Human Growth and Development 1

Question 1

From egg to embryo 
Pregnancy 
Conceptus 
Gestation Period 
Preembryo 
Embryo 
Fetus

Answer 1

Pregnancy
-events that occur from fertilisation until the infant is born
Conceptus
-the developing offspring
Gestation Period
-from the last menstrual period until birth

Preembryo
-conceptus from fertilisation until it is two weeks old
Embryo
-conceptus during the third through the eighth week
Fetus
-conceptus from the ninth week through birth

Question 2

Fertilisation

Answer 2

-oocyte viable for 12-24 hrs
-sperm viable for 24-72 hrs
For fertilisation to occur, coitus must occur no more than:
-3 days before ovulation
-24 hours after ovulation
Fertilization - when a sperm fuses with an egg to form a zygote

Question 3

Sperm penetration

Answer 3

An ovulated oocyte is encapsulated by:
-the corona radiate and zone pellucid (layers of cells, don’t need to know names)
-extracellular matrix
Sperm binds to the zone pellucida
-enzymes are released near the oocyte to digest the zone pellucida

Question 4

Only 1 sperm allowed in

Answer 4

Only one sperm allowed to penetrate the oocyte
Two mechanisms ensure monospermy
1. membrane depolarizes; preventing sperm fusing (changes its electrical charge, changes sodium ion levels causing the other sperm to repel)
2. inhibiting proteins
-destroy sperm receptors
-cause sperm already bound to receptors to detach

Question 5

Completion of Meiosis ll and Fertilization

Answer 5

Upon entry of sperm, the secondary oocyte:

• completes meiosis ll
• casts out the second polar body
• the ovum nucleus swells, and the two nuclei approach each other
• when fully swollen, the twi nuclei are called pronuclei
• fertilization - when the pronuclei come together

Question 6

The pre-embryo

Answer 6

• the first cleavage produces two daughter cells called blastomeres
• morula - the 16 or more cell stage (72 hours old)
• by the fourth or fifth day the preembryo consists of 100 or so cells (blastocyst) (stage where it implants into the uterus)

Blastocyst - a fluid-filled hollow sphere composed of:
-a single layer of trophoblasts
-an inner cell mass
Trophoblasts take part in placenta formation
The inner cell mass becomes the embryonic disc

Question 7

Implantation

Answer 7

• 6-7 days after ovulation
• trophoblasts adhere to the endometrium
• trophoblasts form two distinct layers
  
  • crytotrophoblast
  • syncytiophoblast - cells in the outer layer that invade the endometrium (extends tendrils into the roots of the endometrium, goes to form the beginning of the placenta)
• trophoblasts secrete human chorionic gonadotropin (hCG)
• hCG causes corpus luteum to continue to secrete progesterone and estrogen (levels continue to increase)
• by 12 weeks, the placenta:
  
  • assumes the role of progesterone and oestrogen production
  • is providing nutrients and removing wastes
  • corpus luteum degenerates

Question 8

Placentation

Answer 8

Formation of the placenta from:

• embryonic trophoblastic tissues
• maternal endometrial tissues
• the chorion develops fingerlike villi, which:
  
  • become vascularised
  • extend to the embryo as umbilical arteries and veins
  • lie immersed in maternal blood
• the placenta is fully formed and functional by the end of the first month
• the placenta also secretes other hormones:
  
  • human placental lactogen
  • human chorionic thyrotropin
  • relaxin

Question 9

Embryonic Membranes 1

Answer 9

Amnion

• provides a buoyant environment
• homeostatic temperature
• amniotic fluid comes from maternal blood, and later, feral urine

Question 10

Embryonic membranes 2

Answer 10

Yolk sac - a sac on the ventral surface of the embryo

• forms part of the digestive tube
• produces earliest blood cells and vessels

Question 11

Embryonic membranes 3

Answer 11

Allantois - a small out pocketing of the yolk sac

• structural base for the umbilical cord
• becomes part of the urinary bladder

Chorion - helps form the placenta
-encloses the embryonic body and all other membranes

Question 12

Gastrulation

Answer 12

During the 3rd week:

• two-layered embryonic disc becomes a three-layered embryo
• the primary germ layers are ectoderm (outer skin) , mesoderm (middle) and endoderm (inner)
• primitive streak - raised dorsal groove

Question 13

Organogenesis

Answer 13

• gastrulation sets the stage for organogenesis, the formation of body organs
• by the 8th week, all organ systems are recognizable (not complete and functional)