3. Kinsesology 2Y03 Embrology and Development - Module 3

Hapliod cells

Gamates

• Half sex cells come from males

• Half sex cells come from females

Gonads

• Organs that produce gamates

  • Female: Ovaries

    • In each menstrual cycle, about 14 days after ovulation begins

Process of producting Secondary oocyte

1. One ovary matures a female gamete cell + preps to release (Once released called seconary oocyte)

• The time of release is known as ovulation

  • At this point, the oocyte has not finished meiosis (Secondary)

    • Only finished the first meiotic division

2. Oocyte enters uterine tube/Fallopian tube

• Via Finger like projectors

3. Passes funnel area

4. Enters widened region known as AMPULLA

• Ampulla is where fertilization will occur (SECONDARY OOCYTE DOES NOT MOVE FROM HERE)

Uterus

• Pear shape

• Muscular structure

• Implantation of embryo happens here

• Development + Growth of fetus occurs here

• it connects to Vagina Through opening known as the cervix

  • Vagina: opening to the outside of the body + entry point for male gamete cell (Sperm)

2 uterine tubes connect to the UTERUS

Sperm Killing

• During intercourse, 20 million sperm are deposited into vagine

  • ONLY 1% make it to the uterus

  • Only 100-200 reach the oocyte in the ampulla

WHY????

• Some sperm don’t have good motility (can’t swim well)

• pH in secretions of the female reproductive system kills off many sperm

Overall Goal of Process

• Take male genetic material and combine with female genetic material

Sperm path to Oocyte

1. Initially when sperm enter the vagina:

• They are self-propelled by flagella (tail)

2. Once inside uterus and uterine tube

• Muscular contractions of the walls of structure help push sperm along

Underline: Some are caused by hormones released during intercourse (e.g., oxytocin)

• The system also has prostaglandins from liquid secretions that support sperm (semen)

  • Also stimulate small contractions

Sex cell life span

• Secondary oocyte only stay fertilizable for aproxx 24 hours

• Sperm can live up to 6 days

  • Most die within 24 hours

Window for fertilization

• 6 days before ovulation

• 1 day after

Germinal Period

• first two weeks of development

  • Formation of primitive germ layers (cells multiply)

    • Become body tissue

Embryonic Period

• Week 3 to end of 8th week organs systems develop

Fetal period

9th week until birth; organ systems grow + mature (aproxx 38 weeks - embryonical age)

Clinical/ Medical Events (date set up)

Date since last menstrual period (clinical age of unborn)

Embryologists (date set up)

Post ovulatory age (aproxx 14 days less than clinical age)

oocyte Info

• chromosomes held within the nucleus

• females sex cells don’t divide equally during meiosis

  • big cell (oocyte)

  • small cell (polar body

    • no fucntion in fertilization excepting removing extar genetic material from steps of meiosis

    • Degrade overtime

Oocyte Layers

Around its plasma membrane it has

1. Clear glyoprotein layer

• zona pellucida

2. loosely packed cells

• corona radiata

  • AKA crown of oocyte

    • made of follicular cells

• Maturation of secondary oocyte + release happens in a follicle

  • follicle: Cells surrounding oocyte

    • Supporting them

    • Some of these loose cells stay stuck onto zona pellucida (which is the corona radiata)

Fertilization

1. Once sperm reaches secondary oocyte

• Penetrates corona radiata

• Zona pellucia has a ZP3 glycoprotein (sperm receptor)

  • Sperm must bind to get through

  • ZP3 is species specific

    • NO MAN-HORSE

2. Once sperm acrosome (at head of sperm tip is called acrosome → contains digestive enzymes)

• Acrosomal Reaction starts (activation of digestive enzymes)

3. Zona pellucida starts to break down

• Many sperm must bind to break zona pellucida

  • to allow only one to go through

4. Once sperm is into the oocyte, it binds with the plasma membrane of the oocyte

• Contains receptor “Integrin α6β1”

• As soon as one sperm binds to it, it causes fast depolarization

  • electrical charge in the plasma membrane of the oocyte

  • shocks all other sperm trying to reach the plasma membrane

  • blocks other sperm (fast block to polyspermy)

5. Head of sperm is into Oocyte!!!

6. Female nucleus does 2nd meiotic division

• Ovum Formed

• 2nd Polar body

• Once inside, sperm tail disintegrates (only head left)

• Nucleus undergoes 2nd meiotic division (on one side of cell)

  • Now cells contain:

    • Female pronucleus

    • male pronucleus (sperm head)

7. Fusion of two hybrid pronuclei produces a single nucleus (zygote) - diploid

fast block to polyspermy

• Once sperm is in the oocyte, it binds to the plasma membrane of the oocyte

• Contains the receptor “Integrin α6β1”

