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GAMETES, WHERE DO THEY COME FROM?
https://alphascientists.securesites.com/embryology_images/sperm1.html
https://alphascientists.securesites.com/embryology_images/sperm1.html
http://www.erin.utoronto.ca/~w3bio380/Lectsked/Lect04/Egg2.htm
Three different levels on which this question can be discussed.
1. Organismic - in the adult
organism, what organs give
rise to the gametes.
2. Ontogenetic/embryonic - What
portion of the developing
embryo gives rise to the
gametes.
3. Historical/developmental What is transferred from one
generation to the next that
constitutes the line of cells that
forms the gametes.
Corpus spongiosum
testis
scrotum
3
Human male reproductive tract
Tubuli
recti
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Corpus spongiosum
testis
scrotum
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Corpus spongiosum
What causes an erection at the physiologic level?
1. Sexual arousal ---> action potentials along nerve to corpora cavernosa and
spongiosum ---> stimulate NANC (NonAdrenergic NonCholinergic) nerve cells to
produce nitric oxide.
2. Nitric oxide enhances synthesis of the enzyme guanylate cyclase in tissues of the
corpora.
3. Guanylate cyclase causes production of cyclic guanosine monophosphate (cyclic
GMP) --->
a. Cyclic GMP causes relaxation of smooth muscle in aterial walls of the corpora --->
b. Results in increased blood flow into blood sinuses of the corpora --->
c. Causes erection by engorgement and swelling of sinuses as they fill with blood --->
d. Engorgement produces pressure against the veins in the corpora that greatly reduces
the loss of blood from these tissues thus increasing the erection.
4. Once sexual arousal is lost (e.g. after ejaculation) the action potentials stimulating the
production of cyclic GMP stop --->
a. Cyclic GMP remaining in the corpora tissues is degraded by phosphodiesterase
(PDE) --->
b. This results in contraction of the arterial smooth muscles within the corpora and a loss
of engorgement as the pressure blocking the venous blood flow is relieved, thus
resulting in loss of the erection.
How does Viagra work?
Viagra (sildenifil citrate) blocks the active site on the enzyme phosphodiesterase
(PDE).
Thus, in men with erectile disfunction caused by insufficient cyclic GMP, Viagra
prevents PDE from breaking down the cyclic GMP. As a result, when a male is
sexually aroused cyclic GMP can accumulate in the corpora and cause an erection.
This might be a problem if all PDE in your body was shut down.
However, there are 11 different types of PDE and in the corpora of the penis, PDE5
causes the breakdown of cyclic GMP.
Viagra only blocks the activity of PDE5.
Cialis and Levitra work the same way.
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The Menstrual Cycle
Approximately every 28 days between puberty and
menopause in the human female - however, there is variation
Consists of:
1. Menses
days 0-5
2. Follicular or proliferative phase
Ovulation
days 5-15
day 14 or 15
3. Secretory or luteal phase days
days 15-28
Menstrual Cycle - Players in the game:
Organs:
1. Brain - supraoptic and paraventricular nuclei
of the hypothalamus, median emminence
2. Pituitary gland - pars nervosa, pars distalis
3. Ovary - developing follicles, corpus luteum
4. If pregnancy occurs, the uterus and the
embryo
Major Hormones:
1. Glycoproteins - follicle stimulating hormone
(FSH), luteinizing hormone (LH)
2. Steroids - estrogen (estradiol, est),
progesterone (pro)
3. Peptides - gonadotrophic releasing
hormone (GnRH), chorionic gonadotropin
(HCG, BCG, etc.)
Turn on pic-in-pic
Male reproductive cycle
Hormonal Controlhormones)
of Spermatogenesis (Major
1. Hypothalamus secretes GnRH
2. GnRH stimulates gonadotrophs in pituitary to
secrete FSH and LH
3. LH stimulates interstitial cells Leydig to
produce testosterone
4. Testosterone + FSH stimulate development of
spermatocytes and maturation of sperm
5. Testosterone feeds-back to hypothalamus,
inhibiting further release of GnRH (estrogen
secreted by Sertoli cells may also be involved
in GnRH inhibition)
6. Lack of GnRH causes decrease in LH and
FSH
7. Feed-back inhibition keeps sperm levels
constant
Other hormones involved in the regulation of the
menstrual cycle and sperm production.
Inhibin - secreted by sertoli cells in males and granulosa cells of the
ovarian follicles in females
In both it has the effect of decreasing FSH secretion by the
gonadotrophs.
