Animal Reproduction
Chapter 36
Asexual Reproduction
• Asexual reproduction involves only a single
animal. The animal produces offspring
through repeated mitosis of cells in some part
of its body.
• The offspring are, genetically identical to the
parent.
Sexual Reproduction
• It requires the production of haploid gametes
through meiosis.
• During fertilization, two gametes usually from
separate parents fuse and give rise to a diploid
individual.
• Since the offspring receives genes from each
of its two parents, its genome is not identical
to that of either parent.
Types of Asexual Reproduction
• Budding
• Sponges and cnidarians, such as Hydra, and sea
anemones, reproduce by budding. A bud grows
directly from the body of the adult, drawing
nourishment from its parent. When it has grown old
enough, it breaks off and becomes an independent
individual.
Gemmules
• In this form of asexual reproduction, a parent
releases a specialized mass of cells that can
develop into offspring.
Sponges exhibit this type of reproduction.
Fragmentation
• In this type of reproduction, the body of the
parent breaks into distinct pieces, each of
which can produce an offspring.
Planarians exhibit this type of reproduction.
Regeneration
• In regeneration, if a piece of a parent is detached, it
can grow and develop into a completely new
individual.
Echinoderms exhibit this type of reproduction
• Hydra, planaria, and earthworms also exhibit
regeneration.
Binary Fission
Parthenogenesis
• This type of reproduction involves the
development of an egg that has not been
fertilized into an individual.
• Male honeybees develop from unfertilized eggs
and are haploid; their diploid sisters develop from
fertilized eggs.
• Some fish, amphibians, and reptiles reproduce
parthenogenetically, but restore the diploid
number of chromosomes either before or after
meiosis. All the resulting offspring are females.
Advantages and disadvantages of
asexual reproduction
• Advantages
• Animals that remain in one particular place and are
unable to look for mates would need to reproduce
asexually.
• Numerous offspring can be produced without "costing"
the parent a great amount of energy or time.
• A disadvantage of this type of reproduction is the lack
of genetic variation. All of the organisms are genetically
identical and therefore share the same weaknesses. If
the stable environment changes, the consequences
could be deadly to all of the individuals.
Male Reproductive System
Female Reproductive System
Label the Parts
Structure of female reproductive system:
• Ovaries are a pair of compact bodies located in the
abdominal cavity attached to the dorsal body wall by a fold
of peritoneum.
• It has been estimated that after birth, each ovary contains
about 300,000 ova. But only a few reach maturity.
• The ovaries of the sexually matured female contain ova at
various stages of development. The final stage is a structure
called corpus luteum.
• Fallopian tube: Close to each ovary is a funnel shaped
structure called Fallopian tunnel. It receives the ova as they
are sent out of the ovary. The Fallopian funnel leads to a
narrow tube called Fallopian tube.
• The two Fallopian tubes open behind the uterus.
Contd..
• Uterus: It is a hollow, thick walled muscular organ situated in
the pelvic cavity between the bladder and the rectum. The
inner lining is the mucus membrane the endometrium.
• The main function of the uterus is to prepare the
endometrium to receive the fertilized ovum.
• Vagina: This is a muscular tube lined with mucus membrane
connecting the cervix at the upper end and with external
genitalia at the lower end. Vagina serves as a birth canal.
Spermatogenesis
Spermatogenesis
• Germ cells near the outer wall of the seminiferous
tubules of the testis differentiate into stem cells called
spermatogonia.
• Spermatogonia divide by mitosis, and mature into
primary spermatocytes.
• Each primary spermatocyte undergoes meiosis:
• meiosis I yields 2 haploid secondary spermatocytes
• meiosis II yields 4 equal-sized spermatids.
• The spermatids migrate toward the lumen (central
opening). Sertoli cells supply nutrients for the
spermatids, which mature into motile sperm in the
Spermatogenesis
Structure of the human sperm
The human sperm has a head with a haploid nucleus. Atop the nucleus
lies a specialized lysosome called the acrosome. It contains enzymes that
will dissolve protective layers around the egg and enable the sperm to
enter and fertilize it.
Behind the head is the midpiece, which is packed with mitochondria.
These organelles provide the energy needed to move the tail.
Whiplike miovement of the tail, which is really a long flagellum, propel
the sperm through the female reproductive tract.
Oogenesis
Oogenesis, is the formation of egg cells, begins
in the developing ovaries of a female fetus.
Precursor cell is called oogonia and they divide
my mitosis to form primary oocyte (by the end
of third month of fetal development, no oogonia
remain, as they have all divided by mitosis and
grown into primary oocytes)
The ovaries start out with 2 million primary
oocytes. At puberty only about 400,000 primary
oocyte remain.
Surrounding each oocyte is layer of smaller cells
that nourish the developing oocyte and secrete
female sex hormones.
Oocyte together with these accessory cells make
up the follicle.
During the menstrual cycle, primary oocyte
completes its first meiotic division to form a
single secondary oocyte and a polar body.
Oogenesis
• The accessory cells of the follicle secrete estrogen. As the
follicle matures, it grows, eventually erupting through the
surface of the ovary and releasing the secondary oocyte, a
process called ovulation.
• The secondary oocyte travels through the uterine tube or
Fallopian tube. If the egg/secondary oocyte is fertilized it
may undergo the second meiotic division while in the
uterine tube.
• Some of the follicle cells accompany the egg, but most
remain in the ovary and they enlarge and become glandular,
forming the corpus luteum. The corpus luteum secretes both
estrogen and progesterone. If fertilization does not take
place, corpus luteum breaks down a few days later
Ovulation
Menstrual cycle
• Starting at the age of puberty, human females begin a
hormonal cycle known as the menstrual cycle.
