Reproductive cycle, Pregnancy and labor
Objectives
At the end of this
lecture, student will be able to
• Discuss the process of oogenesis in ovaries
• Describe the major events of each phase of the
reproductive cycle
• Correlate the events of uterine cycle with the events of
the ovarian cycle
• Prepare a labelled diagram of the major hormonal changes
that occur during the uterine and ovarian cycles
• Describe the functions of various hormones involved in
pregnancy
• Explain the events associated with the three stages of
labor
• Describe the structural and functional changes which occur
in mother during pregnancy
• Explain on sex differentiation
Content
• Oogenesis
• Reproductive cycle
• Hormonal changes during reproductive cycle
• Preganacy, Hormones,
Structural and functional changes in mother
• Labor – Stages
• Sex determination
OOGENESIS
• Formation of gametes in the ovaries
• Begins in females before they are even born
During early fetal
development, primordial germ cell in
ovaries
â
Differentiate into oogonia (diploid, 2n)
â
Mitosis
Produce millions of
germ cells
â â
Germ cells degene(Process
Atresia) Develop into larger cells(Primary oocytes)
• At birth 200,000 – 2,000,000 primary oocytes in each ovary
• About 40,000 at puberty
• Around 400 will mature and ovulate
THE FEMALE
REPRODUCTIVE CYCLE
It encompasses the ovarian and uterine cycles
Ovarian cycle
– Series of events in the ovaries that occur during and
after the maturation of an oocyte
– Develop a secondary oocyte
Uterine (menstrual)
cycle
– Concurrent series of changes in the endometrium of uterus
– Prepare the endometrium each month to receive a fertilized
egg
Phases of
the Female Reproductive Cycle
• Encompasses 4 phases
– The menstrual phase
– The preovulatory phase
– Ovulation
– The postovulatory phase
• Cycle typically ranges from 24 to 35 days
• The pre ovulatory phase is more variable in length than
the other phases
Menstrual
Phase
• Also called menstruation or menses
• lasts for roughly the first 5 days of the cycle
Events in the ovaries
• FSH, several primordial follicles develop into primary,
secondary follicles
Events in the uterus
• Stratum functionalis of the endometrium is shed
• Discharges blood, tissue fluid, mucus, and epithelial
cells
• Declining levels of progesterone and estrogens à stimulate release of
prostaglandins à
uterine spiral arterioles constrict. (Cells they supply become oxygen-deprived
and start to die)
Preovulatory
phase
• Time between the end of menstruation and ovulation
• This phase is more variable in length than the other
phases
• It lasts from days 6 to 13 in a 28-day cycle
Events in the ovaries
• A single secondary follicle will outgrow & become
dominant follicle
• Dominant secondary follicle becomes the mature (graafian)
follicle
• Continue to enlarge and produce estrogen
• With reference to the ovarian cycle,
The menstrual + preovulatory = follicular phase, as ovarian
follicles are growing and developing
Events in the uterus
• Estrogens liberated into the blood by growing ovarian
follicles
• Stimulate the repair of the endometrium
• Cells of the stratum basalis undergo mitosis, produce a
new stratum functionalis
• Endometrium thickens, straight endometrial glands develop
• Arterioles coil and lengthen, penetrate the stratum
functionalis
• Also termed the proliferative phase because the
endometrium is proliferating
Ovulation
• Rupture of the mature (graafian) follicle, Release of the
secondary oocyte into the pelvic cavity
• Occurs on day 14 in a 28-day cycle
• Secondary oocyte remains surrounded by its zona pellucida
and corona radiata
• Brought about by a surge of LH
High levels of estrogens exert a positive feedback effect (green
arrows) on the hypothalamus and anterior pituitary, thereby increasing
secretion of GnRH and LH
• Signs and symptoms include
– Increased basal body temperature
– Clear, stretchy cervical mucus
– Changes in the uterine cervix
– Abdominal pain
Postovulatory
phase
• Time between ovulation and onset of the next menses
• Most constant part of the female reproductive cycle
• It lasts for 14 days in a 28-day cycle, from day 15 to day
28
Events in one ovary
• The mature follicle collapses
• A blood clot forms from minor bleeding of the ruptured
follicle, the follicle becomes the corpus hemorrhagicum
• Theca interna cells mix with the granulosa cells,
transforms into corpus luteum cells under the influence of LH
• Corpus luteum secretes progesterone, estrogen, relaxin,
& inhibin
• The luteal cells also absorb the blood clot
• Also called the luteal phase
• If the oocyte is
not fertilized,
– Corpus luteum has a lifespan of only 2 weeks
– Secretory activity declines
– Degenerates into a corpus albicans
– Levels of progesterone, estrogens, and inhibin decrease
– Release of GnRH, FSH, and LH raise
– Follicular growth resumes, a new ovarian cycle begins
• If the secondary
oocyte is fertilized and begins to divide
– Corpus luteum persists past its normal 2-week lifespan.
