Fertilisation and Embedding


The Ovum (egg)


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

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Sperm and seminal fluid

  • 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
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Sperm in the vagina

  • acidic PH (4-5) which can destroy some sperm
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Cervical Mucus and Sperm


  • 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)
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  • 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)
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Pathway of Sperm

  • Vagina 
  • Cervical canal
  • Uterine cavity
  • Fallopian tube

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

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Accessing the Oocyte

  • 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
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Early Embryo Development

  • 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
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Embedding in 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
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Pathway of Cell Division


                 Trophoblast                                                        Inner Cell Mass



Placenta      Chorion     Extra Tissue                   Fetus            Amnion         Umbilical Cord

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The Decidua

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

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