Transcript Anatomy of the female reproductive system

Hormonal cycles:
fertilization and early
development
Dr. Areefa Albahri
Midwifery department
Introduction

Monthly physiological changes take place in the ovaries and
the uterus, regulated by hormones produced by the
hypothalamus, pituitary gland and ovaries. These cycles
commence at puberty and occur simultaneously and together
are known as the female reproductive cycle. The functions of
the cycle are to prepare the egg often referred to as the
gamete or oocyte for fertilization by the spermatozoon
(sperm), and to prepare the uterus to receive and nourish the
fertilized egg. If fertilization has not taken place the inner
lining of the uterus or endometrium and the egg are shed and
bleeding occurs per vagina, and the cyclic events begin again.
Introduction
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The first-ever occurrence of cyclic events is termed menarche,
meaning the first menstrual bleeding. The average age of
menarche is 12 years, although between the ages 8 and 16 is
considered normal. Factors such as heredity, diet and overall
health can accelerate or delay menarche. Interference with the
hormonal-organ relationship during the reproductive years is
likely to cause menstrual cycle dysfunction which may result
in failure to ovulate. The cessation of cyclic events is referred
to as the menopause, and signifies the end of reproductive life.
Each woman has an individual reproductive cycle that varies
in length, although the average cycle is normally 28 days long,
and recurs regularly from puberty to the menopause except
when pregnancy intervenes .
The ovarian cycle
The ovarian cycle (Fig. 10.2) is the name given to the
physiological changes that occur in the ovaries essential
for the preparation and release of an oocyte.
The follicular phase
Throughout the year small primordial follicles
containing primary oocytes, have been
developing into large preovulatory or Graafian
follicles containing secondary oocytes in a
process known as folliculogenesis (oocytes
undergo mitosis (cell division) prior to birth
and until puberty in a process known as
oogenesis).
The follicular phase
Low levels of oestrogen and progesterone
stimulate the hypothalamus to produce
gonadotrophin releasing hormone (GnRH).
This releasing hormone causes the production
of follicle stimulating hormone (FSH) and
luteinizing hormone (LH) by the anterior
pituitary gland. FSH controls the growth and
maturity of the Graafian follicles.
The Graafian follicles begin to secrete
oestrogen, which includes oestradiol, oestrone
and oestriol. Rising levels of oestradiol cause a
surge in LH.
When oestradiol reaches a certain peak, the
secretion of FSH is inhibited. The reduced
FSH &LH hormones secretion causes a
slowing in follicle growth and eventually leads
to follicle death, known as atresia.
This dominant follicle forms a bulge near the
surface of the ovary, and soon becomes
competent to ovulate. The time from the
growth and maturity of the Graafian follicles
to ovulation is normally around 1 week, day
5–14 of a 28-day cycle of events. Occasionally
the follicular phase may take longer if the
dominant follicle does not ovulate and the
phase will begin again.
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Ovulation
 Ovulation is the process whereby the dominant
follicle ruptures and discharges the secondary oocyte
into the uterine tube where it awaits fertilization.
Ovulation is stimulated by a sudden surge in LH
which matures the oocyte and weakens the wall of
the follicle. This LH surge occurs around day 12–13
of a 28-day cycle and lasts 48 hrs.
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Stringy clear mucus appears in the cervix,
ready to accept the sperm.
During ovulation some women experience
varying degrees of abdominal pain known as
mittelschmerz, which can last several hours.
There may be some light bleeding caused by
the hormonal changes taking place. Following
ovulation the fertilized or unfertilized oocyte
travels to the uterus.
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The luteal phase
The luteal phase is the process whereby the
cells of the residual ruptured follicle
proliferate and form a yellow irregular
structure known as the corpus luteum. The
corpus luteum produces oestrogen and
progesterone for approximately 2 weeks, to
develop the endometrium of the uterus, which
awaits the fertilized oocyte. The corpus
luteum continues its role until the placenta is
adequately developed to take over.
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The luteal phase
In the absence of fertilization the corpus
luteum degenerates and becomes the corpus
albicans (white body), and progesterone and
oestrogen, and inhibin levels decrease. In
response to low levels of oestrogen and
progesterone the hypothalamus produces
GnRH. The rising levels of GnRH stimulate
the anterior pituitary gland to produce FSH
and the ovarian cycle commences again
(Stables and Rankin 2004).
