Transcript embryo - Zanichelli online per la scuola

David Sadava, David M. Hillis,
H. Craig Heller, May R. Berenbaum
La nuova
biologia.blu
Anatomia e fisiologia dei viventi S
Animal Reproduction
How Do Animals Reproduce Sexually?
In sexual reproduction two haploid cells (gametes)
form a diploid individual.
Three fundamental steps of sexual reproduction:
• Gametogenesis—making gametes
• Mating—getting gametes together
• Fertilization—fusing gametes
How Do the Human Male and Female Reproductive Systems Work?
Primary sex organs (testes and ovaries) produce
gametes and serve endocrine functions.
Accessory sex organs include external genitalia,
ducts, and glands.
Secondary sexual characteristics are the differences
in external appearance of males and females.
How Do the Human Male and Female Reproductive Systems Work?
Semen is made up of
sperm and other fluids
and molecules.
Sperm are produced in
the paired male gonads,
or testes.
Testes are located in
the scrotum, outside of
the body, to maintain
optimal temperature for
sperm production.
Reproductive Tract of the Human Male
How Do the Human Male and Female Reproductive Systems Work?
Spermatogenesis occurs in the seminiferous tubules
in each testis.
Between the tubules are Leydig cells that produce
testosterone.
Spermatogonia lie
at the outer edges
of the seminiferous
tubules, as sperm
mature they move
toward the lumen.
How Do the Human Male and Female Reproductive Systems Work?
Sertoli cells provide nutrients for developing sperm.
Differentiated
sperm cells are
shed into the
lumen of the
seminiferous
tubule.
How Do the Human Male and Female Reproductive Systems Work?
Sperm are stored in the epididymis, where they
mature and become motile.
The epididymis connects to the urethra via the vas
deferens and the ejaculatory duct.
Semen components are secreted mostly by the paired
seminal vesicles, which empty into the vas deferens.
The prostate gland surrounds the urethra and
contributes to semen.
Prostate fluid is alkaline and neutralizes acidity in male
and female reproductive tracts—more hospitable for
sperm.
How Do the Human Male and Female Reproductive Systems Work?
The bulbourethral glands produce an alkaline
secretion that:
• Neutralizes acidity in the urethra
• Provides lubrication and facilitates sperm movement
during climax
These secretions precede climax yet can carry
residual sperm that are capable of fertilization.
How Do the Human Male and Female Reproductive Systems Work?
Male genitalia include the penis and scrotum.
The sensitive tip of the penis, the glans penis, is
covered by the foreskin.
Sexual stimulation triggers the nervous system to
produce penile erection.
How Do the Human Male and Female Reproductive Systems Work?
Semen is expelled through the vasa deferentia and
urethra in two steps:
Emission—rhythmic contractions move the semen
into the urethra at the base of the penis.
Ejaculation—contractions by muscles at the base of
the penis that force the semen out.
Ejaculation is accompanied by feelings of intense
pleasure called orgasm.
How Do the Human Male and Female Reproductive Systems Work?
In females, when the ovary releases an egg
(ovulation), it moves into the oviduct, or Fallopian
tube, via the fimbria.
The egg is
propelled by cilia
towards the uterus
where it will
develop if fertilized.
The bottom of the
uterus is the narrow
cervix, which
opens into the
vagina.
How Do the Human Male and Female Reproductive Systems Work?
Two sets of skin folds surround the opening of the
vagina and the urethra:
Labia minora—
the inner folds.
Labia majora—
the outer folds.
At the anterior
tip is the clitoris,
erectile tissue
that is important
in sexual
response.
How Do Animals Reproduce Sexually?
Gametogenesis occurs in the gonads.
Males: gonads are the testes and the gametes are
small, motile sperm.
Females: gonads are the ovaries and the gametes
are nonmotile ova, or eggs.
Germ cells undergo mitosis and produce
spermatogonia in males and oogonia in females.
Mitosis continues and produces primary
spermatocytes and primary oocytes, both diploid.
