Whats the cellular process of producing haploid sperm/egg?

Answer 1

its meiosis.
the mature sex cell on the walls of the epididymis doubles its chromosome number, then splits twice to form 4 haploid cells

Answer 2

meiosis or gametogenesis of sperm is called spermatogenesis. 4 sperm (n=23-haploid) are produced from one spermatogonium (2n=46 chromosomes) during two mitosis like divisions. This ocurs in the seminiferous tubules of the testes from puberty til death or castration (chemical or surgical). He can make billions thru life.

gametogenesis of eggs is oogenesis and occurs from puberty til menopause in the follicles of the ovary; 4 cells can be produced, but 3 polar bodies are sacrificed, giving the one ovum-egg the maximum amount of cytoplasm. Her maximum # of eggs, however, is at her fetus stage, when she has about a million. By age 30 she may only have 30,000 eggs left.

Answer 3

Meiosis. I'ts like doubled mitosis, and produces four cells per cell.

Answer 4

Do you mean the process of meiosis?

Check out http://www.emc.maricopa.edu/faculty/farabee/biobk/BioBookmeiosis.html

Answer 5

Fertilization



Fertilization requires five general events

Sperm must find the egg (requires motility)

Contact & recognition (species-specificity)

Fusion of egg & sperm membranes (prevent polyspermy)

Fusion of egg & sperm pronuclei (regenerate diploidy)

Egg activation (stimulate zygote metabolism)



Activation of sperm motility

Mammalian sperm gains motility by passing through the epididymis

become increasingly more motile in the female reproductive tract

Sea urchin sperm become motile upon contact with sea water

Mechanism of activation:

pH of testes is 7.2, sperm are immotile, no cellular respiration

pH of sea water is 8.0

release of sperm into sea water stimulates exchange of extracellular Na+ ions for intracellular H+ ions

loss of H+ ions raises intracellular sperm pH to 7.6, which activates cell respiration & motility

Evidence:

place sperm in Na+-free sea water → no motility

add Na+ ions → sperm immediately become motile



Sperm in many species (echinoderms, mollusks, cnidarians) find the egg by chemotaxis

Arbacia punctulata (purple sea urchin) releases a sperm attractant called RESACT (for respiratory activating peptide)

14 AA peptide in jelly coat

sperm swim up a concentration gradient of resact (see Figure 7.9):

Resact serves an additional function as a sperm-activating peptide. Binding of resact to its receptor in the sperm plasma membrane stimulates a dramatic increase in:

the rate of mitochondrial ATP synthesis

the rate of ATP hydrolysis by flagellar dyneins (stimulates flagellar motility)



The primary events in fertilization in sea urchin are outlined in Figure 7.8A:



Contact with the egg jelly coat stimulates the acrosome reaction (see Figure 7.10):

fusion of the acrosomal vesicle with the sperm plasma membrane releases enzymes to digest a path through the jelly coat

assembly of the acrosomal process -- polymerization of actin filaments creates a cytoplasmic extension coated by the protein BINDIN

Mechanism for the acrosome reaction:

species-specific glycoproteins in jelly coat bind to receptor in sperm cell membrane

receptor/glycoprotein complex activates G proteins

G protein activation stimulates opening of Ca++ ion channels in the sperm plasma membrane, which allows Ca++ influx

Ca++ ions stimulate fusion of acrosomal vesicle & sperm plasma membrane (releases digestive enzymes)

Ca++ ions activate the RhoB protein, which stimulates actin polymerization

Evidence for role of Ca++:

in Ca++-free sea water → no acrosome reaction

increase Ca++ ion concentration in sea water → may cause acrosome reaction in the absence of egg jelly (premature acrosome reaction)



Attachment of sea urchin sperm to the egg cell surface is mediated by the the protein bindin

Bindin is exposed on the extracellular surface of the sperm following the acrosome reaction (see Figure 7.10):

Bindin molecules bind to species-specific receptors on the surface of the egg (exposed in vitelline layer)

