Cleavage

• Events of the 2-week pre-embryonic period


  1. Fertilization

  2. Transportation of the zygote through the uterine tube

  3. Mitotic divisions

  4. Implantation

  5. Formation of primordial embryonic tissue
• Zygote


  Undergoes a series of mitotic divisions once it reaches the two-cell stage with increasing numbers of cells, these cells become smaller with each cleavage division and are known as blastomeres
• Cleavage


  Repeated mitotic divisions of the zygote, resulting in a rapid increase in the number of cells division of the zygote into blastomeres beginning approximately 30 hours after fertilization
• Embryonic cells-blastomeres- cleave normally as the zygote passes along the uterine tube toward the uterus
• Morula formation


  About 3 days after fertilization, cells divide again to form a 16-cell morula (L., morus, mulberry) blastomeres that enters the uterus
• Inner cell mass


  Gives rise to tissues of the embryo proper
• Outer cell mass


  Forms the trophoblast, which later contributes to the placenta
• Compaction

  After the third cleavage, blastomeres maximize their contact with each other, forming a compact ball of cells held together by tight junctions, which segregates inner cells from outer cells
• Compaction is probably mediated by cell surface adhesion glycoproteins
• Blastocyst formation


  Just after the morula enters the uterus (approximately 4 days post-fertilization) blastocystic cavity appears inside the morula, fluid accumulation in the blastocystic cavity separates the blastomeres into two parts: A thin, outer cell layer, the trophoblast, and the inner cell mass called the embryoblast
• Trophoblast


  Gives rise to the embryonic part of the placenta
• Blastocyst


  The conceptus during blastogenesis, the trophoblast forms the wall of the blastocyst
• Hatching and implantation


  Shedding and hatching of zona pellucida permit the hatched blastocyst to increase rapidly in size, approximately 6 days after fertilization, the blastocyst attaches to the endometrial epithelium, the trophoblast starts to proliferate rapidly and gradually differentiates into two layers: An inner layer of cytotrophoblast and an outer layer of syncytiotrophoblast
• Both intrinsic and extracellular matrix factors modulate, in carefully timed sequences, the differentiation of the trophoblast
• Trophoblast invasion


  At approximately 6 days post fertilization, the fingerlike processes of syncytiotrophoblast extend through the endometrial epithelium and invade the connective tissue, by the end of the first week, the blastocyst is superficially implanted in the compact layer of the endometrium and is deriving its nourishment from the eroded maternal tissues
• The highly invasive syncytiotrophoblast expands quickly adjacent to the embryoblast, the area known as the embryonic pole, the syncytiotrophoblast produces enzymes that erode the maternal tissues, enabling the blastocyst to burrow into the endometrium
• Hypoblast formation


  At approximately 7 days, a layer of cells, the hypoblast (primary endoderm), appears on the surface of the embryoblast facing the blastocystic cavity
• Attachment and invasion of the trophoblast involve integrins, expressed by the trophoblast, and the extracellular matrix molecules laminin and fibronectin, integrin receptors for laminin promote attachment, while those for fibronectin stimulate migration
• Extra-embryonic membranes


  • Amnion
  • Yolk sac
  • Allantois
  • Chorion
• Amnion


  A thin membrane, derived from ectoderm and mesoderm, that loosely envelops the embryo, forming an amniotic sac filled with amniotic fluid
• Amniotic fluid


  • Normally swallowed by the fetus and absorbed in the GI tract, the fluid enters the fetal blood, and the waste products it contains enter the maternal blood in the placenta, prior to delivery, the amnion is naturally or surgically ruptured, and the amniotic fluid is released, for the near-term baby, almost 8 liters of fluid are completely replaced each day
• Yolk sac


  Established during the end of the second week as cells from the trophoblast form a thin exocoelomic membrane
• Functions of amnion


  • Ensures symmetrical structural development and growth
  • Cushions and protects by absorbing jolts that the mother may receive
  • Maintains consistent pressure and temperature
  • Eliminates metabolic wastes
  • Permits freedom of fetal movement, important for musculoskeletal development and blood flow
• Allantois


  Forms during the third week as a small outpouching or diverticulum, near the base of the yolk sac, it remains small but is involved in the formation of blood cells and gives rise to the fetal umbilical arteries and vein, it also contributes to the development of the urinary bladder
• Chorion


  The outermost extraembryonic membrane, it contributes to the formation of the placenta as small fingerlike extensions, called chorionic villi penetrate deeply into the uterine tissue, the portion of the chorion associated with the uterine wall is known as the villous chorion or chorion frondosum
• These molecules also interact along signal transduction pathways to regulate trophoblast differentiation so that implantation is the result of mutual trophoblastic and endometrial action, hence, by the end of the first week of development, the human zygote has passed through the morula and blastocyst stages and has begun implantation in the uterine mucosa