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SIGNIFICANCE OF MEIOSIS:
It insures constant chromosome number to all species which reproduce sexually. This is because during gametes formation. The number of chromosomes is reduced to half and restored at fertilization.
(ii) It provide opportunities for new gene combination through chiasmata formation. Hence a mechanism of variation.
DIFFERENCE BETWEEN MEIOSIS AND MITOSIS
STAGE
MITOSIS
MEIOSIS
Prophase
– chromosomes not visible
– Homologous chromosome remain separate
– No chiasmata formation
– chromosomes visible
– Homologous chromosomes pair up
– chiasmata formation take place
Metaphase
– chromaitd pairs line up on the equator of the spindle centromores l
ine up on the equator
– this occur in metaphase II but no metaphase I
– centromere line equidislent aove and below the equater of the spindle
Anaphase
-chromatids separate number of chromosome present as parent cell
– Separated chromatids identical
– chromosomes separate in Anaphase I, chromatids separate in Anaphase II
– The separated chromosomes and chromatids may not be identical
– Half the number of chromosomes is present in daughter cell
Telophase
– Both homologous chromosomes are in each daughter cell
– only one of each pair of homologous chromosome is in each daughter cell
Occurance
– occurs in the formation of somatu cells
– Occurs in the formation of gametes and spores
STAGES OF SEXUAL REPRODUCTION
Sexual reproduction involves the following stages:-
1) gametogenesis
2) Copulation
3) Fertilization
4) Cleavage
5) Implantation
6) Pregnancy
7) Parturition(birth)
8) Parental case.
  1. GAMETOGENESIS.
Definition:
Gametogenesis is the general process of gametes formation in both male and female reproducing sexually.
  • Meiosis is the process by which gametes are formed can also be called gametogenesis literally ‘creation of gametes’.
  • The type of meiosis in male organism forms a spermatogonium to a primary spermatocyte a secondary spermatocyte
    a spermatid and finally a spermatozoid is spermatogenesis.
Definition:
Oogenesis is the process of meiosis in female organism from oogonium to a primary oocyte, a secondary oocyte and then an ovum (egg cell).
  • The primordial germ cells once they have been populated the gonalds proliferate into sperm (in testes) or ova (in the ovary).
SPERMATOGENESIS
  • In male testis there are tiny tubules (seminiferous tubules) containing diploid cells called spermatogonia that develop into mature spermatozoa (spermatozoa are the mature male gametes in many sexually reproducing organisms).
  • In spermatogenesis i.e. a process during which spermatogonia (sperm cells) multiply giving rise to other spermatogonia restoring their population and to other which mature to spermatocyte.
  • Around the periphery of the seminiferous tubules are located specialized cells known as spermatogonia.
Spermatogonia are diploid cells set aside early in embryonic development. They may divide by mitosis to generate more spermatogonia or by meiosis to produce spermatids each of which will differentiate into a mature sperm cell.
  • Spermatogonia destined to undergo meiosis first differentiate into primary spermatocytes which undergo two successive meiosis divisions.
  • After meiosis I the produced c
    ells are called secondary spermatocyte which each in turn undergoes the secondary division become spermatids each containing a unique set of 23 single chromosomes that ultimately mature into four sperm cells (spermatozoa).
  • The seminiferous tubular contain two types of cell
  1. Germ cells these undergo the two division of meiosis to form the spermatozoa
  2. Sertoli Cells: Acts as nerve cells ensuring that the germ cells have adequate nourishment.
EcoleBooks | BIOLOGY A LEVEL(FORM SIX) NOTES - REPRODUCTION 2
Fig: The stages of spermartozoa formation.
  • Spermatids undergo transformation in order to become spermatozoa
EcoleBooks | BIOLOGY A LEVEL(FORM SIX) NOTES - REPRODUCTION 2
Fig: Diagram showing the structure of part of the wall of seminiferous tubule.
EcoleBooks | BIOLOGY A LEVEL(FORM SIX) NOTES - REPRODUCTION 2
SPERMATOGENESIS
  • Occurs in seminiferous tubules.
  • Stored in epididymis.
Process:
  1. Diploid spermatogonia divide by mitosis from germinal epithelium (germinal epithelial cells).
  2. Some of them grow to produce diploid primary spermatocytes.
  3. Diploid spermatocytes undergo first meiotic division to form two haploid secondary spermatocytes.
  4. Haploid secondary spermatocytes undergo second meiotic division to form haploid spermatids.
  5. These grow in shape and become spermatozoan.
  6. The sertoli cells provide nutrition and protection against body immune system.
From the figure
The interior of the testis, site of spermatogenesis within the seminiferous tubules of the testis cells called spermatogonia develop into sperm, passing through spermatocyte and spermatid stages. Each

