4- Early pregnancy physiology (conception, implantation and placental development) Flashcards
primary follicles and primary oocytes
Primordial follicles each contain a primary oocyte.
- The oocytes are the germ cells (first generation of sex cell) that eventually undergo meiosis to become the mature ovum, ready for fertilisation.
- They contain the full 46 chromosomes.
- These primordial follicles and oocytes spend the majority of their lives in a resting state inside the ovaries, waiting for their time to develop.
- The primary oocyte is contained within the pregranulosa cells, surrounded by the outer basal lamina layer.
primordial follicles grow and become
primary follicles
layers of the primary follicle
The primary oocyte in the centre
The zona pellucida
The cuboidal shaped granulosa cells
The granulosa cells secrete
- The material that becomes the zona pellucida
- Oestrogen
as primary follicles grow what do they develop on top of the zona pellucida layer
Theca folliculi.
1. The inner layer of the theca folliculi is called the theca interna.
- The theca interna secretes androgen hormones.
2. The outer layer, called the theca externa
- is made up of connective tissue cells containing smooth muscle and collagen.
Development of the Secondary Follicle
The process of primordial follicles maturing into primary and secondary follicles is always occurring, independent of the menstrual cycle.
- As primary follicles become secondary follicles, they grow larger and develop small fluid-filled gaps between the granulosa cells.
- Once the follicles reach the secondary follicle stage, they have receptors for follicle stimulating hormone (FSH).
- Further development after the secondary follicle stage requires stimulation from FSH.
- At the start of the menstrual cycle, FSH stimulates further development of the secondary follicles.
Development of the Antral Follicles and dominant follicle
- With further development, the secondary follicle develops a single large fluid-filled area within the granulosa cells called the antrum.
- Antrum refers to a natural chamber within a structure. This is the antral follicle stage.
- This antrum fills with increasing amounts of fluid, making the follicle expand rapidly.
- The corona radiata is made of granulosa cells, and surrounds the zona pellucida and the oocyte.
- At this point, one of the follicles becomes the dominant follicle. The other follicles start to degrade, while the dominant follicle grows to become a mature follicle.
- This follicle bulges through the wall of the ovary. **
when does ovulation occur
after a surge in LH from the AP
how does LH surge cause ovulation
- it causes the smooth muscle of the theca externa to squeeze, and the follicle to burst.
- Follicular cells also release digestive enzymes that puncture a hole in the wall of the ovary, allowing the ovum to pass escape.
- The oocyte is released into the area surrounding the ovary.
- At this point, it is floating in the peritoneal cavity, but it is quickly swept up by the fimbriae of the fallopian tubes.
where is the oocyte released into
the peritoneal cavity
but it is quickly swept up by the fimbriae of the fallopian tubes.
after ovulation what happens to the rest of the follicle
becomes the corpus luteum
what does the corpus luteum secrete
progesterone
what maintains the corpus luteum
human chorionic gonadotropin (HCG)released from a fertilised blastocyst when pregnancy occurs
secretion of P from the CL prevents
further ovulation during pregnancy
When fertilisation does not occur, the corpus luteum degenerates after
10 to 14 days.
when does meiosis of the primary oocyte occur
just before ovulation
outline meiosis
his process splits the full 46 chromosomes in the oocyte (a diploid cell) into two, leaving only 23 chromosomes (a haploid cell). The other 23 chromosomes float off to the side and become something called a polar body. It is then a secondary oocyte.
structure of secondary oocyte
outline fertilisation
Occurs in the Ampulla
- sperm swims via the vagina and uterus up into the fallopian tubes
- sperm penetrates corona radiata and zona pellucida to fertilise the egg via the acrosome reaction- the sperm produces digestive enzymes to break through layers
- Penetration of one sperm causes plasma membrane fusion-> egg now shut other sperm out (cortical reaction)
- When a sperm enters the egg, the 23 chromosomes of the egg multiply into two sets. One set of 23 chromosomes combine with the 23 chromosomes from the sperm to form a diploid set of 46 chromosomes, and the other set of 23 chromosomes float off to the side and create the second polar body.
fetilised oocyte is called a
a zygote
the combination of the 23 chromosomes from the egg and 23 chromosomes from the sperm
development of a foetus can be split into
- Pre-embryonic period= Weeks 1-2
- Embryonic = Weeks 3 to 9
- Fetal = 9 to 38
pregnancy weeks calculated from
from date of last menstrual period e.g. conception weeks +2= 40 weeks
outline week 1 of the pre-embryonic period
1) Fertilisation
- Zygote formed
2) Cleavage (Day 1)
- First mitotic division
- Forms morula (32 cells)
3) Blastulation (Day 4)
-Blastocystic contains blastocoel- fluid filled part
- Cell differentiation starts
- Cells of the Embryoblast split into 2: (Hypoblast) Yolk sac and the (epiblast) amniotic cavity
- embryonic disc sits between the yolk sac and amnitotic cavity
4) Cells of the embyronic disc develop into the fetal pole-> later becoming foetus
5) Chorion surrounds the complex
- 2 layers of chorion: cytotrophoblast (inner layer) and synctiotrophoblast (outer layer)
- Synctiotrophoblast becomes embedded in endometrium
implantation
- When the blastocyst arrives at the uterus, 8 – 10 days after ovulation, it reaches the endometrium.
- The cells of the trophoblast (the outer layer of the blastocyst) undergo adhesion to the stroma (supportive outer tissue) of the endometrium.
