Embryology Flashcards
Define germinal age
Define embryonic age
Time since last menstruation
Time since fertilisation (germinal age - 2 weeks)
Define:
Germinal period
Embryonic period
Foetal period
Germinal period: fertilisation - end of the 2nd week
Embryonic period: start of the 3rd week - end of the 8th week
Foetal period: start of the 9th week - birth (38 weeks)
What are the stages of fertilisation?
Oocyte (secondary - paused at M2) is released from the ovaries and carried to the ampulla by fimbriae
Sperm will fertilise the egg in the Fallopian tubes (ampullary region), instigating the remaining meiosis of the oocyte and eventually fusion of the chromosomes producing a diploid (tototent) zygote
The zygote will move from the Fallopian tubes to the uterus dividing as it does so to form a morula (8 cell bundle)
Describe the formation of a blastocyst from a morula?
Morula —> blastocyst
Forms a fluid filled cavity called the blastocoel cavity
What changes also happen to a blastocyst to prepare it for implantation?
Loses its zona pellucida (allows implantation into the endometrium of the uterus)
Inner cell mass differentiates into the embryoblast
How does a blastocyst implant?
Trophoblast (and endometrium) release enzymes that devour the endometrium of the uterus
Blastocyst embeds itself into the endometrium
What other changes does the blastocyst undergo after implantation?
The trophoblast differentiates itself into the inner cytotrophoblast and the outer synctiotrophoblast
The embryoblast differentiates itself into the epiblast and the hypoblast forming a bilaminar embryonic disc
An amniotic cavity forms in the embryoblast between the epiblast and the cytotrophoblast
How is the exocoelomic cavity formed?
The hypoblast forms an exocoelomic membrane around the inner surface of the cytotrophoblast
Blastocoel cavity becomes the Exocoelomic cavity (primitive yolk sac)
How does the embryo develop a uteroplacental circulation?
Holes appear in the synctiotrophoblast (lacunae), these join to form lacunae networks
Capillaries in the uterine wall dilate to become maternal sinusoids
Enzymes from the synctiotrophoblast destroys the lining of sinusoids and glands allowing maternal secretions to enter the lacunae networks
How does the chorionic cavity form?
Rapid growth of the trophoblast creates space around the embryoblast
New cells form between the cytotrophoblast and the embryoblast forming the extraembryonic mesoderm
Cavities forming in the the extraembryonic mesoderm fuse to become the chorionic cavity
What are the final stages of development of a blastocyst?
The exocoelomic cavity (primitive yolk sac) eventually pinches off leaving the secondary yolk sac (embryoblast)
The chorionic cavity increases and the BED connects to the trophoblast by the connecting stalk (extra-embryonic mesoderm) for histiotrophic nutrients
What is the difference between histiotrophic nutrition and haemotrophic nutrition?
Histiotrophic nutrition: nutrition from the trophoblast
Haemotrophic nutrition: nutrition from the maternal blood
What is the normal implantation site of the blastocyst?
Upper posterior wall of the uterus
What is ectopic pregnancy?
What is placenta previa?
Pregnancy at an alternative site in the uterus (most commonly in the Fallopian tubes)
Placenta previa is when the placenta blocks off the cervix
What are the stages in the embryonic period?
Gastrulation
Neuralation
Segmentation
Folding
What is gastrulation?
The formation of the trilaminar embryonic disc from the bilaminar embryonic disc
How is the trilaminar disc formed?
Epiblast cells migrate to the primitive streak, invaginate down the primitive groove and displace the hypoblast layer to create a third layer.
The primitive node (area at the front of the primitive streak) moves caudally to cranially forming the notochord
How is the buccoharyngeal and cloacal membrane formed?
Formed when the epiblast and the hypoblast come into contact with each other
Buccopharyngeal: future mouth
Cloacal: future anus
What do the layers of the trilaminar embryonic disc form?
Ectoderm: external structures (skin, CNS)
Mesoderm: supporting tissues (bone, cartilage, muscle)
Endoderm: internal structures (respiratory tract, GI tract)
How is left-right sidedness coordinated by the primitive node?
Ciliated cells
What is neurulation?
Formation of the neurectoderm from the ectoderm
How is the neurectoderm formed?
Signals from the notochord cause the thickening of the ectoderm forming a neural crest
Neural crest joins forming a neural tube and paraxial mesoderm forms 31 somites
What are somites?
Blocks of mesoderm cells around a cavity
What is segmentation?
Organisation of mesoderm into somites and then degeneration of somites into different tissues
What happens during organised degeneration of somites?
Somites breaks down:
Ventral wall becomes sclerotome
Dorsal wall becomes dermomyotome
Dermomyotome eventually separates into dermatome (outside) and myotome (inside)
What do sclerotome, myotome, dermatome eventually form?
Sclerotome: bone
Myotome: muscle
Dermatome: skin
What is folding?
Process of pulling all the compartments into the right place to form and forming a new cavity (primordium of the GI tract)
What does folding achieve?
Draws together the edges of the trilaminar disc forming a ventral body wall with the connecting stalk
Suspends embryo within amniotic sac
At the end of the embryonic period what has happened?
Nervous system has started forming
Segments have appeared with specific functions
The embryo has been folded with everything in the right place
