
Cells from the epiblast migrate (chemotaxis) and invaginate through centre of primitive steak, form three primitive germ layers. Under complex chemical control.
How are the layers of the trilaminar disk colour coded?
Ectoderm is blue. Mesoderm is red. Endoderm is yellow.
Which direction does mesoderm spread out in?
Laterally and cephalad.
Where is mesoderm not found?
Cloacal membrane
Oropharyngeal membrane
What is the notochord, how does it form?
Solid rod of cells in the midline of the trilaminar disk, important signalling role.
Formed from cells from epiblast, which migrate through cranial part of primitive pit.
What is the role of notochord?
Defines axes
Drives neurulation
Axial skeleton forms around it.
Remnant in adults is nucleus pulposus of intervertebral disks.
What are the axes of an embryo?
Rostral/cephalic and caudal (head and tail)
Ventral and dorsal (tummy and back)
Left and right.
What are the derivatives of the ectoderm?
Structures that keep us in touch with the outside world - cornea, epidermis, nervous system.
What are the derivatives of mesoderm?
Muscle, connective tissue, cartilage, bone, vascular system.
What are the derivatives of endoderm?
Epithelia of GI tract and respiratory tract, parenchyma of glands.
How are the left and right axes determined?
Cilia on node direct signalling molecules to left hand side.
How does situs inversus arise?
Immotile cilia, so signalling molecules for left hand sidedness are not properly directed.
When is gastrulation complete caudally?
By the fourth week.
What is a monozygotic twin?
A fertilised oocyte gives rise to two identical infants.
What are the two ways monozygotic twins form?
Embryo can split after first cleavage - each will have its own placenta.
Or two separate embryoblasts can form in the blastocyst - share a placenta.
What happens if separation is not complete?
Conjoined twins.
What is teratogenesis?
Process by which normal embryonic development is disrupted.
Which period is the most sensitive to teratogenic insult?
Embryonic period - 3-8 weeks
List four possible teratogenic agents.
Therapeutic drugs ACE inhibitors, warfarin, anticonvulsants.
Alcohol
Rubella
Thalidomide