- The outer layer of the trophoblast is called the syncytiotrophoblast.
- This layer forms “finger like” projections into the stroma. The cells of the syncytiotrophoblast mix with the cells of the endometrium (stroma).
- The cells of the stroma convert into a tissue called decidua that is specialised in providing nutrients to the trophoblast.
- When the blastocyst implants on the endometrium, the syncytiotrophoblast starts to produce human chorionic gonadotropin (HCG).
- This HCG is very important for maintaining the corpus luteum in the ovary, allowing it to continue producing progesterone and oestrogen.
Summary: Fertilisation and implantation
basic embryology
Over a short time, a space called the chorionic cavity forms around the yolk sac, embryonic disc and amniotic sac. These structures are suspended from the chorion by the connecting stalk, which will eventually become the umbilical cord.
embryonic disc developes into the
fetal pole -> will become fetus. Three layers:
- ectoderm
- mesoderm
- endoderm
at 6 weeks gestation….
- the fetal heart forms and starts to beat.
- the spinal cord and muscles also begin to develop.
- The embryo (fetal pole) is about 4mm in length.
at 8 weeks gestation
all the major organs have started to develop.
- From this point onwards the fetus matures and grows until birth.
development of the placenta
- the synciotrophoblast (outer layer of the trophoblast) grows into the endometrium and forms finger like projections
- Finger like projections are called chorionic villi
- chorionic villi contain fetal blood vessels
- chorionic villi nearest the connecting stalk are the most vascular containing mesoderm -> area called **chorion frondosum
**- cells of the chorion frondosum become the placenta - the connecting stalk becomes the umbilical cord**
which cells become the placenta
chorion frondosum-> chronionic villi nearest the connecting stalk
what forms the umbilical cord
connecting stalk
development of the lacunae of the placenta
1) Syncytiotrophoblast invasion of the endometrium sends signals to the spiral arteries in that area, reducing their vascular resistance and making them more fragile.
2) The blood flow to these arteries increases, and eventually they break down, leaving pools of blood called lacunae (lakes).
3) Maternal blood flows from the uterine arteries, into these lacunae, and back out through the uterine veins.
4) Lacunae form at around 20 weeks gestation.
5) These lacunae surround the chorionic villi, separated by the placental membrane.
6) Oxygen, carbon dioxide and other substances can diffuse across the placental membrane between the maternal and fetal blood.
Umbilical cord vessel and fetal circulation
Two umbilical arteries
o Deoxygenated blood from fetus to placenta
One umbilical vein
o Oxygenated blood from placenta to fetus
When the process of forming lacunae is inadequate, the woman can develop
pre-eclampsia
key function of the placenta
- respiration
- nutrition
- excretion
- endocrine
- immunity
placenta and respiration
Oxygen source
- fetal haemoglobin has very high affinity for oxygen
- therefore when maternal blood and fetal blood is nearby in the placenta lacunae , oxygen is drawn off the maternal haemoglobin, across the placental membrane and onto the fetal Hb
CO2, H+, HCO3, lactic acid exchange
- allows fetus to maintain healthy acid-base balance
placenta and nutrition
- mostly glucose used for energy and growth
- some vitamins
- potential teratogen transfer
Harmful substances and the placenta
Thalidomide
- Limb defects
Alcohol (small molecule can diffuse across placenta)
- Foetal alcohol syndrome (FAS)
-Alcohol related neurological delay(ARND)
Therapeutic drugs
- Anti-epileptic drugs
- Warfarin
- ACE inhibitors
Drugs of abuse
- Dependency in the fetus and new-born
Maternal smoking
- Intrauterine growth deficiency
placenta and excretion
The placenta performs a similar function to kidneys in a child or adult, filtering waste products from the fetus. These waste products include urea and creatinine.
when does the placenta take over from the CL
at around 10 weeks
- before this the CL is responsible for production of oestrogen and progesterone
HCG is produced by the
syncytiotrophoblast
hCG can cause symptoms of
- nausea and vomiting in early pregnancy.
- Higher levels of hCG occur with multiple pregnancy (e.g. twins) and molar pregnancy.
which hormones does the placenta secrete
oestrogen and progesterone
oestrogen and pregnancy
- softens tissues and make them more flexible- allows the muscles and ligaments of the uterus and pelvis to expand, and the cervix to become soft and ready for birth.
- It also enlarges and prepares the breasts and nipples for breastfeeding.
progesterone and pregnancy
- maintains pregnancy
- causes relation of uterine muscles (preventing contractiona nd labour) and maintains the endometrium
unwanted side effects of increased progesterone
elaxing other muscles, such as the lower oesophageal sphincter (causing heartburn), the bowel (causing constipation) and the blood vessels (causing hypotension, headaches and skin flushing). It also raises the body temperature between 0.5 and 1 degree Celsius.
placenta and immunity
antibody transfer
how can pregnancies be dated
1) Last menstrual period date (Naegeles rule)
2) Crown rump length
Naegele’s rule
the most common method:
- to the first day of the LMP add 1 year,
- subtract 3 months,
- add 7 days).
This can be imprecise, as it requires accurate recall of LMP dates as well as regular menstruation.
the best method for dating pregnancy
crown rump length
when are CRL measurements taken
by ultrasound scan between 10+0 and 13+6.
benefits of crown-rump length
avoid unnecessary inductions for ‘post-dates’ based on LMP recalled later than in reality, and we can monitor labours where the LMP date suggests is over 37+0 but the scan suggests is preterm.