Formation of the bindin/receptor complex activates a sperm protease that degrades the vitelline layer

Once the sperm makes it through the vitelline layer, bindin stimulates fusion of the sperm & egg plasma membranes

Evidence for species-specificity of bindin:

purified bindin from one species will agglutinate eggs of the same species, but not eggs of another species (see Figure 7.14B):

Evidence for the presence of bindin receptors on the egg:

acrosome-reacted sperm adhere to beads coated with purified bindin receptor from eggs (see Figure 7.16B):

Evidence for the fusogenic activity of bindin:

purified bindin stimulates fusion of vesicles in vitro





The primary events in fertilization in mammals are outlined in Figure 7.8B:



Sperm undergo capacitation in the female reproductive tract

Changes in membrane composition (loss of specific glycoproteins)

non-capacitated sperm are unable to pass through the cumulus layer (get stuck by glycoprotein/glycoprotein interactions)

Increased rates of cell respiration & motility



Contact with the zona pellucida facilitates sperm/egg recognition and stimulates the acrosome reaction.

The zona pellucida is a matrix of glycoproteins ZP2 & ZP3 (zona proteins) cross-linked by ZP1 (see Figure 7.17A):

ZP3 is bound by receptors in the sperm membrane

Formation of the ZP3/receptor complex activates G proteins in the sperm plasma membrane

G protein activation stimulates opening of Ca++ ion channels in the sperm plasma membrane, which allows Ca++ influx

Ca++ ions stimulate fusion of acrosomal vesicle & sperm plasma membrane (releases digestive enzymes)

enzymes digest a path through the zona pellucida

Acrosome-reacted sperm remain attached to the zona pellucida through ZP2 receptors that are exposed following the acrosome reaction

ZP3 receptors are lost when the anterior sperm cell membrane is shed during the acrosome reaction (see Figure 7.11):

ZP2 receptors are localized to the posterior region of the acrosomal vesicle, which becomes the leading edge of the sperm following the acrosome reaction

spatial distribution is analogous to that of bindin on sea urchin sperm



Sperm make contact with the egg in the equatorial region of the sperm head, which initiates fusion of egg & sperm plasma membranes (see Figure 7.20E):

fusion is mediated by attachment of protein(s) in the sperm membrane to integrin-associated CD9 proteins in the egg

Evidence for role of CD9 protein:

female mice homozygous for CD9 null allele are infertile due to failure of sperm to fuse with the egg



Evidence that ZP3 is the protein initially bound by sperm:

pre-incubation of sperm with purified ZP3 inhibits binding of sperm to egg zona pellucida (see Figure7.17B):



Prevention of Polyspermy (fusion of more than one sperm with the egg)

Polyspermy is disastrous to the developing embryo because it results in aneuploidy

Sperm that fertilize the egg provide both a haploid nucleus and a centriole to the developing embryo

When greater than one sperm fertilize the egg, the developing embryo contains more than two chromosome sets and excess centrioles



The centriole provided by the sperm normally replicates to produce the two spindle poles associated with the first mitotic cleavage

When additional centrioles are present, they give rise to multipolar spindles at first cleavage (see Figure 7.21):

In the case of dispermic fertilization, three sets of chromosomes are divided among four daughter cells at first cleavage (see Figure 7.21):

this results in dramatic aneuploidy & death of the embryo



Fast Block to Polyspermy (electrical)

Egg resting potential is -70 mV

Fusion of sperm & egg plasma membranes stimulates Na+ influx, which results in membrane depolarization to +20 mV

depolarization is complete within about 3 seconds

persists for about 1 minute

Sperm cannot fuse with eggs that have a positive resting potential

Evidence that a negative resting potential is required for sperm fusion with sea urchin egg

artificially maintain egg resting potential at +20 mV → prevent fertilization

artificially maintain egg resting potential at -70 mV → polyspermy

Evidence that a Na+ influx is responsible for membrane depolarization

decrease Na+ concentration in sea water → increase frequency of polyspermy (see Figure 7.22D):



