Fertilisation to Trilaminar

Fertilisation

  • Fertilisation: union of gametes - fusion of sperm and oocyte to form a zygote
  • Takes place in the ampulla of the uterine tube
 
  1. Sperm binds to zona pellucida glycoprotein
  1. Acrosomal enzymes released from sperm head; sperm digests its way into egg
  1. Egg and sperm plasma membranes fuse and sperm contents enters the egg
  1. Sperm entry triggers completion of meiosis 2 and release of cortical granules by oocyte (make ZP impenetrable to other sperm)

Cleavage

  • One cell embryo undergoes a series of cleavage divisions
  • Rapid mitotic division without increase in size; because cells are confined within the ZP the embryo will change shape (compaction)
  • Form morula (16+ cells) then blastocyst (200-300 cells)
  • The blastocyst is comprised of two different cell types:
    • Outer cell mass (trophoblast): contacts with the endometrium of the uterus to facilitate implantation and the formation of the placenta
    • Inner cell mass (embryoblast): responsible for the formation of the embryo itself
  • Cells maximize available space by coming into closer contact with each other
  • Inner cells form cell junctions, outer cells form gap junctions
  • Fluid enters through ZP producing a blurred cell mass - signals the formation of the blastocyst
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Implantation

  • Embryo implants into endometrial lining on posterior or anterior uterine wall
  • Ectopic pregnancy: implantation outside the uterus
  • Placenta previa: placenta lies low in uterus and partially/completely covers cervix

Development of the bilaminar embryo

Day 7.5

  • Trophoblast divides into cytotrophoblast (individual cells) and syncytioblast (single multinucleated cell which produces hCG)
  • Embryobast divides into epiblast (dorsal surface) and hypoblast (ventral surface)
  • Amniotic cavity begins to form

Day 9

  • Formation of 2 cavities = amniotic cavity (epiblast) and primitive yolk sac (hypoblast)
  • Implantation complete

Day 12

  • Uteroplacental circulation established
  • Extraembryonic mesoderm develops then degenerates, forming chorionic cavity

Day 13

  • Further development of chorionic cavity
  • Presence of existing stalk (later umbilical cord)
  • Second wave of hypoblast cells migrate to form definitive yolk sac

Gastrulation

  • Invagination of the epiblast cells through the primitive stream to form the 3 primary germ layers - ectoderm, mesoderm, endoderm (trilaminar embryo)
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Ectoderm

  • Epidermis of skin, hair, nails
  • Nervous system

Mesoderm

  • Paraxial mesoderm: axial skeleton, skeletal muscle
  • Intermediate mesoderm: urogenital systems
  • Lateral plate mesoderm
    • Somatic layer: dermis, lining of body wall, parts of limbs
    • Visceral layer: cardiovascular system, mesothelial covering of organs, smooth muscle

Endoderm

  • Lining of gut tube
  • Lining of respiratory tract
  • Lining of bladder and urethra