Lecture 15 (DSA): Fetilization and Implantation

Question 1

What triggers the exocytosis of the acrosome (acrosomal rxn), what happens?

Answer 1

• A rise in [Ca2+] inside sperm cell triggers fusion of the outer acrosomal membrane with the sperm cell’s plasma membrane and results in exocyotosis of the acrosomal contents
• Hydrolytic enzymes inside the acrosomal cap are released and act locally to dissolve the zone pellucida

Question 2

How is estrogen involved in the transport of sperm?

Answer 2

• Causes cervix to produce a watery mucus and these mucus channels aid in the passage of sperm through the cervix
• Causes contractions of the myometrium to help propel sperm

Question 3

What is capacitation of spermatozoan and what 3 barriers must be breached in the process of fertilization?

Answer 3

• Spermatozoan is modified so that it becomes capable of fertilizing the egg, including the acrosome reaction
• Must pass through: Corona radiata, Zona pellucida, and Plasma membrane of the egg (aka oolema)

Question 4

What interaction occurs as the sperm head passes the follicular cells and attaches to the zona pellucida surrounding the oocyte?

Answer 4

Sperm-ZP3 (glycoprotein) interaction

Question 5

What is the Cortical reaction and what initiates it?

Answer 5

• Initiated by increased [Ca2+] insidethe oocyte
• Massive exocytosis of cortical granules shortly after sperm-oocyte fusion
• Release enzymes that act on glycoprotein in the zona pellucida and cause them to harden, preventing polyspermy

Question 6

What causes the oocyte to complete its second meiotic division upon sperm entering; what is formed?

Answer 6

• Completion is due to increased [Ca2+]
• Results in the formation of the 2nd polar body (contains a haploid number of unduplicated maternal chromosomes)

Question 7

What happens to the sperm once inside the oocyte; how do we get a zygote?

Answer 7

• The sperm nucleus decondenses and transforms into the male pronucleus
• Male and female pronuclei fuse, forming the zygote

Question 8

What represents the end of fertilization and the beginning of embryonic development?

Answer 8

Mingling of chromosomes

Question 9

What is compaction of the Morula?

Answer 9

Outer cells of the morula become tightly bound together through desmosome and gap junctions

Question 10

Blastocyst forms inside the morula due to which physiological processes?

Answer 10

Active transport of Na+ from trophoblast cells and osmosis of water

Question 11

The inner cells of the blastocyst form what; these cells are known as?

Answer 11

• Inner Cell Mass
• Pluripotent
• Form the “embryo proper”

Question 12

What are the functions of hCG on the corpus luteum?

Answer 12

• Prevents menstruation by sustaining function of corpus luteum
• Leads to increased secretion of progesterone and estrogen

Question 13

How does hCG act as an autocrine growth factor?

Answer 13

• Stimulates trophoblast growth and development
• Stimulates placental growth

Question 14

What do the trophoblasts secrete that helps the blastocyst adhere and implant into the uterine endometrium?

Answer 14

Secrete proteases that break down the extra-cellular matrix of the endometrium and digest the outer-lying zona pellucida

Question 15

What adhesive proteins do Syncytiotrophoblasts express and what is their function?

Answer 15

Express adhesive surface proteins (cadherins and integrins) that bind to uterine surface epithelia

Question 16

What do syncytiotrophoblasts secrete for the breakdown of the extracellular matrix of the endometrium?

Answer 16

Metalloproteases and other hydrolytic enzymes

Question 17

What do the syncytiotrophoblasts secrete at the onset of implantation and what do they make at sufficient levels to maintain pregnancy?

Answer 17

• Secrete hCG at onset of implantation
• Make progesterone at sufficient levels to maintain pregnancy independently of the corpus luteum

Question 18

What is the function of syncytiotrophoblasts as implantation and placentation progress?

Answer 18

• Phagocytosis
• Bidirectional placental transfer of gases, nutrients, and wastes

Question 19

The blastocyst needs to get rid of its zona pellucida, how is this accomplished and where does the precursor for this factor come from?

Answer 19

• Lytic factors, such as plasmin, in the endometrial cavity are essential for the dissolution of the zona pellucida
• Plasminogen, the plasmin precursor, is found in the uterine cavity and blastocyst factors may contribute to its conversion to active plasmin

Question 20

What is the earliest contact between the blastocyst wall, trophoectoderm, and endometrial epithelium called; where does it occur; and what happens to the ICM?

Answer 20

• Apposition stage
• Typically in a crypt in endometrium
• ICM rotates to final correct orientation pointing toward endometrium

Question 21

What occurs during the adhesion stage of the trophoblast to the uterine epithelium?

Answer 21

• Interactions between surface proteins on trophoblasts and epithelial cells
• Initiates decidualization:

- Increased vascular permability

- Intracellular matrix composition

- Stromal cell morphology

Question 22

What is Decidualization?

Answer 22

• Response of maternal cells (stromal cells) to progesterone
• Endometrial stroma is transformed into enlarged and glycogen-filled decidual cells
• Endometrium now called decidua and is ready for the implantation of the embryo

Question 23

What does the decidua form to prevent migration of the implanting embryo?

Answer 23

• Form epithelial-like sheet with adhesive junctions
• Production of signals that prevents embryo from invading myometrium