Lecture 22: Fertilization

Question 1

What is the general transport of sperm in steps?

Answer 1

1. Immediate transport
   -Retrograde loss (gravity)
   -Phagocytosis
   -Entrance into cervix/uterus
2. Cervix
   -“privileged” pathways (during estrus)
   -Removal of non-motile sperm
   -Removal os some abnormalities
3. Uterus
   -Capacitation initiated
   -Phagocytosis on sperm
4. Oviduct
   -Capacitation completed
   -Hyperactive motility
5. Fertilization
   -Acrosome reaction
   -Spermatozoon penetrates oocyte
   -Male and female pronuclei form

Question 2

What is the cervical step of sperm transport?

Answer 2

First trap: Cervical folds
-Glands produce acidic mucus
*prevents microorganism entry
*Physical barrier: fold + muscus= plug
-During estrus E2 act to change mucus composition
*Less acidic
*Thinner and more elastic
*Forms strands or “privilege pathways PP” to allow sperm to pass
-After ovulation. P4 returns mucus to acidic barrier

Question 3

What factors affect AI of cows?

Answer 3

Semen “dose”
-20x106 spermatozoa (or less)

Deposited
-Body of the uterus (first AI)
-Anterior cervix (reperat AI)

Primary sources of loss
-Retrograde flow (still have some with AI but way less)
-Wrong horn

Major reasons for failure
-Cow not in heat
-Poor technique
-Early embryonic mortality

Question 4

What is required for the female reproductive tract to effetely transport sperm?

Answer 4

-Contractions of the female reproductive tract
Steps:
-Mating (or manipulation of cervix per rectum)
-Nervous system stimulation
-Posterior pituitary gland
-Oxytocin (also lead to milk ejection)
-Increased contractions of uterus/oviducts favoured by high ratio E2:P4

Question 5

How is the ejaculated spermatozoa from domestic species “capacitated” in the female reproductive tract?

Answer 5

1.
-Loss of surface proteins (acquired during epididymal transit)= reorganization of plasma membrane lipid composition
2.
-Motility is important for final approach to oocyte= hyperactivation (burst in activation, increases speed to pass utero junction to oocyte

-Initiated in the uterus at the utero-tubal junction
-Following capacitation, require intact acrosomes for acrosome reaction during fertilization
-Must be performed in vitro for IVF (use of high ionic strength medium or heparin)

Question 6

What is happening in the oviduct right before fertilization?

Answer 6

-Internal mucosa and lamina proprietary well developed in the infundibulum and ampulla
*Process ciliated epithelium (under control of E2) beat and move fluid towards the uterus
*Secretory cells (peg cells) secrete nutrients for oocyte, and antimicrobial
-Smooth muscle layers

Question 7

What happens at fertilization?

Answer 7

-Several important steps are required for fusion:
-Penetration of the corona radiata (layer of granulosa cells)
-Perforation of the zona pellucida= acrosome reaction
-Transfer of the nucleus (fuse genetic material)

-Several sperms reach corona radiata then release their enzymes to dissociate granulosa cells (sacrifice themselves)
-One lucky sperm reach the zona pellucida
-Membrane proteins on the sperm cell surface bind glycogpriens of zona pellucida: Acrosomal reaction

Question 8

What happens at the acrosome reaction?

Answer 8

-Release of enzymes= acrosome reaction- fusion of inner and outer acrosomal membranes
-Acrosin (trypsin-like) plus hyaluronidase released in close proximity to oocyte
-Required to penetrate zona pellucida of oocyte
-If no oocyte, acrosome reaction occurs in a progressively larger portion of population
-Must ensure processed semen has a high proportion of intact acrosome

Question 9

What is the summary of fertilization (at junction of ampulla+isthmus)?

Answer 9

-Oocyte arrives and loses cumulus
-Hyperactivated spermatozoa passes through remaining cumulus
-Binds to zona pellucida
-Acrosomal reaction
-Penetrates zona (zona reaction)
-Penetrates vitelline membrane
-Loss of cortical granules
-Extrusion of plolar body 2
-Syngamy

Question 10

What happens directly after fertilization?

Answer 10

-As sperm enters, oocyte completes meiosis
-Within 12h, both haploid pronucleus fuse to form a diploid fertilized egg=zygote
-Zygote=giant cell. divides every 24h to form smaller cells=cleavage
-After 4-5days=morula enters the uterus
-Unattached embryo converts to blastocyst
-Inner cell mass will form the embryo
-Trophoblast will form the amnion and the chorion
-Cavity= blastocoele