– “Rescued” from degeneration by human chorionic
gonadotropin (hCG)
– Produced by the chorion of the embryo, 8 days after
fertilization
– LH, hCG stimulates the secretory activity of the corpus
luteum
• Presence of hCG in maternal blood or urine – indicator of
pregnancy hormone detected by home pregnancy tests
Events in the uterus
• Progesterone and estrogens from corpus luteum promote
– Growth, coiling of the endometrial glands
– Vascularization of the superficial endometrium
– Endometrium thickens in readiness for implantation
• Endometrial glands secrete glycogen
• Also called the secretory phase of the uterine cycle
• If fertilization
and implantation do not occur
– Corpus luteum degenerates
– Discharge of the endometrium
– Initiation of another reproductive cycle
Female reproductive
cycle
Summary of hormonal interactions in the ovarian and uterine cycles
Pregnancy-
its maintenance and parturition
• Corpus luteum can only survive for about 2 weeks in the
absence of a pregnancy
• Corpus luteum will survive for the first 6-10 weeks of a
pregnancy
• Secrete progesterone
• Progesterone secretion is required for the maintenance of
a pregancy
• After implantation, embryo secretes hCG
• Placenta also secretes increasing amounts of estrogen
(mainly estriol)
Function of
progesterone during pregnancy
• Inhibits the contractions of the uterine myometrium
• Helps to keep the embryo/fetus in the uterus
• Maintains the uterine endometrium in a secretory, nutrient
rich state to support the developing embryo/fetus
Function of estrogen
during pregnancy
• Maintains the secondary sexual characteristics of the
mother
• Stimulate mammary gland growth in preparation for
lactation
• Contractions of the myometrium at the time of birth
(parturition)
Human chorionic
gonadotropin (hCG)
• Secreted by chorion in to blood
• Stimulates the corpus luteum to continue production of
progesterone and estrogens
• Placenta is fully established, the secretion of hCG is
greatly reduced
Relaxin
• Hormone produced first by the corpus luteum of the ovary
and later by the placenta
• Increases the flexibility of the pubic symphysis
• Helps dilate the uterine cervix during labor
Human chorionic
somatomammotropin (hCS)
• Also known as human placental lactogen (hPL)
• Rate of secretion of hCS increases in proportion to
placental mass
• Help prepare the mammary glands for lactation
• Enhance maternal growth by increasing protein synthesis
• Regulate certain aspects of metabolism in both mother and
fetus
Hormones during pregnancy
Changes during
Pregnancy
• Near the end of the third month of pregnancy, the uterus
occupies most of the pelvic cavity
• Fetus continues to grow
• End of a full-term pregnancy, the uterus fills nearly the
entire abdominal cavity
• Pregnancy-induced physiological changes also occur,
including
• Weight gain due to the fetus, amniotic fluid, the
placenta, uterine enlargement, and increased total body water
• Increased storage of proteins, triglycerides, and minerals
• Breast enlargement in preparation for lactation
• Lower back pain due to lordosis (hollow back)
• The maternal blood volume increases by over 30%
• Respiratory activity increases by about 20%
• Increased urine output
• Constipation
• Pregnancy amenorrhea
Parturition/
Labor
• Process by which
the fetus is expelled from the uterus through the vagina, also referred to
as giving birth
• True labor
– Dilation of the
cervix
– Expulsion of the fetus
– Delivery of the placenta
• False labor
– Pain in abdomen
at irregular intervals
– No cervical dilation