sperm passes as a long tail coupled to a head which contain a haploid nucleus.
EcoleBooks | BIOLOGY A LEVEL(FORM SIX) NOTES - REPRODUCTION 2
MECHANISM OF SPERMATOGENESIS
The process of spermatogenesis is divided into the following phases (as shown below):-
1. Multiplication phase.
2. Growth phase.
3. Maturation phase.
4. Metamorphosis.
1. MULTIPLICATION PHASE
  • Also known as spermatocytogenesis.
  • Here the sperm mother cells present in the germinal epithelium of the seminiferous tubules divide repeatedly by mitosis to form a large number of diploid rounded sperm mother cells called spermatogonia.
Some of these sex cells move towards the lumen of seminiferous tubules and enter growth phase. These cells are called primary spermatocytes. The primary spermatocytes are diploid and contain (44 + XY)

chromosomes.
Some of these cells produced by the division of spematogonia remain in the original condition and continues to divide giving rise to primary spermatocytes such cells are known as stem cells.
  1. GROWTH PHASE
During this phase, spermatocyte as well as its nucleus enlarges in size. It gets ready to undergo meiotic division.
3. MATURATION PHASE
Each diploid primary spermatocyte undergoes meiosis I which is a reduction division.
Two daughter cells are formed with ‘n’ number of chromosomes. The daughter cells are called secondary spermatocytes are haploid and much smaller comparatively containing (22 + X) or (22 + Y)

chromosomes.
The secondary spermatocyte undergoes the second meiotic division (equational). This results in the formation of four daughter cells known as spermatids
4. METAMORPHOSIS.
The spermatids formed as a result of maturation division in a typical animal cell with all the cell organelles present in it. In this form it cannot function as a male gamete. So many changes take place to change

the non – motile spermatid into motile spermatozoa.
The main aim of the changes is to increase the motility of the sperm. These changes are:-
  1. Nucleus shrinks by losing water and DNA becomes closely packed.
  2. An acrosome is formed from the Golgi complex.
  3. An axial filament of the tail of the spermatozoa is formed from the distal centriole of the spermatid.
  4. Mitochondrial ring is formed from the mitochondria around the distal centrioles and is called.
  5. Much of the cytoplasm of the spermatid is lost and the remaining cytoplasm forms a sheath around the mitochondrial spiral. This is known as manchette.
  • During the process of differentiation, the developing sperms have their head embedded in the sertoli cells which are thought to provide nutrition for the developing sperms because their cytoplasm contains large stores of glycogen which diminish as spermatid mature.
[NB: There is no direct evidence for this nutritional function of the sertoli cells, but some sperms of male sterility are associated with the failure to product normal sertoli cells]
EcoleBooks | BIOLOGY A LEVEL(FORM SIX) NOTES - REPRODUCTION 2
Cellular events in human spermatogenesis
Sertoli cells support the developing gametes in the following ways:-
  1. Maintain the environment necessary for development and maturation via the blood test is barriers.
  2. Secretes substances initiating meiosis.
  3. Secretes supporting testicular fluid.
  4. Secrete the androgen – binding protein which concentrates high quantities of testosterone in close proximity to the developing gametes.
Testosterone is produced by intestinal cells (leydig cells) which reside adjacent to the seminiferous tubules.
  1. Secrete hormones affecting pituitary gland control of spermatogenesis, particularly the polypeptide hormone, inhibin.
  2. Phagocytise residual left over from spermiogenesis.
  3. Release anti – mullerian hormone (AMH) which prevents formation of the mullerian duct/oviduct.
NB: Seminiferous epithelium in sensitive to elevated temperature in humans and will be adversely affected by temperature as high as normal body temperature.
Consequently, the testes are located outside the body in a sack of skin called the scrotum. The optimal temperature is maintained at 20C (man) -80 C below body temperature.
This is achieved by regulation of blood flow and positioning towards and away from the heat of the body by the cremasteric muscle and dartos smooth muscles in the scrotum.
  • Dietary deficiency (such as vitamins B, E and A), anabolic steroids, metals (calcium and lead) X – ray exposure, dioxin, alcohol and infectious diseases will also adversely affect the rate of spermatogenesis.
  • The hormonal control of spermatogenesis varies among species. In humans, the mechanisms are not known completely, however, it is known that initiation of the spermatogenesis occurs at puberty due to the interaction of the hypothalamus pituitary gland and leydig cells.
  • The hormones that are closely related to spermatogenesis are the lutenizing hormone, the follicle stimulating hormone (FSH) and testosterone (T).
  • LH controls spermatogenesis via the secretion of testosterone by leydig cells (3, 4, 5). Testosterone mainly acts onto sertoli cells by increasing their responsiveness to FSH and simultaneously inhibits the secretion of LH by the mechanism of feed back upon the hypothalamus and the pituitary.
  • FSH controls the maturation of the spermatic epithelium by acting directly on the sertoli cells.
  • Finally the protein which binds to the androgens (ABP) is produced by the sertoli cells.
EcoleBooks | BIOLOGY A LEVEL(FORM SIX) NOTES - REPRODUCTION 2

