Embryology

Ovulation Process

1) Ovary releases an Ovum (Matured Oocyte)

2) Fimbriae sweeps Ovum into the fallopian tubes via ciliary beating

3) As Ovum enters fallopian tube, It is now viable to fertilisation for 24hr

What happens if the Ovum remains unfertilised in the Fallopian tube after 24hrs?

The Ovum begins to deteriorate and slowly disintegrate, & will be shed along with the endometrial lining during menstruation

Describe the Fimbriae

Finger-like extenstions of the Fallopian tube (which are lined with cilia).

Fertilisation Process

1) The Entry of sperm triggers the completion of Meiosis II in Ovum, forming female pronuclei.

2) The male & female haploid pronuclei fuse to form a diploid Zygote.

Implantation Window

The period of time that the endometruim is most receptive to a developing embryo.

When does the implantation WIndow peak at

Day 7; The endometrial lining and gland developing peaks so its most receptive to a developing embryo.

After Day 7 of the Implantation Window, what happens to the Developng Embryo in the Endometrium?

The endometrium become less receptive for the embryo, so it needs to implant into the maternal endometrium to receive hormonal support and nutrients for further development.

Human Embryogenesis

1) Zygote is surrounded by zona pellucida, but begins mitotic divisions (cleaving).

2) During cleaving, the embryo size remains the same, but the number of cells increase from 2 to 4 to 8 cells, called Blastomeres.

3) Continuing the division, the embryo becomes a ball of 16-32 cells (Morula), this then enters the uterus.

4) A fluid enters the morula and forms a cavity (blastocoel), The cells now differentiate into a blastocyst; made of an embroblast (Inner cell mass) & a trophoblast (Outer cell)

Hatching

Break up of the zona pellucida surrounding the developing embryo, now having direct contact with the endometrium for implantation.

Implantation (Name of 3 steps

Apposition, Adhesion, Invasion & Embedding

Process of Apposition (Implantation)

The initial contact between blastocyst & endometrial lining. Blastocyst positions the inner cell mass (embryoblast) to enter first.

Adhesion process (Implantation)

Blastocyst becomes more firmly attached as the trophoblast produces ‘integrins’. molecules allow for cells to stick to endometrium

Invasion & Embedding process (Implantation)

Trophoblast differentiates into 2 layers:

• CytoTrophoblast —> Later forms primary chorionic villi

• SyncytoTrophoblast —> Secretes enzymes which digest the endometrial tissue so the blastocyst can embed onto the uterine lining.

Maternal capillaries dilate, forming lacunae

What causes the Decidual Reaction?

The embryo being foreign body to the mother, this will trigger the inflammatory response in the uterus.

So cells begin to proliferate and move around the embryo, to provide stability.

The maternal tissue must not reject the developing embryo.

What is the Decidual Reaction? (process)

progesterone and Oesterogen make the Uterus secrete chemicals which prevents the mothers immune response from destroying the embryo

Placental formation

• The embryo is now securely attached & embedded in the uterus lining.

• Hormones such as hCG are produced in traceable amounts by the Trophoblast.

• Early placenta structures begin to form

Gastrulation

Blastocyst is transformed by mass movements into an embryo with multiple body tissues & distinct body axes.

Ectoderm and what does it do

Outer germ layer, formed from cells remaining on outside of the embryo.

Forms the Nervous System such as skin, & epidermal layers of skin & structures derived from.

Mesoderm & what does it do

Middle germ layer, made of cells between Endoderm & Ectoderm.

Forms many organs:

• Heart

• Bones

• Blood Vesssles

• Muscles

Endoderm & what does it do

Inner most germ layer, forms as the blastomeres enter the embryo.

Gives rise to the epithelial lining of:

Thyroid, Digestive tract, Pancreas & Liver.