Day 13, Lecture 2: Human Development 2: Week 2 in human Development

Question 1

Question 2

Events during second week of development

Answer 2

• Implantation is completed
  
  • Conceptus comes to lie totally within endometrium
• Formation of primitive uteroplacental circulation
• Formation of bilaminar embryo
• Formation of Extraembryonic membranes and cavities

Question 3

Formation of Primitive uteroplacental Circulation

Answer 3

• Second week of development
• Provides means for developing embryo to receive nutrients and get rid of waste products
  
  • Spaces appear in the syncytiotrophoblast to form the lacunar network
  • Endometrial stroma undergoes the decidua reaction
    
    • Stroma becomes hydrated and cells fill with glycogen and lipids
• Maternal blood leaves maternal vascular system and is surrounded by conceptus tissue
• Forms primitive placenta
• No immunological rejection of conceptus
  
  • Conceptus has different genotype than mother
  • Allogenic rejection of conceptus is prevented
  • Conceptus “hides” from immune system
    
    • Reduction in expression of major histocompatibility complex (MHC) class I antigen
  • Conceptus produces locally immunosuppressor molecules

Question 4

Formation of Bilaminar embryonic Disk

Answer 4

• Forms early in second week
• Reorganization of inner cell mass
  
  • Epiblast
    
    • Embryo will form forom epiblast cells
  • Hypoblast
    
    • anterior visceral endoderm
      
      • Thickening of hypoblast at one end the bilamilar disk and this defines the cranial end of disk

Question 5

What defines the cranial end of the bilaminar embryonic disk

Answer 5

• Anterior visceral Endoderm
  
  • Thickening of hypoblast

Question 6

Formation of Extraembryonic membranes and cavities

Answer 6

• Membranes and cavities come to surround developing embryo to provide nutrition and protection
• They form primarily from extraembryonic tissue by a complicated process of migration and cavitation
• By the end of the second week all of the extraembryonic membranes and cavities will be formed
• With further development membranes fuse and cavities are lost but general organization remains
• Amniotic cavity and amniotic membrane
  
  • Develope as inner mass cells reorganize to form the bilaminar disk
• Primative yolk sac
  
  • lined by exocoelomic membrane
    
    • Exocoelomic membrane is made my the migration of hypoblast cells
• Extraembryonic mesoderm
  
  • Majority forms from hypoblast (exocoelomic membrane)
  • some from epiblast during gastrulation
• Extraembryonic Coelom
  
  • forms by splitting of the extraembronic mesoderm
  • Splanchnic Extraembryonic Mesoderm
    
    • Extraembryonic mesoderm associated with yolk sac
  • Somatic extra embryonic mesoderm
    
    • Exra embryonic mesoderm associated with other structures; amniotic membrane, connecting stalk, chorion

Question 7

Chorion

Answer 7

• Somatic Extraembryonic Mesoderm
• Cytotrophoblast
• Syncytiotrophoblast
• Forms fetal portion of placenta

Question 8

Allantois

Answer 8

• Endodermally lined projection from yolk sac into the connecting stalk

Question 9

Molar Pregnancy

Answer 9

• Abnormal embryonic development with excessive development of trophoblast – extraembryonic structures
• Complete hydatidiform molar pregnancy
  
  • Excessive development of trophoblast
  • Little or no embryonic tissue
  • genome is entirely paternal
    
    • fertilization of oocyte, which is lacking its nucleus, either by one sperm that duplicates its DNA to form a diploid or by two sperm (with separate DNA)
  • May produce benign or malignant tumor
• Partial or incomplete hydatidiform molar pregnancy
  
  • Excessive development of trophoblast
  • Some development of iner cell mass/embryonic tissue
  • Genetic analysis revealed triploidy
    
    • one maternal and two paternal sets of chromosomes

Question 10

Genomic Imprinting

Answer 10

• Paternal genome drives Trophoblast
• Maternal genome drives inner cell mass development
• Spermatogenesis and oogenesis have different patterns of methylation of DNA
• Molar pregnancy believe to be the result of genomic imprinting
• Suggests non-equivalence of maternal and paternal genomes with respect to regulating extraembryonic and embryonic development

Question 11

_____ genome drives trophoblast development

Answer 11

Paternal

Question 12

_____ genome drives inner cell mass development

Answer 12

Maternal

Question 13

Incomplete or Partial Hydratidiform Molar Pregnancy

Answer 13

• Excessive development of trophoblast
• Some development of inner cell mas
  
  • Embryonic tissue
• Genetic analysis revealed triploidy
  
  • one maternal and two paternal sets of chromosomes

Question 14

Complete Hydatidiform Molar Pregnancy

Answer 14

• Excessive Development of trophoblast
• Little or no embryonic tissue
• Genome is entirely paternal
• May produce benign or malignant tumor