Transcript Human reproduction - Zanichelli online per la scuola

1
Reproduction
and development
2
© Zanichelli editore 2016
Different reproductive strategies
Vertebrates have three types of reproductive strategies:
• in oviparous species, the mother lays eggs wherein the
embryo develops;
• in ovoviviparous species, the embryo develops inside an
egg which remains inside the mother’s body;
• in viviparous species, the embryo develops inside the
mother’s body.
3
© Zanichelli editore 2016
Human reproduction
Human reproduction has three
characteristics:
egg cell
sperm cell
FERTILIZATION
zygote
© Zanichelli editore 2016
1.it is a sexual reproduction – one
egg cell and one sperm cell unite
during fertilization, thereby mixing
their genetic material;
2. fertilization occurs inside the
female genital tract;
3.it is a viviparous reproductive
strategy.
4
Male reproductive system
The external male genitals include:
• the testicles, that secrete androgens and stimulate the
production of sperm cells;
• the penis, the organ that facilitates sexual intercourse.
The corpora cavernosa are formed by erectile tissue that fills
with blood during an erection.
5
© Zanichelli editore 2016
Spermatogenesis
Spermatogenesis is the production of sperm cells in men
which occurs inside seminiferous tubules in the testicles. It is
controlled by androgens.
spermatogonium
primary
spermatocyte
secondary
spermatocyte
Diploid spermatogonia
divide by mitosis, producing
primary spermatocytes.
The primary spermatocyte
divides by meiosis to
produce haploid spermatids
which, in turn, develop into
mature sperm cells.
spermatid
6
© Zanichelli editore 2016
The structure of the sperm cell
Sperm cells consist of:
• a head containing the nucleus of the
cell and the acrosome;
• a midpiece;
• a tail, formed by a flagellum.
7
© Zanichelli editore 2016
Female reproductive system
The female reproductive system consists of
• ovaries (the gonads);
• internal genital organs (uterus, fallopian tubes, vagina)
• external genital organs (clitoris and labia).
8
© Zanichelli editore 2016
Oogenesis
oogonium
primary oocyte
polar body
secondary
oocyte
polar body
Oogenesis is the production of
female gametes – the egg cells.
The process starts from
oogonia which divide by
meiosis to produce primary and
secondary oocytes.
If the secondary oocyte is
fertilized, meiosis is then
completed with the production
of an egg cell.
egg cell
9
© Zanichelli editore 2016
Differences between
spermatogenesis and oogenesis
Spermatogenesis
Oogenesis
4 gametes from one primary
spermatocyte
1 gamete from 1 primary oocyte
continous production
cyclic production
from puberty, all life long
limited in time
10
© Zanichelli editore 2016
Ovulation and fertilization
Ovarian follicles in ovaries are the structures in which the
primary oocyte develops.
During ovulation, the follicle releases an egg cell.
If fertilization occurs (in the fallopian tubes), the zygote
implants into the internal wall of the uterus – the
endometrium.
11
© Zanichelli editore 2016
The effect of estrogens
Estrogens are sexual hormones that control:
• the development of reproductive organs in females;
• the development of secondary sexual characteristics;
• the menarche (first menstruation in puberty);
• the ovarian cycle;
• the uterine cycle.
FSH and LH stimulate the primary oocyte to develop into a
mature egg cell.
Progesterone induces the thickening of the endometrium.
12
© Zanichelli editore 2016
Fertilization and the first phases
of development
During fertilization, the sperm cells enter the oocyte,
thereby activating it. The two haploid nuclei of the gamete
fuse together, generating a diploid zygote.
A few hours after fertilization, the cleavage takes place: a
series of mitotic divisions that lead to the formation of the
embryonic stage called the morula, and then to blastocyst,
which implants in the uterine endometrium.
13
© Zanichelli editore 2016
Gastrulation and neurulation
Gastrulation is the second phase of embryonic development.
In this phase, three different embryonic layers are formed:
• endoderm;
• mesoderm;
• ectoderm.
During neurulation, the embryo forms the notochord and the
neural tube.
The coelom is the main body cavity in which many organs will
develop.
14
© Zanichelli editore 2016
Placenta
The placenta is an organ formed by a spongy tissue which
allows communication between the mother and the embryo
through the umbilical cord.
Its functions are to:
• favor the exchange of respiratory gases and the transfer of
nutrients between the mother and the embryo;
• allow the embryo to eliminate waste materials;
• produce hormones.
15
© Zanichelli editore 2016
Pregnancy
Pregnancy in humans lasts 9 months and is divided into three
trimesters.
Tissues differentiate in the first trimester. At the end of the first
trimester, the embryo becomes a fetus.
In the second trimester, the fetus grows and starts moving.
In the third trimester, organs start working autonomously.
16
© Zanichelli editore 2016
Delivery and lactation
Pregnancy ends with the delivery of the child – a process
consisting of three phases:
• contractions of the uterus and opening of the cervix;
• descent of the baby through the vaginal channel and birth;
• expulsion of the placenta.
After birth, the prolactin hormone stimulates milk production
which is necessary for lactation.
17
© Zanichelli editore 2016