Reproduction In Animals Flashcards
(138 cards)
1
Q
What is Oviparity?
A
- Here animals deposit fertilized eggs in the external environment for development e.g. in all birds some reptiles and some fish.
2
Q
What is Ovoviviparity?
A
- Here animals retain eggs in the mother’s body to complete development, but embryos still obtain all of their nourishment from the egg yolk.
The young are hatched from the mother’s body when fully developed. E.g. in many reptiles and some fish
3
Q
What is Viviparity?
A
- Here eggs develop to advanced stage in the mother’s body and the embryo obtains nourishment directly from the mother’s blood, rather than just from the egg yolk.
E.g. in mammals
4
Q
Define Internal fertilization
A
- This is where fusion of male and female gametes occurs inside the body of the female animal.
5
Q
Define External fertilization
A
- This is where fusion of male and female gametes occurs outside the body of the female animal.
6
Q
What are Isolecithal eggs?
A
- These are eggs with very little yolk that is evenly distributed in the egg e.g. human eggs.
7
Q
What are Mesolecithal eggs?
A
- These are eggs with moderate amount of yolk concentrated in the vegetal pole e.g. in amphibians.
8
Q
What are Telolecithal eggs?
A
- These are eggs contain an abundance of yolk that is densely concentrated at the vegetal pole of the egg. E.g. in birds, reptiles, most fishes.
9
Q
Define Cleidoic eggs
A
- These are shelled eggs e.g. eggs of birds, reptiles
10
Q
Define Gametogenesis
A
- This is the series of transformations that result into the formation of mature gametes.
11
Q
Define Spermatogenesis
A
- This is the series of transformations that result into the formation of male gametes.
12
Q
What is Oogenesis?
A
- This is the series of transformations that result into the formation of female gametes.
13
Q
Define Menopause
A
- This is a period when ovulation and menstruation cease in human females.
14
Q
Describe some of the mechanisms leading to fertilization and development in mammals that are of evolutionary advantage.
A
- Fertilisation and development are internal to limit wastage of gametes and provide protection to the young respectively
- The breeding seasons coincide with the breeding cycle so that birth occurs at a time when environmental conditions are most favourable for growth of young
- Feeding young ones on nutritious milk enables them to prepare for adult food as the digestive system develops
- Secondary sexual characteristics enable easy identification of mating partners
- Parental care provides protection from predation and harsh environmental conditions to the young
- Development of placenta enables gaseous exchange and the young to excrete wastes
- Females are often more receptive to males during ovulation or the act of copulation stimulating ovulation.
15
Q
What are the main features of sexual reproduction in mammals?
A
- Fertilisation is internal
- Females go through a sexual cycle known as menstrual cycle
- Sexual cycle is restricted to the breeding season, except in humans and other primates, which are sexually receptive throughout the year
- Young ones are born at an advanced stage
- There is display of courtship behaviour that leads to mating
- Development of embryo is internal and completely dependent on the mother for food and protection
- The young are fed on milk
- Parental care to the young is prolonged
16
Q
What are Primary sex organs?
A
- These are organs, which produce gametes and secrete sex hormones i.e. the gonads (testes in males and ovaries in females)
17
Q
What are Secondary sex organs (accessory organs)?
A
- These are organs associated with testes or ovaries which play some roles in reproduction but other than gamete production and hormone secretion.
E.g. penis, prostate, seminal vesicles, sperm duct in males, and fallopian tubes, uterus, vagina, mammary glands in females.
18
Q
What are Accessory or external sex characters?
A
- These are external characters, which do not play any direct role in reproduction but are distinct and enable sexes to be distinguished as male and female.
E.g. low pitch voice and facial hair (males) and high pitch voice (females)
19
Q
Differentiate between primary sex organs and secondary sex organs
A
Primary:
- Produce gametes
- Secrete sex hormones
- Development is under the control of FSH and LH
Secondary:
- Do not produce gametes
- Do not secrete sex hormones
- Development is under the control of Oestrogen and progesterone in females and testosterone in males
20
Q
What is the function of the Penis?
A
- Delivers sperm to the neck of the cervix, as close to the site of ovulation as possible.
21
Q
What is the function of the scrotum?
A
- Regulates teste’s temperature at 30C lower than body temperature for proper sperm formation.
When cold, the cremaster muscle elevates the testes to absorb heat from the body, this’s reversed at high temperature.
22
Q
What are the functions of the testes?
A
- Contain seminiferous tubules that produce sperm
- Produce the male sex hormone testosterone.
23
Q
What is the function of the ovaries?
A
- Are sites for egg production
- Secrete the hormones oestrogen and progesterone
24
Q
What is the function of the funnel of oviduct?
A
The finger-like projections sweep the egg into oviduct.
