Early Development

• Fertilization


  A process by which the sperm & ovum meet and fuse→ zygote

  
• Site of fertilization


  In the ampullary part of the uterine tube, for 2 reasons: It is close to the ovary and It is the widest part of the uterine tube
• Sperm


  • Length = about 50µ, and highly motile
  • Parts→ head, neck and a motile tail
  • Head contains the nucleus that is covered by the acrosome (an organelle containing enzymes)
  • Two types: Y-bearing sperm (22 autosomes + Y) and X bearing sperm (22 autosomes +X)

  
• Secondary Oocyte (ovum)


  • Has large size & immotile
  • Covered by zona pellucida and corona radiate
  • The cytoplasm is abundant (containing yolk granules for nutrition of the embryo during the 1wk of deve.)
  • The nucleus contains 22 chromosomes + one X chromosome

  
• Movement of sperm & oocyte


  1. Sperm: By own propulsion →tail, By the movement of uterine cilia

  2. Egg: By the movement fimbria of the uterine tube, By peristaltic movement of the uterine tubes

  
• Capacitation


  1. The glycoprotein coat is removed from the surface of the sperm's acrosome (about 7 hours)

  2. Only the capacitated sperms can penetrate the corona radiata

  
• Acrosome reaction


  Proteolytic enzymes (acrosin, esterases, hyaluronidase, and neuraminidase) released by sperm cells →digest & penetrate the zona pellucida

  
• Phases of Fertilization

  Several sperm reach the site of fertilization

  • Sperm bind to the zona pellucida & triggers release of acrosomal contents → zona digestion
  • Sperm contacts the plasma membrane of the egg,
  • Immediate change in the zona pellucida (release of cortical granules- lysosomal enzymes (depolarizes- zona reaction) – blocks polyspermy
  • Cell membrane adhere and fuse
  • Both head and tail enter the cytoplasm of the oocyte
  • The sperm plasma membrane → left on oocyte surf
  • Fusion of the sperm's pronucleus with the pronucleus of the egg = zygote

  
• Fertilization


  Once the sperm cell enters the cytoplasm of the oocyte, the oocyte will complete its second meiotic division, The sperm nucleus becomes swollen and the tail →degenerate as a male pronucleus is formed, Both pronuclei will come in close contact, lose nuclear envelopes and fuse

  
• Results of fertilization

  
• Week 1: days 1-6


  Fertilization, day 1, Cleavage, day 2-3, Compaction, day 3, Formation of blastocyst, day 4, Ends with beginning of implantation, day 6
• Cleavage


  The repeated mitotic divisions of the zygote, resulting into a rapid increase in the number of cells that are called blastomeres
• Site of cleavage


  The uterine tube medial to the ampula
• Cleavage


  The zygote 1st divide →two-celled zygote, Further cleavage divisions →the two-celled zygote forms 4 cells, The 4 cells will form 8 cells, 8 will form 16…..until we reach a 32 cell stage known as a morula, The resulting cells are called blastomeres – the cell → smaller with each cleavage division due to limiting zona pellucida
• Morula


  • A 16 -32 cells, has mulberry appearance, surrounded by Zona pellucida, enters the uterus nearly 3 days after fertilization
• Compaction


  The centrally placed cells of the morula → the inner cell mass, The cells on the periphery of the morula →outer cell mass or trophoblast → form extra-embryonic membranes and placenta, The inner cell mass →the embryo proper
• Blastocyst formation


  Fluid from the uterine cavity penetrate the zonz pellucida, accumulates in the inner-cullular spaces of the inner cell mass →creates pressure among the blastomeres/inner cell mass, inner cell mass →pushed to one pole →called embryonic pole, The opposite pole is the abembryonic pole, A single cavity is formed →the blastocele or blastocytic cavity, The morula after the formation of blastocele →called blastocyst, The blastocyst consists of: the inner cell mass (called embryoblasts) and the outer cell mass (future trophoblast → extra-embryonic membranes – contribute to placenta)
• The break of the zona pellucida occurs under the influence of two forces: mechanical pressure of the growing blastocyst on the zone and chemical dissolution of the zone material with secreted lytic enzymes secreted by cells of the trophectoderm
• Totipotency


