Embryology (Lec 13/14)

Question 1

Phases of Prenatal Development

Answer 1

1. Period of the Zygote: conception through to implantation
2. Period of the Embryo: 3rd to 8th week (organ formation, heart beat)
3. Period of the Fetus: 9th week to birth

Question 2

Period of the Zygote

Answer 2

Day 2: Cell stage
Day 3: Morula
Day 4-5: Early blastocyst enters uterus
Day 6-7: Blastocyst implantation (Cytotrophoblast and Syncytiotrophoblast formation)
Day 12: Implantation complete
Week 2: Bilaminar Embryo

Question 3

Week 1

Answer 3

• Implantation begins day 6-7, attached by Adhesion to Endometrial wall
• Nourished by uterine secretions

Question 4

Week 2

Answer 4

Blastocyst consists of:

1. Cytotrophoblast: Inner layer of cells
2. Syncytiotrophoblast: Outer layer anchored into endometrium by long protrusions

Question 5

Day 12

Answer 5

• Embryo completely embedded in endometrium
• Formation of embryonic discs occurs:
• > Epiblast and Hypoblast
• > Amnion and Yolk visible

Question 6

Week 2: Embryonic Development

Answer 6

Inner cell mass divides into 2 layers (bilaminar):

1. Epiblast
2. Hypoblast

Question 7

Amnion

Answer 7

Epiblast cells form transparent sac filled with amniotic fluid that envelopes embryo. Used for:

• cushioning
• temp regulation
• weightless environment for movement

Question 8

Yolk Sac

Answer 8

Sac that hangs from ventral surface of embryo

• forms part of digestive tube
• source of earliest blood vessels/cells

Question 9

Allantois

Answer 9

Small vessel at caudal end of yolk sac

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

Question 10

Chorion

Answer 10

Helps form placenta

• encloses embryonic body and all other membranes
• Syncytiotrophoblast and Cytotrophoblast

Question 11

Period of the Embryo

Answer 11

Week 3: formation of trilaminar embryo
Week 4: embryonic folding, heart beat, budding of limbs
Week 4-5: Placentation
Week 5: Alimentary becomes continuous tube
Week 7: Feet and hands emerge, rudimentary skeleton, eyes, nose and mouth shape
Week 7-8: Sexual development

Question 12

Formation of Chorionic Villi

Answer 12

1. Anchoring Villi: penetrate into decidua basalis and anchor chorion vesicle of wall of uterus
2. Absorbing Villi: branch in intervillus spaces (primary, secondary, tertiary)

Question 13

Primary Villi

Answer 13

Syncytiotrophoblast and Cytotrophoblast

Question 14

Secondary Villi

Answer 14

Primary Villi, Extraembryonic mesoderm

Question 15

Tertiary Villi

Answer 15

Secondary Villi, Afferent/Efferent capillaries

Question 16

Week 3: Changes in Embryonic Disc

Answer 16

Migration of totipotent, superficial epiblast cells to interior creates 3rd layer (Gastrulation) = Ectoderm, Mesoderm, Endoderm

Question 17

Gastrulation (during 3rd week)

Answer 17

Conversion of bilaminar embryo to trilaminar embryo.

• Epiblast cells push through Primitive Streak and form Mesoderm
• Epiblast forms Ectoderm
• Hypoblast forms Endoderm

Question 18

Neurulation (during 3rd week)

Answer 18

Some cells migrate cranially from the primitive cord to form a rod-like structure (Notochord):
- Notochord plays major inductive role in the subsequent development of the nervous and musculoskeletal systems

Question 19

Neural Tube Development

Answer 19

• Notochord induces ectodermal cells above it to thicken and form Neural Plate
• Neural folds move together and fuse -> Neural Tube

Question 20

Somite Development

Answer 20

• Embryonic mesoderm adjacent to the notochord and neural tube = paraxial mesoderm
• End of week 3: Paraxial mesoderm undergoes craniocaudal condensation to form segmental blocks of mesoderm = somites
• Somites will form axial skeleton and its musculature

Question 21

The Germ Layers: Ectoderm

Answer 21

Outer layer (forms from epiblast cells)

• nervous system
• skin, epidermis
• hair

Question 22

The Germ Layers: Mesoderm

Answer 22

Middle layer

• muscles
• bones
• circulatory system

Question 23

The Germ Layers: Endoderm

Answer 23

Inner layer (formed from hypoblast cells)

• GIT and associated glands (pancreas, gall bladder)
• lungs
• urinary tract

Question 24

Week 4: Embryonic Folding

Answer 24

Folding of cranial and caudal ends and lateral folding of the embyro occur simultaneously

• head fold brings primitive heart and oral membrane to ventral surface
• lateral fold forms the abdominal walls

Question 25

Placentation (4 1/2 weeks)

Answer 25

Placenta: organ that forms from Chorion and endometrium to allow embryo/fetus to exchange nutrients and waste

• Chorionic villi provide surface area for exchange
• Umbilical cord contains 2 arteries and 1 vein

Question 26

Week 5 Development

Answer 26

Alimentary canal is continuous from mouth to anus

Question 27

Period of the Fetus

Answer 27

End of 8th week: Fetal development
Weeks 9-12: External genitalia form
Week 12: all organ systems laid out
3rd Month: digestive/excretory systems functioning, repro systems contain immature ova or sperm
4th-6th Months: movements (sucking, swallowing, breathing), visual and auditory senses functional
7th-9th Months: 3rd trimester mostly for size increase and maturity

Question 28

Lung Development

Answer 28

Lungs and surfactant develop last in the fetus as they are not required while fetus is in the womb.