25
What is the function of the oviducts or fallopian tubes?
- Walls are muscular and lined with ciliated epithelium for moving egg from ovary towards uterus.
26
What is the function of the uterus?
- Site of implantation of fertilized egg, development of foetus during pregnancy and origin of muscular contractions that precede parturition.
27
What is the function of the prostate gland?
- Secretes an alkaline fluid that neutralizes the acidic vaginal secretions to avoid reduction in sperm motility at low PH.
28
What is the function of the seminal vesicles?
- Secrete an alkaline mucous fluid rich in fructose – the respiratory substrate for sperm motility.
29
What is the function of the Cowper’s (bulbourethral) gland?
- Produces a mucous secretion for lubricating the penis during intercourse and neutralizing the acidity of any remaining urine.
30
What is the function of the Epididymis?
- Sperm maturation site (1-10 days)
- Stores spermatozoa (up to 4wks)
31
What is the function of the Vas deferens?
- Stores sperm (up to many months) before ejaculation.
32
What is the respiratory substrate for sperm motility?
- Fructose
33
What is the function of the Vagina?
- Passage for menstrual flow, receptacle for penis during coitus and lower part of birth canal.
34
What is the function of the clitoris?
- Tactile stimulation excites the female sexually during intercourse.
35
What is the function of the labia minora and labia minora?
- Produce a lubricant mucus secretion during intercourse and protect the clitoris from abrasion.
36
What is the clitoris homologous to?
- The glans penis of males
37
Define Spermatogenesis
- The process by which spermatogonia in seminiferous tubules of testes develop into sperm that can leave the male’s body.
38
Define Oogenesis
- Production of eggs in the ovary of females
39
Describe the main events during Spermatogenesis in humans
- At puberty, diploid germinal epithelial cells (primordial germ cells) of seminiferous tubules undergo repeated mitotic divisions to form a number of diploid spermatogonia
- Each spermatogonium increases in size and becomes a primary spermatocyte
- Each primary spermatocyte undergoes the first meiotic division to form two haploid secondary spermatocytes, which undergo second meiotic division to form four haploid spermatids, connected to each other by cytoplasm
- The spermatids get embedded into sertoli cells (loosely called “nurse cells”) to be transformed into sperm by:
- Losing part of cytoplasm,
- Condensation of nucleus into head
- Formation of flagellated tail
- The mature spermatozoa (sperms) finally detach from sertoli cells and are released into the lumen of seminiferous tubules.
40
Describe the main events during development of ova in humans
- During embryonic development, diploid oogonia (germinal epithelial cells of ovary) undergo repeated mitotic divisions to increase in number
- Some oogonia undergo mitosis to form primary oocytes, which remain at prophase I of meiosis, while the rest (now called follicle cells/granulosa cells) enclose the primary oocytes
- At puberty, granulosa cells multiply to form primary follicle & other cell layers around the primary oocyte
- The primary oocyte undergoes meiosis up to metaphase II only to form a secondary oocyte and 1st polar body
- The primary follicle develops to form fluid filled secondary follicle and later Graafian follicle, which enclose secondary oocyte & 1st polar body
- At fertilization, the secondary oocyte completes meiosis II to form a large ootid (ovum) and second polar body
- The first polar body also undergoes meiosis at the same time to form two small polar bodies
- All the three polar bodies degenerate and only one functional egg remains
41
What are the functions of sertoli (sustentacular) cells?
- Provide nourishment to developing spermatids
- Phagocytise (eat off) the cytoplasm of spermatids
- Secrete a fluid that carries spermids through the tubules
- Phagocytise foreign particles that invade the tubules.
42
What is the egg released from the Graafian follicle during ovulation?
- It is a secondary oocyte, which has undergone meiosis up to metaphase II only.
43
When does the secondary oocyte turn into an egg?
- When meiosis II is completed at the time of fertilization and turns the secondary oocyte into an egg.
44
Explain the significance of formation polar bodies during oogenesis.
- Polar bodies take the extra chromosomes resulting from meiosis in order for the ovum to carry haploid number of chromosomes
- The unequal cytoplasmic division results into the formation of a large egg with the cytoplasm containing sufficient yolk for the development of the embryo.
45
What are the similarities between spermatogenesis and oogenesis in humans?
- Both begin with diploid germinal epithelial cells
- Mitosis and meiosis are involved in both
- Both yield haploid gametes
- Both occur in gonads
46
What are the differences between spermatogenesis and oogenesis in humans?