  The ability of a single cell to divide and produce all of the differentiated cells in an organism
• Pluripotent cells


  Can give rise to all of the cell types that make up the body
• Hemotrophic nutrition


  Exchange of blood-borne materials between maternal and fetal circulations, facilitated by the extensive and intimate apposition of the maternal and fetal tissues - the placenta
• Histotrophy


  Nutrition in which the developing embryo receives nutrition from its mother in the form of uterine secretions, known as histotroph (or "uterine milk")
• Monozygotic twinning typically occurs during cleavage/blastocyst stages
• "Hatching" of the blastocyst: preparation for implantation


  Hatching of the embryo from the zona pellucida occurs just prior to implantation, Occasionally, the inability to hatch results in infertility, and premature hatching can result in abnormal implantation in the uterine tube

  
• Ectopic Implantation


  Implantation somewhere other than upper portion of uterus, "Rupture" can lead to life-threatening hemorrhage
• Week 2: days 7-14 implantation

  Implanted embryo becomes more deeply embedded in endometrium, Further development of trophoblast into placenta, Development of a bi-laminar embryo, amniotic cavity, and yolk sac
• Implantation


  Penetration of the blastocyst into the superficial (compact) layer of the endometrium, Occurs at the 6th day after fertilization and is completed – about 11th day, Normal site is the endometrium of the posterior wall of the uterus, The endometrium after implantation is called decidua
• Implantation


  About 6 days after fertilization: attaches to endometrium, Orients inner cell mass toward endometrium, 7 days after fertilization: attaches more firmly and burrows in, Endometrium becomes more vascularized and glands enlarge, Decidua – modified portion of endometrium after implantation, Regions named relative to site of implantation
• Trophoblast


  Cells that form the outer layer of the blastocyst and develop into the placenta
• Syncytiotrophoblast


  Invasive fused cells (syncytium) derived from cytotrophoblast
• Cytotrophoblast


  Stem cell population of the trophoblast
• Embryoblast


  Cells that develop into the embryo
• Implantation


  1. Implanted embryo becomes more deeply embedded in endometrium

  2. Further development of trophoblast

  3. Development of a bi-laminar embryo, amniotic cavity, and yolk sac
• Early placentation


  • Development of trophoblast
  • Development of bilaminar embryo
  • Development of amnion
  • Development of yolk sac
  • Development of sinusoids
• Maternal changes


  • Endometrial capillaries dilate to form maternal sinusoids
  • Maternal blood enters the lacunar system
• Week 2 of development


  Days 7-14
• Development of trophoblast


  1. Blastocyst becomes buried in endometrium and inner 1/3 of myometrium

  2. Trophoblast develops into 2 layers

  3. Cytotrophoblast: stem cell population

  4. Syncytiotrophoblast: invasive fused cells
• Development of bilaminar embryo

  1. Cells of embryoblast differentiate into epiblast (primitive ectoderm) and hypoblast (primitive endoderm)

  2. Bilaminar embryo formed
• Development of amnion


  1. Fluid filled cavities develop at embryonic pole

  2. Amniotic cavity lined by amniotic ectoderm

  3. Amnion eventually surrounds entire embryo

  4. Amniotic cavity filled with amniotic fluid
• Development of yolk sac


  1. Cells at edge of hypoblast migrate to cover inner surface of blastocyst wall

  2. Form exocoelomic (Heuser's) membrane

  3. Relatively small and empty, nutrition still derived from endometrium

  4. Supplies early nutrients, source of blood cells, contains primordial germ cells, forms part of gut, functions as shock absorber, prevents desiccation