UTERINE DEVELOPMENT AND MUSCULATURE
In response to the ebb and flow of estrogenic secretions, the uterus exhibits two patterns of growth and development—one a lifelong curve from infancy to senility, and the other a transient recurrent increment and recession due to the swings of ovarian activity in each menstrual cycle.
Up to the seventh month of fetal life, the uterus grows in proportion to
the rest of somatic development. Thereafter, a disproportionate acceleration in
size takes place; this is considered to be a specific response to the high level
of estrogens present in the mother as she approaches term.
Within a few days after birth, the infant’s uterus shrinks somewhat
because of the abrupt withdrawal of maternal hormones, which may even, at
times, be sufficient to result in some vaginal spotting or frank bleeding.
Thereafter, the size of the uterus remains static until, as a prelude to the
menarche, the ovaries start to produce hormones.
Uterine growth is one of the earliest signs of puberty and generally
precedes the menarche by 1 or 2 years. In 60% of girls, the uterus reaches
adult size by the 15th year. By this time, a difference in proportion of length
of the cervix to that of the fundus becomes evident. In the newborn and
prepuberal uterus, the ratio of cervical length to that of the corpus is
approximately 1:1. However, in the adult this ratio becomes 1:2 or greater.
Measuring the distance from the external to the internal cervical os using a
uterine probe and then measuring the total length of the uterine canal may
confirm the diagnosis of an infantile organ in the adult.
When mature, the uterus is about 8 to 10 cm long. The uterine width at
the top measures 7 by 5 cm. It narrows to a diameter of 2 to 3 cm at the
cervix.
Repeated pregnancies leave the uterus larger than in the nulliparous
woman. After the menopause, and its associated loss of hormonal stimulation,
shrinking and atrophy progress. The senile uterus, with its thinned-out
myometrium, often retrogresses to the size of the preadolescent stage. The
ratio of cervical length to overall size often regresses to that found before
puberty. Because the uterus is formed by fusion of the müllerian ducts, its
muscular structure is rather complex. The external longitudinal and internal
circular fibers in the tubes are confluent with those in the uterus. Indeed,
although the deep, spiraling, circular fibers sweep around the uterus in both
clockwise and counterclockwise directions, each set is motivated independently
by contractions that originate in each of the tubes. It appears that
peristaltic waves initiated in the tubal walls act as lateral pacemakers for
uterine contractions, which sweep down the fundus to the cervix. During labor,
an element of rhythmicity results from a wandering
pacemaker system of independent foci of initiator signals and strong
interconnections between the cells through cellular gap-junctions.
Smooth muscle bundles contained in the supportive ligaments interdigitate
with the circular muscle system of the uterus. The importance of this
complicated and interlaced system, embracing tubes, uterus, and ligaments, is
realized during the menstrual cycle, where smooth muscle contractions are found
to be a function of ovarian activity. They
occur with greater force and frequency during the preovulatory peak of estrogen
production. This coordination of muscular contractions in the three different
structures may also serve to orient properly the ovary with the infundibulum of
the tube at the time of ovulation. Any pathologic condition interfering with
the coordination or strength of contractions from the tubes may be an important
cause for uterine dysrhythmia,
inertia, or dysmenorrhea.
