Fertilization and Gastrulation

Question 1

What are the structural features of a human ovum?

Answer 1

Corona radiate= follicular cells from the uterus

Zona pellucida= cellular amorphous proteoglycan meshwork

Nucleus= haploid 23 X chromosomes

Perivitelline space

Plasma Membrane

Question 2

What are the structural features of the mature sperm?

Answer 2

Head made of the acrosome on the anterior 2/3 which contains enzymes and a nucleus with 23 X or 23 Y chromosomes

Neck

Tail with flagella and mitochondrial sheath for energy

Question 3

What events take place during fertilization? What are their timing?

Answer 3

1. Capacitation= glycoprotein (acrosome lid) and seminal plasma proteins removed and sperm passes through the corona radiata via Hyaluronidase, movement of sperm, and tubal mucosal proteins
   - takes 7 hours as going through uteran tube
2. Penetration of the zone pellucida via esterases, actosin, and neuramindiase, zona reaction to block polyspermy
3. Fusion of plasma membranes of oocyte and sperm, head and tail of sperm enter oocyte cytoplasm
4. Completion of second meiotic division, formation of male pronucleus, fusion of male and female pronuclei

Can take up to 24 hours to complete

Question 4

What are the parts of the blastocyst? How does it differ between Day 5 and Day 7

Answer 4

Inner cell mass, Blastocyst cavity, Trophboblast

There is degeneration of the zona pellucida- embryo hatched so cells can get bigger because implantation occurs at Day 6

Question 5

What is the difference between the trophoblast and inner cell mass (embryoblast)?

Answer 5

The trophoblast is the stem cells for the placenta whereas the inner cell mass aka embryoblast is the stem cells for the embryo

Question 6

What are the events of cleavage? What is the timing of these?

Answer 6

Increase in cell number and decrease in cell size
- 2 cell –> 4 cell –> 8 cell and development of morula –> early blastocyst –> late blastocyst

Occurs about 30 hours after fertilization, happens as traveling into the uterus

Question 7

What are the derivatives of the trophoblast?What are their functions?

Answer 7

Cytrotrophoblast= stem cell layer of placenta, miotically active

Gives rise to Syncytiotophoblast= proteolytic enzymes, hCG aka initial signal for pregnancy and immunosuppressant so don’t reject cells, multi nuclear, have lost cell walls

Question 8

What are the early derivatives of the inner cell mass (embryoblast)?

Answer 8

In week two derivives into the epiblast which gives rise to the ectoderm, amnion, and amniotic cavity

Question 9

What is the difference between the hypoblast and embryonic endoderm?

Answer 9

The hypoblast is just the places holder for the endoderm during week 2

Question 10

How does the prechordal plate develop?

Answer 10

Develops from hypoblast during week 2, columnar cells that are the future mouth
From endoderm along with primary and secondary yolk sacs

Question 11

How does the extraembryonic mesoderm develop? What are its parts?

Answer 11

Hypoblast –> endoderm –> extraembryonic mesoderm

Extraembryonic coelom= cavity outside of embryo

Extraembryonic somatic mesoderm= lining the trophoblast and covering the amnion

Extraembryonic Splanchnic Mesoderm= lining the yolk sac (umbilical vesicle)

Question 12

What are patient signs and symptoms that can be used to diagnose placenta previa? What are the 3 types?

Answer 12

vaginal bleeding beyond 20 weeks
antepartum bleeding should prompt sonographic evaluation before digital because palpation can cause severe hemorrhage

Marginal
Partial
Total/ central= totally covering the cervix

Question 13

What are the 3 germ layers?

Answer 13

• ectoderm from epiblast (“ec”tratctive tings that make up your appearance)
• mesoderm (m for muscle)
• endoderm from hypoblast –> GI, lungs, bladder, pharynx, thyroid tonsils

Question 14

How do teratomas form?

Answer 14

A sacrocoxcegeal forms due to persistence of the primitive streak in the caudal region

Oropharyngeal forms due to primordial germ cells not being sequestered early so they abnormally migrate

Question 15

How does the notochord form? What is its role?

Answer 15

Comes from the mesoderm
Extends from the primitive node anterior to the prechordal plate

Template (DOES NOT BECOME) for the vertebral column

Induces neural phase via Sonic Hedgehog

Question 16

What are the major classes of the mesoderm and what are their derivatives?

Answer 16

Paraxial= somites –> muscle, skeleton, dermis

Intermediate= urogenital –> kidneys and gonads

Lateral plate= connective tissue –> blood, lymph, mesnteries, cardiovasculat

• has 2 layers somatic which is adjacent to the ectoderm and the splanchnopleure adjacent to the endoderm
• supports viscera and organs

Question 17

What happens during the second week of human development?

Answer 17

Formation of epiblast and hypoblast

Question 18

What happens during the third week of human development?

Answer 18

Gastrulation

Question 19

What are the derivatives of the extraembryonic mesoderm?

Answer 19

Connecting stalk= Somatic mesoderm
- forms embryonic pole, precursor to umbilical cord, position can change

Primitive Blood= sphlanic mesoderm
- forms in wall of yolk sac

Chorion= somatic mesoderm
- cytrotrophoblast and syncytiotrophoblasts aka layers of the placenta

Question 20

What are chordomas?

Answer 20

Slow growing agressive neoplasms of bone
Arise from remnants of the notochord
Occur in axial skeleton in the spheno-occipital region

Question 21

What are chordomas?

Answer 21

Slow growing agressive neoplasms of bone
Arise from remnants of the notochord
Occur in axial skeleton in the spheno-occipital region

Question 22

What is the allantois?

Answer 22

Residual piece of endoderm

posterior midline

Diverticulum from wall of yolk sac

Template for umbilical arteries and vein

Becomes urachus attached to bladder

Question 23

What is the allantois?

Answer 23

Residual piece of endoderm

posterior midline

Diverticulum from wall of yolk sac

Template for umbilical arteries and vein

Becomes urachus attached to bladder

Question 24

What is the process of gastrulation?

Answer 24

Formation of primitive streak= cut in epiblast caudal end to head end

• the cells of primitive streak going from the epiblast and moving down so they end up between the original epiblast layer and the hypoblast
• layer closest to hypoblast aka deepest is endoderm then mesoderm then ectoderm

Question 25

What is the process of the formation of the notochord?

Answer 25

The cells forming the notochord migrate from the primitive pit in a cephalic direction
• Lie between ectoderm and endoderm they form a tube called the notochordal process
• Notochordal process will fuse with the endoderm forming the notochordal plate
– At the time the amniotic cavity and the
primary yolk sac are in communication
• Notochordal cells will round up and form the
notochord proper

Question 26

What secretes hCG to maintain pregnancy?

Answer 26

Syncytiotophoblasts

also responsible for degradation of endometrial epithelium during implantation

Question 27

What cells are mitotically active during implantation?

Answer 27

Cytotophoblast cells

Question 28

Where is ectopic pregnancy most commonly found?

Answer 28

proximal uterine tube

Question 29

What is the primitive streak?

Answer 29

Identifies site where the formation of the embryonic mesoderm is initiated, where cells of embryonic mesoderm invaginate

Site of formation of the notochord

Determines body axis of the embryo

Question 30

What is the cloacal membrane?

Answer 30

Located between notochord and primitive streak, immediately caudal to the primitive knot

composed of meso, ecto, and endo derm

Question 31

What population of mesodermal masses contributes to the body wall?

Answer 31

Somatic layer of lateral mesoderm plate