Reproductive Physiology - Lecture 3

Fertilization and Pregnancy

what is fertilization?

the fusion of male and female gametes to form a zygote

what is fertilized egg called for the first two months

embryo

what is the fertilized egg called for the next 7 months

fetus

how long does the gestational period

40 weeks

what is the site of fertilization

female oviduct

how long is sperm viable for

5 days

how long is the oocyte viable for

12-24 hours

is the sperm initially capable of fertilization

no, it requires it capacitation

what is the survival time of sperm in the reproductive track

5 days

what are the two reaons only a few hundred sperm make it to the uterine tubes

1. damage due to acidic pH in the female tract

2. some sperm are lost due to leakage from cervix

what is capacitation

Capacitation is a process that sperm undergo in the female reproductive tract, which enhances their motility and ability to fertilize an egg. This prepares the sperm to successfully penetrate the zona pellucida of the oocyte.

what is the acrosome reaction

The acrosome reaction is a vital process during fertilization where the sperm's acrosome releases enzymes that digest the zona pellucida, facilitating the sperm's entry into the oocyte.

• enzymes poke hole into the zona pellucida so the nuclear male parts can get to female nuclear parts

why is polyspermy not favoured in fertilization

Polyspermy is not favored in fertilization because it can lead to an abnormal number of chromosomes, resulting in failed embryo development.

how is polyspermy prevented during fertilization

polyspermy is prevented by block

describe the polyspermy block mechanism

when sperm make contact with the egg membrane, a change in membrane potential occurs

• corticle granulles surrounding the egg release its enzymes to harden the zona pellucida

• since hardended, the proteins in the zona pellucida are inactivated, so sperm cannot bind to the zona pellucida

describe the pathway of fertilization

Fertilization begins with sperm traveling through the female reproductive tract, meeting the egg in the fallopian tube. The sperm then penetrates the zona pellucida, triggering the cortical granule release and fusion with the egg, culminating in the formation of a zygote.

what stimulates the 2nd meiotic division of the oocyte

fusion of sperm with the oocyte, causes it to become the ovum

what occurs after the second meiotic division

the sperm plasma membrane disintegrates, chromome somes from sperm and ovum migrate to center resulting in DNA replication leading to a zygote

what do the mitotic division after fertilized ovum result in?

the formation of the morula

• same size as zygote

• can now reach the uterus and begin implantation

the morula is totipotnet at the 16-32 cell stage. what does totipotent mean?

Totipotent refers to the ability of a cell to differentiate into any cell type, including all embryonic and extraembryonic cell types.

• each cell has the capacity to grow into the individual

division of the totipotent morula cells results in _____

identical twins

• ‘egg splits into two’

fertilization of two oocytes released during the same cycle result in ….

fraternal twins

what forms after the morula phase?

blastocyst stage

what is the outer layer of the blastocyst

the trophoblast

• which will become the fetal placenta

what is the inner layer of the blastocyst

the inner cell mass that floats within the blasocoele cavity

• will become the embryo

does a blastocyst have a zona pellucida?

no, it is replaced by a trophoblast

are cells still totipotent in the blastocyst stage?

no, they become pluripotent

• each cell will differentiate into specific cell types to make the organs

what hormone is high in the uterus during the blastocyst stage

progesterone is high to allow for implantation

when does implantation occur

6-7 days after fertilization

describe the process of implantation

trophoblast layer has enzymes that can dig itself into endometrial layer by eating through endometrial layer

• syncytiotrophoblast are the fused trophoblasts that attach to endometrium

• cytotropblasts are differentiated trophoblasts

amniotic cavity forms during implantation as blastocyst burrows

what is the decidual response

The decidual response refers to the changes in the endometrium (uterine lining) that prepare it for and support pregnancy, specifically in response to implantation of an embryo.

after how many weeks is the placenta completely functioning and the embryo has a heart beat

5 weeks

what is the function of the amniotic cavity

a cavity made of fluid that cushions the fetus and avoid bugs and germs from mother’s body

where does all the food from the mother enter the embryo, and where does the embryo’s waste leave?

from the umbilical cord

• all waste is then excreted via mother’s kidneys or GI system

what are the three functions of the placenta

1. endocrine organ

   • secrete hCG and three other hormones

2. exchange tissue

   • respiratory gases, nutrients and waste products

3. filter/immunological protection

what is the function of chorionic villus

area of exhange with the placental capillaries to exhance nutrients with the baby

do maternal and baby blood ever come in direct contact?

