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Animal Reproduction
Sexual and Asexual Reproduction
Asexual Reproduction involves the formation of individuals whose genes come from
a single parent
...

Sexual Reproduction is the formation of offspring by the fusion of haploid gametes
to form a diploid zygote
...
The male gamete is the sperm which is small and
motile
...


Mechanisms of Asexual Reproduction
Some reproduce asexually by fission, in which a parent separates into two or more
equal-sized individuals
...

Fragmentation is another form in which the body breaks into several pieces, some or
all of which develop into complete adults
...
Some can also replace appendages
...


Fertilization
Fertilization is the union of sperm and egg; this plays an important part in sexual
reproduction
...
g
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The
environment has to be wet so that the eggs do not dry out and the sperm can
swim to the eggs
...
It is adapted to terrestrial life
and enables the sperm to reach an egg in dry environment
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Mating animals may use pheromones, chemical signals released by one organism
that influence the behaviour or physiology of other individuals of the same species
...
They act as male attractants
...
However, the survival rate is higher for internal fertilization
...

Marsupial mammals retain their embryos for a short period in the uterus
...

The embryos of eutherian mammals develop entirely within the uterus, nourished
through the placenta
...
External reproductive
structures consist of two sets of labia surrounding the clitoris and vaginal opening
...

The role of the ducts and chambers is to conduct the gametes and house the embryo
and fetus
...
Each ovary is enclosed in a tough protective capsule and contains many
follicles
...

These produce a primary female sex hormone, oestrogen
...
After ovulation, the remaining follicular tissue forms the
corpus luteum
...
If fertilisation does not occur,
the corpus luteum disintegrates and a new follicle matures during the next cycle
...
They convey the egg to the
uterus
...
The
neck of the uterus, the cervix opens into the vagina, which is a thin-walled chamber
that forms the birth canal and is the repository for sperm during copulation
...

Vagina and Vulva

The vaginal opening is covered by a thin sheet of tissue, hymen
...
The labia majora
enclose and protect the labia minora and vestibule
...

Mammary Glands

Present in both, but only function in females
...
Adipose tissue forms the main mass of the mammary gland of a nonlactating mammal
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Male Reproductive Anatomy
External: scrotum and penis
Internal: gonads that produce sperm & hormones, accessory glands that secrete
products essential to sperm movement and ducts to carry the sperm & glandular
secretions
...
The tubes are seminiferous tubes, where sperm is produced
...

The scrotum, a fold in the body wall, holds the testes outside the body cavity at a
temperature about 2°C below that of the abdomen
...

Ducts
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From the seminiferous tubules, the sperm pass through the coiled tubules of the
epididymis
...

Ejaculation propels sperm from the epididymis to the vas deferens, which runs from
the scrotum and behind the urinary bladder and joins with a duct from the seminal
vesicle to form an ejaculatory duct
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Glands

Accessory sex glands add secretions to semen
...
Seminal fluid is thick, yellowish and alkaline
...

The prostate gland secretes directly into the urethra a thin and milky fluid
...

The bulbourethral glands are a pair of small glands along the urethra below the
prostate
...
It carries some sperm released before ejaculation
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Semen in the Female Reproductive Gland

Prostaglandins in semen thin the mucus at the opening of the uterus and stimulate
uterine contractions that help the semen to move up
...

Anticoagulants then liquefy the semen, and the sperm begin to swim
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Penis

Composed of 3 layers of spongy erectile tissue
...
The increased pressure seals off the veins that
drain the penis, causing it to engorge with blood
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The main shaft is covered by a thick skin
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The glans is covered by the foreskin, which may be
removed by circumcision
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Two types of physiological reaction predominate in both sexes:
- Vasocongestion, filling of tissue with blood, caused by increased blood flow
- Myotonia is increased muscle tension
Both smooth & skeletal muscles show sustained and rhythmic contractions
...
Myotonia may result in nipple erection or tension in the arms
and legs
...
Stimulation of the autonomic nervous
system increases breathing and heart rate
...

Orgasm is the shortest phase; it is characterized by rhythmic, involuntary
contractions of the reproductive structures in both sexes
...
Ejaculation occurs with the contraction of the
urethra and expulsion of semen
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Resolution completes the cycle and reverses the responses of earlier stages
...


Gametogenesis
Gametogenesis is based on meiosis
...
It is a
continuous and prolific process in the adult male
...
Primordial germ cells of the embryonic testes differentiate into
spermatogonia, the stem cells that give rise to sperm
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As spermatogenesis
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progresses, the developing sperm cells move from the wall to the lumen of a
seminiferous tubule and then to the epididymis, where they become motile
...
A head containing the haploid nucleus is
tipped with an acrosome, which contains enzymes that help the sperm penetrate to
the egg
...

Oogenesis is the production of ova from oogonia
...
At birth an ovary may contain all of the
primary oocytes it will ever have
...

