What are the preimplantation hormones?
Gonadotrophin Releasing Hormone (GnRH)
Luteinizing Hormone (LH)
Follicle Stimulating Hormone (FSH)
Estrogen (from the follicle)
Progesterone (from the corpus luteum)
What are the postimplantation hormones?
Human Chorionic Gonadotrophin (hCG)
Human Placental Lactogen (hPL)
Estrogen (from the placenta)
Progesterone (from the placenta)
What is the role of LH?
LH is a hormone produced by the anterior pituitary gland. In females, an acute rise of LH called the LH surge triggers ovulation and development of the corpus luteum. It acts synergistically with FSH.
In females, at the time of menstruation, FSH initiates follicular growth, specifically affecting granulosa cells. With the rise in estrogens, LH receptors are also expressed on the maturing follicle that produces an increasing amount of estradiol. Eventually at the time of the maturation of the follicle, the estrogen rise leads via the hypothalamic interface to the "positive feed-back" effect, a release of LH over a 24- to 48-hour period. This 'LH surge' triggers ovulation, thereby not only releasing the egg but also initiating the conversion of the residual follicle into a corpus luteum that, in turn, produces progesterone to prepare the endometrium for a possible implantation. LH is necessary to maintain luteal function for the first two weeks. In case of a pregnancy, luteal function will be further maintained by the action of hCG (a hormone very similar to LH) from the newly established pregnancy.
The release of LH at the pituitary gland is controlled by pulses of gonadotropin-releasing hormone (GnRH) from the hypothalamus. Those pulses, in turn, are subject to the estrogen feedback from the gonads
What is FSH?
Follicle-stimulating hormone (FSH) is secreted by the anterior pituitary gland. FSH secretion begins to rise in the last few days of the previous menstrual cycle, and is highest and most important during the first week of the follicular phase.
The rise in FSH levels recruits five to seven tertiary-stage ovarian follicles (this stage follicle is also known as a Graafian follicle) for entry into the menstrual cycle. These follicles, that have been growing for the better part of a year in a process known as folliculogenesis, compete with each other for dominance.
FSH induces the proliferation of granulosa cells in the developing follicles, and the expression of luteinizing hormone (LH) receptors on these granulosa cells. Under the influence of FSH, aromatase and p450 enzymes are activated, causing the granulosa cells to begin to secrete estrogen. This increased level of estrogen stimulates production of gonadotropin-releasing hormone (GnRH), which increases production of LH. LH induces androgen synthesis by thecal cells, stimulates proliferation, differentiation, and secretion of follicular thecal cells and increases LH receptor expression on granulosa cells.
Throughout the entire follicular phase, rising estrogen levels in the blood stimulates growth of the endometrium and myometrium of the uterus. It also causes endometrial cells to produce receptors for progesterone, which helps prime the endometrium to respond to rising levels of progesterone during the late proliferative phase and throughout the luteal phase.
What is hCG?
hCG is made by the developing embryo after conception and later by the syncytiotrophoblast (part of the placenta). Human chorionic gonadotropin interacts with the LHCG receptor and promotes the maintenance of the corpus luteum during the beginning of pregnancy, causing it to secrete the hormone progesterone.
What is hPL?
It modifies the metabolic state of the mother during pregnancy to facilitate the energy supply of the fetus. hPL has anti-insulin properties. hPL decreases maternal insulin sensitivity, and, therefore, raises maternal blood glucose levels, whilst decreasing maternal glucose utilization, which helps ensure adequate fetal nutrition (the mother responds by pancreatic endocrine upregulation; this is thought to represent an evolutionary 'arms-race' resulting from the conflict of interest between mother and fetus). hPL is a hormone secreted by the syncytiotrophoblast during pregnancy.
This hormone is lactogenic and growth-promoting properties. It promotes mammary gland growth in preparation for lactation in the mother.
What is the role of estrogen preimplantation?
Two or three days before LH levels begin to increase, usually by day seven of the cycle, one (or occasionally two) of the recruited follicles has emerged as dominant. Many endocrinologists believe that the estrogen secretion of the dominant follicle has increased to a level that GnRH production is suppressed, which lowers the levels of LH and FSH. This slowdown in LH and FSH production leads to the atresia (death) of most of the recruited follicles, though the dominant follicle continues to mature.
Estrogen levels will continue to increase for several days (on average, six days, but this varies widely).
These high estrogen levels initiate the formation of a new layer of endometrium in the uterus, histologically identified as the proliferative endometrium.
Crypts in the cervix are also stimulated to produce fertile cervical mucus. This mucus reduces the acidity of the vagina, creating a more hospitable environment for sperm. It also has a characteristic texture that helps guide sperm through the cervix and to the fallopian tubes, where they wait for ovulation. In addition, basal body temperature may lower slightly under the influence of high estrogen levels.
