2 - Fertilization and Implantation

Question 1

How does the embryo control regulation of the corpus luteum?

What type of receptor does it act through?

Answer 1

Secreting Human Chorionic Gonadotropin (hCG) into maternal circulation

Acts through LH Receptor

hCG maintains the corpus luteum intil placenta has developed to eventually take over endocrine function

Question 2

What hormone serves to acidify the vaginal lumen?

What protects sperm against this?

Answer 2

Estradiol

Sperm are protected by high buffering capacity of seminal fluid and alkaline pH of cervical mucus

Question 3

What occurs to semen as it is deposited close to the opening of the cervix?

What occurs following this?

What enzymes assist in these processes?

Answer 3

It coagulates in response to a clotting enzyme produced by the prostate

This restricts exit of the sperm cells

Proteolytic enzymes in the semen liquify the coagulum; Prostate Specific Antigen (PSA) also aids in process

Question 4

What changes occur to the cervix during the estrogen cycle?

What can enhance this passage?

Answer 4

Interstitial spaces expand as mucin production and water accumulation occur in response to the mid-cycle extrogen peak

Sperm can more easiy pass through the cervix

Sperm passage is enhanced by uterine contractions

Question 5

Explain the process of capacitation

What slows this process?

Answer 5

Before sperm can fertilize an egg, they much undergo this process in female reproductive tract; occurs 5-7 hrs after ejaculation

Binding of sperm cells to epithelial cells lining fallopian tubes slows the process, but extends the life of the sperm/prevents too many from reaching the egg

• Capacitation:
  
  • ​Non-covalent surface components removed
  • Uterine Wall/Fallopian tube secretions destabilize the plasma membrane around the acrosome
  • Sperm cell plasma membrane Ca²⁺ permeability increases
  • Spermatozoa undergo hyperactivation (increase movement)

Question 6

What is the site of fertilization (where sperm meet the egg)?

Answer 6

Ampulla

Question 7

Explain the acrosomal reaction of fertilization

What important receptor proteins modulate this reaction?

What triggers the acrosomal reaction?

What ion movement induces released of key enzymes?

Answer 7

Cell surface proteins on the head of the spermatozoa interact with receptor glycoprotein in zona pellucida called ZP3

This triggers the Acrosomal Reaction; rise in intracellular Ca²⁺ induces release of hydrolytic enzymes from acrosome; these digest path through zona pellucida

Question 8

What two reactions block polyspermy?

Answer 8

• Fast Block - The oocyte membrane depolarizes after fusing with the spermatozoa, preventing other spermatozoa from fusing with it
• Cortital Reaction - Wave of Ca²⁺ spreads through oocyte; causes fusion of plasma membrane w/electron dense granules which release substances that induce hardening of zona pellucida

Question 9

The cortical reaction releases a wave of Ca²⁺, what does this trigger in the oocyte?

What fuses to form a diploid zygote?

Answer 9

Triggers completion of the second meiotic division of the oocyte; develops into female pronucleus

Contents of spermatozoa that enters the oocyte develops into male pronucleus

These will fuse to form a diploid zygote

Question 10

How are progesterone levels around the time of fertilization?

What role does this have on the endometrium and implantation?

Answer 10

Progesterone levels are beginning to rise due to secretory activity of newly formed corpus luteum

The endometrium differentiates into a structure that is receptive to implanation and capable of sustaining the developing embryo

Question 11

After fertilization, what is the immediate location of the embryo?

Where does it move to next, and what important structures will it become?

Answer 11

• Days 0-3: Embryo remains within Fallopian Tube (mostly ampulla)
  
  • Undergoes mitotic division; does NOT increase in size, retained w/in zona pellucida
• Days 3 ~ 6: Uterus: Solid ball of cells (morula); will develop into hollow, fluid filled sphere (blastocyst).

Question 12

What is a morula?

What is a blastocyst?

What cells are responsible for implantation/formation of the detal part of placenta?

Answer 12

• Morula: Solid ball of cells following mitotic divisions contained within the zona pellucida
• Blastocyst: Hollow, fluid filled sphere; composed of two distinct cell types
  
  • Inner Cell Mass: Fetus / Outer layer of trophoblastic cells
• Trophoblastic Cells are responsible ofr implanation and the formation of the fetal part of the placenta

Question 13

Before implanation, what must occur to the blastocyst?

How does this occur?

Answer 13

The blastocyst must escape from te zona pellucida

Trophoblastic cells secrete proteases which digest the zona pellucida to facilitate the escape of the embryo

Question 14

What does the trophoblast differentiate to?

Answer 14

• Inner layer of continuosly dividing cytotrophoblasts, which provides cells that fuse into:
• Syncytiotrophoblast - Multinucleated structure which binds to the endometrium

Question 15

What does the syncytiotrophoblast secrete to affect the endometrium?

What occurs as these effects are finalized?

Answer 15

Proteolytic enzymes which digest:

1. Matric proteins holding epithelial cells of endometrium together
2. Matrix proteins of the stroma

The syncytiotrophoblast pushes through the epithelial cells and the basement membrane and into the stroma

Question 16

Why does implanation beyond the stroma not occur?

What nourishes the blastocyst as it is implanted?

When is implantation completed?

What forms the initial blood supply to the syncytiotrophoblast?

Answer 16

The proteolytic enzymes secreted by the syncytiotrophoblast are inhibited (invasion stops at stroma)

Uterine Gland secretions nourish the blastocyts

Implantation is secreted by day 9

Lacunae develop in the syncytiotrophoblast; ~ Day 11/12, maternal capillary walls are eroded and maternal blood fills the lacunae

Question 17

What is the decidual reaction?

What triggers it?

What transformations occur?

Answer 17

• Invasion of the endometrium by the trophoblast triggers a decidual reaction
• This reaction is a continuation of the process initiated by progesterone during the luteal phase of the reproductive cycle
• Endometrial stromal cells are transformed into enlarged rounded (polygonal) shaped cells that form epithelioid like compact layer

Question 18

What is the purpose of the compact layer created by the decidual reaction?

After implantation, what is the endometrium reffered to as?

Answer 18

The compact layer limits the migration of the implanted embryo; inhibits the hydrolytics enzymes released by the syncytiotrophoblast

After implantation, the endometrium is referred to as the decidua

Question 19

What are the key roles of decidual cells?

Answer 19

They are fulled with glycogen / lipids to serve as nutrients for developing embryo until placental is formed

Question 20

What alterations do cytotrophoblasts cause to the maternal structure?

What is the result?

Answer 20

• Invasion of uterine spiral arteries, causing destruction of smooth muscle layer and elastic fibers
  
  • Replaced with fibrous tissue
• Partial replacement of endothelial lning cells by cytotrophoblast
• These alterations create low resistance vessels that are unresponsibe to normal mediators of vascular tone
• Result: Enhanced blood flow to placenta