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The menstrual or endometrial cycle
 The menstrual cycle is the name given to the
physiological changes that occur in the uterus,
and which are essential to receive the
fertilized oocyte. The menstrual cycle consists
of three phases.
1.The menstrual phase
 This phase is often referred to as
menstruation, bleeding, menses, or a period.
The term eumenorrhea denotes normal, regular
menstruation that lasts for typically 3–5 days,
although 2–7 days is considered normal. The
average blood loss during menstruation is 50–150
mL. The blood is inhibited from clotting due to the
enzyme plasmin contained in the endometrium. The
term menorrhagia denotes heavy bleeding.

Some women experience uterine cramps caused by muscular
contractions to expel the tissue. Severe uterine cramps are
known as dysmenorrhea.
2.The proliferative phase
 There is the formation of a new layer of endometrium in the
uterus, referred to as the proliferative endometrium. This
phase is under the control of oestradiol and other oestrogens
secreted by the Graafian follicle and consist of the re-growth
and thickening of the endometrium in the uterus.
 During the first few days of this phase the endometrium is reforming, described as in the regenerative phase.
3.The secretory phase
 This phase follows the proliferative phase and
is simultaneous with ovulation. It is under the
influence of progesterone and oestrogen
secreted by the corpus luteum. The functional
layer of the endometrium thickens to
approximately 3.5 mm and becomes spongy in
appearance. The blood supply to the area is
increased and the glands produce nutritive
secretions such as glycogen. These conditions
last for approximately 7 days, awaiting the
fertilized oocyte.
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Fertilization
 Human fertilization, known as conception, is the
fusion of the sperm. The process takes
approximately 24 hrs and normally occurs in
the ampulla of the uterine tube.
 In the fertile male approximately 300 million
sperm are deposited in the posterior fornix of
the vagina. Some are destroyed by the acid
medium of the vagina. Once inside the uterine
tubes the sperm undergo a process known as
capacitation.
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Development of the zygote
 The development of the zygote can be divided
into three periods. The first 2 weeks after
fertilization referred to as the pre-embryonic
period includes the implantation of the zygote
into the endometrium; weeks 2–8 are known as
the embryonic period; and weeks 8 to birth, are
known as the fetal period.
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The pre-embryonic period
During the first week the zygote travels along the
uterine tube towards the uterus. At this stage a strong
membrane of glycoproteins called the zona pellucida
surrounds the zygote. The zygote receives
nourishment, mainly glycogen, from the goblet cells of
the uterine tubes and later the secretory cells of the
uterus. During the travel the zygote undergoes mitotic
cellular replication and division referred to as cleavage,
resulting in the formation of smaller cells known as
blastomeres. The zygote divides into two cells at 1 day,
then four at 2 days, eight by 2.5 days, 16 by 3 days,
now known as the morula.
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The cells bind tightly together in a process known as
compactation. Next cavitation occurs whereby the outermost
cells secrete fluid into the morula and a fluid-filled cavity or
blastocele appears in the morula. This results in the formation
of the blastula or blastocyst, comprising 58 cells. The process
from the development of the morula to the development of the
blastocyst is referred to as blastulation and has occurred by
around day 4 (Fig. 10.5). The surrounding zona pellucida
begins to disintegrate and the volume of fluid increases.
Around days 3–5 the blastocyst enters the uterus. The
blastocyst possesses an inner cell mass or embryoblast, and
an outer cell mass or trophoblast. The trophoblast becomes
the placenta and chorion, while the embryoblast becomes the
embryo, amnion and umbilical cord (Carlson 2004).
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Chromosomes
Each human cell has a complement of 46
chromosomes arranged in 23 pairs, of which two
are sex chromosomes. The remainder are known
as autosomes. During the process of maturation,
both gametes shed half their chromosomes, one of
each pair, during a reduction division called
meiosis. Genetic material is exchanged between
the chromosomes before they split up. In the male,
meiosis starts at puberty and both halves redivide
to form four sperm in all. In the female, meiosis
commences during fetal life but the first division is
not completed until many years later at ovulation.
