11.4 Sexual Reproduction (+ 6.6 Reproduction) Flashcards
External fertilisation
sperm swims to the egg which happens easily in a water environment
Internal fertilisation
needed for organisms living on land
special structures are needed to deliver the sperm into the female
“Soil and seed” theory
One of the earliest theories as to how animals reproduce sexually was the ‘soil and seed’ theory proposed by Aristotle
According to this theory, the male produces a ‘seed’ which forms an ‘egg’ when mixed with menstrual blood (the ‘soil’)
The ‘egg’ then develops into a fetus inside the mother according to the information contained within the male ‘seed’ alone
Falsifying the “soil and seed” theory
debunked by William Harvey
William Harvey studied the sexual organs of female deer after mating in an effort to identify the developing embryo
He was unable to detect a growing embryo until approximately 6 – 7 weeks after mating had occurred
He concluded that Aristotle’s theory was incorrect and that menstrual blood did not contribute to the development of a fetus
Harvey was unable to identify the correct mechanism of sexual reproduction and incorrectly asserted that the fetus did not develop from a mixture of male and female ‘seeds’
Female sex hormones
oestrogen, progesterone, follicle stimulating hormone and lutenising hormone
Ovary
The ovary is where oocytes mature prior to release (ovulation) – it also responsible for estrogen and progesterone secretion
Fimbria
Fimbria (plural: fimbriae) are a fringe of tissue adjacent to an ovary that sweep an oocyte into the oviduct
Fallopian tube (oviduct)
transports the oocyte to the uterus – it is also typically where fertilisation occurs
Uterus
is the organ where a fertilised egg will implant and develop (becoming an embryo)
Endometrium
The mucous membrane lining of the uterus, it thickens in preparation for implantation or is otherwise lost (via menstruation)
Vagina
Passage leading to the uterus by which the penis can enter (uterus protected by a muscular opening called the cervix)
Cervix
Passageway between the vagina and the uterus. During childbirth this dilates to allow the baby to exit the uterus.
Follicle stimulating hormone (FSH)
produced by the pituitary gland
stimulates the ovaries to produce oestrogen
also causes the final development of follicles (fluid filled sacs that contain an egg cell)
Luteinising hormone (LH)
produced in the pituitary gland
stimulates follicles to become mature, release their egg and develop into the corpus luteum
Estrogen and progesterone
They promote the pre-natal development of the female reproductive organs
They are responsible for the development of secondary sex characteristics (including body hair and breast development)
They are involved in monthly preparation of egg release following puberty (via the menstrual cycle)
The menstrual cycle
The menstrual cycle describes recurring changes that occur within the female reproductive system to make pregnancy possible
Each menstrual cycle lasts roughly one month (~28 days) and begins at puberty (menarche) before ending with menopause
There are two key groups of hormones which control and coordinate the menstrual cycle:
Pituitary hormones (FSH and LH) are released from the anterior pituitary gland and act on the ovaries to develop follicles
Ovarian hormones (estrogen and progesterone) are released from the ovaries and act on the uterus to prepare for pregnancy
Key events:
1. Follicular phase
2. Ovulation
3. Luteal phase
4. Menstruation
Follicular phase
Follicle stimulating hormone (FSH) is secreted from the anterior pituitary and stimulates growth of ovarian follicles
The dominant follicle produces estrogen, which inhibits FSH secretion (negative feedback) to prevent other follicles growing
Estrogen acts on the uterus to stimulate the thickening of the endometrial layer
Ovulation
Midway through the cycle (~ day 12), estrogen stimulates the anterior pituitary to secrete hormones (positive feedback)
This positive feedback results in a large surge of luteinizing hormone (LH) and a lesser surge of FSH
LH causes the dominant follicle to rupture and release an egg (secondary oocyte) – this is called ovulation
Luteal phase
The ruptured follicle develops into a slowly degenerating corpus luteum
The corpus luteum secretes high levels of progesterone, as well as lower levels of oestrogen
Estrogen and progesterone act on the uterus to thicken the endometrial lining (in preparation for pregnancy)
Estrogen and progesterone also inhibit secretion of FSH and LH, preventing any follicles from developing
Menstruation
If fertilisation occurs, the developing embryo will implant in the endometrium and release hormones to sustain the corpus luteum
If fertilisation doesn’t occur, the corpus luteum eventually degenerates (forming a corpus albicans after ~ 2 weeks)
