11 .4 R e p r o d u c t i o nOverview of Spermatogensis
Spermatogenesis describes the production of spermatozoa (sperm) in
the seminiferous tubules of the testes The first stage of sperm
production requires the division of germline epithelium by mitosis
These cells (spermatogonia) then undergo a period of growth This is
followed by two meiotic divisions that result in four haploid
daughter cells These haploid cells then differentiate to form sperm
cells The developing sperm cells are nourished throughout by the
Sertoli cells
The ovary contains follicles in various stages of development
Egg cells within primordial follicles have been arrested in
prophase I and have yet to undergo meiotic division Egg cells
within mature follicles have begun meiotic division and are
released from the ovary as secondary oocytes (arrested in prophase
II) The ruptured follicle develops into a corpus luteum that will,
in time, degenerate into a corpus albicans The germline epithelium
functions as an epithelial layer separating ovarian tissue from the
rest of the body - it is not involved in oocyte development
LH:Stimulates the interstitial cells (Leydig cells) to produce
testosteroneFSH:Stimulates the (first) meiotic division of
spermatogoniaTestosterone:Stimulates the (second) meiotic division
of spermatogonia and the maturation of spermatozoa through
differentiation
Structure of the ovaryTestis Tissue The testes are composed of
seminiferous tubules which produce sperm Each tubule is surrounded
by a basement membrane which is lined by germ line epithelium cells
The germ line epithelium will divide by mitosis to make
spermatogonia (which divide by meiosis to make spermatozoa) The
developing spermatozoa are nourished by Sertoli cells Outside of
the tubules are blood capillaries and interstitial cells (Leydig
cells), which produce the male sex hormone, testosterone
Similarities between spermatogenesis and oogenesis Both
processes result in the formation of haploid gametes Both processes
involve mitosis, growth and meiosis
Epididymis Testicular fluids are removed, concentrating the
sperm Sperm mature and develop the ability to swim
Seminal Vesicle Adds nutrients (including fructose) for
respiration Secretes prostaglandins, causing contractions to the
female system and helping sperm move towards the egg
Prostate Gland Secretes alkaline fluid which neutralises vaginal
acids (changes pH from 4 to 6 which aids sperm motility)
* O v e r v I e w o f O o g e n s I s* Oogenesis describes the
production of female gametes (ova) within the ovary The process
begins during foetal development, when a large number of cells
(oogonia) are formed by mitosis before undergoing a period of
growth These cells begin meiosis but are arrested in prophase I
until puberty At puberty, some follicles continue to develop each
month is response to FSH secretion These follicles complete the
first meiotic division to form two cells of unequal size The cell
with less cytoplasm is a polar body (which degenerates), while the
larger cell forms a secondary oocyte The secondary oocyte begins
the second meiotic division but is arrested in prophase II (until
fertilisation) It is released from the ovary (ruptured follicle
develops into corpus luteum) and, if fertilisation occurs, will
complete meiosis The second meiotic division will produce an ovum
and a second polar body
Mature sperm and egg
Differences between spermatogenesis and oogenesis
Early Embryo Development After fertilisation, the zygote
undergoes several mitotic divisions to create a solid ball of cells
called a morula (at around 4 days) Unequal divisions beyond this
stage cause a fluid-filled cavity to form in the middle - this
makes a blastocyst (at around 5 days) The blastocyst consists of:
An inner mass of cells (this will develop into the embryo) An outer
layer called the trophoblast (this will develop into the placenta)
A fluid filled cavity (called the blastocoele) These developments
all occur as the developing embryo is moving from the oviduct to
the uterus When the blastocyst reaches the uterus, it will embed in
the endometrium (implantation)
The endometrium is a blood-rich environment in which an
implanted zygote can grow and it is sustained by the hormone
progesterone If progesterone levels aren't maintained (i.e. the
corpus luteum degenerates), then the endometrium will be sloughed
away (menstruation) A fertilised zygote develops into a blastocyst
that secretes human chorionic gonadotrophin (hCG) hCG maintains the
corpus luteum post-ovulation so that the blastocyst can remain
embedded in the endometrium and continue to develop Gradually the
placenta develops and produces progesterone (at around 8 - 10
weeks), at which point the corpus luteum is no longer needed
*Role of hCG*Process of Fertilization When the sperm enters the
female reproductive tract, biochemical changes to the sperm occur
in the final part of its maturation (capacitation) The sperm is
attracted to the egg due to the release of chemical signals from
the secondary oocyte (chemotaxis) Fertilisation generally occurs in
the oviduct (fallopian tube) To enter the egg membrane, the sperm
must penetrate the protective jelly coat (zona pellucida)
surrounding the egg via the acrosome reaction The acrosome vesicle
fuses with the jelly coat and releases digestive enzymes which
soften the glycoprotein matrix The membrane of the egg and sperm
then fuse and the sperm nucleus (and centriole) enters the egg To
prevent other sperm from penetrating the fertilised egg
(polyspermy), the jelly coat undergoes biochemical changes via the
cortical reaction The cortical granules release enzymes that
destroy the sperm-binding proteins on the jelly coat Now
fertilised, the nucleus of the secondary oocyte completes meiosis
II and then the egg and sperm nuclei fuse to form a diploid
zygote
Material exchange The fetus relies on the exchange of materials
across the placental wall to grow and develop:
Hormonal Role The placenta also takes over the hormonal role of
the ovary (at around 12 weeks) Estrogen stimulates growth of the
muscles of the uterus (myometrium) and the development of the
mammary glands Progesterone maintains the endometrium, as well as
reduces uterine contractions and maternal immune response (no
antibodies against fetus) Both estrogen and progesterone levels
drop near time of birth
Structure and Function The placenta is a disc-shaped structure
that nourishes the developing embryo It is formed from the
development of the trophoblast upon implantation and eventually
invades the uterine wall The umbilical cord connects the fetus to
the placenta and maternal blood pools via open ended arterioles
into intervillous spaces (lacunae) Chorionic villi extend into
these spaces and facilitate the exchange of materials between the
maternal blood and fetal capillaries Nutrients, oxygen and
antibodies will be taken up by the fetus, while carbon dioxide and
waste products will be removed The placenta is expelled from the
uterus after childbirth
Placenta
Amniotic sac and Amniotic fluid The fetus develops in a
fluid-filled space called the amniotic sac Amniotic fluid is
largely incompressible and good at absorbing pressure, and so
protects the child from impacts to the uterine wall The fluid also
creates buoyancy so that the fetus does not have to support its own
body weight while the skeletal system develops Finally, amniotic
fluid prevents dehydration of the tissues, while the amniotic sac
provides an effective barrier against infection
The process of childbirth is called parturition and is
controlled by the hormone oxytocin After nine months, the fetus is
fully grown and takes up all available space in the uterus,
stretching the walls of the uterus This causes a signal to be sent
to the brain, releasing oxytocin from the posterior pituitary
Oxytocin inhibits progesterone, which was inhibiting uterine
contractions Oxytocin also directly stimulates the smooth muscle of
the uterine wall to contract, initiating the birthing process The
contraction of the uterine wall causes further stretching, which
triggers more oxytocin to be released (causing even more
contraction) Additionally, the fetus responds to the cramped
conditions by releasing prostaglandins which cause further
myometrial contractions As the stimulus causing oxytocin release is
increased by the effects of oxytocin, this creates a positive
feedback pathway Contractions will stop when labour is complete and
the baby is birthed (no more stretching of the uterine wall)
The Hormonal Control of Child Birth