• Each cycle lasts on an average, 28 days.
• The purpose of this cycle is to time the release of an egg
or an ovum (ovulation) for possible fertilization and later
implantation into the inner lining of the uterus. This
implantation must occur when the uterine lining (the
endometrium ) is rich with blood vessels.
• The highly vascular endometrium is not maintained if
there is no implantation.
• The breakdown of the blood vessels leads to the
menstrual bleeding.
Hormones controlling menstrual cycle
• Hypothalamus is the regulatory centre of the
menstrual cycle.
• The hypothalamus produces a hormone known as
gonadotropin releasing hormone (GnRH).
• The target tissue of this hormone is the near by
pituitary gland and it results in the pituitary gland
producing two hormones into the blood streamfollicle stimulating hormone and luteinizing
hormone. The target tissues for these two
hormones are the ovaries.
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The outer ring of follicle cells remains within the ovary. The cells of this outer ring
begins to divide and fill in the area left by ovulation and this forms a glandular
structure known as corpus luteum.
Corpus luteum is hormonally active and produces a hormone-progesterone.
Progesterone is a hormone that maintains the thickened, highly vascular
endometrium and the embryo will be able to implant.
Estrogen levels rise to a peak and stimulate LH secretion by the pituitary gland.
High levels of estrogen and progesterone are a negative feedback signal to the
hypothalamus. The hypothalamus does not produce GnRH when these hormone
levels are high. So FSH and LH remain at a level not conducive to the production
of another Graffian Follicle during this time.
IF there is no pregnancy, the corpus luteum begins to break down and then there
is a decline in both progesterone and estrogen levels. As both of these levels fall,
the highly vascular endometrium can no longer be maintained. The capillaries
and small blood vessels begin to rupture and menstruation begins.
The drop in progesterone and estrogen levels also signals the hypothalamus to
begin secreting GnRH and thus normal menstrual cycle begins.
• Effects of hormones (FSH , LH, Progesterone, estrogen)on the
ovaries:
• One of the effects of FSH and LH on the ovaries is to increase the
production and secretion of another reproductive hormone by the
follicle cells of ovaries.
• The hormone is estrogen. Estrogen enters the blood stream. It’s
target tissue is the endometrium of the uterus. The result is the
highly vascularized endometrium.
• Another effect of FSH and LH is the production of structures
within the ovaries called the Graffian follicles.
•
• A spike in the level of FSH and LH leads to ovulation. (release of
oocyte from the Graffian follicle) This enters the fallopian tube
soon after ovulation.
Review
• Sexual maturity in women is marked by the beginning of the
menstrual cycles. These cycles coordinate the development and
release of an egg with the conditions required in the uterus to
support a pregnancy.
• The cycle is controlled by hormones from both the brain (FSH and
LH) and the ovary(oestrogen and progesterone).
• The natural cycle repeats until there is either a pregnancy or the
woman reaches menopause and the end of the reproductive
phase of her life.
• FSH and LH are two hormone that are active at significant phases
of human development including primary and secondary sexual
characteristics. They are both significant hormones in the primary
sex determination. Following puberty human become fertile and
in females this is manifest as the menstrual cycle.
Anterior Pituitary Hormones
• Follicle Stimulating hormone (FSH)
• Stimulates the development of a primary follicles (oocytes).
• Increases the number of follicular cells which in turn produce
oestrogens.
• Produces follicular fluids.
• Develops the oocyte in the follicle.
• Luteinising Hormone (LH):
• surges in mid cycle (12 days) to bring about ovulation.
• high LH is associated with a resumption of meiosis in the oocyte.
Meiosis has been arrested in Prophase I since the embryonic
stage. Only at the point of fertilisation does meiosis complete.
• stimulates the development of the corpus luteum.
Ovarian Hormones
• Oestrogen:
• Stimulates the development of the endometrium (lining of the
uterus) and it associated blood supply.
• During the first half of the cycle there is positive feedback through
increased sensitivity of the follicle cells to FSH (Up-regulation of
receptors on the follicular cell plasma membrane).
• During the second half of the cycle (high oestrogen) there is
negative feedback on FSH and LH.
Progesterone:
• maintains the lining of the endometrium
• negative feedback on FSH and LH
Menstrual cycle and hormonal
interaction
Menstrual cycle is generated by
interactions among the hormones
of the hypothalamus, the anterior
pituitary, and the ovaries.
Fertilization
• Fertilization is internal. Sperms and eggs live only for a few
days if fertilization does not occur. Fertilization can succeed
only if copulation occurs within a couple of days before or
after ovulation. The cells surrounding the egg called corona
radiata, and an inner jelly like layer called zona pellucida for a
barrier between the sperm and the
egg.
Fertilization
• In the uterine tube, hundreds of sperm reach the egg and encircle the
corona radiata, each sperm releasing enzymes from its acrosome. These
enzymes weaken both the corona radiata and the zona pellucida, allowing
the sperm to wriggle to the egg.
• Why do you think may sperms approach one egg?
• When the first sperm finally contacts the surface of the egg, the plasma
membranes of egg and sperm fuse, and the sperm’s head is drawn into
the egg’s cytoplasm
• Two changes take place as the sperm enters
• Vesicles near the surface of the egg release chemicals into the zona
pellucida, reinforcing it and prevents polyspermy.
• The egg undergoes second meiotic division forming the haploid gamete.
• Fertilization occurs as the haploid nuclei of sperm and egg fuse forming a
diploid nucleus that contains all the genes of a new human being.
Family planning
Vasectomy
Tubectomy
Family Planning
Family planning
• Read page number 736-737 and go over the
Table 36-3