Spermatogenesis:
- Occurs in seminiferous tubules in testes of males
- Begins only at puberty
- It is a continuous process and occurs all the time
- During growth phase, primary spermatocyte shows only double the increase
- Four spermatids are formed from one primary spermatophyte
- Equal cytoplasmic divisions during meiosis I and meiosis II and no formation of polar bodies
- All stages are completed and sperms are formed in the testes only
- Male gamete or sperm is comparatively very small
- Spermatid undergoes spermiogenesis to become sperm
- Takes a longer time to complete
Oogenesis:
- Occurs in ovaries of females
- Begins during embryonic development. A baby girl is born with the set number of primary oocytes already in prophase stage of 1st meiotic division
- It is a discontinuous process, only one egg matures in about 28 days
- Primary oocyte may show the increase of about four to eight times
- Only one ovum is formed from one primary oocyte
- There is unequal cytoplasmic division during meiosis I and meiosis II and resulting into formation of polar bodies
- The secondary oocyte leaves the ovary and final second meiotic division at fertilization in the fallopian tube
- Female game te is very large comparatively
- No such stage after the formation of ootid or ovum
- Takes a shorter time to complete
47
What is the function of the Acrosome of the human sperm?
- Contains hydrolytic enzymes which facilitate the penetration of the egg membranes prior to fertilization.
48
What is the function of the nucleus of the human sperm?
- Contain a haploid set of chromosomes, which on fusion with the egg restores the diploid state of organisms.
49
What is the function of the mitochondria of the human sperm?
- They complete aerobic to release ATP required for contraction of filaments during the sperm’s movement.
50
What is the function of the tail piece (flagellum) of the human sperm?
- Enables motility of the sperm.
51
What is the function of the centriole of the human sperm?
- One of a pair produces microtubules that
52
What is the function of the yolky cytoplasm of the human ovum?
Contains fat and protein which nourish the developing embryo.
53
What is the function of the cortical granules (lysosomes) of the human ovum?
Contain enzymes that alter the structure of vitelline membrane to prevent polyspermy at fertilization, to avoid upsetting the diploid state of the zygote.
54
What is the function of the vitelline membrane of the human ovum?
Undergoes structural changes that prevent polyspermy at fertilisation
55
What is the function of the nucleus of the human ovum?
Contains 23 chromosomes that complete meiosis II at fertilization to provide female haploid nucleus
56
What is the function of the polar body of the human ovum?
Contains 23 chromosomes, but is non-functional and degenerate
57
Outline the hormonal control of spermatogenesis in humans.
Interaction of hormones from the hypothalamus and anterior pituitary gland working together controls spermatogenesis
- From the hypothalamus, gonadotrophin-releasing hormone (GnRH) stimulates the anterior pituitary gland to secrete two gonadotrophins (gonad stimulating hormones), i.e. follicle stimulating hormone (FSH) and luteinising hormone (LH)/interstitial cell stimulating hormone (ICSH).
- FSH stimulates spermatogenesis by causing sertoli cells to complete the development of spermatozoa from spermatids.
- FSH also causes sertoli cells to release a peptide hormone inhibin that specifically inhibits FSH secretion.
- LH (ICSH) stimulates the leydig cells (interstitial cells) of the testes to secrete testosterone.
- Testosterone stimulates the growth and development of germinal epithelial cells (spermatogonia) to form sperm, and also works with FSH to stimulate the sertoli cells.
- However, increased testosterone level inhibits the secretion of GnRH and LH.
58
Distinguish between oestrous and menstrual cycles.
Oestrous cycle: series of hormone controlled changes in the non-primate reproductive cycle characterized by females experiencing a period of heightened sexual excitement just before ovulation.
While
Menstrual cycle: series of hormone controlled changes in the primate female reproductive system that result in monthly discharge of blood and uterine materials when fertilization fails.
59
Outline the four main phases of the menstrual cycle
The four main phases of the menstrual cycle:
- Follicular phase
- Ovulation
- Luteal phase
- Menstruation
60
What is the general name for male sex hormones?
Androgens (e.g. testosterone)
61
What is the general name for female sex hormones?
Oestrogens
62
Describe the hormonal, physiological and structural changes that occur during the human menstrual cycle. (hormonal control of menstrual cycle)
Follicular phase:,
- At puberty (about 12 years) the hypothalamus:
1. Secretes Gonadotrophin-releasing hormone (GnRH)
- This stimulates the anterior pituitary to secrete follicle stimulating hormone (FSH)
2. FSH stimulates:
(i) The development of primary follicles in the ovary
(ii) The secretion of oestrogen.
Ovulation:
3. Oestrogen
(i) Causes the repair and healing of the uterine wall following menstruation.
(ii) Inhibits the secretion of FSH.
(iii) Causes the secretion of LH from the anterior pituitary.
Luteal phase:
4. LH stimulates:
(i) Ovulation i.e. Meiosis I resumes in the primary oocyte to form polar body and secondary oocyte, which is released by rupturing of Graafian follicle.
