2010 BGD Practical 3 - Fertilization

This page covers the process of mammalian fertilization. A complex interaction between the two haploid gametes (oocyte and spermatozoa) resulting in a single diploid cell (zygote). Follwong entry of the spermatazoa into the oocyte a series of changes occur within the oocyte and zona pellucida that block further fertilization by additional bound spermatozoa (polyspermy)

Due to both scientific and medical research on this process, this can now occur outside the body (in vitro fertilization) as well as fertility therapies to aid normal (in vivo fertilization). In addition, our understanding of fertilization has led to the development of a number of alternative fertility control methods.

File:Fertilization.jpg

Fertilization Dynamics

White arrowhead - sperm

White exclamation mark - time of fertilization (short cytoplasmic contraction)

Black asterisk - site of second polar body formation

White asterisk - site of fertilization cone formation

(Reference: Motosugi N, Dietrich JE, Polanski Z, Solter D, Hiiragi T. Space asymmetry directs preferential sperm entry in the absence of polarity in the mouse oocyte.)

Fertilization of the Mouse Oocyte (More? [../Movies/week1/fertilization.htm Mouse Fertilization Movie])

][../Movies/week1.htm Week 1 Movies])

Sperm Events

Capacitation - removal of glycoprotein coat and seminal proteins, alteration of sperm mitochondria

Binding - ZP3 acts as receptor for sperm

Acrosome reaction - exyocytosis of acrosome contents (Calcium ion mediated), enzymes to digest the zona pellucida, exposes sperm surface proteins to bind ZP2

Membrane fusion - between sperm and egg, allows sperm nuclei passage into egg cytoplasm

Egg Events

Sperm membrane fusion - causes depolarization of egg membrane, primary block to polyspermy

Cortical reaction - IP3 pathway elevates intracellular Calcium, exocytosis of cortical granules, enzyme alters ZP3 so it will no longer bind sperm plasma membrane (cortical reaction)

2nd meiotic division - completion of 2nd meiotic division, forms second polar body

Sperm Penetration

The animation shows:

sperm moving between granulosa cells to contact the zona pellucida.
sperm releasing acrosomal contents to breakdown zona pellucida.
sperm fusing with egg membrane.
egg releasing cortical granule contents.
egg completing meiosis.

Pronuclear Fusion

The animation shows:

zona pellucida (yellow).
polar bodies (green) which contain excess DNA.
two pronuclei (blue) which loose their nuclear membranes and fuse together.
chromosomes pairing to form the diplod zygote

File:Pronucfu.jpg [../Movies/larsen/pronucfu.mov Pronuclear Fusion] (264Kb)

(Links: [../Movies/humemb.htm Movies page] | sperm and egg pronuclei after mammalian fertilization)

Male Sperm Genome - light greenFemale Egg Genome - dark blue

In the mouse zygote, separation of chromatin according to parental origin is preserved up to the four-cell embryo stage and then gradually disappears.

Fertilization Overview

Capacitation

The mammalian sperm must remain for a time in the female genital tract before being capable of fertilization - [week1_3c.htm#top Capacitation].

Acrosome Reaction

Penetration of egg by sperm is initiated by the acrosome reaction which takes different forms in different species. Mammalian acrosomal lysins contain proteinases which lyse the glycoproteins of the zona pellucida.

The central part of the acrosome elongates into a tube which extends form the head of the spermatozoon. On contact with the egg the acrosomal membrane fuses with the sperm plasma membrane thus opening the acrosomal vesicle and liberating the granules containing acrosomal lysins. The inner portion of the acrosomal membrane everts and lengthens to form the acrosomal tubule through which the sperm nucleus enters the egg.

Sperm Contact

The act of fertilization changes the egg from a stage of slow structural and metabolic decline to one of renewed activation. Morphologically egg activation is a series of surface changes immediately following sperm contact.

Mammals - No phenomenon comparable to the raising of the fertilization membrane is displayed. Mammalian eggs are surrounded by the zona pellucida which undergoes a structural change known as the zonal reaction after sperm penetration. On sperm contact with the egg plasma membrane, cortical granules break down as in above forms, substances liberated into the perivitelline space rapidly modify the zona pellucida resulting in a block to further sperm penetration.

