Development Anatomy - Exam 2

Fertilization definition

Fusion of sperm and secondary oocyte to form a zygote

Sperm pathway to egg layers

Corona radiata → zona pellucida → oocyte membrane

How sperm penetrates egg

Acrosome reaction releases enzymes that digest zona pellucida

How polyspermy is blocked

Cortical reaction hardens zona pellucida after first sperm enters

What triggers meiosis II completion

Sperm entry into secondary oocyte

Result of meiosis II in oocyte

One ovum and one polar body

When do nuclei fuse in fertilization

After sperm enters the oocyte

What is nuclear fusion in fertilization

Combination of maternal and paternal genetic material

What is formed after nuclear fusion

Diploid zygote

What is cleavage

Rapid cell division without growth

Cell number change during cleavage

2 → 4 → 8 → 16 cells

Why do cells get smaller in cleavage

No G1 or G2 growth phases

What are blastomeres

Cells produced during cleavage

Why is cleavage important

Forms early embryo structure

Difference: cleavage vs somatic division

Cleavage skips growth phases and is rapid

What is a morula

Solid ball of 16–32 cells (day 3)

What is a blastocyst

Fluid-filled embryo with inner and outer cell layers

What is a gastrula

Embryo with three germ layers

Three germ layers

Ectoderm

Where fertilization occurs

Fallopian tube

Where morula forms

Traveling to uterus

Where blastocyst forms

Uterine cavity

Where gastrulation occurs

Uterus/endometrium

What is zona pellucida

Protective layer around early embryo

When is zona pellucida lost

Days 5–6 (hatching stage)

What is hatching

Blastocyst escapes zona pellucida

Why is hatching important

Allows implantation

What is trophoblast

Outer layer of blastocyst

Function of trophoblast

Implantation and placenta formation

Why do cleavage patterns differ in animals

Different yolk amounts

What is incomplete cleavage

Division that doesn’t fully cut through yolk

Why do chicks have incomplete cleavage

High yolk content blocks full division

Function of yolk sac

Early nutrition and early blood cell formation

Does yolk sac remain in pregnancy

No

What are blastomeres

Cleavage cells forming early embryo

What is blastocoel

Fluid-filled cavity in blastocyst

What is inner cell mass

Cells that form embryo proper

What is blastula

Basic hollow ball of cells

Blastula vs blastocyst

Blastocyst has trophoblast + inner cell mass

What is embryonic pole

Side with inner cell mass

What is implantation

Blastocyst embedding into uterus

First step of implantation

Trophoblast contacts endometrium

Stages of implantation

Hatching → apposition → adhesion → invasion

What happens in invasion

Trophoblast penetrates endometrium

What enzymes are used in invasion

Proteolytic enzymes

What do these enzymes do

Break down tissue for embedding

Trophoblast layers

Cytotrophoblast and syncytiotrophoblast

Function of cytotrophoblast

Cell division and growth

Function of syncytiotrophoblast

Invasion into uterus

Function of amnion

Forms fluid-filled protective sac

Function of chorion

Forms placenta

Function of yolk sac

Early nutrition and germ cells

What is closing plug

Seals endometrium after implantation

Purpose of closing plug

Prevents infection and bleeding

What forms bilaminar disc

Inner cell mass

Layers of bilaminar disc

Epiblast and hypoblast

What follows bilaminar disc

Gastrulation

Function of hCG

Maintains pregnancy and corpus luteum

Who produces hCG

Syncytiotrophoblast

What happens to hCG later in pregnancy

It decreases

What is gastrulation

Formation of three germ layers

Ectoderm forms

Skin and nervous system

Mesoderm forms

Muscle

Endoderm forms

Digestive and respiratory systems

What is embryonic induction

One group of cells directs another’s development

What are monozygotic twins

Identical twins from one egg

What are dizygotic twins

Fraternal twins from two eggs

Do monozygotic twins share placenta

Sometimes

Do dizygotic twins share placenta

No

Di/Di twins

Separate placenta and sacs

Mono/Di twins

Shared placenta

Mono/Mono twins

Shared placenta and sac

What are conjoined twins

Incomplete separation of embryo

What increases chance of twins

Genetics