(ii) The remains of Graafian follicle to develop into corpus luteum (yellow body)
(iii) The corpus luteum to secrete progesterone and oestrogen.
Menstruation:
5. Progesterone:
(i) Causes increased thickness (muscularisation) and vascularization of the uterus.
(ii) Inhibits the release of LH and FSH by negative feedback.
- Decreased level of FSH prevents development of Graafian follicles, hence secretion of oestrogen stops.
- Decreased level of LH prevents ovulation, hence the corpus luteum degenerates and progesterone decreases.
- The sudden decrease of progesterone level in blood completes menstrual cycle, as the hypothalamus resumes the secretion of GnRH.
- GnRH stimulates the anterior pituitary to secrete FSH as menstruation occurs, characterized by breakdown and shedding of endometrial materials.
63
What is Sperm capacitation?
The process of activation of mammalian sperm to fertilise the egg, during which the acidity and enzymes in the female genital tract cause perforation of the sperm head by removal of cholesterol and glycoprotein to allow entry of Ca2+ and the release of acrosome enzymes.
64
Give the significance of sperm capacitation
Entry of Ca2+ increases the beating activity of the sperm tail and also promotes acrosome reaction to enable sperm penetrate the egg.
65
What is fast block?
- A process during which contact of the first sperm with the egg membrane is instantly followed by an electrical potential change in the egg membrane to prevent entrance of more than one sperm.
66
Give the significance of fast block
- Prevents entrance of more than one sperm into the egg (polyspermy) that would upset the diploid state of the embryo.
67
What is acrosome reaction?
- A process that occurs in the sperm head on making contact with a secondary oocyte, during which the cell and acrosome membranes rupture to release hydrolytic enzymes e.g. hyaluronidase and proteases.
68
Give the significance of acrosome reaction
- Enables sperm head to penetrate the egg membranes.
69
What is cortical reaction?
- A process that occurs following sperm penetration of the secondary oocyte during which lysosomes (cortical granules) fuse with the plasma membrane and release their contents, causing the vitelline membrane to harden and form the fertilization membrane to prevent polyspermy.
70
Give the significance of cortical reaction
- Formation of fertilization membrane prevents multiple sperm entry into the egg (polyspermy) that would upset the diploid state to cause death of mammalian embryo.
71
Define Fertilization
- This is the fusion of sperm and egg nuclei to form a diploid zygote.
72
Outline the events which lead to fertilization of an egg by a sperm.
- On entering the vagina, sperm spend about 7 hours being capacitated, after which they move towards the oviducts, aided by muscular contractions of the uterus and oviducts, and lashing of tail.
- A spermatozoon comes into contact with the oocyte by random movement.
- Acrosome enzymes hydrolyse a path in the granulosa layer of egg until the sperm head makes contact with zona pellucida.
- Sperm acrosome membrane ruptures to release hydrolytic enzymes (acrosome reaction) and the acrosomal filament pierces through the oocyte membranes up to the plasma membrane of the oocyte.
- An electrical potential change in the oocyte membrane occurs (fast block), followed by fusion of cortical granules with plasma membrane to discharge their contents (cortical reaction), which creates an osmotic gradient that draws water into the space between the plasma membrane and vitelline membrane.
- The two membranes are lifted away and the vitelline membrane hardens (fertilization membrane) to block polyspermy.
- While the sperm tail is lost and disintegrates, the nucleus expands and is now known as pronucleus.
- Entry of a sperm stimulates completion of second meiotic division of the secondary oocyte to form the second polar body, which disintegrates, and an egg.
- The haploid male and female pronuclei fuse to form a diploid zygote, which divides immediately by mitosis to form two diploid cells.
73
After their release, how long do sperm and secondary oocyte remain viable for fertilization?
Approximately 48 hours for sperm and about 10-15 hours for secondary oocyte.
74
Outline the events that occur in the egg immediately following the entry of the spermatozoon.
- Sperm acrosome membrane ruptures to release hydrolytic enzymes and the acrosomal filament pierces through the oocyte membranes up to the plasma membrane of the oocyte.
- An electrical potential change in the oocyte membrane occurs followed by fusion of cortical granules with plasma membrane to discharge their contents which creates an osmotic gradient that draws water into the space between the plasma membrane and vitelline membrane.
- The two membranes are lifted away and the vitelline membrane hardens to block polyspermy.
- While the sperm tail is lost and disintegrates, the nucleus expands and is now known as pronucleus.
- Entry of a sperm stimulates completion of second meiotic division of the secondary oocyte to form the second polar body, which disintegrates, and an egg.
- The haploid male and female pronuclei fuse to form a diploid zygote, which divides immediately by mitosis to form two diploid cells.
75
What is meant by negative feedback?
