Embryology (Cole) Lecture 1

Question 1

What is Wilms’ tumor?

Answer 1

Cancer of the kidney predominately in children.

Question 2

Gametogenesis

Answer 2

Process by which gametes (oocytes and spermatozoa) are produced in the adult

• Employs a specialized process of cell division, meiosis, to distribute chromosomes among gametes

Oogenesis – 1 ovum (3 polar bodies which are lost)

Spermatogenesis – 4 sperm

Question 3

2 layers of the Oocyte

Answer 3

Corona Radiata (outside) - Protects the oocyte.

Zona Pellucida - Helps protect the ovum. Also helps keep ovum from attaching to the wrong area.

Question 4

Where does Fertilization occur?

Answer 4

Occurs in lateral third of the uterine tube

Question 5

Zona (Cortical) Reaction

Answer 5

Zona contains
sperm receptors that
recognize integral
proteins of sperm
plasmalemma

Question 6

What events occur to help prevent polyspermy?

Question 7

How does Fertilization occur?

Answer 7

• Coordinated series of events

• Phase 1 – penetration of the
corona radiata

• Phase 2 – sperm binding
(SED1 protein help sperm bind to egg) and
penetration of zona

• Acrosome reaction – lysis by
enzymes

sperm cells go through a process known as theacrosome reaction which is the reaction that occurs in the acrosome of the sperm as it approaches the egg. The acrosome is a cap-like structure over the anterior half of the sperm’s head.

As the sperm approaches the zona pellucida of the egg, which is necessary for initiating the acrosome reaction, the membrane surrounding the acrosome fuses with the plasma membrane of the oocyte, exposing the contents of the acrosome. The contents include surface antigens and numerous enzymes which are responsible for breaking through the egg’s tough coating and allowing fertilization to occur.

Question 8

Fast Component vs. Slow Component of Oocyte

Answer 8

• Fast component – change
in resting potential of oocyte
plasma membrane prevents
further binding of sperm

• Slow component - release
of cortical granules
containing enzymes that
destroy sperm receptors
(ZP3)

Question 9

What are the Immunoglobin superfamily protein Izumo?

Answer 9

Required for sperm to fuse with eggs.

A mutation in Izumo creates the inability for the sperm to fuse to the egg and undergo this acrosome reaction.

Question 10

Second meiotic division with production of second polar body

Answer 10

Unequal contribution of cytoplasm, equal contribution of genetic material

Question 11

What are Monozygotic Twins (identical)?

Answer 11

form when a single
fertilized egg splits
into two genetically
identical parts.

MZ -share
the same
DNA set, but
can actually
look very
different

Question 12

What are Dizygotic Twins (fraternal)?

Answer 12

develop when two separate eggs are fertilized and implant in the uterus.
DZ - genetic connection is no more or less the same as siblings born at separate times. They may look alike, or they may not.

Question 13

What are Polar (Half-identical) Twins?

Answer 13

• Polar body twins are believed to occur when the
mother’s egg splits before fertilization and the
polar body is also fertilized by a different sperm.

• Twins share half their genes in common (from
the mother) and the other half different (from
the two sperm). They share some features of
identical twins and some features of fraternal
twins and are so-called half-identical twins.

• DNA-based zygosity tests are currently not
available to determine if twins are polar-body
twins.

Question 14

What is Mosaicism?

Answer 14

Cells within
one individual have
different genetic makeup.

• Some cells have a normal
  number of chromosomes,
  others do not.
• Somatic mosaicism - mitotic
  non-dysjunction early in
  cellular division
  • Gonadal or tissue mosaicism
  – Mosaic Down Syndrome
  – Segmental Neurofibromatosis: (NF1)
  – McCune-Albright Syndrome: GNAS1
  mutations

Question 15

Mosaicism vs. Chimerism

Answer 15

• Mosaic – Genetically
different cells, same
zygote

• Chimera – genetically
different cells, different
zygotes that fused together.

Question 16

Chimerism

Answer 16

• Multiple cell lines (genotypes) with different set of
chromosomes in a single individual
• Fraternal twins that “fuse” to form a single individual
• Individual with bilateral “asymmetry” penis/testicle &
ovary/uterine tube; mix of XY & XX (IVF)
• 40 cases of no genetic “relationship” between mother
and offspring

Question 17

What is lyonization?

Answer 17

Early in embryogenesis in mammals, one X
chromosome is functionally inactivated through a
process called X chromosome inactivation (lyonization).

• Because this inactivation occurs randomly, all normal
females have roughly equal populations of two
genetically different cell types and are therefore a
type of mosaic.

Question 18

How long does the Fusion of pronuclei take?

Answer 18

12 hours.

Return to diploid chromosome number

Mitotic division of the zygote begins
Embryogenesis begins

Question 19

What is cleavage

Answer 19

Series of mitotic divisions

Question 20

What happens at the eight-cell stage?

Answer 20

At the eight-cell stage, the embryo goes through some changes. Most of the blastomeres in this stage become polarized and develop tight junctions with the other blastomeres. This process leads to the development of two different populations of cells: Polar cells on the outside and apolar cells on the inside. The outer cells, called the trophoblast cells, pump sodium in from the outside, which automatically brings water in with it to the basal (inner) surface to form a blastocoel cavity in a process called compaction. The embryo is now called a blastocyst.

Question 21

What is the fluid filled cavity in a Blastocyst called?

Answer 21

Blastocoele.

Question 22

What happens to the Zona Pellucida at the Blastocyst stage?

Answer 22

Zona pellucida begins to dissipate.

Question 23

What is the Trophoblast?

Answer 23

The trophoblast cells will eventually give rise to the embryonic contribution to the placenta called the chorion.

Question 24

What are the Inner cell mass (ICM)?

Answer 24

The inner cells are pushed to one side of the cavity (because the embryo isn’t getting any bigger) to form the inner cell mass (ICM) and will give rise to the embryo and some extraembryonic membranes. At this stage, the embryo is called a blastocyst.

Question 25

What is a hatched Blastocyst?

Answer 25

Cells of Blastocyst secrete proteases that digest through the zona. The embryo hatches through the digested zona and becomes a hatched Blastocyst.

Question 26

What are the parts of a Blastocyst called?

Answer 26

Inner Cell Mass = Embryoblast

Outer Cell Mass = Trophoblast

Cavity = Blastocoele

Question 27

What is the cell potency of a Zygote up until 16 cell compated Morula?

Answer 27

It is Totipotent until it becomes a Blastocyst. Once it is a Blastocyst then it is Pluripotent.

Question 28

Totipotent vs. Pluripotent

Answer 28

Totipotent - (Blastomere)can
grow into an
entire organism
and even
produce extraembryonic
tissues

Pluripotent - (embryoblast) stem cells
cannot grow
into a whole
organism, but
they are able
to
differentiate
into cells
derived from
any of the
three germ
layers.

Question 29

What are Conjoined twins?

Answer 29

Incomplete division of monozygotic twins
Due to secondary fusion of ICM or development of 2 primitive streaks
• Type of Conjoined Twins: Ischiopagus/Omphalopagus conjoined twins. They are joined at the abdomen and the pelvis, sharing a kidney, liver, bladder, and large intestine, as well as other organs
• Dicephalus tribrachius: Two heads, one body with two legs and two, three, or four arms

Question 30

What is Craniofacial duplication (diprosopus)?

Answer 30

Not due to abnormal twinning, but rather a protein
abnormality in the Shh (Sonic Hedgehog) Protein that leads to facial features being duplicated.