The cycle of life : Fertilization

Fertilization

the process whereby the gametes fuse together to begin the creation of a new organism

fertilization accomplishes two separate ends

sex and reproduction

Sex

the combining of genes derived from two parents

Reproduction

the generation of new organisms

the first function of fertilization

to transmit genes from parents to offspring

the second function of fertilization

to initiate in the eggs cytoplasm the reactions that permit development

Type of fertilization

1. external

2. double

3. internal

External fertilization

occurs externally in the environment

Example of External fertilization

fish and aquatic animals

Example of Double fertilization

in some plants

Internal fertilization

the egg cells remain inside the female reproductive system until they fertilized by sperm

Example of Internal fertilization

most wild animals

Types of Internal Fertilization

1. Oviporous

2. Ovoviviparous

3. Viviparous

Oviporous

Eggs laid and hatch outside the body

Example of Oviparous

birds and reptiles

Ovoviviparous

eggs develop and hatched inside of female body but the developing embryos get their food from the stored yolk in eggs, and there is no organic contact between the embryos and the mothers body

Example of Ovoviviparous

some sharks and lizards

Viviparous

Embryos develop in female body and receive nourishment directly from her, often using a placenta

Example of Viviparous

mammals

Steps of Fertilization

1. Capacitation

2. Interaction between spermatozoa and the zona pellucida

3. Arcosomal reaction

4. Gamete fusion and the prevention of polyspermy

5. Fusion of genetic material

Capacitiation

the set of physiological changes by which mammalian sperm become capable of fertilizing an egg

Gaining of Capacity

The cells of the oviduct induce the sperm to mature, and mammalian sperm complete their development in the female oviducts

the capacities the sperm gain are those for the next step

1. recognizing the cues that will guide them to egg

2. undergoing the acrosome reaction

3. and fusing with the egg cell membrane

Interaction between spermatozoa and the zone pellucida

1. Capacited spermatozoa binds with the ZP

2. Acrosome reaction and penetration of ZP

3. Modification of the ZP that prevent polyspermy

zone pellucida (ZP)

an extracellular matrix surrounding the oocyte and the early embryo

what provides the sperm with directions

1. Thermal

2. Chemical gradients

thermotaxis

the ability to sense temperature and preferentially swim from cooler to warmer sites

Chemical gradients

detect and responds to picomolar amounts of the hormone progesterone, which is secreted by the cumulus cells surrounding the egg

Acrosomal reaction is induced by

the higher levels of progesterone

Acrosomal reaction perform by following steps

1. the spermatozoa adhere with the ZP

2. acrosomal cap secretes hydrolytic enzyme

3. above two enzymes digest the jelly coat and vitelline layer of ZP

4. by the propulsive action of the tail and actin filament the spermatozoa will enters the oocytes

Equatorial region

the junction between the inner acrosomal membrane and the sperm cell membrane and is where membrane fusion begins

fusion involves several proteins

Izumo

Juno

an oocyte membrane protien

mutation in Juno or Izumo

block fertilization

monospermy

only one sperm is allowed to penetrate the oocyte

types of monospermy

1. 
   

Fast block to polyspermy

Na+ diffuse into the oocyte from extracellular space membrane depolarization prevents addition sperm from fussing with the oocyte membrane by switching the negative charge to a positive charge

Slow block to polyspermy type 1

Ca2+ released by oocyte endoplasmic reticulum as part of preparation for cell division switching the negative charge of the egg to a positive charge

Slow block to polyspermy type 2 (Cortical granule reaction)

they release protein-digesting enzyme that modify the zone pellucida proteins so they can no longer bind sperm.

the result of Cortical granule reaction

the cortical granules rupture and their contents comes out to collect under the plasma membrane, forming a new membrane known as the fertilization membrane

Slow block to polyspermy type 3

Juno appears to be release from the oocyte cells membrane where the soluble protein can bind sperm in the perivitelline space thereby preventing the sperm from seeing any juno that may still reside on the oocyte membrane

Slow block to polyspermy type 4

release of billions of zinc ions that is induced by the entry of the first sperm and are seen to bind to the ZP forming a zinc sheild that prevants further sperm entry

Fusion of genetic material step 1

Sperm entry activates second meiotic division of oocyte to mature oocyte and polar body

Fusion of genetic material step 2

oocyte choromsomal decondensation leads to female pronuci formation

Fusion of genetic material step 3

Nucleus of sperm enlarges to form male pronuclei

Fusion of genetic material step 4

Both pronuclei replicates its DNA

Fusion of genetic material step 5

the two pronuclei migrate towards each other

Fusion of genetic material step 6

upon meeting, the two nuclear envelopes break down

Fusion of genetic material step 7

the two pronuclei have fused and the male and female chromosome had arranged on the spindle

fertilized egg =

zygote

in mammals, the nucleus formed is

not active until the maternal mRNA occurs in the 2- cell stage in the mouse and after two to four division in humans

Fertilization in amphioxns

takes place externally, where sperm penetrates the egg toward the vegetative pole, which leads to the stimulation of egg to complete its mutation.

Fertilization in amphibian

takes place externally, when sperm penetrates the egg, a rapid flow of peripheral cytoplasm of the egg occurs towards the point which sperm entered as a result, some black pigment granules flow to the area of entry of the sperm, so this region appears light in color and form of a crescent

Fertilization in birds

takes place internally , more than one sperm may enter the egg, but only the nucleus of one sperm fuses with the nucleus of the egg.

fertilization in mammals

when the ovum is released from the ovary, it is picked up by the cilia of the fallopian tube, and the egg enters the oviduct until it reaches the first third of the tube, and there it is penetrated by a single sperm.

largest cell in the human body

the female ovum (diameter of 200 microns)

1

Removal of the glycoprotein coat and seminal plasma protein from its surface

2

Alteration in the flagella motility that are necessary to penetrate the zone penllucida

3

Development of the capacity to fuse with the plasma membrane of the oocyte

Acrosomal reaction performed

1

Sperm contains jelly layer

2

Acrosome reaction

3

Digestion of jelly layer

4

Binding to Vitelline envelope

5

Fusion of Acrosomal process membrane and egg membrane

Gamete fusion and the prevention of polyspermy

Cortical granule reaction

A

Before fertilization

B

After fertilization

Grey crescent