A mechanism in which the effect of deviation from the normal condition triggers a response that eliminates its deviation in order to reduce further corrective action of the control system once the set point value has been reached.
76
Briefly explain how negative feedback operates in the control of testicular hormone secretion.
- The hypothalamic hormone, gonadotrophin-releasing hormone (GnRH) stimulates the anterior pituitary gland to secrete both follicle stimulating hormone (FSH) and luteinising hormone (LH).
- FSH stimulates spermatogenesis by causing sertoli cells to complete the development of spermatozoa from spermatids.
- FSH also causes sertoli cells to release a peptide hormone inhibin that specifically inhibits FSH secretion.
- LH stimulates leydig cells of the testes to secrete testosterone.
- Testosterone stimulates the growth and development of spermatogonia to form sperm, also inhibits the secretion of LH by feeding back, both directly at the anterior pituitary gland and indirectly by reducing GnRH release.
77
Briefly explain how negative feedback operates in the control of the menstrual cycle.
- The hypothalamic Gonadotrophin-releasing hormone (GnRH) stimulates the anterior pituitary to both FSH and LH.
- FSH stimulates the secretion of oestrogen in the ovary.
- Oestrogen in increased levels inhibits FSH secretion and causes secretion of LH from the anterior pituitary.
- LH stimulates ovulation and development of corpus luteum, which secretes progesterone and also continues to secrete oestrogen.
- Progesterone inhibits the release of LH and FSH thus arresting development of any further follicles.
78
What hormonal controlled changes occur in the endometrium during the menstrual cycle?
(Effect of ovarian hormones on the endometrium during the menstrual cycle)
- During the follicular phase, oestrogen (estradiol) from the ovary causes the uterine endometrium to repair and heal.
- During the luteal phase, progesterone secreted by the corpus luteum in the ovary causes the endometrium to become highly muscular and vascular.
- As the corpus luteum degenerates, the rapid fall in oestrogen and progesterone levels at the end of the cycle causes the endometrium to be sloughed off in menstruation.
79
Define pregnancy (gravidity or gestation).
This is the period between conception (fertilisation) and birth.
80
What are the highlights of human pregnancy up to 9 days of conception?
30 hours after fertilisation:
- first cleavege
3-4 Days after Conception:
- The zygote, now called morula arrives at the uterus after a 4 inch journey through the fallopian tube.
- In the uterus the morula burrows itself into the endometrium (inner lining of uterus).
- The outside cells of the morula eventually grow to form the placenta.
6-7 Days after Conception:
- The morula, now called blastocyst attaches to the uterus, causing some women to feel implantation cramps.
7-9 Days After Conception:
- Pregnancy tests can detect the levels of HCG (human Chorionic Gonadotropin) hormone in the body.
- HCG, a protein hormone, is first produced in the second week of gestation to prevent menstruation and is most concentrated at 8 weeks gestation.
- Levels gradually decline after the 8th week.
81
What is the major occurrence at 2 weeks of pregnancy?
Most women can test positive for HCG urine pregnancy tests, at 95% accuracy.
82
What is the major occurrence at 3 weeks of pregnancy?
Baby-in-the-making is a ball of cells called a blastocyst.
Gastrulation occurs
83
What is the major occurrence at 4 weeks of pregnancy?
Organogenesis
84
What is the major occurrence at 5 weeks of pregnancy?
Heart begins to beat – at twice the rate of adults.
85
What is the major occurrence at 6 weeks of pregnancy?
Facial features (e.g. eyes and nostrils) begin to form, and little buds appear where arms and legs will develop
86
What is the major occurrence at 8 weeks of pregnancy?
Arms and legs are growing, as well as a nose and upper lip are formed
Notochord degenerates
87
What is the major occurrence at 9 weeks of pregnancy?
Eyes have developed, though eyelids are still fused and shut.
88
What is the major occurrence at 10 weeks of pregnancy?
The embryo has become a foetus
Vital organs – such as kidneys, intestines, brain, and liver – are starting to function.
Tiny fingernails and toenails are forming.
89
What is the major occurrence at 11 weeks of pregnancy?
Foetus is almost fully formed.
Bone templates are formed, external genitalia are developing.
90
What is the major occurrence at 12 weeks of pregnancy?
Baby’s heartbeat can be felt.
91
What is the major occurrence at 14 weeks of pregnancy?
Kidneys can release urine into the amniotic fluid.
92
What is the major occurrence at 15 weeks of pregnancy?
Baby can see light that filters in from outside the womb, even though the eyelids are still shut
93
What is the major occurrence at 16 weeks of pregnancy?
Baby's sex can be detected.
94
What is the major occurrence at 19 weeks of pregnancy?
Baby can hear mother’s heartbeat and sounds that come from outside the body, such as father's voice.
95
What is the major occurrence at 23 weeks of pregnancy?
