Embryonic development occurs after Fertilisation (Previous Post), but what are the different processes involved towards developing an embryo ? In this IBDP Biology Blog post we will learn
- The role of hCG in pregancy
- Embryonic development
- Maintenence of the placenta
- Hormonal concentrations during birth
Role of hCG in early pregnancy
- The endometrium is a
- blood-rich environment
- in which an implanted zygote can grow and it is
- sustained
- by the hormone
- progesterone
- If progesterone levels aren't maintained (i.e. the
- corpus luteum degenerates
), then the endometrium will be sloughed away (menstruation)
- A fertilised zygote develops into a
blastocyst that secretes human chorionic gonadotrophin (hCG)
- hCG maintains the
corpus luteum post-ovulation so that the blastocyst can remain embedded in the endometrium and continue to develop
- Gradually the
placenta develops and produces progesterone (at around 8 - 10 weeks), at which point the corpus luteum is no longer needed
Early embryo development
- After fertilisation, the zygote undergoes several
- mitotic divisions to create a solid ball of cells called a morula (at around 4 days)
- Unequal divisions beyond this stage cause a
- fluid-filled cavity
- to form in the middle - this makes a blastocyst (at around 5 days)
- The blastocyst consists of:
- An inner mass of cells (this will develop into the embryo)
- An outer layer called the
- trophoblast
(this will develop into the placenta)
- A fluid filled cavity (called the
blastocoele)
- These developments all occur as the developing embryo is moving from the
oviduct to the uterus
- When the blastocyst reaches the uterus, it will embed in the
endometrium (implantation)
How the the placenta maintains pregnancy
Structure and Function:
- The placenta is a disc-shaped structure that
- nourishes
- the developing embryo. It is formed from the development of the
- trophoblast
- upon implantation and invades the uterine wall.
- The umbilical cord connects the
- fetus to the placenta
- and maternal blood pools via open ended arterioles into
- intervillous spaces
- (lacunae).
- Chorionic villi extend into these spaces and facilitate the
- exchange of materials
between the maternal blood and fetal capillaries.
- Nutrients, oxygen and antibodies
will be taken up by the fetus, while carbon dioxide and waste products will be removed. The placenta is expelled from the uterus after childbirth.Hormonal Role: The placenta also takes over the hormonal role of the ovary.
- Estrogen stimulates growth of the muscles of the uterus (myometrium) and the development of the mammary glands.
- Progesterone maintains the endometrium, as well as reduces uterine contractions and maternal immune response (no antibodies against fetus). Both estrogen and progesterone levels drop near time of birth.
The process of birth and Hormonal concentrations
- The process of childbirth is called parturition and is controlled by
- oxytocin
- After nine months, the fetus is fully grown and takes up all available space in the uterus, stretching the walls of the uterus
- This sends a signal to the brain, releasing oxytocin from the
- posterior pituitary
- Oxytocin
- inhibits progesterone, which was inhibiting uterine contractions
- Oxytocin also directly stimulates the smooth muscle of the uterine wall to
- contract
- The contraction of the uterine wall causes further stretching, which triggers
- more oxytocin to be released (causing even more contraction)
- Additionally, the fetus responds to the cramped conditions by
- releasing prostaglandins which cause further myometrial contractions
- As the stimulus causing oxytocin release is increased by the effects of oxytocin, this creates a
positive feedback pathway
- Contractions will stop when labour is complete and the uterus is no longer stretched
And we're done with this sub topic! We should know:
- hCG maintains the corpus luteum
- Villi helps with nurtient uptake near placenta
- Oxytocin controls the process of childbirth
References:
- https://ib.bioninja.com.au/higher-level/topic-11-animal-physiology/114-sexual-reproduction/embryo-development.html
Drafted by Venetia (Biology)