The division is unequal; the larger part will eventually
go on to form the oocyte while the remainder forms
the first polar body. At fertilization the second division
takes place and results in one large cell, which is
now mature, and a much smaller one, the second
polar body. At the same time, division of the first
polar body creates a third polar body.
 When the gametes combine at fertilization to form
the zygote, the full complement of chromosomes is
restored. Subsequent division occurs by mitosis
where the chromosomes divide to give each new
cell a full set.
1 Sex determination
Females carry two similar sex chromosomes, XX; males carry two
dissimilar sex chromosomes, XY. Each sperm will carry either an X or
a Y chromosome, whereas the oocyte always carries an X chromosome.
If the oocyte is fertilized by an X-carrying sperm a female is
conceived, if by a Y-carrying one, a male.
During week 2, the trophoblast proliferates and differentiates into 2
layers: the outer syncytiotrophoblast or syncytium and the inner
cytotrophoblast (cuboidal dividing cells) (Fig. 10.6). Implantation of
the trophoblast layer into the endometrium now known as the decidua
begins (see Ch. 11). Implantation is usually to the upper posterior wall.
At the implantation stage the zona pellucida will have totally
disappeared
The syncytiotrophoblast layer invades the decidua by forming
finger-like projections called villi that make their way into the
decidua and spaces called lacunae that fill up with the
mother's blood. The villi begin to branch, and contain blood
vessels of the developing embryo, thus allowing gaseous
exchange between the mother and embryo. Implantation is
assisted by hydrolytic enzymes secreted by the
syncytiotrophoblast cells that erode the decidua and assist
with the nutrition of the embryo. The syncytiotrophoblast
cells also produce human chorionic gonadotrophin (hCG), a
hormone that prevents menstruation and maintains pregnancy
by sustaining the function of the corpus luteum.
Simultaneously to implantation, the embryo is
developing from the embryoblast. The cells of the
embryoblast differentiate into two types of cells: the
epiblast (closest to the trophoblast) and the hypoblast
(closest to the blastocyst cavity). The epiblast cells
give rise to cells of the embryo. Each layer of
epiblast cells, of which there are three, will form
particular parts of the embryo. The first appearance
of these layers, collectively known as the primitive
streak, is around day 15.
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The ectoderm is the start of tissue that covers most
surfaces of the body: the epidermis layer of the skin,
hair and nails. Additionally it forms the nervous system.
The mesoderm forms the muscle, skeleton, dermis of
skin, connective tissue, the urogenital glands, blood
vessels and blood and lymph cells.
•
The endoderm forms the epithelia lining of the
digestive, respiratory and urinary systems, and
glandular cells of organs such as the liver and
pancreas.
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The epiblast separates from the trophoblast and forms a cavity,
known as the amniotic cavity. The amniotic cavity derives from
the ectoderm layer. The cavity is filled with fluid, and gradually
enlarges and folds around the developing structures of the
embryo to enclose it. The amnion forms from the lining of the
cavity. It swells out into the chorionic cavity and eventually
obliterates it when the amniotic and chorionic membranes come
into contact.
The hypoblast layer of the embryoblast gives rise to extraembryonic structures only, such as, the yolk sac. Hypoblast cells
migrate along the inner cytotrophoblast lining of the blastocele
secreting extracellular tissue which becomes the yolk sac. The
yolk sac is lined with extraembryonic endoderm, which in turn is
lined with extraembryonic mesoderm.
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The yolk sac serves as a primary nutritive function, carrying
nutrients and oxygen to the embryo. The endoderm and mesoderm
cells contribute to the formation of some organs, such as the
primitive gut arising out of the endoderm cells; blood islands which
later go on to develop blood cells arise from the mesodermal layer;
the remainder resembles a balloon floating in front of the embryo
until it atrophies by the end of the 6th week when blood forming
activity transfers to embryonic sites. After birth, all that remains of
the yolk sac is a vestigial structure in the base of the umbilical
cord, known as the vitelline duct.
The pre-embryonic period is crucial in terms of initiation and
maintenance of the pregnancy and early embryonic development.
Inability to implant properly results in miscarriage. Additionally
chromosomal defects and abnormalities in structure and organs
can occur during this time (Moore & Persaud 2003).
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