Pregnancy & Birth : IB Biology HL

Fertilization

Union of egg and sperm to form a diploid zygote

Where does fertilization occur?

The oviduct

Prior to fertilization, the egg that is released from the ruptured follicle is referred to as

The secondary oocyte, as it has not yet completed meiosis II

Once fertilization occurs, the secondary oocyte…

Completes meiosis II

What is the cytoplasm of the gamete composed of

There are many cortical granules filled with enzymes

The Zona Pellucida is:

A non-cellular jelly-like layer composed of glycoproteins that prevent the entry of sperm

Outer to the plasma membrane of the gamete, there are two layers:

• Zona Pellucida

• Corona Radiata

Corona Radiata

Located around the Zona Pellucida, the Corona Radiata consists of follicular cells which provide nourishment to the egg

Capacitation is when

Sperm undergo physiological changes in the female reproductive system that lead to their activation

Binding of sperm activates the oocyte in a step called egg activation, which includes:

• Rise of intracellular calcium levels

• Completion of meiosis II

• Cortical reaction

During fertilization:

Complementary receptors on the head of the sperm bind to the proteins on the oocyte plasma membrane

Acrosome reaction

Enzymes from the acrosome digest the Zona Pellucida

Cortical reaction

Enzymes released from the cortical granules digest the receptor proteins and harden the Zona Pellucida

When the fertilizing sperm binds to the oocyte…

The head enters the cytoplasm of the egg while the rest is destroyed

The sperm nucleus undergoes a series of changes to become …

The male pronucleus

The male and female pronucleus both have a:

Haploid genome

The diploid zygote undergoes mitotic division when

• The nuclear membranes of the male and female pronuclei dissolve

• Chromatin from both nuclei condense to form chromosomes, resulting in a single-celled diploid organism

In Vitro Fertilization (IVF)

Artificial reproductive technology where egg and sperm are fused in a laboratory environment before the embryos are transferred to the uterus to increase chances of pregnancy

Steps of IVF are:

1. Normal mensuration is suppressed using medication. This lasts about 2 weeks

2. Fertility drugs containing FSH are taken, stimulating super ovulation (production of multiple eggs)

3. Person is injected with human chorionic gonadotropin (HCG), which causes follicles to mature. Before follicles rupture, eggs are collected through follicular aspiration.

4. Eggs are placed into a Petri dish with sperm from the donor. After 16-18 hours, the sample is assessed to see if fertilization occurred.

5. If fertilization has occurred, fertilized eggs are allowed to grow in a specially formulated culture medium in a lab

6. Healthy embryos are selected and transferred to the uterus (day 3-5) This is called implantation. Usually multiple embryos are transferred to increase the chances of implantation

7. A pregnancy test 2 weeks after implantation determines if the process was successful or not

Polyspermy

Fusion of more than 1 egg with the sperm, resulting in an non-viable zygote

Soon after fertilization, the single-celled diploid zygote begins dividing rapidly in a process called:

Cleavage

The 16 cell stage of the zygote is the …

Morula, which is still surrounded by the Zona Pellucida

Cleavage (rapid cell division) prevents..

Growth

Around day 4, the dividing cells rearrange themselves into 2 layers:

• Outer trophoblast

• Inner cell mass

• Fluid filled cavity develops in the center called the blastocoel

Around day 4, the embryo is called:

A blastocyst (hollow ball, 32 cells)

In the blastocyst, the structures will differentiate into:

Outer trophoblast: Structures that help in attachment or implantation of embryo to the endometrium of the uterus

Inner cell mass: Structures of the embryo

To have a successful pregnancy, the endometrial lining must be maintained, this is done through:

Continued production of estrogen and progesterone in early pregnancy days

Soon after implantation, cells of the trophoblast (which will eventually form the placenta) secrete..

HCG, this hormone maintains the corpus luteum which secretes progesterone

By the third month of pregnancy

The placenta takes over the role of the corpus luteum and produces estrogen and progesterone to maintain the pregnancy

Pregnancy tests are

Designed to detect the presence of HCG in a woman’s urine

Pregnancy tests work by

• The test is coated in monoclonal antibodies specific to HCG

• When the individual pees on the stick, HCG, if present in the urine, binds to the monoclonal antibodies on the stick

• This results in a color change indicating pregnancy

Pregnancy tests are often done

A few days after implantation since the embryo secretes enough HCG to give a positive test

Once the blastocyst is implanted…

The extraembryonic membrane starts forming

One of the extraembryonic membranes forms the

Umbilical cord

The umbilical cord

Connects the embryo to the wall of the uterus

Another extraembryonic membrane forms

The chorion

By day 14..

The chorionic villi start forming

Placenta

Spongy tissue composed of both maternal and embryonic tissue that begins to form the 4th week of pregnancy, all exchanges between mother and embryo take place through here

Maternal blood is rich in oxygen and nutrients.. these diffuse from maternal blood that

Collects in the intervillous spaces to fetal blood vessels in this chorionic villi of the placenta. The oxygen and nutrients travel down the umbilical cord, entering the fetal circulation.

Waste and CO2 produced by the developing embryo travel from the umbilical blood vessels to the placenta… here they

Diffuse into maternal umbilical blood vessels to the placenta, then they diffuse into maternal blood in the intervillous space of the placenta. The waste is then removed by mom

The placenta acts as a…

Endocrine gland, secreting progesterone and estrogen to maintain the pregnancy

The term embryo is used up until

11 weeks

After 11 weeks, it is called a

Fetus

A typical pregnancy lasts:

40 weeks

By the 7th month of pregnancy, in anticipation of birth

Progesterone levels plateau and then begin declining

The growth of the baby causes..

The walls of the uterus to stretch, which is detected by receptors present on the uterine wall

The stretching of the baby..

Exerts a physical stress on the mom and the baby, secreting stress hormones like cortisol.

The rise in the level of stress hormones stimulates the release of…

Oestriol, which inhibits progesterone production by the placenta

Oxytocin is released by..

The posterior pituitary