Embryology

• The embryonic period spans the first 8 weeks and the fetal period encompasses the remaining 30 weeks
• The fertilized egg is called a zygote and while it moves towards the uterus it divides repeatedly to produce two cells, then four, then eight, etc.
• The early division of the zygote on the way to the uterus is called cleavage and provides a large number of cells needed as building blocks for the embryo
• After around 72 hours after fertilization, cleavage has generated a solid cluster of 12-16 cells called morula. During day four, the late morula now consisting of around 60 cells enters the uterus. It takes up fluid, which gathers into a central cavity. This new fluid-filled structure is called a blastocyst
• Two distinct types of cells are obvious in the blastocyst stage

  • inner cell mass on one side of the blastocyst cavity
  • outer cell mass - layer of cells surrounding the cavity called the trophoblast
• Implantation of the blastocyst occurs around day 6 - this is where it burrows into the endometrium of the uterus. Implantation takes about a week to complete
• The highest risk of birth defects occurs during the 5th week of pregnancy
• The inner cell mass will form the embryo and the trophoblast will help form the placenta
• Scenarios that produce monozygotic twins

  • blastocyst splits into two during early stages of cleavage (forms 2 separate embryos with their own amniotic sacs and placentas)
  • complete split of inner cell mass (share placenta but have their own amniotic sac)
  • split of inner cell mass late in development (shares placenta and amniotic sac - can be dangerous)
• Week two of development is the formation of the bilaminar embryo (2 layers)
• About 9 days after fertilization, the inner cell mass has divided into two sheets of cells, the epiblast and the hypoblast. They will form two fluid filled sacs. together, they make up the bilaminar embryonic disc
• The sac formed by the epiblast is the amniotic sac. The outer membrane of the amniotic sac is called the amnion and the internal amniotic sac cavity is filled with amniotic fluid
• The yolk sac is formed by an extension of the hypoblast
• Label the bi-laminar embryo

  A) epiblast

  B) amnion

  C) amniotic sac cavity

  D) bilaminar embryonic disc

  E) hypoblast

  F) yolk sac cavity

  
• Week three of gestation is known as the formation of the tri-laminar embryo/gastrulation
• Week 3 is when the three primary germ layers are formed

  • ectoderm
  • mesoderm
  • endoderm
• The germ layer begins to form around week 3 when a raised groove called the primitive streak appears on the dorsal surface of the epiblast. Epiblast cells migrate inward at this streak and will displace the cells of the underlying hypoblast
• The epiblast becomes the ectoderm, the invagination of the epiblast cells into the primitive streak become the mesoderm, and the hypoblast becomes the endoderm
• The ectoderm forms the outer layer of the skin (epidermis), the brain, and the spinal cord
• The mesoderm forms muscle, bone, and connective tissues
• The endoderm forms the innermost lining of the inner tube (epithelial lining)
• At one end of the primitive streak is a swelling called the primitive node. The epiblast cells that move through the primitive node migrate straight anteriorly. These mesodermal cells, along with a few cells from the underlying endoderm form the notochord
• The notochord defines the body axis. It extends the length of the body and is the site of the future vertebral column. It appears on day 16 and by day 18 it reaches the future head region. It will signal the formation of the CNS
• As the notochord develops, it signals the overlying ectoderm to start forming the spinal cord and brain, this is called neurulation. The ectoderm in the dorsal midline thickens into a neural plate, and thenstarts to fold inward as a neural groove. This groove deepens until a hollow neural tube is pinched odd into the body.
• The closure of the neural tube begins at the end of week 3. It starts in the neck region and moves cranially and caudally. Complete closure occurs by the end of week 4
• Label this image from during neurulation

  A) surface ectoderm

  B) neural plate

  C) notochord

  
• Label this image from during neurulation

  A) neural fold

  B) neural groove

  C) neural crest

  D) notochord

  
• Neural crest cells are pulled into the body along with the invaginating neural tube. The neural crest cells originate from ectodermal cells on the lateral ridges (neural folds) of the neural plate. They form the sensory nerve cells and some other important structures
• Neural tube defects result from failure of the neural tube to close completely. The most common defects being spina bifida and anencephaly
• During closing of the neural tube, two pores are formed cranially and caudally; the anterior neuropore and the posterior neuropore. The anterior neuropore is the head end and closes a little earlier than the posterior neuropore
• In the middle of week 3, the mesoderm lies lateral to the notochord on both sides of the body and extends cranially to caudally. By the end of this week, the mesoderm is divided into three regions, somites, intermediate mesoderm (segmented and form the segmented structures of the outer tube), and lateral plate mesoderm (unsegmented and is associated with inner tube organs)
• The ectoderm becomes the brain, spinal cord, and epidermis of the skin. The early epidermis, produces hair, nails, and sweat glands. Neural crest cells from the ectoderm give rise to the sensor nerve cells
• The endoderm becomes the inenr epithelial lining of the gut tube and its derivatives: the respiratory tubes, digestive organs, and the urinary bladder. Also the thyroid, thymus, and parathyroid glands, liver and pancreas
• The formation of the cylindrical body plan occurs in week 4 of development
• Label the embryonic layer derivatives

  A) epidermis

  B) brain and spinal cord

  C) sensory nerve cells

  D) notochord

  E) nucleus pulposus

  F) somite

  G) intermediate mesoderm

  H) lateral plate mesoderm

  
• After closure of the neural tube the embryo begins folding and the begins forming forebrain (prosencephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon). Between the midbrain and hindbrain there is a folding called the cephalic flexure and between the hindbrain and what will be the spinal cord is a folding called the cervical flexure.

  A) cephalic flexure

  B) cervical flexure

  
• Between week 5 and 6 of development the neuroepithelial cells of the neural tube begin to differentiate. They will differentiate into neuroblasts that form into 2 major regions, the alar plate and the basal plate. The neuroblasts in the alar plate become interneurons. The neuroblasts in the basal plate become motor neurons, and the neural crest cells become sensory neurons
• Spinal nerves begin to sprout around 36 days into development
• At around 49 days into development, the forebrain differentiates into two regions

  • diencephalon
  • telencephalon
• Around 3 months of development, the midbrain begins to be folded into the cerebral hemisphere and the cerebral hemisphere completely covers the diencephalon. The hindbrain also differentiates into the cerebellum and the medulla oblongata