04: Fertilization & Embryogenesis

Ways that sperm are lost/screened on the way to the oviduct

• Retrograde transport

• Phagocytosis

• Non-privileged pathway through the cervix

T/F: sperm is deposited in the vagina

Depends on the specie

How do some species prevent or decrease the amount of sperm that are lost to retrograde transport

Males ejaculate a vaginal plug

Cells that are responsible for phagocytosing sperm

Neutrophils sequestered in the female reproductive tract

Phases of sperm transport

Rapid phase and sustained phase

Rapid phase of sperm transport

Quick movement of sperm up the repro tract in minutes, facilitated by increased tone and motility of the myometrium under E₂ influence

Sustained phase of sperm transport

Slower movement of sperm up the repro tract in hours to days

Which phase of sperm transport is more important and why

Sustained phase, gives sperm time to undergo capacitation before meeting the oocyte

Cervical barriers to sperm transport

Cervical folds and cervical mucus

How does cervical mucus change during estrus

There is low and high viscosity types of mucus present during estrus

Characteristics and location of low viscosity cervical mucus

Rich in sialomucins, lives in the cervical crypts to form the privileged pathway

Characteristics and location of high viscosity cervical mucus

Rich in sulphomucins, lives at the apex of the cervical folds to catch sperm with abnormal motility

Characteristics of sperm that reach the site of fertilization

Alive, hyperactive motility, normal or very subtle head abnormalities

When do sperm acquire maximal fertility

Only after undergoing capacitation in the female reproductive tract

Capacitation of sperm

Contact with uterine factors that strip off membrane proteins on the sperm

Change in sperm that happens because of capacitation

Sperm gain hyperactive motility and the zona pellucida binding sites are unmasked

T/F: all the sperm in the repro tract are capacitated at the same time

False

Function of LH surge on the oocyte

Induces nuclear maturation and resumption of meiosis just before ovulation

What stage of development are oocytes arrested in during development

Prophase I

What is created once the oocyte is done undergoing meiosis

1 daughter cell with most of the cytoplasm and machinery and 3 polar bodies

When does sperm have the best chance of fertilizing an oocyte

Within 6 hours of ovulation (except in the dog)

When does sperm have the best chance of fertilizing an oocyte in a dog

4 days after ovulation

If we are planning breeding/AI, when do we want the sperm to be sitting in the repro tract

Before ovulation

Why is it beneficial that a capacitated sperm exhibits frenzied motility

It is though to increase the chance of encountering the egg

How does the sperm find the ovulated oocyte

It is thought that the oocyte has a chemoattractant

Zona pellucida

Meshwork of glycoproteins that form a shell outside the oocyte plasma membrane

Functions of the zona pellucida

• Site for binding and triggering of the acrosome reaction in sperm

• Barrier to polyspermy

Zona binding sites on the sperm and function

• Primary zona binding region: adherence

• Acrosome reaction promoting region: triggers acrosome reaction

Which zona pellucida glycoprotein on the oocyte does the sperm bind to initially

ZP3

Function of the acrosome reaction

Allows the sperm to digest through the zona pellucida

Zona pellucida glycoprotein that stabilizes binding as sperm break through the ZP

ZP2

Once the sperm make it through the ZP, where are they

Perivitelline space

What happens if a sperm undergoes the acrosome reaction early or late

Can’t penetrate the ZP

Reaction happens once the sperm makes it into the oocyte

Oocyte releases cortical granules → induces zona block and vitelline block to further prevent polyspermy

Change in the oocyte that happens when the sperm enters the oocyte

Completes metaphase II → emits second polar body

Syngamy

Fusion of male and female pronuclei

When does the first cell division happen after fertilization

24 hours

How long are embryonic cells (blastomeres) totipotent

Until there are 8-16 blastomeres, depends on specie

Stages of embryonic development that can be supported in vitro

Zygote to early blastocyst stage (this is why we can do embryo flushes!)

Stage of embryonic development that requires the uterine environment

Anything after the early blastocyst

At what stage does the embryo enter the uterus

Morula, in most species

Cellular change that results in a distinct inner cell mass and blastocele → blastocyst

Outer cells pump sodium towards the inner cells, pulling fluid with it

Fate of the inner cell mass

Becomes the embryo proper

Fate of trophoblasts (outer embryonic cells)

Becomes the chorion

Shape of ruminant hatched blastocyst

Long, threadlike structure

Shape of mare hatched blastocyst

Remains spherical

What is responsible for the RAPID growth immediately after the blastocyst hatches

Largely the development of extraembryonic membranes

Embryonic tissue that becomes the yolk sac

Primitive endoderm

Embryonic tissues that make up the amnion and chorion

Mesoderm and trophectoderm

Embryonic tissues that make up the allantois

Mesoderm and extension of the embryonic gut