Early human development

Question 1

Oogenis

Answer 1

• process involving meiotic division of a diploid germ cell, resulting in a single haploid female gamete (egg) and to two polar bodies
• cytoplasm is kept in 1 developing oocyte and the 2 polar bodies (nuclear biproducts of meoiosis) extruded and ulyimately degrated
• Fertilizationoccurs externally in fish/frogs (lower vertebrates) internally in higher vertebrates

Question 2

Fertilization and first cell division in humans

Answer 2

• Once a singlr sperm enters the oocyte, the zona reaction (hardening of zona pellucida) occurs to block polyspermy
• The second meiotic division of the oocyte is completed at fertilization (generation of the 2nd polar body)
• formation of pronuclei and DNA replication follows
• pronuclei fuse into a single (diploid) zygote nucleus (genetically unique)
• Miotic division now occurs

Question 3

Preimplantation

Answer 3

*The blastocyst is in the uterus for 2 days (day 4 and 5) prior to implantation
*During this time the blastocyst derives nourishment from secretions of the uterine glands
*The zona pellucida must degrade before implantation
*An immunosuppressant, early pregnancy factor is secreted
by the trophoblast (detected in the mother’s serum- helps to prevent rejection of the embryo by the mother’s immune system)

Question 4

Early stages of implantation

Answer 4

The inner cell mass contains pluripotent stem cells
The trophectoderm is extraembryonic and is made up of:
* Cytotrophoblasts (individual cells)
* Syncytiotrophoblasts (invades the endometrial epithelium)

Question 5

fertilization in human occurs at ?

Answer 5

the falopian tubes

Question 6

When things go wrong: ectopic pregnancies

Answer 6

• X is the “normal”
  implantation site

Question 7

Second week of development overview

Answer 7

I. Implantation of the blastocyst is completed (6-10 days after fertilization)
II. Embryoblast becomes a bilaminar disc ( epiblast and hypoblast )
III. Amnion and amniotic cavity forms IV. Umbilical vesicle (yolk sac) forms

Question 8

Hypoblast =

Answer 8

primitive/primary

Question 9

What does extraembryonic mean?

Answer 9

outside the embryon

Question 10

Third and fourth week of development overview
(here we can see the distinct parallels to lower vertebrate development)

Answer 10

I. Gastrulation: Establishment of the three germ layers II. Development of the notochord
III. Development of the somites
IV. Neurulation: formation of the neural tube

Question 11

Gastrulation:

Answer 11

• This process gives rise to the three primary germ layers
• Epiblast cells migrate through the primitive streak (extends anteriorly)
  forming the underlying mesoderm and definitive endoderm
• The primitive streak than regresses posteriorly, while the forming
  notochord (forms in the anterior to posterior direction)
• Cells that remain in the epiblast will form ectoderm

Question 12

Gastrulation: formation of three definitive germ layers

Answer 12

• The primitive streak identifies the craniocaudal axis and left/right sides of the embryo
• Epiblastic cells move through the primitive streak (ingression) to either form the mesoderm -or- displace hypoblast cells to form the definitive endoderm
• The remaining epiblastic cells form the ectoderm of the embryo
• Primitive streak diminishes in size by the fourth week

Question 13

Formation of the neural plate
Elongation and regression of the primitive streak

Answer 13

• The primitive streak reaches an anterior limit forming a primitive node and begins to regress posteriorly
• The developing notochord induces the overlying ectoderm to thicken and form the neural plate
• The neural plate develops craniocaudally. The expanded cranial portion gives rise to the brain, and the tapered portion gives rise to the spinal cord

Question 14

gastrulation starts at the

Answer 14

dorsal lips

Question 15

When things go wrong: Sacrococcygeal teratoma

Answer 15

• due to the failure of the
  primitive streak to regress *1:35,000 live births
• male/female disparity (76% female)
• can include many different tissue types (e.g., neural tissue, skin, teeth)
  Why do you think this happens? Most are benign- when is not regress

Question 16

Neurulation: formation of the neural tube (NT)
The NT gives rise to the central nervous system and neural crest

Answer 16

o The neural plate invaginates to form a neural groove along the central axis
o The neural folds on both sides of the groove fuse to form the neural tube
o Cells at the crest of the neural folds (neural crest cells) detach and migrate dorsolaterally
Neurulation is completed by the end
Neural crest cells
of the fourth week

Question 17

When things go wrong:

Answer 17

Neural Tube Defects (NTDs)
(neural tube fails to close

Question 18

Disorders:

Answer 18

spina bifida occulta (least severe –tuft of hair)
spina bifida with myeloschisis (most severe –neural tissue is expose

Question 19

Risk Factors

Answer 19

• use of anti-seizure medications (e.g. valproic acid)
• neural tube defect in a previous child (genetics)
• lack of folic acid
• maternal diabetes
• maternal hyperthermia (hot tubs, etc.)

Question 20

Prevention:

Answer 20

Folic acid supplements before conception and during neural tube developmental period

Question 21

At fertilization a single sperm must pass through the zona pellucida

Answer 21

yes, zone of reaction, prevents polysperm

Question 22

The zona reaction prevents polyspermy

Question 23

The zona pellucida persists until day 4 of development; once it degrades, the embryo can implant

Answer 23

because prevents inmanute implanation

Question 24

what happens is something goes wrong with neurolation

Answer 24

neurotube is not going to cloose- spina bifida

Question 25

The inner cell mass contains pluripotent stem cells, which can form any cell of the embryo proper

Answer 25

- extra embryonic, becomes part of the placenta - neuro cell mass -

Question 26

The fallopian tube is the most common site for ectopic pregnancies

Answer 26

fertilization happens, implantation happens

Question 27

Cells of the epiblast migrate through the primitive streak during gastrulation to form the three germ layers

Answer 27

migrate, during the process of fertilization

Question 28

Sacrococcygeal teratoma results from a failure of the primitive streak to regress

Question 29

Neural tube defects result when the neural tube fails to close properly

Answer 29

neurotube effects- spina bifida