• As soon as one sperm binds to it, it causes fast depolarization

  • electrical charge in the plasma membrane of the oocyte

  • shocks all other sperm trying to reach the plasma membrane

  • blocks other sperm

Slow Block to Polyspermy

• Once sperm enters the oocyte

  • Release of Ca2+ Intracellularly

    • Ca2+ moves out of the cell, taking water with it

      • Fluid and ions make their way between the zona pellucida and plasma membrane

        • Causes the oocyte to shrink

        • Zona pellucida denatures along with ZP3 → inactive receptor

        • Space now between zona pellucida and membrane

Formation of Blastocyst

1. Zygote undergo clevage

Cleavage: Series of cellular divisions

• Very first division

  • takes the longest

  • Usually happens in the first 24 hours

    • Can take 6-36 hours to occur

2. Divisions become faster

• 2 cells in 24 hours

• 4 cells in 48 hours

• Solid ball of cells (12 + cells)

• Up until this point, all cells are totipotent

  • One single cell can divide + differentiate into every cell type in body (up-until Morula)

3. Morula Formed (day 5)

• Size of morula is the same size as the original cell

  • Cells get smaller with each cleavage

• Denatured zona pellucida still exists around the cells

• At this point, all cells are now pluripotent

  • Started to Differentiate

    • Cant form all cells but still can differentiate into many

4. Hatching

• Morula breaks free of the zona pellucida

• Creates fluid which pushes cells to the side to create a fluid-filled cavity

5. Blastocyst Born (day 6)

• Outer layer (single layer of cells (trophoblast)

• Blastocele

• Inner cell mass → Becomes Embryo

  • Cells that become embryo proper

    • Everything else → some extra embryonic membrane

While Blastocyst is forming….

• As the disviosn occur

  • cells move closer to uterine cavity

• By the time it is a blastocyst, it is in the uterine cavity

  • undergoes implantation into the wall of uterine cavity

Implantation (8-12 days)

Until now:

• Cells survided on nutrients that were part of originals cells cytosol

  • + secretions by walls of uterus

    • Uterine milk

      • glycogen rich

• As cells get bigger, diffusion of nutrients won’t work

  • needs connection to mother’s circulatory system

IMPLANTATION and formation of the placenta (8-12 days)

1. Blastocyst reaches Uterine Endometrium

• When the blastocyst reaches this layer, it is very thick and vascular

  • Large blood supply

• Blastocyst orients region of inner cell mass towards wall of endometrium

  • Starts this on DAY 7

    • Made of both the embryo proper + trophoblast

      • Trophoblast differentiate into two cell types

trophoblast Differentiations:

1. Syncytiotrophoblasts (away from embryo proper)

• Invade into uterine wall

• Digest Blood vessels in that area

• Aim for the maternal Ateriole so they can break down walls

  • Get blood connection

• These are multinucleated

  • Lose plasma membrane

    • one mass with many nuclei

• They release hormones

  • Human chorionic gonadotropin into the blood supply

    • Circulates in blood

      • Once in sufficient concentration can be detected in the urine of women

      • Used for pregnancy test

      • Helps maintain the thickened vascular wall of endometrium

      • Stops the menstrual cycle

2. Cytotrophoblast (close to Embryo proper)

• Layer of separation for embryo proper

Uterine endometrium

Wall that is closest to the cavity of the uterus

• Changes in thickness with each monthly cycle (menstrual)

Implantation and formation of the placenta (14-20 days)

1. Syncytiotrophoblast invades further into the wall

2. Changes in Embryo proper

• Connection made

  • Connection stalk develops

  • Which later becomes umbillical cord

  
  

3. Blastocyst made its way into wall

Cytotrophoblast:

• Continues to expand and form finger-like structures

Syncytiotrophoblast:

• further invade

• As they digest more arterioles, they form pools of blood

  • called lacuna

    • will become suplly of nutrients + oxygen for embryo

Implantation and formation of the placenta (14-20 days)

• More lacune

• Cytotrophoblast cord forms

  • surrounds the syncytiotrophoblast + lacunae on the maternal side

    • creates a protective layer for

      • maternal blood

      • new embryonic blood

        • they dont like to mix

• embryo makes its own blood + blood vessels

Mesoderm cells:

• Extar embryonic mesoderm cells

• not part of embryo

• builiding own blood + vessels

Fetus

• Cytotrophoblast chords form chorionic villi

  • Fill with blood vessels from the fetus

• ONE BIG LACUNA surrounding chorionic villi

• Materal aterioles bring fresh blood

  • Venules take away used blood

• Chorionic villi connected via umbelical chord

NOTE:

• Umbilical arteries carry deoxygenated blood away from the fetus (arteries move blood away from the heart)

  • in this case the oxygen comes from the maternal blood supply and goes into the heart of the baby (veins of baby), and then the deoxygenated blood is carried away from the heart by ateries back to refuel.

Chorion

• barrier created by chorionic villi

• made of

  • Syncytiotrophoblast (remaining cells)

  • Basement membrane (thin)

    • Made of protein fibres

• Chorion is the final division between

  • maternal blood supply

  • Embryo blood supply

Cytotrophoblast cells disintegrated by this stage.