• Control of labor contractions during parturition occurs
via a positive feedback cycle
True labor can be
divided into three stages
1. Stage of dilation
• Time from the onset of labor to the complete dilation of
the cervix
• Lasts 6–12 hours
• Regular contractions of the uterus
• Usually a rupturing of the amniotic sac, and complete
dilation (to 10 cm) of the cervix
2. Stage of expulsion
• The time (10 minutes to several hours) from complete
cervical dilation to delivery of the baby
3. Placental stage
• The time (5–30 minutes or more) after delivery until the
placenta or “afterbirth” is expelled by powerful uterine contractions
• Constrict blood vessels that were torn during delivery,
reducing the likelihood of hemorrhage
• Labor lasts longer with first babies, about 14 h
• After delivery of the baby, maternal reproductive organs
and physiology return to the prepregnancy state (Puerperium, about 6 weeks)
• Tissue catabolises, the uterus undergoes a remarkable
reduction in size, Involution
• For 2–4 weeks after delivery, women have a uterine
discharge called lochia
• Consists initially of blood and later of serous fluid
derived from the placenta
SEX
DIFFERENTIATION
• There are 46 human chromosomes (of 23 different pairs) in
a normal somatic cell
• Karyotype- An
entire set of chromosomes arranged in decreasing order of size and according to
the position of the centromere
• 22 pairs are called autosomes- homologous chromosomes look
alike and have the same appearance in both males and females
Human karyotype showing autosomes and sex chromosomes
• Two members of the 23rd pair are the sex chromosomes
• They look different in males and females
• In females, the
pair consists of two chromosomes called X
chromosomes
• In males, X
chromosome + Y chromosome (smaller)
• Y chromosome has only 231 genes
• Spermatocyte undergoes meiosis, gives rise to
– Two sperm that contain an X chromosome
– Two sperm that contain a Y chromosome.
• Oocytes have no Y chromosomes, produce only X-containing
gametes
• If the secondary
oocyte is fertilized by an X-bearing sperm, the offspring normally is female (XX).
• Fertilization by a
Y-bearing sperm produces a male (XY)
• Individual’s sex is determined by the father’s chromosomes
• Sex is determined at the time of fertilization by the
presence or absence of a Y chromosome in the sperm
Summary
• Oogenesis is the formation of gametes in the ovaries
• Female reproductive cycle comprises of uterine and ovarian
cycle
• Function of the ovarian cycle is to develop a secondary
oocyte
• Function of the uterine (menstrual) cycle is to prepare
the endometrium each month to receive a fertilized egg
• Uterine and ovarian cycles are controlled by GnRH from the
hypothalamus
• If fertilization and implantation do not occur, the corpus
luteum degenerates
• If fertilization and implantation do occur, the corpus
luteum is maintained by hCG
• Pregnancy is maintained by human chorionic gonadotropin
(hCG), estrogens, and progesterone
• During pregnancy, several anatomical and physiological
changes occur in the mother
• During pregnancy, some joints become less stable, and
certain physical activities are more difficult to execute
• Labor is the process by which the fetus is expelled from
the uterus through the vagina to the outside
• Oxytocin stimulates uterine contractions via a positive
feedback cycle
• Each somatic cell has 46 chromosomes—22 pairs of autosomes
and 1 pair of sex chromosomes
• In females, the sex chromosomes are two X chromosomes
• In males, they are one X chromosome and a much smaller Y
chromosome