Fertilisation and Embedding

Sperm clustering around the human ovum.    - Oocyte = unfertilised egg cell

• normally only one sperm penetrates the oocyte (egg)
• each sperm contains 23 chromosomes
• each oocyte contains 23 chromosomes
• hair, skin and eye colour and sex is determined

Composition of ejaculate

• 40-250 million sperm
• contains fructose (energy source for sperm) 
• is alkaline - PH 7-8.3
• volume of 2-6mls

• acidic PH (4-5) which can destroy some sperm

IN THE FEW DAYS PRECEESGIN AND DURING OVULATION

• provides protection and nourishment for sperm
• filters morphologically abnormal spern
• 99% of sperm will not pass this stage and enter the cavity of the uterus
• the first stage of the sperms movement can depend on the rhythmic muscular movements of the female tract (but this is not essential for fertilisation)
• a chemo-attractant (substance which attracts cells of a particular type) guides the sperm to the oocyte (egg)

• Sperm need to undergo a biochemical and functional change in the uterus and fallopian tubes in order to be ready to fertilise the oocyte (egg)
• This preperation is sped up by the female enzymes and the oestrogen-stimulated high salt concentration in the uterus
• The sperm tails become more able to thrust vigorously through the fallopian tube
• The sperm are able to make lateral head movements which allow them to bore into the oocyte (egg)

• Vagina 
• Cervical canal
• Uterine cavity
• Fallopian tube

*Fertilisation occurs in the widest portion of the tube - the Ampulla

• sperm have to penetrate to layers which surround the ooctye (egg)

Corona Radiata

• outer layer of cumulus cells which the sperm must penetrate
• Hyalurinidase is released from the head of the sperm which breaks down the corona radiate cells
• the sperm can then pass through to the second layer

Zona Pellicuda

• a layer of glycoproteins produces by the oocyte
• most sperm bing to the glycoprotein on this layer but one of these will penetrate it and fuse wirth the oocyte plasma membrane
• after binding to this layer that sperm undergoes th acrosome reaction
• the enzymes released from the acrosome in the sperms head allow it to digest a path to the oocyte

• The zygote (fertilised egg) spends 4-6 days travelling down to the uterine cavity
• it moves by the peristaltic action of the fallopian tubes and the sweeping movement of the fluid produced by the ciliated epithelium
• cellular devision occurs during the passage to the uterus - day 1 = 2 cell embryo

                                                                                         day 2 = 4 cell embryo

                                                                                         day 3 = Morula

                                                                                         day 4 = Blastocyst

                                                                                         day 6 = Early trophoblast

                                                                                         day 6-8 = Trophoblast (start of                                                                                                                 implantation)                                                               

• a fertilised ovum contains 46 chromosomes 
• embeds into the fundus at the top of the uterus

• The blastocyst accumulates fluid
• Zona pellicuda layer is shed which allows the egg to directly contact the epithelium of the uterus which allows the egg to embed in the inner cell mass
• embedding normally occurs in the fundus
• a layer of cells called a syncytiotrophoblast secreted enzymes which break down some of the endometrial surfuce creating a depression in which the blastocyst can embed
• Once the egg is implanted the lining of the uterus closes over it and pregnancy is established
• The cytotrophoblast (a layer of cells) switches off FSH and rpoduces HCG (nourishes the egg) which maintains the corpus luteum and produces hormones which support pregnancy

                                                        Blastocyst

                 Trophoblast                                                        Inner Cell Mass

               Cytotrophoblast

              Syncytiotrophoblast

Placenta      Chorion     Extra Tissue                   Fetus            Amnion         Umbilical Cord

In pregnancy the endometrium is refered to as the Decidua which is divided into 3 sections:

Decidua Capsular  - covers over the top of the embryo

Decidua Badalis - the trophoblast rests on this (fertilised egg)

Decidua Parietalis - the rest of the uterine cavity