Section 1 — Pregnancy Basics → Fertilization → Pre-Embryonic Stage

What is pregnancy and how is it divided

events from fertilization to birth
Divided into 3 three month intervals called trimesters

Gestation period and how long is it

Time period from last menstrual period (LMP) until birth

around 280 days = 1 full term

How long is conception to childbirth
compare to gestation period (timeline)

38 weeks → 266 days

Last Menstrual period → 14 days (2weeks) → ovulation → conception → 266 days (38 weeks) → childbirth

What is conceptus and what are the 3 stages

Conceptus = the products of conception / fertilization

Pre embryonic stage, embryo, fetus

Pre embryonic stage

From fertilization → first 16 days

Embryo

conceptus from day 16 - week 8

Fetus

conceptus from week 9 through birth

What is the pathway of conceptus

Fertilization → pre embryo (till day 16) → embryo (till week 8) → fetus (9 - birth)

How long is oocyte viable for
How long is sperm viable for

after ovulation → sent to uterine tubes → if not penetrated by sperm within 12-24 hrs → degenerates

sperm is viable for 24-48 hours after ejaculation

For fertilization to occur when must coitus / intercourse occur

no more than 2 days before ovulation and 24 hours after ovulation

What is fertilization

Sperm’s chromosome combines with secondary oocyte chromosome → fertilized egg (zygote)

What hormone triggers ovulation

LH → luteinizing hormone

How is sperm transported to oocyte (3)

1. whiplike tail movement of flagella → move

2. endometrial (inner most layer of uterus) cilia + uterine contractions → disperses sperm throughout uterine cavity

What are ejaculated sperm (40-500 million)

(4)

1. leaked out of vagina

2. destroyed by acidic vaginal environment

3. stopped by cervical mucus

4. dispersed in uterine cavity → destroyed by phagocytes

How many reach the uterine tubes and how long is the whole trip

100-few thousand
5 inch trip

What are the two barriers that the sperm has to reach the oocyte

corona radiata → zona pellucida

label the 2 barriers that the sperm has to pass through here

Ground the green is the corona radiata

Green is the zona pellucida

Blue - fluid filled antrum

What must happen to the sperm before it can penetrate the oocyte

The sperm must be capacitated

Capacitation

female tract → secretions → weakens acrosome membrane → makes sperm membrane fragile to → allow release of hydrolytic enzymes

What are two other things that capacitation does

It makes sperm membrane more permeable to calcium
Activates receptors on sperm for chemical attractants (attracts the sperm)

What happens once the sperm reaches the oocyte (part 1 of penetration)

acrosomal process forms → binds to receptor of oocyte membrane → sperm and oocyte membrane fuses together → sperms nucleus is pulled into the oocytes cytoplasm

Monospermy vs polyspermy

monspermy → only one sperm can penetrate the oocyte

Polyspermy → multiple sperm penetrates oocyte

Why is polyspermy not desired

too many sperms → too many chromosomes for zygote

What happens once sperm enters oocyte and what does it activate (2)

Waves of Ca2+ are released into oocyte’s cytoplasm

→ oocyte prepares for 2nd meiotic division (continue from metaphase II)

→ cortical reactionC

cortical reaction

oocyte’s cortical granules release → Zonal inhibiting proteins (ZIPS) are released → blocking other sperms from entering

What is the full pathway of sperm getting to and penetration of oocyte

1. Sperm surfance enzymes help sperm get past granulosa cells of corona radiata

2. Sperm binds to sperm binding site of zona pellucida → increase Ca2+ in sperm → acrosomal reaction → forms acrosomal process + releases enzymes that digest through zona pellucida

3. acrosomal process binds to receptors on oocyte membrane → oocyte and sperm membranes fuse

4. Sperm nucleus enters oocyte cytoplasm → Ca2+ increases in oocyte cytoplasm → cortical reaction (coritcal granules release cortical granules / zona inhibiting proteins → hardens zona pellucida blocking off other sperm

What happens after sperm penetrates secondary oocyte

Secondary oocyte → completes meiosis 2 from when it was last arrested → ovum + second polar body

→ sperm and ovum nuclei swell → male and female pronuclei → pronuclei approach each other and mitotic spindles form between each other → chromosomes intermix → fertilization compeltes → zygote

Pre embyronic stage and what are the three stages

first 16 days of development that results in embryo

• cleavage → implantation → embryogenesis

Cleavage

rapid mitotic division of zygote without increase in size → increases number of cells but not size of entire structure

For what reason does cleavage increase in number of cells but not in size

increases surface area → easier to uptake O2/ nutrients, easier to remove waste

Development sequence during cleavage

Zygote → blastomeres → morula → blastocysts

each division → cells get smaller

Blastomeres
+ how long

2-8 cells → 36 hrs

morula + how long

16 or more cells

72 hrs

blastocyts

+ how long

4-5 days → fluid filled hollow sphere reaches uterus

as the cells divide → get smaller → creating more space

morula → blastocysts : sodium uptake → followed by water

Monozygotic

identical twins

• one egg + 1 sperm → 1 zygote → embryocyst → embryoblast divides into 2 within 2 weeks of fertilization

Dizygotic

fraternal twins

• 2eggs + 2 sperm → 2 zygotes

What are the two parts of blastocysts

trophoblast cells + inner cell mass/ embryoblasts

Trophoblast cells

single outer layer of flat cells thats part of placenta formation

Inner cell mass

“embryoblasts” → becomes part of embryonic disc that eventually becomes embyroI

What happens after blastocyst reaches uterus

after reaching uterus, embryocyst floats for 2-3 days being nourished by uterine secretions

Implantation and when does it happen

6-7 days after ovulation

trophoblasts of blastocysts attaches/adheres to endometrium of uterus → secretes enzymes that break irritate the endometrium (break down some endometrium tissue so blastocyst can burrow deeper)

What happnes if implantation fails

blastocyst is aborted

ectopic pregnancy

instead of implanting in uterus, the implantation occurs in abnormal location such as uterine tube