The Pre-Embryonic Period: Cleavage to Blastocyst

What is the stage after fertilization?

Cleavage

What is cleavage?

Cell division without growth

What is a zygote?

The first single cell; it’s one complete nucleus formed from the fusion of male and female cells; it’s a complete set of chromosomes

What is a pronuclei?

A single nucleus; two of them move together to form a complete nucleus

How many chromosomes does each pronuclei have?

23 chromosomes

What is meiosis II?

DNA of egg cell is split into two groups of 23 DNA molecules (one group disintegrates, the other condenses into 23 chromosomes and nucleus forms around this)

What is formed when the female pronucleus fuses with the male pronuclues?

Zygote

What are microtubles?

The long part of a cell’s cytoskeleton that act as “little train tracks” that guide the two pronuclei together

How many chromosomes does the zygote have?

46 chromosomes

After the zygote is formed, the nuclear membranes dissolve what helps the zygote divide?

Cage-like spindles direct the subsequent chromosome movements into two new cells

What are the spindles made from?

Microtubles

What is cleavage?

The beginning of the creation of a multicellular human body by partitioning/dividing the zygote into a ball of many smaller cells

What are the physical properties of the zygote during cleavage?

They increase in quantity but do not increase in size

Do all cells contain the same region of the original egg cell’s cytoplasm?

No, each new cell has different regions of the original cell’s cytoplasm — they each have a different fate

What are cytoplasmic determinants?

Aka maternal determinants that take on different fates

What occurs during day 1 of development?

Fertilization occurs and the single-cell zygote forms

After mitotic cell division, what does the zygote become?

Two-celled embryo

What occurs during day 2 of development?

Cleavage (mitosis) occurs and the two-celled pre-embryo forms. Cleavage continues rapidly and the pre-embryo becomes multicellular

What is epigenetics?

The study of changes in gene activity during development, caused by environmental factors

What are monozygotic twins?

Identical twins that develop from one zygote; the zygote splits into two groups

What are dizygotic twins?

Fraternal twins that develop when two eggs are released during ovulation and fertilized by two separate sperms

Which set of twins are always genetically identical (including same gender)?

Monozygotic twins

What is a conjoined twin?

When the dividing mass of cell doesn’t complete its split

What are parasitic twins?

When one conjoined twin stops developing (there is a whole human that has extra body parts)

What occurs around day 4 of development?

The pre-embryo reaches the uterus and the morula is formed

What is the morula?

A solid ball with 12+ cells that are tightly bound within the zona pellucida; it’s the same size as a zygote

What occurs around day 5 of development?

The morula enters the uterus; spaces appear between cells at the centre of the ball and fluid accumulates; the zona pellucida breaks apart

What occurs around day 6 of development?

The morula is now the blastocyst

What is a blastocyst?

A hollow ball of cells that contains:

• inner cell mass

• trophoblast

When does the blastocyst find a set position in the uterus (when does it stop floating freely)?

Implantation

What is the outer trophoblast layer?

The layer that gives rise to the extraembryonic membrane that will form the embryo’s placenta; the part that will become the placenta

What is the inner cell mass?

The part of the blastocyst that becomes the embryo and some of the extraembryonic membranes that will extend from or surround the embryo; aka you!

What is the blastocoel?

The fluid-filled cavity

Around what day does the zona pellucida disappear?

Around day 4