Embryology| From Implantation to Birth

Last time we looked at how you get from a single cell to a multicellular organism. This time we will look at the events that happen from Implantation to Birth.

Implantation – syncytiotrophoblast continues to grow in the stroma of the endothelium, a small space begins to appear in the inner cell mass, and the Cytotrophoblast begins to form the amniotic cavity. Cytotrophoblast begins to form a membrane that encloses the amniotic cavity.

  • At some time the embryonic disk forms from the inner cell mass the disk are made up two layers THE EPIBLAST – which is related to the amniotic cavity and has high columnar cells  and the
  • HYPOBLAST – ( HIGH CUBOIDAL CELLS) facing the blastocyst cavity other cells that originate the Cytotrophoblast form a membrane is continued with the hypoblast that forms the yolk sac
  • More epiblast cells around the trophectoderm from an extraembryonic mesoderm around both amniotic and primitive yolk sac
  • LACUNAE – in the syncytiotrophoblast that is filled maternal blood and glandular secretion provide nutrition for the embryonic disc. This is beginning of uteroplacental circulation, vessels from the mother contact the lacunae and circulation begins at day 10. Implantation is now complete with the endometrium now called DECIDUA
  • As this happens the extraembryonic mesoderm increases – spaces form and fuse to form extraembryonic coelom – this is fluid-filled cavity surrounding the amnion and the yolk sac
  • CHORIONIC VILLI – are formed by the end of the second week, Cytotrophoblast proliferates and pushes through the syncytiotrophoblast which forms the chorionic villi
  • Embryonic disc – HYPOBLAST thickens in once area to form the PROCHODIAL PLATE which will become the oral membrane of the embryo

Trilaminar EMBRYO- WEEK 3

  • The primitive streaks begin to appear as the thickening of the epiblast at the opposite end of the precordial plate in the midline of the dorsal parts of the embryonic disc. It elongates as the cells are added to the caudal end. Primitive end develops at the cranial end and the groove forms in the streak
  • the cells migrate across the epiblast pushing into the grove to form primitive mesenchymal cells that migrate into the regions between the epiblast and the hypoblast called MESODERM. EPIBLAST IS CALLED ECTODERM HYPOBLAST IS NOW ENDODERM
  • some mesenchymal cells move towards the cranial regions from the NOTOCHORD process that stops at the prechordal plate.
  • Some of the mesenchymal cells migrate cranially and laterally they then unite with the embryonic mesoderm covering the amnion and the yolk
  • at the cranial end of the primitive streak is the cloacal which is the future ANUS
  • ECTODERM – epidermis of the skin, nervous system
  • ENDODERM – epithelium of the respiratory system and the digestive tract, liver and pancreas
  • MESODERM – smooth muscle, connective tissue, blood vessels and the blood supply, skeleton and skeletal muscle and the organs of reproduction and excretion
  • Primitive streak – disappear
  • Oral membrane – becomes the mouth
  • Cloacal membrane becomes the anus
  • Notochord process become the notochord around which vertebral column develops
  • Notochord then disintegrates
  • Notochord also causes the formation of the neural plate from the ectoderm which then develops into the nervous system


  • This is the formation of the nervous system, starts with the formation of the neural plate, fold and tube
  • Notochord forms and the overlying ectoderm thickens forms neural plate. The neural plate then invaginates along midline forming the GROVE with the neural fold along each side
  • Grove deepens folds move close together and fuse forming the neural tube
  • The notochord induces and influences the overlying ectoderm causing proliferation and differentiation
  • Overlying ectoderm on the dorsal surface some together and fuse to eventually become the epidermis
  • At the same, the neural tube separate from the ectoderm and some neuroectodermal cells migrate away from the tube and form NEURAL CREST between tube ectoderm
  • The crest splits and becomes the sensory ganglia, spinal and cranial ganglia, Schwan pia and arachnoid matter
  • Other neural crests migrate to the edges and the ventral surfaces of the embryo to become pigment cells medulla of the adrenal and mesenchymal cells of the head
  • Teratogens during neurulation results in the lack of fusion of neural fold most common result are spinal Bifida


  • Formed from paraxial mesoderm either side of the notochord become the vertebrae, ribs and the Musculature
  • Lateral mesoderm will develop spaces with that fuse to form horse hole shape which will become body cavities


  • First seen in the chorion and yolk sac then develops in the embryo, mesenchymal cells in extraembryonic mesoderm form clumps blood islands and cords
  • The spaces form in the islands and angioblast line these primitive vessels that form by endothelial budding into another area this is known angiogenesis fusing with other cells forming the primitive vascular system
  • Blood cells are initially made by primitive endothelial cells and are made with the embryo at about week 5 of the development
  • Primitive heart forms from the longitudinal endothelial channels called endocardial heart tubes. Which fuse, and by the end of week 3 have formed a tubular heart joined to blood vessels in the connecting stalk chorion and yolk sac with the circulating heart and the beating heart


  • Develop around the whole blastocyst, primary chorionic villi starts to branch at day 15, mesenchymal grows into them – a core of loose CT now called secondary villi some mesenchymal cells develop into capillaries then called tertiary and mature villi in the region of the connective stalk will remain to form the fetal part of the placenta other villi degenerate to form the smooth chorion


  • Gas exchange – the total surface area of the placenta 11m2 of oxygen and carbon dioxide exchange by simple division
  • Fetal nutrition – different nutritive component transported by different means. Fatty acids by diffusion. Amino acid ascorbic acid by active transport through placental membrane. But fetus syntheses most of its protein through amino acids except for maternal protein IgG
  • Also performs their endocrine function


  • Head/ tail and lateral, head/ tail principally because of the rapid growth of the neuroectodermal. Lateral growth from the rapid growth of the somite
  • Folds eventually meet in the midline to form the gut tube
  • Midgut temporally continues with yolk sac through the yolk stalk
  • From now and until the 8th week it is embryonic period and it is tia stage that has a high degree of teratogenicity

Development is divided into trimesters

  1. 1-12 is called the embryonic period
  2. 12 to 24 rapid growth of the fetus
  3. 24 to birth
  • 0-2 week – implantation and bilaminar embryo is formed
  • 3 weeks trilaminar, neurulation, somites, blood vessels and chorionic villi formation of the heartbeat
  • 4 weeks is the folding of the embryo, limb buds are visible
  • 8 weeks embryogenesis is complete
  • 20 weeks toenails
  • 24 weeks the fetus is viable
  • 28 weeks the eyes open
  • 38-weeks birth


  • BMR increase by 15%
  • Thyroid enlarges by 50%
  • Maternal deficiencies if there is not enough in the diet eg iron
  • Cardiac output increase by 30-40% by 27 week
  • Blood volume increases by 30% due to fluid retention but the blood volume increases
  • Oxygen consumption increases by 20% near-term and minute ventilation increase by 50% but tidal also increase so reabsorption rate remains stable
  • Urine out increases because of an increase in the intake of fluids. GFR also increases by 50% this is counteracted 50% increase in reabsorption of NaCl and H2O by the renal tubes


Sadler, T. W. (2011). Langman’s medical embryology. Lippincott Williams & Wilkins.


The notes above are bachelor degree level, and where used as study notes for reproductive biology.  If there are inaccuracies, send us an email and will change it as soon as possible


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