Fertilisation to Trilaminar Embryology

Question 1

What is fertilisation?

Answer 1

Process by which male and female gametes fuse and occurs in the AMPULLARY REGION of the UTERINE TUBE

Question 2

Stages of fertilisation?

Answer 2

1. SPERM binds in a human-specific interaction with the ZONA PELLUCIDA glycoprotein of the oocyte
2. ACROZOMAL ENZYMES are released from the sperm head (acrosome) and sperm digests its way into the egg
3. Egg and sperm plasma membranes fuse and sperm contents enter the egg
4. Sperm entry triggers the:
   - Completion of meiosis 2 (when egg is released from ovary, meiosis is not complete)
   - Release of CORTICAL GRANULES by the oocyte (vesicle under the oocyte cell membrane which are released and HARDEN the zona pellucida against further sperm entry)

Question 3

Describe the formation and use of blastomeres

Answer 3

Zygote undergoes a series of mitotic division which subdivide the fertilised egg into many smaller daughter cells called BLASTOMERES (up to the 8 cell stage, blastomeres are thought to be TOTIPOTENT - embryonic stem cells retrieved from here)

Question 4

What occurs at 6 days (implantation)?

Answer 4

At 6 days - trophoblastic cells begin to penetrate between the epithelial cells of the uterine mucosa

Question 5

Describe the structure of the blastocyst

Answer 5

Cells lining the zona pellucida are TROPHOBLASTS - go on to produce the placenta

Inner cell mass is the embryoblast - goes on to form the embryo proper

Question 6

Function of the zona pellucida in week 1?

Answer 6

Zona pellucida provides a tough coat that protects the oocyte and prevents sticking to the uterine tube

Question 7

Summary of week 1?

Answer 7

Within the first 30 hours, the first cell division occurs

Zona pellucida disappears once the uterine cavity is reached (called BLASTOCYST HATCHING) - blastocyst can make contact with maternal cells (endometrial lining of the uterus) and begin IMPLANTATION

Question 8

Implantation sites?

Answer 8

Normal sites - upper and lower dorsal uterine wall

Abnormal sites - ovary, ampulla, uterine tube, lower part of the uterus, cervix or the peritoneum

Usually, implantation occurs in the posterior wall of the uterus - uterine walls are muscular and can support placenta development

Question 9

What is an ectopic pregnancy?

Answer 9

Implantation site is not ideal causing problems; blastocyst will still establish a placenta in abnormal sites but, in narrow uterine tubes, it can cause ruptured blood vessels and heavy bleeding - potentially fatal

Question 10

What occurs at 7.5 days (implantation)?

Answer 10

Most of the blastocyst is in the uterine wall

In the area over the embryoblast, the trophoblast has differentiated into 2 LAYERS:

An inner layer of mononucleated cells (cyto-trophoblast)

An outer multinucleated zone (as cells divide, membranes are lost); called a syncytiotrophoblast

Cytotrophoblast cells divide and migrate into the syncytiotrophoblast - where they fuse and lose cell membranes. Cells of the embryoblast also begin to differentiate into two layers:

HYPOBLAST layer

EPIBLAST layer

Simultaneously, a small cavity forms in the epiblast and enlarges to form the AMNIOTIC CAVITY

Question 11

What occurs at 9 days (implantation complete)?

Answer 11

COAGULATION PLUG - only indication of implantation

Entire structure now surrounded by syncytiotophoblasts but the amniotic cavity is surrounded by the epiblast (cavity is above the bilayered embryonic disc and DEFINITIVE YOLK SAC)

Associated with the yolk sac is the HYPOBLAST and HEUSER’S MEMBRANE (thin membrane lining the yolk sac that originates from the hypoblast)

Question 12

What occurs at day 11 and 12 (implantation)?

Answer 12

Blastocyst completely embedded in endometrial stroma and surface epithelium almost entirely covers the original defect in the uterine wall

Cell of syncytiotrophoblast penetrate deeper into stroma and erode the endothelial lining of the maternal capillaries

CHORIONIC CAVITY develops and encircles entire embryo

Question 13

What occurs at day 13 (implantation)?

Answer 13

Primary yolk sac is pushed away and broken down - replaced by secondary yolk sac

With blood vessel development, the connecting stalk will become the umbilical cord

Question 14

What occurs at the end of week 2 (implantation)?

Answer 14

Oropharyngeal membrane is where future opening of oral cavity will be

Below, epiblast and hypoblast are in contact with each other and PRIMITIVE STREAK forms a shallow groove in the caudal region of the embryo - extends along a third of the embryo (driving force of gastrulation)

Within epiblast, cells lose connection and crawl across epiblast surface to the?

Question 15

What occurs during gastrulation?

Answer 15

Two layers (hypoblast and epiblast) become three

Epiblast cells migrate through primitive streak and form the 3 PRIMARY GERM LAYERS:

Endoderm - replaces hypoblast

Intraembryonic mesoderm - layer formed between endoderm and epiblast

Ectoderm - epiblast changed name to this

Question 16

Fates of the germ layers?

Answer 16

Endoderm - produces lining of gut tube and RT

Mesoderm - becomes subdivided into 3 regions either side of the notochord:

Paraxial mesoderm (yellow) - forms axial skeleton, parts of dermis and voluntary muscle

Intermediate mesoderm (red) - forms the urogenital system

Lateral plate mesoderm - consists of:

Somatic mesoderm (blue) - forms lining of body wall, most of dermis and part of limbs

Visceral mesoderm (purple) - forms mesothelial organ coverings and CVS

Ecotderm - forms epidermis of skin and nervous system

Question 17

Function of notochord?

Answer 17

Solid chord of cells underlying the neural tube and serve as basis of axial skeleton; also important in induction signalling

Question 18

Label areas of female reproductive system?

Answer 18