Slow Block to Polyspermy (mechanical)

Cortical granule reaction is activated shortly after sperm/egg fusion

Fusion of cortical granules with the egg plasma membrane results in the release of cortical granule contents in the space between the cell membrane and the vitelline envelope

The cortical granule reaction is propagated from the point of sperm entry around the entire egg surface in about 1 minute (see Figure 7.23):



Functions of cortical granule contents (see Figure 7.24A):

Cortical granule serine protease

cleaves proteins that connect the vitelline envelope to the egg cell membrane

cleaves bindin receptors & releases any additional sperm attached to them

Mucopolysaccharides

creates an osmotic gradient that causes water to flow into the space between the egg cell membrane & the vitelline envelope -- raises the fertilization envelope

Peroxidase

cross-links proteins in the vitelline layer, thus making it more rigid (hardens the fertilization envelope)

Hyaline & additional proteins

provide a protective support matrix for the developing embryo



In mammals, the cortical granule reaction does not create a raised fertilization envelope, but it nonetheless prevents polyspermy

N-acetylglucosaminidase cleaves N-acetylglucosamine from ZP3 → prevents binding of non-acrosome-reacted sperm

ZP2 proteinase cleaves ZP2 → prevents acrosome-reacted sperm binding & hardens the zona pellucida



The cortical granule reaction is activated by the release of Ca++ from the E.R. (not from Ca++ influx)

When eggs are pre-injected with a fluorescent dye that binds Ca++ ions, a wave of Ca++ release is propagated across the egg beginning at the point of sperm entry (see Figure 7.25):

Evidence that a Ca++ is responsible for propagation of the cortical granule reaction

inject eggs with Ca++ → stimulate cortical granule reaction

treat eggs with Ca++ ionophore (A23187) → stimulate cortical granule reaction even in Ca++-free water

pre-inject eggs with EGTA (Ca++ chelator) → prevent cortical granule reaction following fertilization



Ca++ release is stimulated by a signal transduction pathway that ultimately activates the enzyme phospholipase C (PLC)

Active PLC hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP2), a membrane phospholipid, producing

diacylglycerol (DAG) → serves as a signal molecule in the membrane

inositol triphosphate (IP3) → serves as a soluble signal molecule in the cytoplasm

IP3 binds to receptors in the E.R. membrane → stimulates release of Ca++ (see Figure 7.28):

Ca++ binds to Ca++-sensitive channels in the E.R. membrane → stimulates release of more Ca++

Ca++ propagates the cortical granule reaction around the egg beginning from the point of sperm binding/entry

Evidence that IP3 is responsible for Ca++ release from the E.R.

inject eggs IP3 → stimulate Ca++ release from the E.R.

inhibit PLC → prevent IP3 synthesis → no Ca++ release from the E.R.

Note: IP3 receptors may be purified from egg E.R. membranes



The absolute mechanism by which phospholipase C (PLC) becomes activated is not entirely clear

All pathways rely upon the activation of a src-family kinase as the enzyme that directly activates PLC by phosphorylation (see Figure 7.29A):



Pathway 1:

Sperm binding to a transmembrane receptor activates the receptor

Active receptor activates a G protein

Active G protein activates intracellular src-family kinase

Active kinase activates phospholipase C (PLC) by phosphorylation



Pathway 2:

Sperm binding to a transmembrane receptor activates the receptor

Active receptor directly activates intracellular src-family kinase

Active kinase activates phospholipase C (PLC) by phosphorylation (see Figure 7.29B):



Pathway 3:

Soluble factor in sperm cytoplasm activates intracellular src-family kinase after fusion of sperm & egg plasma membranes

Active kinase activates phospholipase C (PLC) by phosphorylation (see Figure 7.29D):

Evidence for soluble factors in sperm cytoplasm

inject human egg with a single human sperm → stimulate Ca++ release, egg activation, and normal development