Meiosis then produces haploid cells that mature into
sperm and ova.
How Do Animals Reproduce Sexually?
Spermatogenesis:
Throughout the male life span, spermatogonia continue
to divide by mitosis.
One daughter cell remains a spermatogonium; the
other daughter cell becomes a primary spermatocyte.
The primary spermatocyte undergoes meiosis.
The first meiotic division results in two secondary
spermatocytes; the second division produces four
haploid spermatids.
Spermatids differentiate into sperm.
Spermatogenesis
How Do Animals Reproduce Sexually?
Oogenesis:
Primary oocytes immediately enter prophase I of
meiosis.
After prophase I development stops in many species.
In humans this state lasts until puberty.
During this period, the primary oocyte grows larger
and acquires nutrients.
When meiosis I resumes, the two daughter cells are of
unequal sizes.
How Do Animals Reproduce Sexually?
The daughter cell with more cytoplasm is the
secondary oocyte and the smaller one forms the
first polar body.
The polar bodies degenerate.
How Do Animals Reproduce Sexually?
The secondary oocyte enters another period of
arrested development.
It may be expelled from the ovary in this condition.
In humans, the
second meiotic
division is not
completed until
the egg is
fertilized by a
sperm.
How Do the Human Male and Female Reproductive Systems Work?
Hormones control male
sexual function:
GnRH (gonadotropinreleasing hormone) is
released by the
hypothalamus at puberty.
GnRH increases release of
LH (luteinizing hormone)
and FSH (follicle-stimulating
hormone) by the anterior
pituitary.
How Do the Human Male and Female Reproductive Systems Work?
The female reproductive cycle consists of two linked
cycles:
•The ovarian cycle that produces eggs and
hormones
•The uterine, or menstrual cycle that prepares the
endometrium for a blastocyst.
If a blastocyst does not arrive, the thickened
endometrium sloughs off.
How Do the Human Male and Female Reproductive Systems Work?
How Do the Human Male and Female Reproductive Systems Work?
Menstruation is the
sloughing off of the
endometrium through
the vagina.
A human female is
born with about one
million primary oocytes
in each ovary.
At menopause, the end of fertility, there may be only a
few oocytes left in each ovary.
How Do the Human Male and Female Reproductive Systems Work?
The ovarian and uterine cycles are controlled by
hormones:
• At puberty increased GnRH
stimulates production of FSH
and LH
• FSH and LH stimulate
ovarian tissue to grow and
produce estrogen, which
causes maturation of
accessory sex organs and
secondary sexual
characteristics
How Do Animals Reproduce Sexually?
Fertilization is the union of a haploid sperm and a haploid
egg.
It creates a single diploid cell, called a zygote, which will
develop into an embryo.
Steps in fertilization:
• Sperm and egg recognize each other
• Activation of sperm
• Plasma membranes fuse
• Egg blocks entry of additional sperm
• Egg is activated metabolically
• Egg and sperm nuclei fuse
How Do Animals Reproduce Sexually?
Species with internal fertilization have egg-sperm
recognition mechanisms.
A mammal egg is surrounded by the cumulus, cells
in a gelatinous matrix.
The zona pellucida is a
glycoprotein envelope
beneath the cumulus.
How Do Animals Reproduce Sexually?
The acrosome is a
membrane-enclosed
structure on the sperm
head.
Egg and sperm contact
causes substances in the
jelly coat to trigger an
acrosomal reaction.
Membranes in the sperm
head and acrosome
break down, enzymes
are released and digest
the jelly coat.
How Do Animals Reproduce Sexually?
Sperm and egg plasma membranes fuse to form a
fertilization cone that engulfs the sperm head, bringing it
into the egg.
Fusion of sperm and egg membranes initiate a sequence
of events:
• Blocks to polyspermy—mechanisms to prevent more
sperm from entering the egg.
• Cortical granule enzymes dissolve the bonds between
the vitelline envelope and the plasma membrane.
• Water enters and swells to form the fertilization
envelope.