Baby's sense of movement has developed, so s/he can feel the motion if mother dances.
96
What is the major occurrence at 27 weeks of pregnancy?
Baby can “practice breathing” by inhaling and exhaling amniotic fluid, and also open and close eyes.
97
What is the major occurrence at 34 weeks of pregnancy?
Baby is now considered full-term, lungs can work fine if born now.
98
What is the major occurrence at 40 weeks of pregnancy?
Baby is due and fully ready for life outside the womb.
99
Which hormone is used to confirm pregnancy?
Human Chorionic Gonadotropin (HCG) hormone
100
Why is HCG (pregnancy hormone) concentration very low at conception?
Before conception, HCG is secreted by the anterior pituitary and functions in a Luteinizing Hormone-like manner to promote ovulation and progesterone production during the menstrual cycle.
101
Why does HCG concentration increase rapidly at about 1 - 2 weeks after fertilization to a maximum at about 10 - 11 weeks of gestation?
At implantation, trophoblast cells secrete HCG to:
(i) maintain the corpus luteum.
(ii) stimulate the corpus luteum to continue secreting oestrogen and progesterone.
(iii) cause the blockage of any immune or macrophage action by mother on foreign invading placental cells.
(iv) cause uterine growth parallel to fetal growth.
(v) suppress any contractions by uterine wall during the course of pregnancy.
(vi) cause growth and differentiation of the umbilical cord
102
Why does HCG concentration decrease rapidly to a minimum at about 19 – 20 weeks, and remains relatively constant after the 20th week until about 40th week when it drops to zero?
As the embryo grows, the placenta increases in size causing increased secretion of progesterone, which takes over some of the roles of HCG causing its secretion to decrease.
A decrease in HCG causes degeneration of corpus luteum.
At the 40th week the foetus is expelled therefore HCG secretion stops.
103
Why is there a slight rise to a maximum at about 4-6 weeks after conception of Luteal progesterone followed by a rapid decrease thereafter to zero at 11 – 12 weeks?
A decrease in HCG causes degeneration of corpus luteum.
Without luteal progesterone, the lining of the uterus would slough off, ending the pregnancy.
104
What hormone which if deficient ends pregnancy?
(Luteal) progesterone; without it, the lining of the uterus would slough off, ending the pregnancy.
105
Why is placental progesterone absent at conception but concentration increases first slowly up to about 8-10 weeks, then rapidly to a maximum just before birth (40th week)?
As the corpus luteum and ovaries become inactive in the later stages of pregnancy, progesterone secretion is by the placenta.
As the pregnancy progresses, there is increased growth of the placenta, causing increased secretion of placental progesterone which:
(i) Inhibits contraction of the myometrium (promotes relaxation)
(ii) Increases mucus secretion in the cervix of the womb, forming a protective plug (promotes glandular activity in uterus)
(iii) Stimulates growth of maternal part of placenta.
(iv) Stimulates enlargement of the uterus.
(v) Inhibits FSH release, thus prevents ovulation and menstruation.
(vi) Causes enlargement of the breasts and growth of mammary glands.
106
Why should blood tests for pregnant women be done early morning before eating?
After a meal, progesterone levels drop greatly (about 50%)
107
Why is estrogen concentration very low at conception?
After ovulation, Oestrogen is initially secreted by the corpus luteum up to 12 weeks, hence the low and constant concentration.
108
Why does estrogen concentration remain relatively constant from conception to about 12 weeks and then increase rapidly?
The placenta takes over oestrogen secretion, therefore increased growth of the placenta causes increased secretion of oestrogen which:
109
Why aren’t prolactin and oxytocin secreted until after the 30th week?
High levels of oestrogen and progesterone inhibit the secretion of Prolactin from the anterior lobe of pituitary gland and Oxytocin from the posterior pituitary until birth.
110
Why does prolactin secretion start at about 31-32 weeks and increase, first slowly up to about 42 weeks then rapidly thereafter?
When levels of Oestrogen and Progesterone decrease after birth, prolactin causes lactation.
Towards parturition (birth), high levels of oestrogen promote uterine contractions and increased sensitivity of uterine wall to oxytocin, which causes the uterine muscle (myometrium) to contract.
111
Why does relaxin peak during the 14 weeks of the first trimester and at delivery?
To cause increased:
(i) Relaxation of ligaments, softening of cervix and inhibition of muscle contractions.
(ii) Cardiac output, renal blood flow, and arterial compliance.
112
Give an account of the role of the placenta as an endocrine organ in mammals.
It secretes various hormones which control development of the foetus:
- HCG (human chorionic gonadotrophin) causes the corpus luteum to continue secreting progesterone and oestrogen necessary for endometrial development for the first 3-4 months of pregnancy.