Unequal cytokinesis during meiosis results in the formation of a single secondary
oocyte and 3 small polar bodies
...
In spermatogenesis, all 4 products of meiosis become mature sperm
...
Spermatogenesis produces mature sperm from
spermatogonia in an interrupted sequence
...
An oogonium multiplies by mitosis and
begins meiosis, but the process stops at prophase 1
...
At puberty, FSH stimulates a follicle to
grow and induces its primary oocyte to complete meiosis I and start meiosis II
...

The secondary oocyte is released when the follicle breaks open at ovulation
...
Oogenesis is completed,
producing an ovum
...

The ruptures follicle develops into the corpus luteum
...


Female Reproductive Cycles
Secretion of hormones and reproductive events they regulate are a complex cycle
...
If pregnancy does not occur, the endometrium is
shed through the cervix and vagina in menstruation
...
It is caused by cyclic events that occur
in the ovaries; ovarian cycle
...

FSH stimulates follicles growth, aided by luteinizing hormone and the cells of the
follicle start to makes oestrogen
...
The low level of oestrogen inhibits
secretion of the pituitary hormones, keeping FSH and LH levels low
...

The high level of oestrogen stimulates the secretion of the gonadotropins by acting
on the hypothalamus to increase the output of GnRH, which stimulates secretion of
FSH and LH
...

LH induces ovulation
...
During the luteal phase, LH stimulates the transformation of
the follicle into the corpus luteum, a glandular structure
...
As these increase in level, they exert negative feedback on the
hypothalamus and pituitary, inhibiting secretion of LH and FSH
...
The pituitary and hypothalamus are liberated from the
inhibitory effects
...

The Uterine/Menstrual Cycle

The follicular phase is coordinated with the proliferative phase of the menstrual
cycle
...
The oestrogen and progesterone in the luteal phase stimulate
development and maintenance of the endometrium, including the enlargement of
arteries and the growth of endometrial glands, which secrete a nutrient fluid that
can sustain an early embryo before it implants in the uterine lining
...

When the corpus luteum disintegrates, it causes spasms in the uterine lining,
depriving it of blood
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Oestrogen is also responsible for female secondary sex characteristics, including
deposition of fat in the breasts and hips, increased water retention and stimulation of
breast development
...

Menopause, the cessation of ovarian and menstrual cycles is when ovaries lose their
responsiveness of FSH & LH and a decline of oestrogen production by the ovary
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Hormonal Control of the Male Reproductive System
Androgens are steroid hormones produces mainly by the Leydig cells of the testes,
interstitial cells near the seminiferous tubules
...

Primary sex characteristics are associated with the development of the vas deferens
and other ducts, development of the external reproductive structures, and sperm
production
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Androgens also affect behaviour and increase general aggressiveness
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Conception, Embryonic Development and Growth
Pregnancy or gestation is the condition of carrying one or more embryos
...
24 hours later a cleavage begins
...
By one week past fertilization the blastocyst forms as a sphere of cells
containing a cavity
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The embryo secretes hormones to signal its presence and control the mother’s
reproductive system
...
Some is excreted in the urine, which gives a positive pregnancy
test
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The outer layer of the blastocyst called the trophoblast invades the
endometrium, eventually helping to form the placenta
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Blood from the embryo travel to the placenta via the umbilical vein
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Organogenesis occurs during the first trimester
...
The rapidity of development makes this a time when the
embryo is especially sensitive to environmental insults such as radiation or drugs
...

These include increased mucus in the cervix to form a protective plug, growth of the
maternal part of the placenta, enlargement of the uterus and cessation of ovarian
and menstrual cycling
...
It provides anchorage, hormonal support,
immunological protection and acts as a barrier to maternal pathogens
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2 n d Trimester

Fetus grows rapidly to 30cm and is very active; the mother may feel movements
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3 rd trimester

Fetus grows rapidly to about 3-4 kg and 50 cm
...
Maternal abdominal organs become compressed
and displaced, leading to frequent urination, digestive blockages and back strain
...
The mechanism is not fully understood
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Oxytocin, produced by the fetus and the mother’s posterior pituitary, stimulates
powerful contractions by the smooth muscles of the uterus
...

The physical and emotional stress of the contractions stimulates the release of more
oxytocin and prostaglandins, a positive feedback system that underlies the process of
labour
...
The
process has three stages:
- Opening up and thinning of the cervix, ending in complete dilation
- Expulsion of the baby as a result of a strong uterine contraction
- Expulsion of the placenta

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Lactation
After birth, decreasing levels of progesterone free the anterior pituitary from
negative feedback and allow prolactin secretion
...
The release of milk from mammary glands is
controlled by oxytocin
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These include HCG, a
variety of protein factors, prostaglandins, several interleukins and an interferon
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