no never. chorion creates a seperation

• oxygen and gas can diffuse, but not blood itself

what are the four major hormones of the placenta

1. human chorionic gonadotropin (glycoprotein)

2. human placental lactogen (protein)

3. Progesterone

4. Estrogen

function of hCG

maintains corpus luteum in early pregnancy

function of hPL

Growth hormone and anti-insulin action in mother

• prevents mother from taking up sugars so they can be transfered to the embryo at placenta

4 functions of placental progesterone

decreases uterine contraction (no labor)

inhibition of LH and FSH

growth of mammary glands

secrete sperm unfriently mucus

3 functions of placental estrogen

1. growth of myometrium

2. growth of mammary ducts

3. inhibition of LH and FSH

when the trophoblast layer attaches to uterine wall, what hormone is secreted

hCG

why does hCG drop off after 2.5 months

hCG is essential to maintain the corpus luteum for estrogen and progesterone secretion for first trimester, however second trimester and onward, the placenta increases estrogen and progesterone levels

why does hPL increase over term of pregnancy

to increase glucose available for fetus

theoretically, how could you abort an embryo in the first trimester

damage/destroy the corpus luteum

Effects of estrogen and progesterone on uterus, anterior pituitary, and breasts

Describe the production of estrogen and progesterone by the placenta

the mother transfers cholesterol to the placenta, where it is coverted into the progesterone

• the progesterone can either be given back to the mother, or:

• progesterone is delivered to the fetus and converted into androgens which are then given back to the placenta

the placenta then takes the androgens and converts it to estrogen with P450 aromatase and gives it to mother

describe the positive feedback loop of parturition

oxytocin is secreted from the posterior pituitary gland which strengthends uterine contractions

• pressure of the fetus against the cervix causes a positive feedback loop on to the posterior pituitary to release more oxytocin. This increased oxytocin amplifies contractions, facilitating labor and delivery.

what 4 hormones increase myometrial contractions

1. estrogen

2. oxytocin

3. prostaglandins

4. stretch

what two hormones inhibit myometrial contractions

1. progesterone

2. relaxin

what two hormones increase cervical ripening

1. prostaglandins

2. relaxin

what hormone inhibits cervical ripening

progesterone

what are the two sources of relaxin in a human

1. corpus luteum

2. placenta

What other organ does relaxin act on, and what does it do?

Relaxin will also work on the kidneys to reduce eGFR increase renal vasodilation to reduce workload of mother as she is filtering blood for both herself and baby

4 steps of partuition

1. end of pregnancy

   • cervix ripens

2. beginning of partuition

   • cervix dilates and labor is induced

3. birth

   • baby’s head wedges cervix open and his born head first

4. afterbirth

   • expulsion of placenta

what is afterbirth

The expulsion of the placenta following the birth of the baby. This process occurs after delivery and is crucial for maternal recovery.

what do the mammary glands look like from birth to puberty

ruidmentary ducts with very few alveoli

what does estrogen do to the mammary glands at puberty

causes ducts to grow and branch out

what does progesterone do to mammary glands at puberty

causes alveoli to develop and mature

what else occurs to mammary glands in puberty, not controlled by hormone regulation

deposition of fat and alveolar tissue

what does prolactin do to mammary glands during pregnancy

lactogenesis: initiation of milk synthesis

• low E and P

what does oxytocin do to mammary glands during pregnancy

required for milk ejection

what hormone is needed for galactopoesis (maintenance of lactation)

prolactin

Describe how milk is released from the mammary gland

Milk release is stimulated by oxytocin, which contracts the myoepithelial cells surrounding the mammary glands, pushing milk into the ducts and out of the nipple for breastfeeding.

• the milk itself is made by the alveolar cells

• myoepithelial cells contract to push milk out of gland

what type of secretions are milk?

apocrine secretions: partial loss of apical cytoplasm

describe the suckling reflex

tactile receptors of nipples stimulate the hypothalamus to decrease dopamine in the anterior pituitary to secrete prolactin to cause milk secretion

• dopamine = prolactin inhibiting hormone

• thyroid releasing hormone, ANG II may help prolactin secretion (but we don’t know)

hypothalamus also increases neurosecretory cells in posterior pituitary to increase oxytocin to cause milk ejection