- Oestrogen prevents ovulation and menstruation, stimulates growth of mammary glands and increase in uterine muscle cells, and increases myometrium sensitivity to oxytocin
- Progesterone also stimulates growth of mammary glands, inhibits the contraction of uterine muscles and inhibits the release of prolactin (a hormone that stimulates milk production).
- Relaxin hormone relaxes the connective tissue in pelvic girdle to enlarge the cervix in preparation for birth.
113
What is the function of estrogen during pregnancy?
Oestrogen prevents ovulation and menstruation, stimulates growth of mammary glands and increase in uterine muscle cells, and increases myometrium sensitivity to oxytocin
114
What is the function of progesterone during pregnancy?
Progesterone also stimulates growth of mammary glands, inhibits the contraction of uterine muscles and inhibits the release of prolactin (a hormone that stimulates milk production).
115
What is the function of relaxin hormone?
Relaxin hormone relaxes the connective tissue in pelvic girdle to enlarge the cervix in preparation for birth.
116
Give an account of the role of the placenta as a non-endocrine organ in mammals.
- Digested food and other nutrients are transported through umbilical vein to link up with the foetal blood
- Waste foetal products diffuse from umbilical artery to maternal blood.
- Oxygen diffuses from umbilical vein to the foetal blood while carbondioxide moves in opposite direction.
- Antibodies cross the placenta from mother to foetus hence providing means by which passive immunity is acquired.
- It serves as a barrier to the transfer of solutes and blood components from maternal to foetal circulation.
- It prevents direct contact of maternal and foetal blood systems enabling them to operate at different pressures
117
How is the placenta suited for providing the developing fetus with nutrients?
- The finger-like projections which grow into the endometrium increase the surface area for exchange of substances.
- Closeness of maternal and foetal blood vessels facilitates faster diffusion of substances.
- Continuous flow of blood at the placenta ensures replacement of substances to maintain diffusion gradients for easy diffusion of these materials.
- Chorionic villi cells contain numerous mitochondria to provide energy required for active transport
118
Outline the transport mechanisms involved in the exchange of substances between mother and developing fetus.
Osmosis:
- Water
Simple diffusion:
- Respiratory gases (oxygen and carbondioxide)
- Nitrogenous wastes (urea) from the foetus
- Ions (of sodium, potassium, calcium) to a small extent diffuse.
Facilitated diffusion:
Glucose
Active transport:
- Ions (of sodium, potassium, calcium) largely move by active transport
- Amino acids
- Iron
- Vitamins
119
Outline the stages in the process of parturition (birth).
- The onset of birth is triggered by decreased progesterone and increased oestrogen levels during the last stages of pregnancy.
- The posterior pituitary produces Oxytocin, which causes contraction of the uterus that increase in force and frequency.
- Cervix dilates to allow passage of baby’s head into the vagina while embryonic membranes rupture.
- Foetus is expelled in down face position, followed by afterbirth (umbilical cord and placenta) expulsion.
120
Distinguish between contraception and birth control.
Contraception: use of methods which act to prevent fertilization of an egg by sperm
While
Birth control: a wide range of methods that prevent development of egg into foetus, whether it is already fertilized or not.
121
Give an outline of birth control methods in man.
Condom (for males and females):
Inserted on erect penis or into vagina before sexual intercourse
Diaphragm (cap):
Inserted into vagina before sexual intercourse
Spermicide:
Cream, foam or gel placed into vagina to kill sperm
Pill:
Combination of oestrogen and progesterone prevents ovulation and implantation
Morning after pill (emergency pill):
Used within 48 hours after sex.
Rhythm method:
Sex is avoided during ovulation period
Penis withdrawal (coitus interruptus):
Penis is withdrawn from vagina before ejaculation occurs
Vasectomy (males):
Sperm duct is cut and tied permanently
Ligation of oviducts:
Both oviducts are cut and tied permanently
Other e.g. intra-uterine device (IUD), plastic or copper device:
Prevents implantation
122
What is infertility?
This is the failure of a couple to conceive a pregnancy after trying to do so for at least one full year
123
Define primary infertility.
Here pregnancy has never occurred.
124
Define secondary infertility.
Here one or both members of the couple have previously conceived, but are unable to conceive again after a full year of trying.
125
What are the main causes of infertility?
1. Complex changes in the hypothalamus, pituitary gland and ovaries can cause hormone imbalance to cause ovulation disorders. It's the most common cause of female infertility.
2. Excess physical or emotional stress can disrupt the pattern of secretion of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) and affect ovulation – evidenced by irregular or absent periods.
3. Excessive overweight or underweight can disrupt the pattern of secretion of FSH and LH and affect ovulation.
4. Auto-immune response - the body mistakenly attacks ovarian tissues.
5. Premature loss of eggs from the ovary due to genetic problems or environmental insults such as chemotherapy causing ovulation failure, as well as a decreased estrogen secretion below 40 years.
