Fertilisation and Implantation

Fertilisation and Implantation

1. Passage of sperm from the vagina to the unfertilised ovum

   • Spermiated spermatozoa released in the lumen of seminiferous tubule containing a fluid secreted by Sertoli cells and washed towards rete testis
   • Spermatozoa are incapable of movement and cannot attach nor fertilize an oocyte upon insemination in a female
   • Spermatozoa maturation consists of::
     • 100 fold concentration of sperm (oestrogen-dependant)
     • Completion of sperm modelling
     • Changes in metabolism
     • Acquisition of forward motion
     • Changes in membrane surface proteins, charge and fluidity.
     • Coating of sperm plasma membrane with glycoproteins
     • Depends on adequate stimulation of epididymis by androgens
   • Coitus
     • COMPLETE

2. Cellular events of fertilisation, early embryological development and implantation

   • During ejaculation, the sperm is deposited into the upper vagina near the external os of the cervix
   • Ovulated oocyte surrounded by cumulus oophorus cells is released from the ovary into the peritoneal cavity where it is collected by the fimbria of the oviduct / fallopian tube
   • Fertilisation occurs in the ampulla of the fallopian tubes / oviducts
   • HURDLES
     • finish this
     • Survival of spermatozoa in the vagina
       • Physical loss of spermatozoa from the vagina
         • Forced retention of sperm in the vagina
         • After ejaculation, semenogelin I, II and fibronectin in the semen aggregate to form a loose gel (coagulum) helping spermatozoa remain of at the site of deposition.
         • The coagulum is broken down within ~ 20 min. via proteolytic cleavage of semenogelins by a serine protease called prostate-specific antigen (PSA).
     • Penetration through the cervix
       • The characteristics of the cervical canal and the quality of the cervical mucus change during the ovarian cycle, affecting spermatozoa progression to the uterus.
       • Follicular phase: Oestrogen relaxes the muscles of the cervix and increases secretion and hydration of cervical mucus  facilitates sperm penetration into the uterus and select sperm with good motility and hydrodynamic profile
       • Luteal phase: Progesterone decreases cervical mucus secretion and hydration level -> impermeable to sperm and pathogens to penetrate, hence protecting the potential fertilized embryo from infection and interference.
     • Journey through the uterus
       • Several mechanisms are involved in uterine transport of spermatozoa
       • Self-propulsion of sperm
       • Ciliated endothelial cells of endometrium create a current of fluid
       • Oestrogen-driven contractions of the myometrium towards the fundus during follicular phase (inhibited by progesterone)
     • Entering the Fallopian tube leading to the ovulated oocyte
       • Two fallopian tubes, one oocyte: how does sperm enter the “right” one?
         • In late follicular phase, labelled spheres are preferentially transported to the Fallopian tube ipsilateral to the ovary hosting the dominant follicle
         • Dominant follicle secretes progesterone which disperses to ipsilateral Fallopian tube causing relaxation of muscles in the ipsilateral utero-tubal isthmus allowing sperm penetration
       • How do sperms find the oocyte in the Fallopian tube?
         • Specific receptors on capacitated sperm responding the chemo-attractive substances (e.g. ANP or progesterone from follicular fluid or secreted by cumulus oophorus cells)
         • Creation of concentration gradients that attract capacitated sperms
       • Fertilisation

3. Contraception