Embryology Flashcards
Zygote
Fertilised ovum
Embryo
2-8 weeks
Fetus
9 weeks - term
Day 2
Zygote (2 cell)
Day 3
Morula (16 cell) - reaches uterine cornu
Day 4
Blastocyst - enters uterine cavity
Day 6
Implantation
Blastocyst
inner cell mass (embryoblast) - forms embryonic tissues
outer cell mass (trophoblast) - forms extra-embryonic tissues
70-100 cells
Fluid-filled cavity called blastocoele
Gastrulation
Gastrulation is a phase early in the embryonic development of most animals, during which the single-layered blastula is reorganized into a trilaminar (“three-layered”) structure known as the gastrula
Prior to gastrulation, the trophoblast differentiates in to
- Syncitiotrophoblast - outer
2. Cytotrophoblast - inner
Week 2: Day 8
Decidual reaction
Bilaminar disc - epiblast and hypoblast
Amniotic cavity forms
Week 2: Day 9
Lacunar stage
Hypoblast forms Exocoelomic membrane
Week 2: Day 12
Lacunae formation - communicate with endometrial sinusoids
Extra-embryonic mesoderm appears from yolk sac
Extra-embryonic coelum forms - chorionic cavity
Week 2: Day 13
Implantation is complete Cytotrophoblast forms primary chorionic villi Yolk sac to secondary yolk sac Syncitiotrophoblast excretes hCG Uteroplacental circulation begins
Week 3: Day 15
Primitive streak appears caudal end
Primitive node appears at cephalic end
Ectodermal cells migrate towards the streak and detach from it - intra-embryonic mesoderm
Mesoderm is between endoderm and ectoderm except at prochordal plate and cloacal plate
Week 3: Day 16
Allantois appears - diverticular that forms from the posterior wall of the yolk sac and extends in to the connecting stalk
Week 3: Day 18
Neurulation - neural plate formed at day 18
Week 3: Day 20
Primitive heart tube
Week 3: Day 21
First heart beat
Gastrulation
formation of three germ layers
3rd week
ectoderm to intra-embryonic mesoderm
Embryo folding
Occur in two planes
- longitudinal (enlarged cranial end)
- transverse (enlarged somites)
day 21-24
Components of ectoderm
Epidermis
Nervous system
Components of mesoderm
Muscles
Skeletal system
Connective tissues
Components of endoderm
GIT Resp Endocrine Auditory Urinary
Week 4
Septum premium appears Branchial arches Neural tube closes Optic pits appear Pulmonary primordium appear Hepatic plate appears Cystic diverticulum appear Dorsal pancreatic bud forms Spleen forms Urorectal septum appears Ureteric buds form
Week 5
Lens pit and optic cups
Nasal pits
Leg buds
Rudimentary blood starts moving
Week 6
Lungs start to form
Lymphatic system begins to form
Gonadal ridges begin to be perceptible
Week 7
Fetal heart tone audible
Nipples form
Hair follicles
Spontaneous limb movements
Week 8-10
eyelids close
Tooth buds
RBCs produced in liver
Week 11-14
Lanugo hair
Fetal skin transparent
Men forms in GIT
Week 17
Eyebrow and eyelashes
Nails
Week 21
Alveoli
Mesoderm - 3 parts
Paraxial mesoderm - segmental, forms somites
Intermediate - forms urogenital system
Lateral plate - forms the somatic and splanchnic mesoderm
Somites
Either side of neural tube First pair at D20 Max 42-44 pairs 1. Dermomyotomes - form connective tissue and muscles 2. Sclerotomes - form bones
Lateral plate mesoderm
- continuous with extra-embryonic mesoderm
- somatic mesoderm (lines the amnion)
- Splanchnic mesoderm (lines the yolk sac)
Intra-embryonic coelom
D19
merging of the clefts within the lateral plate
forerunner of serous cavities