When the corpus luteum degenerates, estrogen and progesterone levels drop and the endometrium can no longer be maintained
The endometrial layer is sloughed away and eliminated from the body as menstrual blood (i.e. a woman’s period)
As estrogen and progesterone levels are too now low to inhibit the anterior pituitary, the cycle can now begin again
IVF
refers to fertilisation that occurs outside of the body (in vitro = ‘in glass’)
It involves using drugs to suspend normal ovulation (down regulation), before using hormone treatments to collect multiple eggs (superovulation)
- FSH is given for about 2 weeks to stimulate follicles to grow as many eggs as possible
- Gonadotropin releasing hormone (GnRH) is given week 1 of cycle to prevent ovulation too early
- Human chorionic gonadotropin (hCG) is given on day 12 to stimulate ovulation
- At ovulation eggs are removed from the ovary and fertilised with sperm invitro
- One of two embryo are implanted into the uterus
Testis
The testis (plural: testes) is responsible for the production of sperm and testosterone (male sex hormone)
Epididymis
Site where sperm matures and develops the ability to be motile (i.e. ‘swim’) – mature sperm is stored here until ejaculation
Vas deferens
Long tube which conducts sperm from the testes to the prostate gland (which connects to the urethra) during ejaculation
Seminal vesicle
Secretes fluid containing fructose (to nourish sperm), mucus (to protect sperm) and prostaglandin (triggers uterine contractions)
Prostate gland
Secretes an alkaline fluid to neutralise vaginal acids (necessary to maintain sperm viability)
Urethra
Conducts sperm / semen from the prostate gland to the outside of the body via the penis (also used to convey urine)
Scrotum
where the testes are located
keeps the testes at a slightly lower temperature than body temperature
SRY gene
Sex Determining Region Y), which leads to male development
The SRY gene codes for a testis-determining factor (TDF) that causes embryonic gonads to form into testes (male gonads)
In the absence of the TDF protein (i.e. no Y chromosome), the embryonic gonads will develop into ovaries (female gonads)
Roles of testosterone
It is responsible for the pre-natal development of male genitalia
It is involved in sperm production following the onset of puberty
It aids in the development of secondary sex characteristics (including body hair, muscle mass, deepening of voice, etc.)
It helps to maintain the male sex drive (libido)
Gameotogenesis
the process by which diploid (2n) cells underto meiotic division to become haploid (n) sex cells
- in males it is spermatogenesis
- in females it is oogenesis
both involve:
- multiple mitotic divisions and cell growth
- two meiotic divisions to produce haploid daughter cells
- differentiation of the haploid daughter cells to produce functional gametes
Sperm
A typical human spermatozoa can be divided into three sections – head, mid-piece and tail
The head region contains three structures – a haploid nucleus, an acrosome cap and paired centrioles
The haploid nucleus contains the paternal DNA (this will combine with maternal DNA if fertilisation is successful)
The acrosome cap contains hydrolytic enzymes which help the sperm to penetrate the jelly coat of the egg
The centrioles are needed by a zygote in order to divide (egg cells expel their centrioles within their polar bodies)
The mid-piece contains high numbers of mitochondria which provide the energy (ATP) needed for the tail to move
The tail (flagellum) is composed of a microtubule structure called the axoneme, which bends to facilitate movement
Egg
A typical egg cell is surrounded by two distinct layers – the zone pellucida (jelly coat) and corona radiata
The zona pellucida is a glycoprotein matrix which acts as a barrier to sperm entry
The corona radiata is an external layer of follicular cells which provide support and nourishment to the egg cell
Within the egg cell are numerous cortical granules, which release their contents upon fertilisation to prevent polyspermy
Although diagrams of egg cells commonly include a haploid nucleus, no nucleus will form within the egg until after fertilisation has occurred (the egg cell is arrested in metaphase II until it becomes fertilised by a sperm)
Oogenesis
Starts when thousands of oogona (germinal cells) are formed by mitosis become primary oocytes. This is completed either before or shortly after birth. The first meiotic division of the primary oocytes stops at prophase I.
After puberty the primary oocytes continue to develop, by finishing meiosis, although only a few do so every menstrual cycle. As a result of meiosis I, the primary oocyte becomes the secondary oocyte and the first polar body.
Immediately after meiosis I, the haploid secondary oocyte initiates meiosis II. This is halted at metaphase II until fertilisation occurs (if it does). When meiosis II is completes an ootid and another polar body is formed.
