blastulation to gastru

• Blastulation


  A fundamental stage of embryonic development characterized by the formation of a hollow ball of cells called the blastula or blastocyst

  
• Blastulation


  1. Fertilization

  2. Morula formation

  3. Blastula/blastocyst formation

  
• Morula


  A solid ball of about 16 undifferentiated, spherical cells
• Blastula/Blastocyst


  A hollow ball of cells with an inner cell mass and an outer trophoblast layer

  
• Blastula/Blastocyst


  • Inner cell mass (embryoblast) forms the embryo
  • Trophoblast forms the placenta

  
• Blastocyst formation


  1. Compaction of morula cells

  2. Differentiation into inner cell mass and trophoblast

  3. Formation of blastocoel cavity

  
• Trophoblast


  Outer layer of blastocyst that forms the placenta
• Inner cell mass (embryoblast)


  Source of embryonic stem cells, forms the embryo
• Syncytiotrophoblast


  Outer layer of trophoblast that forms a syncytium
• Cytotrophoblast


  Inner layer of trophoblast with well-defined cells
• Implantation


  1. Blastocyst embeds in uterine endometrium

  2. Trophoblast secretes enzymes to erode endometrium

  3. Villi form for nutrient absorption and attachment

  
• Decidua


  Uterine lining that forms during implantation, divided into 3 layers
• Human chorionic gonadotropin (hCG) is secreted by trophoblast, used for pregnancy tests
• Progesterone secreted by corpus luteum maintains pregnancy
• Blastulation and cell differentiation

  1. Cells in inner cell mass become pluripotent

  2. Epiblast and hypoblast layers form

  3. Amniotic cavity and yolk sac form

  
• Epiblast


  Layer facing amniotic cavity, forms embryo
• Hypoblast


  Layer facing blastocoel, does not contribute to embryo

  
• Bilaminar disc


  Where epiblast and hypoblast meet, splits blastocyst into cavities

  
• Embryonic disk


  Floor of amniotic cavity, composed of embryonic ectoderm
• Gastrulation


  Stage when gastrula forms from blastula by inward migration of cells
• Eutherian


  Mammals that give birth to live young nourished by placenta
• Blastocyst


  Possesses an inner cell mass (ICM) or embryoblast, which forms the embryo, and an outer layer of cells or trophoblast, which forms the placenta. The trophoblast surrounds the ICM and a fluid-filled blastocyst cavity called the blastocoele.

  
• Embryoblast

  The source of embryonic stem cells and gives rise to all later structures of the adult organism

  
• Trophoblast


  Combines with the maternal endometrium to form the placenta in eutherian mammals
• Before gastrulation


  The cells of the trophoblast become differentiated into two strata: the outer syncytiotrophoblast and the inner cytotrophoblast or layer of Langhans
• Yolk sac


  A small sac formed by the flattened endoderm cells on the deep surface of the inner cell mass
• Amniotic cavity


  A cavity gradually developed by the enlargement and coalescence of spaces between the remaining cells of the inner cell mass, with its floor formed by the embryonic disk
• Ectoderm


  Gives rise to neural tissue and the epidermis
• Mesoderm


  Develops into somites that differentiate into skeletal and muscle tissues, the notochord, blood vessels, dermis, and connective tissues
• Endoderm


  Gives rise to the epithelium of the digestive and respiratory systems and the organs associated with the digestive system, such as the liver and pancreas
• Somite


  Paired masses of mesoderm, distributed along the sides of the neural tube, that will eventually become dermis, skeletal muscle, or vertebrae
• Gastrulation


  The stage of embryonic development at which a gastrula is formed from the blastula by the inward migration of cells

  
• Notochord


  A structure found in the embryos of vertebrates from which the spine develops
• Epiboly


  One of many movements in the early embryo that allow for dramatic physical restructuring and is characterized by a thinning and spreading of cell layers

  
• Gastrulation in amniotes (humans)

  1. The embryo becomes asymmetric

  2. The primitive streak forms

  3. Cells from the epiblast at the primitive streak undergo an epithelial to mesenchymal transition and ingress at the primitive streak to form the germ layers
• Following gastrulation, the cells in the body are either organized into sheets of connected cells (as in epithelia), or as a mesh of isolated cells, such as mesenchyme
• Five basic types of cell movements during gastrulation

  • Invagination
  • Involution
  • Ingression
  • Delamination
  • Epiboly
• Initiation of Gastrulation


  Signaling cues from specific regions of the embryo trigger a cascade of events leading to the onset of gastrulation
• Cell Movements during Gastrulation

  Invagination, involution, ingression, delamination, and epiboly
• Epithelial-to-mesenchymal transition (EMT)


  Allows epithelial cells to transition to a mesenchymal state, enabling them to migrate more freely through tissues