Cervical Softening - Anatomy & Physiology

Introduction=

The basic function of the cervix during pregnancy is to retain the foetus within the uterus and to maintain the internal environment of the uterus by preventing the external environment accessing the uterus. Therefore the cervix regulates the passage to and from the uterine cavity. This ensures that any micro-organisms within the external environment are unable to access the uterus. During pregnancy the uterine body becomes distended whilst stretch resistant tissues allow the cervix to be maintained in a closed state. For the foetus to move out of the uterus, the cervix must soften or 'ripen'.

Cervix Structure

The cervix has only a small amount of musculature and is mainly composed of collagen fibre bundles and proteoglycan matrices. The collagen fibres are helical strands of amino acids bound together to form fibrils whilst the proteoglycan matrix are made up of a protein core with Glycosaminoglycan branches or GAGs. GAGs determine the degree of collagen fibre aggregation. An increased GAG content reduces collagen aggregation and vice versa.

The texture of the cervical tissues is influenced by the relative levels of oestogen and progesterone and therefore is changable dependant on the stage of the oestrous cycle. The mucous membrane of the cervix is highly folded and contains mucin producing cells. Mucin production increases under the influence of oestradiol and helps to lubricate the vagina in preparation for copulation. Mucin is also responsible for the transport of bacteria and foreign bodies away from the uterus. During the luteal phase and during pregnancy small levels of mucin are produced under the influence of progesterone to help form a 'cervical plug'. This plug ensures that the external environment is unable to penetrate the uterus.

Cervical Species Differences

In ruminants the cervical mucus is expelled from the vagina during oestrous and is known as 'bulling string'. Bulling string therefore indicates that animal is in oestrous and should be mated.

Cervical Softening

Endocrinology

Immediately prior to birth, the pre-parturition cervix looses firmness. Cervical softening involves two changes in the intracellular matrix; firstly a reduction in the number of collagen fibres and secondly an increase in GAGs to decrease aggregation of the remaining collagen fibres.

Several hormones are known to exhibit an effect on the cervix resulting in pre-parturition softening; 'prostaglandins' and 'relaxin'. Prostaglandin levels increase markedly in the days prior to parturition, peaking at parturition. There are three main types and sources of prostaglandin that are important in cervical softening; prostaglandin E2 (PGE2), prostacyclin (PGI2) and Prostaglandin F2α (PGF2α).

PGF2α is produced by the placenta in response to the production of fetal corticoids. (PGF2α also helps to remove the progesterone block pre-parturition.) PGF2α is not thought to act directly on the cervix and instead causes the myometrium of the uterus to become more active resulting in increased cervical stimulation and therefore softening and dilation. PGE2 is maternally derived and is the main driver of cervical softening. The production of PGE2 coincides with reductions in progesterone levels. PGE2 also acts on the uterus resulting in increased myometrial contractions, increased uterine pressure and therefore also cervical stimulation. PGI2 is also maternally derived and acts as a vasodilator and as an anticoagulant, playing another important role in cervical relaxation. With all types of prostaglandin, isoforms are produced locally and act locally and are therefore not strictly classed as hormones.

Relaxin is produced by the ovaries and the placenta and together with progesterone prevent uterine contractions throughout the pregnancy. However, relaxin also aids in the loosening of tissues in the cervix and pelvic ligaments to loosen pre-parturition. Relaxin and PGE2 work in combination on the cervix.

Softening Mechanisms

Prostaglandins are thought to cause cervical softening, production increases at term and during parturition.

• Prostaglandins may induce cervical softening by:
  • Inducing collagen breakdown
  • Altering GAG/Proteogylcan composition