Embryology Flashcards
How does a sperm cell develop?
Spermatogenesis = Stem cell –> primary spermatocyte –> meiosis I –> 2 secondary spermatocyte –> meiosis II –> 4 spermatids –> maturation –> mature sperm
Describe the development of an oogonia
FETAL: Oogonia –> mitosis –> Oogonia (2n) –> meiosis stops at prophase I –> primary oocyte (2n)
PRIOR TO OVULATION: monthly 1 primary oocyte finishes meiosis I –> becomes secondary oocyte (n), released from ovary –>
AFTER FERTILISATION: fertilisation –> meiosis II –> ovum (2n)
Outline the female reproductive anatomy
Ovary
Ampulla
Fallopian tube
Uterus
Internal Os - Cervix - External Os
Vagina
Labium Minus
Where does fertilisation normally occur?
ampullary region of the fallopian tube
Define embryonic age
time since fertilisation
Define gestational age
time since last menstration (embryonic age minus 2 weeks)
What is the germinal stage?
time from fertilisation to end of 2nd week
What is the embryonic period?
time from 3rd week to end of 8th week = all major structures/systems are formed, greatest risk for congenital malformation here
What is the foetal period?
time from beginning of 9th week to birth at 38 weeks
The process of a blastocyst losing the zona pellucida is called?
hatching
Why does the blastocyst have to lose the zona pellucida before implanting?
zona pellucida is thought to prevent the blastocyst adhering to the oviduct
Where is the normal implantation site and what does that determine?
superior and posterior wall of uterine cavity, on the 6th day in the endometrium determines site of placenta formation
What is a morula?
a solid ball of cells resulting from division of a fertilised ovum, and from which a blastula is formed
What is a blastocyst?
a mammalian blastula in which some differentiation of cells has occurred zona pellucida dissappearing from the outside, trophoblast now on the outside, inner cell mass on the inside
In regards to the morula what does the zona pellucida do?
stops the morula enlarging
What does the trophoblast become?
the yolk sac and placenta
What is the difference between histiotrophic and haemotrophic nutrition?
histioyrophic: nutrition provided not from mother blood up to 12th week haemotrophic: nutrition provided by mothers blood from 12th week onwards
Outline the development of a zygote
Zygote
2-cell stage
4-cell stage
8-cell stage
Morula: ~16 cells
Early blastocyst: ~32cells
Mid blastocyst: ~64-100 cells
Late blastocyst: >100 cells, no zona pellucida, embryoblast, primative endoderm inside trophoblast
An abnormal site of implantation is called?
an ectopic pregnancy
What is placenta previa?
the placenta is lying unusually low in the uterus, next to or covering your cervix
What happens in the 2nd week of development?
blastocyst has burrowed into the endometrium
trophoblast diff to 2 layers: cytotrophoblast (inner, amniotic cavity) and syncytiotrophoblast (outer)
embryoblast forms 2 layers: epiblast and hypoblast (= bilaminar embryonic disk)
hypoblast contributes to the formation of 2 cavities: yolk sac and chorionic cavity
Outline ovulation
endometrium thickens, mid menstrual cycle pituitary gland stimulates release of egg, wall of ovarian follicle ruptures at surface of ovary releasing egg, fimbriae sweep egg to fallopian tube, propelled by cilia and contractions of walls, ampullary region egg may be fertilised by sperm = zygote, begins dividing, travels to uterus, implants in endometrium. If egg isn’t fertilised it is reabsorbed, endometrium sheds through menstration
What is the lining of the uterus called?
endometrium
During intercourse how many sperm enter the vagina?