Both polar bodies disintegrate at the end of meiosis II leaving the ootid which undergoes maturation and eventually matures into an ovum
Spermatogenesis
The process begins at puberty when the germline epithelium of the seminiferous tubules divides by mitosis
These cells (spermatogonia) then undergo a period of cell growth, becoming spermatocytes
The spermatocytes undergo two meiotic divisions to form four haploid daughter cells (spermatids)
The spermatids then undertake a process of differentiation in order to become functional sperm cells (spermatozoa)
The mature sperm are then released into the tubule and transported to the epididymis
Sertoli cells
span the wall of the seminiferous tubules and provide nutrients to male germ cells
Leydig cells
are found adjacent to the seminiferous tubules in the testicle
They produce testosterone in the presence of luteinising hormone
Hormones involves in spermatogenesis
FSH - pituitary gland - stimulates primary spermatocytes to undergo the first division of meiosis, to form secondary spermatocytes
Testosterone - interstitial cells in the testis - stimulates the development of secondary spermatocytes into mature sperm
LH - pituitary gland - stimulates the secretion of testosterone by the testis
Differences between spermatogenesis and oogenesis
- Number of cells produced
In spermatogenesis, the cells divide equally during meiosis to produce four functional gametes
In oogenesis, the cells do not divide equally and as a result only one functional gamete is formed (plus 2 – 3 polar bodies)
- Size of cells produced
In spermatogenesis, the cells that are formed following differentiation are all of equal size with equal amounts of cytoplasm
In oogenesis, one daughter cell (the ovum) retains all of the cytoplasm, while the other daughter cells form polar bodies
The polar bodies remain trapped within the surrounding layer of follicle cells until they eventually degenerate
- Timing of the process
In spermatogenesis, the production of gametes is a continuous process that begins at puberty and continues until death
In oogenesis, the production of gametes is a staggered and finite process:
It begins before birth (prenatally) with the formation of a fixed number of primary oocytes (~40,000)
It continues with the onset of puberty according to a monthly menstrual cycle
It ends when hormonal changes prevent the further continuance of the menstrual cycle (menopause)
Human fertilisation
The process of fertilization in humans involves a number of key processes, including:
Capacitation – biochemical changes which occur post ejaculation to improve sperm motility
Acrosome reaction – the release of hydrolytic enzymes which softens the zona pellucida (jelly coat)
Cortical reaction – hardening of the jelly coat post fertilization to prevent potential polyspermy
Capacitation
Capacitation occurs after ejaculation, when chemicals released by the uterus dissolve the sperm’s cholesterol coat
This improves sperm motility (hyperactivity), meaning sperm is more likely to reach the egg (in the oviduct)
It also destabilises the acrosome cap, which is necessary for the acrosome reaction to occur upon egg and sperm contact
Acrosome reaction
When the sperm reaches an egg, the acrosome reaction allows the sperm to break through the surrounding jelly coat
The sperm pushes through the follicular cells of the corona radiata and binds to the zona pellucida (jelly coat)
The acrosome vesicle fuses with the jelly coat and releases digestive enzymes which soften the glycoprotein matrix
The sperm then pushes its way through the softened jelly coat and binds to exposed docking proteins on the egg membrane
The membrane of the egg and sperm then fuse and the sperm nucleus (and centriole) enters the egg
Cortical reaction
The cortical reaction occurs once a sperm has successfully penetrated an egg in order to prevent polyspermy
Cortical granules within the egg’s cytoplasm release enzymes (via exocytosis) into the zona pellucida (jelly coat)
These enzymes destroy sperm binding sites and also thicken and harden the glycoprotein matrix of the jelly coat
This prevents other sperm from being able to penetrate the egg (polyspermy), ensuring the zygote formed is diploid
Blastocyst formation
Following the fusion of an egg and sperm (fertilization), an influx of Ca2+ into the ova prompts the completion of meiosis II
The egg and sperm nuclei combine to form a diploid nuclei and the fertilized cell is now called a zygote
The zygote will undergo several mitotic divisions to form a solid ball of cells called a morula
As the morula continues to divide, it undergoes differentiation and cavitation (cavity formation) to form a blastocyst
A blastocyst is comprised of three distinct sections:
An inner cell mass (that will develop into the embryo)
A surrounding outer layer called the trophoblast (this will develop into the placenta)
A fluid filled cavity called the blastocoele
Implantation of the blastocyst
The final stage of early embryo development is the implantation of the blastocyst into the endometrial lining of the uterus
The blastocyst breaches the jelly coat that was surrounding it and preventing its attachment to the endometrium
Digestive enzymes are released which degrade the endometrial lining, while autocrine hormones released from the blastocyst trigger its implantation into the uterine wall
Only once the blastocyst is embedded within the uterine wall can the next stage of embryogenesis occur
The growing embryo will gain oxygen and nutrients from the endometrial tissue fluid, ensuring its continued development