Sperm Activation of Egg

During fertilization sperm activates the egg by induction of a calcium ion (Ca2+) oscillation within the egg's cytoplasm. Induction occurs by a sperm protein factor (unidentified) which can stimulate only once Ca 2+ oscillations in metaphase eggs. Another sperm derived factor is then responsible for the inactivation of this oscillation. The activation of the egg by this Ca2+ oscillation is essential for entry of the egg into the first mitotic cycle.

Zygote - Sperm Contribution

What the fertilizing sperm contributes in addition to the genetic material to the zygote differes between species.

Centriole - most mammalian species, sperm contribute a centriole to reconstitute the zygotic centrosome. In rodents, only a maternal centrosomal inheritance occurs.

Sperm Mitochondria - may enter the zygote, but are eliminated by a ubiquitin-dependent mechanism.

Perinuclear Theca - located in the sperm head perinuclear region. Contains a cytoskeletal element to maintain the shape of the sperm head and functional molecules leading to oocyte activation during fertilization.

Note that intracytoplasmic sperm injection techniques may introduce sperm components normally lost during in vivo fertilization.

Zona Pellucida

After fertilization, the zona pellucida:

blocks polyspermic fertilization
physically protects the preimplantation embryo during early embryonic development (aided by an initial "hardening" after fertilization)
aids uterine tube transport
impacts upon blastocyst development

Zygote Contributions

Maternal - mitochondria, nucleolus, oocyte contributes one centriole during fertilization

Paternal - spermatozoon contributes one centriole during fertilization

Both Parents - chromosomes (inherent epigenetic differences between the paternal and maternal pronuclei), plasma membranes spermatozoon/oocyte mingle to form a mosaic plasma membrane

Online Textbooks

Dev Biol - Fertilization

MBoC - Fertilization

Glossary

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Terms

adplantation - Initial adhesion of blastocyst (released from zona pellucida) to uterine wall. Adplantation is followed by implantation.
ampulla &endash; longest segment (approximately 2/3 of overall length) of uterine tube (oviduct or Fallopian tube). Medial segment forming the remainder of the tube is called the isthmus.
antrum- (L. a cave), cavity; a nearly-closed cavity or bulge. In the ovary this refers to the follicular fluid-filled space within the follicle.
blastocyst &endash; the developmental stage following morula, as this stage matures, the zona pellucia is lost allowing the coceptus to adplant and then implant into the uterine wall.
capacitation &endash; the process of activation of sperm, requires removal of surface glycoproteins and increased motility. The sperm now become capable of fertilizing an egg.
cavitates- to form a space within a solid object.
conceptus &endash; the entire structure generated from the zygote.
fertilization- sperm and oocyte fusion process resulting in formation of the zygote.
fimbriae- ( L. = a fringe) fingerlike projections at the ovarian end of uterine tube.
follicular fluid- (or follicular fluid) the fluid found in the antrum of a secondary follicle. Secreted by cells in the wall of the follicle. This fluid is released along with the oocyte at ovulation.
infundibulum- funnel-shaped initial segment of uterine tube (oviduct or Fallopian tube) opening into peritoneal cavity and connected to the ampulla. The peritoneal opening sitting over the ovary.
morula &endash;(L. morus = mulberry) early stage of development (12-15 cells) when conceptus is a solid ball of cells, further cell division forms the blasocyst.
oocyte &endash; (egg or ovum) female germ cell.
ovulation- release of the oocyte from the mature follicle.
uterine tube- (also called oviduct or Fallopian tube) the laterally paired tubes that connect the ovary to the uterus. Is the site for oocyte fertilization and initial development of the conceptus.
uterine wall &endash;the site of normal blastocyst implantation.
zona pellucida- glycoprotein shell that surrounds the oocyte through to blastula stage of development

uterus- site of embryo implantation and development. Uterine wall has 3 layers; endometrium, myometrium, and perimetrium.

zona pellucida- extracellular layer lying directly around the oocyte underneath follicular cells. Consists of glcosaminoglycans and glycoproteins (ZP1, ZP2, ZP3).

Online Resources

Glossary Links

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Cite this page: Hill, M.A. (2024, May 4) Embryology 2010 BGD Practical 3 - Fertilization. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/2010_BGD_Practical_3_-_Fertilization

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