6. Too much prolactin secretion which reduces oestrogen production and may cause infertility due to pituitary malfunction or medications taken for another disease.
7. Damage or blockage of fallopian tubes hence preventing sperm from getting to the egg or block the passage of the fertilized egg into the uterus.
8. Implantation failure due to fibroids/tumors, inflammation, abnormally shaped uterus, cervical narrowing.
9. Sometimes the cervix can't produce the best type of mucus to allow the sperm to travel through the cervix into the uterus.
10. Low sperm count: less than 5 million sperm per ml of semen 11.
11. Impotence: failure of the penis to erect or ejaculate
126
Describe the changes in blood before and after birth.
Before birth:
- Foetal haemoglobin has a higher affinity for oxygen than maternal haemoglobin
- In the foetus, blood bypasses the lungs via the ductus arteriosus, which connects the pulmonary artery to the aorta.
- Blood also bypasses the lungs, which are functionless by going through the foramen ovale connecting the two atria of the foetal heart.
- Blood from the left atrium passes into the left ventricle and into the aorta, which supplies blood to the body and the umbilical artery.
- Pressure in the foetal circulatory system is greatest in the pulmonary artery and this determines the direction of blood flow through the foetus and placenta.
After birth:
- In a few weeks of life, foetal haemoglobin is replaced by adult haemoglobin since it is less suitable as a means of gaseous exchange with air.
- at birth when the baby takes the first breath, there is increased partial pressure of oxygen in its blood together with the nervous reflexes occurring in its body results in the closure of ductus arteriosus.
- As a result of this, most of the blood vessels and the opening of pulmonary circulation results in the blood pressure in the left atrium exceeding that of the right atrium, causing the foramen ovale to close with the aid of a valve in its passage.
- Blood then passes from the right ventricle and pulmonary artery to the lungs.
127
What is the result of the failure of the closure of foramen ovale?
This is the reason why some children called blue babies bear a hole in the heart, where a portion of blood continues to bypass the lungs resulting in inadequate oxygenation of the tissues.
128
What major change would occur in the fetal circulation if blood pressure were highest in the aorta?
Blood would flow in the reverse direction along the ductus arteriosus.
129
What is the chorion?
It completely surrounds the foetus and is the foetal contribution to the placenta.
130
What is the amnion?
Forms a fluid filled amniotic cavity that cushions the foetus from shock and mechanical damage.
131
What is the yolk sac?
Contains little or no yolk, it is a temporary site for red blood cell formation.
132
What is the allantois?
Derived from embryonic hind gut, it contributes blood vessels that form the umbilical cord.
133
What are the main features of reproduction in birds?
- Fertilization is internal
- Mating is preceded by elaborate courtship displays
- Hard shelled eggs (cleidoic/amniotic eggs) are laid in the external environment
- Eggs are incubated usually by the mother as the embryo develops
- Newly hatched young ones are fed and cared for by the parents
134
How are birds suited for reproduction on land?
- Production of hard-shelled eggs for protection from mechanical damage
- Fertilization is internal to avoid drying up of eggs and wastage of gametes
- Newly hatched young ones are fed and cared for by the parents e.g. nest building, brooding e.t.c.
- Zygote develops within the amniote (cleidoic egg), which provides the embryo with a fluid-filled cavity in which it can develop on land.
135
Give similarities between embryo development in birds and mammals.
- Both contain yolk sac
- In both the embryo is surrounded by extra-embryonic membranes, which develop from tissues outside the embryo
- In both the embryo is cushioned in the fluid-filled amniotic cavity
- Embryo development is preceded by internal fertilization in both
- Allantois is involved in gaseous exchange.
136
Give differences between embryo development in birds and mammals.
In birds:
- Yolk sac is well developed nourish the foetus
- Allantois is a depository organ for nitrogenous wastes e.g. uric acid.
- Embryo is protected from damage by an outer shell.
- Yolk sac transfers digested food to the embryo.
- Allanto-chorion is lacking.
In mammals:
- Yolk sac is poorly developed since the foetus derives nourishment from the mother.
- Nitrogenous wastes e.g. urea diffuse into maternal blood.
- Outer shell is lacking around the developing embryo.
- Digested food is transferred by placenta.
- There is a developed allanto-chorion
137
State the forms of parental care provided by mammals.
- Protection from predators
- Feeding
- Provision of shelter
- Training of offspring.
138
What is the function of HCG hormone?
HCG (human chorionic gonadotrophin) causes the corpus luteum to continue secreting progesterone and oestrogen necessary for endometrial development for the first 3-4 months of pregnancy.