How Does Mitosis Divide Up the Early Embryo?
Cleavage: early cell divisions with no cell growth.
Embryo becomes a solid ball of small cells.
Blastocoel—a central fluid-filled cavity that forms in
the ball.
The embryo becomes a blastula and its cells are
called blastomeres.
How Does Mitosis Divide Up the Early Embryo?
At the 32-cell stage, cells separate into two groups:
• Inner cell mass—becomes the embryo;
• Trophoblast—sac that forms from the outer cells. Its
cells secrete fluid to create the blastocoel cavity, with
the inner cell mass at one end.
Embryo is now called a
blastocyst.
How Does Mitosis Divide Up the Early Embryo?
In mammals, fertilization
occurs in the oviduct; cleavage
occurs as the zygote travels
down the oviduct to the uterus.
Implantation occurs when the
trophoblast adheres to the
endometrium, or uterine
lining.
Implantation that occurs in the
oviduct is an ectopic, or tubal,
pregnancy.
How Does Gastrulation Generate Multiple Tissue Layers?
Gastrulation: massive movements of cells transform
the blastula into an embryo with multiple tissue layers
and distinct body axes.
In triploblastic animals, three germ layers (tissue
layers) form.
• Endoderm: innermost layer;
• Ectoderm: outer layer;
• Mesoderm: middle.
How Is the Growing Embryo Sustained?
Extra-embryonic membranes surround the
vertebrate embryo.
They function in nutrition, gas exchange, and waste
removal.
• Amnion—surrounds the embryo
• Yolk sac—encloses yolk within
the egg
• Allantoic membrane—forms the
allantois, a sac for waste storage
• Chorion—reduces water loss
and exchanges gases
How Is the Growing Embryo Sustained?
In mammals, they interact with the mother’s tissues to
form the placenta.
The placenta develops from the chorion and uterine
tissues.
It exchanges
nutrients, gases, and
wastes between
mother and embryo.
With the umbilical
cord the embryo is
attached to the
chorionic placenta.
What Are the Stages of Human Development?
Gestation, or pregnancy, in humans is about 266
days (9 months) and is divided into trimesters.
In the first trimester the embryo becomes a fetus:
• Heart begins to beat by week four
• Limbs form by week eight
What Are the Stages of Human Development?
During the second trimester the limbs elongate and
fingers, toes, and facial features form.
The fetus’s nervous system develops rapidly.
First fetal movements are felt by the mother.
During the third trimester internal organs mature.
The brain goes through sleep-wake cycles.
Birth occurs when the lungs are mature.
How Can Fertility Be Controlled?
The only failure-proof methods to prevent pregnancy
are complete abstinence and surgical removal of the
gonads.
Contraception methods prevent fertilization or
implantation, and vary widely in effectiveness.
Table 43.1 Methods fo Contraception (Part 1)
Table 43.1 Methods fo Contraception (Part 2)
Table 43.1 Methods fo Contraception (Part 3)
Table 43.1 Methods fo Contraception (Part 3)
How Do Animals Reproduce Without Sex?
Asexual reproduction requires no energy for mating
but does not result in genetic diversity—all offspring
are genetically identical to their parents.
Asexually reproducing species are mostly
invertebrates, sessile, and tend to live in constant
environments.
How Do Animals Reproduce Without Sex?
Budding produces new individuals that form from the
bodies of older animals.
A bud grows by mitotic cell division—cells
differentiate before the bud breaks away.
The bud is genetically identical to the parent.
Regeneration can
replace damaged
tissues or lost limbs,
or form a complete
individual.
How Do Animals Reproduce Without Sex?
Parthenogenesis is the development of offspring
from unfertilized eggs; arthropods, some fish,
amphibians, reptiles.
Most species also engage in sexual reproduction at
other times.
Parthenogenesis may determine the sex of the
offspring (e.g., in honey bees).
Adapted from
Life: The Science of Biology, Tenth Edition, Sinauer Associates, Sunderland, MA, 2014
Inc. All rights reserved