5.4 - Embryogenesis

Question 1

Define Teratology.

Answer 1

• the study of causes and biological processes leading to abnormal development
• become prominent in 1960s due to Thalidomide

Question 2

Define critical periods

Answer 2

• time of increased sensitivity in development
• during weeks 1-2, the embryo is not susceptible to teratogens, which is beneficial because this is usually when the mother is unaware that she is pregnant
• weeks 3-8 (Embryonic period) - generally a period of high sensitivity
• weeks 8-38 (Fetal period) - organs are undergoing maturatio, so they’re not quite as vulnerable to major congenital anomalies
  
  • ​exception: CNS - longer period of high sensitivity

Question 3

Define congenital birth defects

Answer 3

• leading cause of infant mortality in US
• causes:
  
  • genetics
  • “environment” (intrauterine environment, infection, drugs, diet)
  • unknown (65% of defect have no known or identifiable cause)
  • multifactorial
• risks: periods o higher/lower susceptibility

Question 4

Define embryonic stem cells

Answer 4

• derived from inner cell mass of embryo
• are pluripotent: can form virtually any cell or tissue type

Question 5

Describe Fertilization

Answer 5

• when male gamete (sperm) uniting with female gamete (oocyte)
• resulting in fertilized egg (zygote)
• occurs typically in ampullary region of uterine tube

Question 6

List the 3 results of fertilization

Answer 6

1 - restoration of diploid number of chromosomes

• determined by sperm

#3 - initiation of cleavage

• necessary for zygote to persist
• without fertilization. the oocyte degenerates 24 hours after ovulation

Question 7

What is the morula?

Answer 7

• 16-cell stage of zygote
• forms after several rounds of mitosis, which follows fertilization

Question 8

What is the blastocyst and when does it form?

Answer 8

• around day 5
• a hollow ball of cells formed from morula

Question 9

What is the inner cell mass?

Answer 9

• cluster of cells inside blastocyst
• cells that will eventually form body of embryo
• not yet specialized; phenoenal ability to differentiate

Question 10

What is (normal and abnormal) implantation and when does it occur?

Answer 10

• day 6
• when blastocyst implants into endometrial layer in lateral or posterior uterine wall (normally)
• (abnormally) - implants outside of uterus
  
  • “ectopic” or “extra-uterine” pregnancy
  • 95% implant in ampullary region of uterine tube
  • 5% implant in abdominal cavity, specifically attach to peritoneum in rectouterine cavity

Question 11

What is a teratogen?

Answer 11

Any substance that causes birth defects.

Question 12

What is an example of a teratogen that caused phocomelia?

Answer 12

Thylidamide that was used to treat morning sickness.

Question 13

What are the two periods and timeframes of development?

Answer 13

Fertilization and gestational

Question 14

How many weeks is a full term child considered in the context of the Fertilization timeline?

Answer 14

38 weeks, begins at conception, Embryology is taught using days “postfertilization”. Thus pregnancy is concidered 38 weeks.

Question 15

How many weeks is a full term child considered in the timeframe Gestational?

Answer 15

40 weeks, begains two weeks earlier at the begining of last menstrual period.

Question 16

Define Trophoblast

Answer 16

Differentiates into 2 layers; the cytotrophoblast and the syncytiotrophoblast during the second week.

Question 17

What two layers does the embryoblast form?

Answer 17

Epiblast and hypoblast

Question 18

What 2 layers does the extraembryonic mesoderm split into?

Answer 18

The somatic and splanchnic layers

Question 19

Which layer produces hCG?

Answer 19

Produced by the syncytiotrophoblast.

Question 20

What two cavities are large during the first 2 weeks but completely disapper by the 3rd month?

Answer 20

Chorionic cavity and the yolk sac cavity.

Question 21

When and what is Gastrulation?

Answer 21

Occurs during the thrid week.

Gastrulation converts the bilaminar embryo into trilaminar embryo.

Question 22

What is Holoprosencephaly?

Answer 22

• Injury to the anterior midline of germ disc.
• Cyclops (one eye)
• High doses of alcohol or genetics.

Question 23

What is Caudal dysgenesis?

Answer 23

Injury to the caudal aspect of the germ disc.

Mermaid/fused legs

-Genetics and environmental insults largest causes.

Question 24

What is Sacrococcygeal tumors?

Answer 24

Remnant of the primitive streak.

Question 25

How do conjoined twins normally occur?

Answer 25

Splitting of the primitive node

Question 26

What three germ layers form the adult tissues?

Answer 26

Ectoderm, Mesoderm, and Endoderm

Question 27

What does the Ectoderm form?

Answer 27

Neural system, skin, appendages that relate to external environment.

Question 28

What does the mesoderm later form?

Answer 28

Musculoskeletal tissue, genitourinary system, body wall and membranes that line cavities.

Question 29

What does endoderm later form?

Answer 29

Foregut, midgut and hindgut, GI tract, and appendages (liver, pancreas), respiratory tract, bladder.

Question 30

Dizygotic (fraternal twins)

Answer 30

2/3 of twins

Result from two oocytes fertilized by differnet spermatozoa

Totally differnet genetic constitutions

May be same or opposite sex

Question 31

Monozygotic twins (identical)

Answer 31

Result from splitting of the zygote at various stages of development.

Can happen prior to implantation

Often the inner cell mass (blastocyst stage) splits

Will be same sex and have same genetic constitution.

Question 32

Conjoined twins

Answer 32

• At later stages of development, partial splitting of the primitive node and streak may result in formation of conjoined twins.
• The type of twins formed depends on when and to what extendt abnormalities of the node and streak occur.

Question 33

Define the placenta

Answer 33

The placenta is an organ that connects the developing being to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mothers blodd supply.

Question 34

Do the blood types of the mother and child mix?

Answer 34

The blood supplies of the mother and embryo do not mix.

Blood types may not be compatible

Mothers blood pressure might damage embryo

Question 35

Define the Umbilical Cord

Answer 35

Connects the embryo with the placenta

Takes blood from the embryo to the placenta and back again. Cut at birth to separate the mother and baby.

Question 36

Define embryology

Answer 36

The branch of biology and medicine concerned with the study of embryos and their development.

Question 37

Define Embryogenesis

Answer 37

The process by which the embryo is formed and develops, until it develops into a fetus.