Estrogen levels are highest right before the LH surge begins.
What is the role of progesterone preimplantation?
After ovulation, the egg only lives for 24 hours or less without fertilization while the remains of the dominant follicle in the ovary become a corpus luteum; this body has a primary function of producing large amounts of progesterone. Under the influence of the steroid hormone progesterone, the endometrium (uterine lining) changes to prepare for potential implantation of an embryo to establish a pregnancy. Specifically, it is responsible for the decidualization of the endometrium (its development) and maintenance, respectively.If pregnancy occurs, human chorionic gonadotropin is released maintaining the corpus leuteum allowing it to maintain levels of progesterone. However, after the 8th week, production of progesterone shifts to the placenta.
What is the role of prolactin?
PRL, produced by the anterior pituitary, stimulates the mammary glands to produce milk (lactation): Increased serum concentrations of prolactin during pregnancy cause enlargement of the mammary glands of the breasts and prepare for the production of milk. However, the high levels of progesterone during pregnancy suppress the production of milk. Milk production normally starts when the levels of progesterone fall by the end of pregnancy and a suckling stimulus is present.
Prolactin also has a number of other effects including contributing to surfactant synthesis of the fetal lungs at the end of the pregnancy and immune tolerance of the fetus by the maternal organism during pregnancy.
What is the role of oxytocin?
Oxytocin, produced by the posterior pituitary, acts at the mammary glands, causing milk to be 'let down' into subareolar sinuses, from where it can be excreted via the nipple. It is important for cervical dilation before birth and causes contractions during the second and third stages of labor.
Oxytocin release during breastfeeding causes mild but often painful contractions during the first few weeks of lactation. This also serves to assist the uterus in clotting the placental attachment point postpartum.
What is the role of relaxin?
In the female, it is produced by the corpus luteum of the ovary, the breast and, during pregnancy, also by the placenta, chorion, and decidua. Relaxin is produced mainly by the corpus luteum, in both pregnant and nonpregnant females; it rises to a peak within approximately 14 days of ovulation, and then declines in the absence of pregnancy, resulting in menstruation. During the first trimester of pregnancy, levels rise and additional relaxin is produced by the decidua.
Relaxin's role or necessity in human pregnancy remains under investigation, as in humans its peak is reached during the 14 weeks of the first trimester and at delivery. It is believed to soften the pubic symphysis. Evidence is presented that suggests that relaxin may have significant roles in sperm motility, fertilisation, implantation, uterine growth and accommodation, the control of myometrial activity to prevent preterm labour, cervical ripening and the facilitation of labour.
What is the role of estrogen postimplantation?
Estrogen has a major role during pregnancy of building tissue. It accomplishes this by either increasing the size or the number of tissues, vessels, and blood cells.
- a major factor in the increased size of the uterus and thickening of the uterine wall (through hyperplasia (increase in the number of cells), hypertrophy (increase in the size of cells) of muscle cells, and stretching of the uterus) --> the uterus grows from 60 grams in weight to 1200 grams (20 times it's nonpregnant weight), with a volume increasing from 10 cc to 2-10 liters of fluid
- there is an increase in the size of the external genitalia; the vaginal walls are thick and easily distensible; the genitalia is also very vascular
- Estrogen is responsible for an increased blood, lymphatics and nerve supply to the uterus, and throughout the body
- Estrogen also contributes to the increase in breast size, especially the alveolar ductile tissue; the numbers and size of ducts and lobes increase, vascularity increases, and the nipples and areolae become larger
Another function of estrogen is change in the character of tissue:
- change in composition of the blood (increase in clotting factors, increase in leukocytes, increase in plasma volume (40-50%) and RBC count (20-30%)
- there is a softening of connective and collagen tissues; estrogen, along with the hormone relaxin, relaxes pelvic joints and ligaments
- estrogen is also responsible for skin changes (striae gravidarum, telangiectases, palmar erythema, hyperpigmentation)
- Estrogen also causes increased acidity (lower pH) in the vagina
- decrease in gastric secretion of hydrochloric acid and pepsin under the influence of estrogen
- increases uterine oxytocin receptors
What is the role of progesterone postimplantation?
Progesterone has a number of physiological effects that are amplified in the presence of estrogen. Estrogen through estrogen receptors upregulates the expression of progesterone receptors.
- maintains myometrial quiescence
- constricts myometrial vessels
- inhibits prolactin secretion
- helps suppress immunological responses to fetal antigens
- relaxes smooth muscle in GI and urinary systems
- increases basal body temp
- increases sodium and chloride excretion
- stimulates appetite and fat storage
- stimulates respiratory centre