The Embryo: Early Development

Question 1

In males, what cells stay dormant until puberty?

Answer 1

Primordial germ cells (PGCs)

Question 2

What changes occur in spermiogenesis?

Answer 2

Formation of acrosome
Condensation of nucleus
Formation of neck, middle piece and tail
Shedding of cytoplasm

Question 3

Events that take place in 4th week and 5th month of oogenesis?

Answer 3

4th week - PGCs arrive and differentiate into oogonia

5th month - all oocytes are formed (meiosis I) and arrested in prophase of meiosis I until puberty

Question 4

What occurs after puberty in oogenesis?

Answer 4

Each monthly cycle, primary oocyte completes meiosis I to form secondary oocyte and polar body

Secondary oocyte enters meiosis II, 3 hours after ovulation but arrests at metaphase of meiosis II

Only if fertilized will it complete meiosis II to form a mature oocyte and a second polar body

If no fertilization, cell degenerates after 24 hours

Question 5

What and where (oocyte penetration)

Answer 5

Where: Ampulla of uterine tube

Determination of sex

Initiation of cleavage

Question 6

When do cleavage form and what is important about morula?

Answer 6

Cleavage: 30 hours after fertilization

Morula: divided into inner cell mass (tissues of embryo) and outer cell mass (trophoblase [placenta formation])
Develops 72 hours from fertilization

Question 7

What happens after morula enters uterus?

Answer 7

Fluid accumulates and form blastocyst

Question 8

How are inner/outer cell mass and blastocyst related?

Answer 8

1. outer cell mass is trophoblast - forms placenta
2. Innter cell mass is embryoblast - forms embryo
3. At the end of week 1, trophoblast differentiate into cytotrophoblast and syntiotrophoblast
4. Implantation begins

Question 9

What is the progestational/secretory phase?

Answer 9

phase in menstrual cycle where implantation of blastocyst occurs

Question 10

What are the important factors in implantation?

Answer 10

Layer of endometrium: functional layer (compact and spongy)

Normal site of implantation: anterior or posterior wall of uterus

Zone pellucida must degenerate for implantation to occur

Embryonic pole implants first

Question 11

What are the clinical signs for abnormal implantation/ectopic pregnancy?

Answer 11

Abnormal uterine bleeding

Sudden abdominal pain with missed period

Question 12

Partial Mole vs Complete Mole

Answer 12

Partial mole:
one oocyte fertilized by 2 sperm cells

Complete mole:
enucleated ovum fertilized by normal sperm cell

Moles produce high levels of hCG

Question 13

Which germ layers are partially formed on 8th day of development?

Answer 13

Partial Endoderm (hypoblast)
Partial Ectoderm (epiblast)

Question 14

Importance of hCG?

Answer 14

Produced by syncytiotrophopblast

stimulates progestorone by corpus luteum

Causes endometrium to grow and proliferate

Too low: spontaneous abortion or ectopic pregnancy

Too high: multiple pregnancies, mole pregnancy

Question 15

What occurs in week 3 and week 4?

Answer 15

Week 3: Gastrulation and formation of 3 germ layers

Week 4: Neurulation

Question 16

What is gastrulation?

Answer 16

Early development of nervous and cardiovascular system

Appearance of primitive streak

Forms endoderm, mesoderm, ectoderm
These cells will give rise to all tissues and organs in embryo

Question 17

Importance of prechordal plate?

Answer 17

Cranial end of embryonic disc

Establishes bilateral symmetry

Induces forebrain development

Becomes oropharyngeal membrane which gives rise to opening of oral cavity (mouth)

Question 18

Importance of notochord?

Answer 18

Days 16-18
Future site of vertebral column
Induces formation of neural plate - primordium of CNS

Remnants of notochord in adults are nucleus pulposus

Question 19

How is notochord formed?

Answer 19

Prenotochordal cells migrate through primitive streak

Imbed in endoderm to form plate

Question 20

What occurs during neuralation?

Answer 20

week 4

neural plate to neural groove to neural tube

Question 21

Caudal dysgenesis / sirenomelia

Answer 21

Think problems with caudal region (lower limbs)

Insufficient mesoderm (contributes to formation of lower limbs)

Causes fusion of limb buds

Question 22

Holoprosencephaly

Answer 22

High doses of alcohol kill anterior midline of the germ disc

Forebrain is small, fails to develop 2 hemispheres
eyes are closer together

Question 23

Neural tube defects

Answer 23

At cranial region - anencephaly

At lumbosacral region - spina bifida cystica

Can be prevented by taking folic acid

Question 24

sclerotome, myotome, dermatome (somite differentiation)

Answer 24

Scleratome - forms cartilage and bone component

Myotome - segemental muscle component

dermatome - segmental skin component

Question 25

What happens to embryo at the end of week 3? (week 3-4)

Answer 25

Folding of embryo
Head, tail, lateral folds

Head and tail fold caused by growth of neural tube

Lateral folds - caused by growth of somites

Question 26

Components of placenta

Answer 26

Maternal - decidua basalis (portion of endometrium)

Fetal part - villous chorion (tertiary chorionic villi)

Question 27

Sides of placenta

Answer 27

Fetal Side - chorionic plate and umbilical cord covered by amnion

Maternal side - shows cotyledone

Question 28

Major formation timeline of placenta

Answer 28

End of week 2:
Establishes utero-placental circulation
(through synctiotrophoblast)
Formation of primary chorionic villus

Beginning of 3rd:
formation of secondary chorionic villus

End of 3rd:
formation of tertiary chorionic villus

Question 29

Functions of placenta

Answer 29

Exchange of gases
Exchange of nutrients and electrolytes
Transmission of maternal antibodies
Hormones: progesterone, hCG, somatomammotopin 
Detoxification of some drugs

Question 30

Structure and function of amnion

Answer 30

Large cavity filled with amniotic fluid

Fetus is suspended by umbilical cord in amniotic fluid

Question 31

Structure and function of amniotic fluid

Answer 31

Absorbs jolts
Allows for fetal movements
Prevents adherence of embryo to surrounding tissues

Question 32

Umbilical cord features

Answer 32

1-2 cm in diameter and has false knots

Right and left umbilical arteries: carries deoxygenated blood from fetus to placenta

One umbilical vein: carries oxygenated blood from placenta to fetus

Wharton’s Jelly: protective for the vessels