In males, inhibin is also involved in regulating testosterone secretion decreases it
Assay of inhibin A concentrations in the blood
during pregnancy is part of the “Quad Test” or
“Quad screen” (used to be the “Triple Test”)
This test measures the concentrations of inhibin
A, estriol, beta - HCG, and alpha-fetoprotein.
Elevated inhibin A
Elevated beta-HCG
Reduced estriol
Reduced alpha-fetoprotein
suggests the possibility of Down’s syndrome.
In such cases, more definitive tests will be done.
Ontogenetic/embryonic - where do gametes arise from in the embryo?
Amphibians
8 cell stage, vegetal view
Xpat at vegetal pole
Stage 26 tadpole
Xpat cells in tail endoderm
Xpat - Xenopus primordial germ cell associated transcript
http://www.imolbio.oeaw.ac.at/xenopus/Marker_pages/germ/Xpat.html
zygote
Stage 37 tadpole
Xpat cells at genital ridges
Origin of primordial germ cells in primates, including humans
It is now known that the PGCs of
mammals originate in the outside tissue
layer of the early embryo (epiblast) and
then migrate to the endoderm of the
yolk sac. They then migrate via the
circulatory system to the genital ridge.
Historical/developmental - What is transferred from one generation to the
next that constitutes the line of cells that form the gametes?
Germ plasm theory - August Weismann 1893
Recognized the difference between somatic and germ cells.
Since only germ cells could pass on hereditary information to the next
generation, Weismann proposed his “Germ plasm Theory,”
i.e. that a germ plasm that was responsible for inherited characteristics was
passed from generation to generation via the germ cells.
This was before the function of DNA was understood.
A germ plasm of a sort is formed as the result of gene
activity that causes the synthesis of RNAs that
specify primordial germ cell differentiation.
In some species, such as amphibians, these RNAs
are segregated into a specific group of cells during
cleavage and these cells form the primordial germ
cells.
In other species (e.g., mammals) unkown factors
cause the genes that specify primordial germ cells to
turn on at a later point in development.
The germ cell line forms the thread of
continuity between generations.
Sexual Differentiation
Male? Female?
ABSOLUTES
Or
SHADES OF GRAY
?
What is sex? How is it determined?
What are the characteristics that define a person’s sex?
At the molecular level - Genotype
At the organismal level
Phenotype
Behavior
How do we (you and I) determine a person’s sex?
Phenotype
Behavior
What is sex? How is it determined?
What ultimately determines sexual phenotype?
Is the XX or XY genotype the only determining factor?
No
What are the various types of sexual behavior in
terms of human-human interactions?
Heterosexual
Homosexual
Bisexual
Asexual
What sorts of things affect a person’s sexual behavior?
Genotype
Phenotype
Hormones
Social/cultural Factors
In humans, up to the 7th week of gestation male and female
embryos are identical. Rudiments of both the male and
female reproductive systems are present.
Changes that will determine whether
an embryo will be male or female
begin at the 7th week of gestation.
Default differentiation is female.
So, what determines whether an
embryo is male or female?
The SRY gene - Peter Goodfellow and Robin
Lovell-Badge (1990)
SRY - Sex determining Region of the Y
chromosome
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Is it really that simple?
Usually, but not always.
Do XX and XY always determine a person’s
genetic sex?
No!
On rare occasions the SRY gene is on the X
chromosome (i.e XX can sometimes be
male) or the SRY gene on the Y
chromosome is missing or defective and
non-functional (i.e. XY can sometimes be
female).
Does the presence or absence of a
functional SRY gene and the results of its
activity always determine a person’s sex?
Genetic sex - YES!
Phenotypic sex - NO!
The presence or
absence of
hormones and their
effects on tissues are
critical for
determining
phenotypic sex!
 dihydrotestosterone
Testosterone
 -ketosteroid reductase
Genital tubercle, genital swellings give rise to the penis and the scrotal sac in males.
Testosterone and dihydrotestosterone dependent male structures
Androgen insensitivity syndrome (AIS)
Catch-all term that is used for conditions where, for whatever
reason, in genetically male embryos, the tissues that form male
genitalia don’t respond to 5 -dihydrotestosterone.
Examples:
XY testicular feminism
Defect in the cytoplasmic receptor for 5 -dihydrotestosterone
Has effect on cells of the genital tubercle and genital swellings
5a-ketosteroid reductase deficiency
Defect in the enzyme that converts testosterone into 5 dihydrotestosterone. It’s not as efficient in converting the
testosterone.