Life Sciences Term 1

Implantation

• The blastula  forms two sac-like structures
• Amnion
• Yolk sac
• The outer most covering is called the chorion
• The yolk sac shrinks
• The amnion fills with fluid and fuses with the chorion after 2 months
• Villi develop from the chorion
• These become larger and more vascular
• Called attachment villi
• The region of the endometrium undergoes further changes and forms a placenta.

Development of the placenta

• Large spaces or sinuses  develop in the decidua or endometrium where attachment occurs
• Attachment villi fit into theses spaces
• Where maternal blood bathes the villi
• NB!! Maternal and foetal blood never mix
• One vein and two arteries form the umbilical cord
• Nutrients diffuse from maternal blood across the membranes into foetal blood.

Functions of the placenta

• Nutrition
• Nutrients including vitamins and minerals diffuse from the maternal blood to the foetus
• Gaseous exchange
• Oxygen and carbon dioxide are exchanged between maternal and foetal blood
• Excretion
• Nitrogenous wastes are carried away from the foetus by maternal blood
• Protection
• The placenta acts as a barrier or filter for certain harmful pathogens and chemicals
• Maternal antibodies cross the placenta to the foetus
• However, HIV and alcohol are able to cross this barrier
• Endocrine function
• After two months the placenta takes over the secretion of progesterone and oestrogen to maintain the endometrial lining
• Secretes relaxin to relax joints and ligaments to assist delivery of the foetus

Amnion and amniotic fluid

• The amnion is a membranous structure in which the foetus develops
• It is impenetrable and secretes fluid in which the foetus lives and moves

Functions of amniotic fluid

• Shock absorber – protects the foetus from mechanical injury
  
• Thermoregulatory – prevents extremes temperature changes
  
• Moisturising – prevents  desiccation (drying out) of the foetus
  
• Environment – provides the medium in which the foetus moves, breathes and swallows
  

Amniocentesis

• Test carried out at 12 weeks to diagnose any genetic disorders the foetus may have e.g. Down Syndrome
• Amniotic fluid consists of 99% water as well as a mix of foetal cells, micro-organisms and waste products

Pregnancy or gestation

• The period between fertilization and birth when the foetus develops in the womb / uterus