The ovarian cycle refers to the series of changes in the ovary during which
the
follicle matures, the ovum is shed, and the corpus luteum develops.
The follicular phase describes the development of the follicle in response to
follicle stimulation hormone ( FSH ). As luteinizing hormone ( LH ) and FSH
levels increase they stimulate ovulation, or the release of a mature oocyte into
the fallopian tubes.
In the luteal phase, the corpus luteum forms on the ovary and secretes many
hormones, most significantly progesterone, which makes the endometrium of
the uterus ready for implantation of an embryo.
If implantation does not occur, the corpus luteum will be degraded, resulting
in menstruation.
If implantation occurs the corpus luteum is maintained.
The Follicular Phase
The follicular phase (or proliferative phase) is the phase of the menstrual cycle in
humans and great apes during which follicles in the ovary mature, ending with
ovulation. The main hormone controlling this stage is estradiol. During the
follicular phase, follicle-stimulating hormone (FSH) is secreted by the anterior
pituitary gland. FSH levels begin to rise in the last few days of the previous
menstrual cycle and peak during the first week of the follicular phase. The rise in
FSH levels recruits five to seven tertiary-stage ovarian follicles (also known as
Graafian or antral follicles) for entry into the menstrual cycle. Only one dominant
follicle can continue to maturity and complete each menstrual cycle. As oestrogen
levels rise, negative feedback reduces FSH levels, and only one follicle can
survive, with the other follicles forming polar bodies
Ovulation
Ovulation is the phase in which a mature ovarian follicle ruptures and discharges
an ovum (also known as an oocyte, female gamete, or egg). Ovulation also occurs
in the estrous cycle of other female mammals, which differs in many fundamental
ways from the menstrual cycle. The time immediately surrounding ovulation is
referred to as the ovulatory phase or the periovulatory period.
Ovulation
In response to the LH surge, the follicle ruptures and the mature oocyte is assisted
to the fallopian tube by fimbria. Here it remains viable for fertilisation for around
24 hours.
Following ovulation, the follicle remains luteinised, secreting oestrogen and now
also progesterone, reverting back to negative feedback on the HPG axis. This,
together with inhibin (inhibits FSH) stalls the cycle in anticipation of fertilisation.
The Luteal Phase
The luteal phase (or secretory phase) is the latter part of the menstrual or estrous
cycle. It begins with the formation of the corpus luteum and ends in either
pregnancy or luteolysis. The main hormone associated with this stage is
progesterone, which is significantly higher during the luteal phase than in other
phases of the cycle. Some sources define the end of the luteal phase as a distinct
ischemic phase.
In the luteal phase, the corpus luteum forms on the ovary and secretes many
hormones, most significantly progesterone, which makes the endometrium
of the uterus ready for implantation of an embryo.
If implantation does not occur, the corpus luteum will be degraded, resulting
in menstruation.
At the end of the cycle, in the absence of fertilisation, the corpus
luteum spontaneously regresses after 14 days. There is a significant fall in
hormones, relieving negative feedback, resetting the HPG axis ready to
begin the cycle again.
If fertilisation occurs, the syncytiotrophoblast of the embryo produces
human chorionic gonadotropin (HcG), exerting a luteinising effect,
maintaining the corpus luteum. It is supported by placental HcG and it
produces hormones to support the pregnancy. At around 4 months of
gestation, the placenta is capable of production of sufficient steroid hormone
to control the HPG axis.
The Uterine Cycle
Proliferative Phase
Following menses, the proliferative phase runs alongside the follicular phase,
preparing the reproductive tract for fertilisation and
implantation. Oestrogen initiates fallopian tube formation, thickening of the
endometrium, increased growth and motility of the myometrium and
production of a thin alkaline cervical mucus (to facilitate sperm transport).
Secretory Phase
The secretory phase runs alongside the luteal phase. Progesterone stimulates
further thickening of the endometrium into a glandular secretory form,
thickening of the myometrium, reduction of motility of
the myometrium, thick acidic cervical mucus production (a hostile
environment to prevent polyspermy), changes in mammary tissue and other
metabolic changes.
Menses
Menses marks the beginning of a new menstrual cycle. It occurs in the
absence of fertilisation once the corpus luteum has broken down and the
internal lining of the uterus is shed. Menstrual bleeding usually lasts
between 2-7 days with 10-80ml blood loss.
https://teachmephysiology.com/reproductive-system/development-maturation/menstrual-
cycle/
Hypothalamic-Pituitary-Gonadal (HPG) Axis
The hypothalamus, anterior pituitary gland and gonads (ovaries) work
together to regulate the menstrual cycle. For more information on how this
begins, visit Puberty. Gonadotropin releasing hormone (GnRH) from the
hypothalamus stimulates luteinising hormone (LH) and follicular stimulating
hormone (FSH) release from the anterior pituitary gland. LH and FSH are
gonadotropins that act primarily on the ovaries in the female reproductive
tract:
FSH binds to granulosa cells to stimulate follicle growth, permit the
conversion of androgens (from theca cells) to oestrogens and stimulate
inhibin secretion
LH acts on theca cells to stimulate production and secretion of androgens
The menstrual cycle is controlled by feedback systems:
Moderate oestrogen levels exert negative feedback on the HPG axis
High oestrogen levels (in the absence of progesterone) positively feedback
on the HPG axis
Oestrogen in the presence of progesterone exerts negative feedback on the
HPG axis
Inhibin selectively inhibits FSH at the anterior pituitary