The entire process (from fertilization to implantation) takes roughly 6 – 8 days
Human chorionic gonadotropin (hCG)
When a blastocyst becomes implanted in the endometrial lining it begins to secrete human chorionic gonadotropin (hCG)
hCG promotes the maintenance of the corpus luteum within the ovary and prevents its degeneration
As a consequence of this, the corpus luteum survives and continues to produce both oestrogen and progesterone
Oestrogen inhibits FSH and LH production by the pituitary gland, preventing the release of more eggs from the ovaries
Progesterone also functions to maintain the endometrium (which is nourishing the embryo) and thicken the cervix
The levels of hCG are maintained for roughly 8 – 10 weeks while the placenta is being developed
After this time, the placenta becomes responsible for progesterone secretion and nourishing the embryo
At this point the corpus luteum is no longer required and begins to degenerate as hCG levels drop
Structure of the placenta
The placenta is a disc-shaped structure that nourishes the developing foetus
It is formed from the development of the trophoblast upon implantation and eventually invades the uterine wall
Maternal blood pools via open ended arterioles into intervillous spaces within the placenta called lacunae
Chorionic villi extend into these pools of blood and mediate the exchange of materials between the foetus and the mother
Exchanged material is transported from the villi to the foetus via an umbilical cord, which connects the foetus to the placenta
Upon birth, the placenta is expelled from the uterus with the infant – it is then separated from the infant by severing the umbilical cord (the point of separation becomes the belly button)
Placenta - material exchange
The chorionic villi extend into the intervillous space (lacuna) and exchange materials between the mother and foetus
Chorionic villi are lined by microvilli to increase the available surface area for material exchange
Foetal capillaries within the chorionic villi lie close to the surface to minimise diffusion distance from blood in the lacunae
Materials such as oxygen, nutrients, vitamins, antibodies and water will diffuse from the lacunae into foetal capillaries
Foetal waste (such as carbon dioxide, urea and hormones) will diffuse from the lacunae into the maternal blood vessels
Placenta - hormonal role
The placenta takes over the hormonal role of the ovaries (at ~12 weeks) and begins producing estrogen and progesterone
Estrogen stimulates the growth of uterine muscles (myometrium) and the development of the mammary glands
Progesterone maintains the endometrium, as well as reducing uterine contractions and potential maternal immune responses
Both estrogen and progesterone levels drop near the time of birth
Childbirth - feedback process
Positive feedback involves a response that reinforces the change detected (it functions to amplify the change)
In the case of childbirth, fetal growth eventually causes stretching of the uterine walls, which is detected by stretch receptors
This triggers the release of hormones (oxytocin) that induce uterine muscles to contract, further reducing space in the womb
This causes more stretching and hence more contraction until the origin stimulus (the foetus) is removed (i.e. birth)
Childbirth - Hormonal control
The chemical regulators of the birthing process include oxytocin, oestrogen, progesterone and prostaglandin
After 9 months, the baby is fully grown and stretches the walls of the uterus – placing a strain on both mother and infant
This stress induces the release of chemicals which trigger a rise in the levels of estrogen (estriol in particular)
Estriol prepares the smooth muscle of the uterus for hormonal stimulation by increasing its sensitivity to oxytocin
Estriol also inhibits progesterone, which was preventing uterine contractions from occurring while the foetus developed
Now that the uterus is primed for childbirth, the brain triggers the release of oxytocin from the posterior pituitary gland
Oxytocin stimulates the uterine muscles to contract, initiating the birthing process (it also inhibits progesterone secretion)
The foetus responds to this uterine contraction by releasing prostaglandins, which triggers further uterine contractions
As the uterine contractions trigger the release of chemicals that cause further contractions, a positive feedback loop ensues
Contractions will stop when labour is complete and the baby is birthed (no more stretching of the uterine wall)
Altrical mammals
give brith to relatively helpless, undeveloped offspring that need extended care
require shorter gestation periods
Precocial mammals
give birth to more developed offspring that are mobile and independent and require minimal rearing
Factors that contribute to length of gestation periods
animal size/mass - larger animals tend to have longer gestation periods (as they tend to have larger offspring)
level of development at birth - more developed infants require a longer gestation period
Production of semen
the epididymis, seminal vesicle and the prostate gland are involved
when the sperm arrive in the epididymis they are unable to swim so they undergo maturation while being stored to gain mobility.
The fluid from the seminal vesicle contains nutrients from the sperm including fructose and mucus (which protects sperm in the vagina).
The fluid from the prostate gland contains mineral ions and is alkaline so protects the sperm from the acidic conditions in the vagina.