~300 million
Outline the sperms journey to the egg
ejaculation, millions die in vaginas acidic environment, passes through cervix = open for few days during ovulation, cervical mucus also thinned, millions get trapped in mucus/folds, muscular uterine contractions assist sperm towards fallopian tubes, immune system start to attack sperm, inside fallopian tube sperm must swim against cilia current, chemical cause change in sperm membrane = hyperactive, sperm reach egg
Outline fertilisation
sperm reach egg, egg covered by corona radiata, sperm pushed through this layer, reaches zona pellucida, attach sperm receptors = acrosomes release digestive enzymes, sperm burrows through zona pellucida, attaches to egg cell membrane, outer membranes fuse, egg pulls sperm inside = changes in egg cell membrane = no other sperm can get in, chemicals produced by egg = pushed sperm away from egg, inside male and female pronucleus is formed, microtubules pull towards each other = fuse, cila move zygote to uterus
How long is the sperm vs the secondary oocyte viable for?
sperm: 5 days secondary oocyte: 12-24hrs
How many days post-fertilisation does implantation take place?
6
What is the diff between monozygotic (identical) twins and dizygotic (fraternal) twins?
monozygotic: morula splits into 2 embryos dizygotic: 2 eggs fertilised by 2 sperm
What happens during implantation?
endometrium swells = increasing glycogen stores trophoblast secrete enzymes, dissolve endometrium paracrines increase no. capillaries = more O2/nutrients
What connects the circulation of the mother and foetus?
placenta
What is the structure of a late blastocyst
>100 cells, no zona pellucida, embryoblast, primative endoderm inside trophoblast
What is the connecting stalk?
Yolk sac diverticulum, derived from extraembryonic mesoderm. Connects chorionic cavity and yolk sac. Eventually it forms umbilical cord
What is the function of the yolk sac?
acts as the preliminary circulatory system providing nutrients, is eventually absorbed into the gut
Define gastrulation
single-layered blastula is reorganized into 3 embryonic germ layers = gastrula
What are the stages in early embryonic development?
fertilisation, implantation, gastrulation, neurulation, segmentation, folding
During the third week what new feature appears on the epiblast and where?
primitive streak on the dorsal surface, with the primitive node at the apex
At what point is an embryo not allowed to be kept in culture?
formation of the primitive streak
What happens in the migration and invagination stages of gastrulation?
cell of the epiblast layer migrate towards edge of primitive streak, push underneath the epiblast above the hypoblast = 3 newly named layers
Name the layers of the bilaminar disk
epiblast (outside), hypoblast (inside)
Name the layers of the trilaminar disk
ectoderm (outside), mesoderm (middle), endoderm (inside)
The 2 zones of missing mesoderm function as?
the future mouth and future anus
What lineages arise from the ectoderm?
structures that maintain contact with outside world: nervous epidermis
What lineages arise from the mesoderm?
supporting tissues: muscle, cartilage, bone, vascular
What lineages arise from the endoderm?
internal structures: epithelial lining of GI and respiratory
What is situs inversus?
heart and other organs of the body are transposed through the sagittal plane to lie on the opposite (left or right) side from the usual
How does right to let asymmetry occur?
at the primitive node a left-ward flow (by cilia) of signalling molecules is releases = signalling cascade
What is the notochord?
feature formed by gastrulation, solid clump of cell that runs down the midline = important signalling role to ectoderm above = conversion of overlying ectoderm to neurectoderm = thickening and formation of slipper shaped neural plate
How is the neural tube created?
notochord signals ectoderm to thicken to neurectoderm = neural plate from which the edges elevate out, curl towards each other and fuse = neural tube
During the embryonic period what happens to the mesoderm?
organises itself into distinct regions = paraxial mesoderm, intermediate mesoderm, somatic mesoderm, splanchnic mesoderm
What are the distinct regions/blocks of mesoderm referred to as?
somites
How do embryologists age embryos?
looking at the somite structure present
What do somites transform into?
myotome =building blocks of the MSK, dermatome = dermis of the skin, sclerotome = ribs
How many somites are there?
31 = which corresponds to the amount of spinal nerves
Folding of the trilaminar disk when the neural tube and cardiogenic area are present is driven by what?
Growth of the neural tube and somite segments
What is cephalocaudal folding?
Folding of the trilaminar disk at the head and tail end
What does folding achieve?
Creates ventral body wall, pulls amniotic membrane around disk, creates gut, puts heart in the right place