Hormonal interaction in the hypothalamus pituitary
  • FSH is necessary to develop the ABP production by the sertoli cells and to develop the blood testis barrier and other functions of these cells.
Once the sertoli function is developed, testosterone alone will maintain spermatogenesis. The yield of spermatozoa is increased if FSH is present.
  • The FSH is known to increase the yield of spermatogonia by preventing atresia of differentiating spermatogonia.
Normally 50% of spermatogonia can also be reduced by increased sexual activity.
FSH levels in males are environmentally influenced, increased by sexual activity and decreased by inhibin.
  • Androgens are transported from the site of production (leydig cells) to influence the developing germ cells.
  • ABP produced by the sertoli cells and shed into the adluminal compartment, assists in the role as well as transporting large amount of androgens to epididymis.
First stimulates ABP synthesis under the action of androgen influence.
Testosterone induces and maintains spermatogenesis acting through the sertoli cells or through spermatogenetic cells.
  • The testis also secretes some other hormones that participate in the regulation of spermatogenesis, but their cells are not closely understood. These include:-
i) Estradiol formerly known as female sex hormone. These estradiol receptors are widely distributed in testicular cells, suggesting a role of oestrogens in the regulation of testicular function.
The receptors are localized in the nuclei of spermatogonia, spermatocytes and early developing spermatids of adult men.
ii) Inhibin – (Inh – b), this is produced by the sertoli cells and controls the secretion of FSH from the pituitary and consequently the spermatogenesis, via a negative feedback mechanism. Low blood concentration of inh – b of ten reflect in a disorder of spermatogenesis.
iii) Antimullerian hormone
Exclusively secreted by the sertoli cells and represents a prelocious hormonal index of their function.
Its production is influenced by transcriptional factors testosterone, FSH and spermatocytes at prophase I. It prevents formation of mullerian duct.
SUMMARY:
Mechanism of hormonal control of spermatogenesis:
  • The hypothalamus secretes gonadotrophin releasing hormone (GnRH) which travels in a small vein from the hypothalamus to the pituitary gland.
  • GnRH stimulates in turn the anterior pituitary gland to secrete two hormones know as gonadotrophins. (A gonadotrophin is a hormone that stimulates a gonad, in this case the testis). These gonadotrophins are follicle stimulating hormone (FSH) and lutenizing hormone (LH). Also secreted in female they are glycoproteins.
  • FSH acts by stimulating spermatogenesis by stimulating the sertoli cells to complete the development of spermatozoa from spermatids.
  • LH stimulates the synthesis of the hormone testosterone by the leydig cells (interstitial cells) of the testis. It is therefore known interstitial cells stimulating hormone (ICSH) in the male
  • Testosterone stimulates growth and development of the germinal epithelial cells (spermatogonia) to form sperms and also work with the FSH to stimulate the sertoli cells.
The negative feedback mechanism operates where by an increase in the level of testosterone results in a decrease in secretion of GnRH from the hypothalamus, this in turn results in declining levels of LH and FSH.
The testosterone also acts directly on the anterior pituitary gland to reduce LH secretion but this effect is weaker.
When the rate of spermatogenesis in high, inhibin (a glycoprotein hormone) is released, it acts on the anterior pituitary gland to reduce the secretion of FSH by negative feedback mechanism.
It also has a slight effect in the hypothalamus reducing GnRH secretion. When the rate of spermatogenesis low, inhibin is not secreted and FSH stimulates spermatogenesis.
THE ROLE OF CYCLIC AMP
Both FSH and LH acts by causing the release of cyclic AMP (Adenosine monophosphate) within the cells they stimulate.
Cyclic AMP is the second messenger system. It is released into the cytoplasm and then passes to the nucleus where it stimulates the synthesis of enzymes. In the case of LH, for example enzymes are

involved in the synthesis of testosterone from cholesterol.
EcoleBooks | BIOLOGY A LEVEL(FORM SIX) NOTES - REPRODUCTION 2

STRUCTURE OF MATURE HUMAN SPERMATOZOANS
Structurally, a spermatozoan is divided into three pieces:-
I) Head piece
  • This consists of a nucleus and small portion of
    cytoplasm.
  • At the tip of the head, there is a special structure called acrosome. Acrosome is a collection of lysosomes; it thus contains very powerful hydrolytic enzymes known as proteases and hyaluronidases.
II) Middle piece
This is largely consisting of mitochondria. These provide energy for propelling the spermatozoans towards the egg cell. The head and middle peace together constitute the principal peace.
III) The tail piece
  • It consists of the flagellum made of axial fillaments that continue from middle peace. The flagellum serves in:-
  • Propelling the spermatozoans towards the egg cell.
  • Orienting the spermatozoans so that it properly binds itself into the egg cell.
  • At the end of the flagellum is a hair like extension called the end piece.
EcoleBooks | BIOLOGY A LEVEL(FORM SIX) NOTES - REPRODUCTION 2


Structure of mature human sperm
ROLE OF SPERMATOZOAN.
  • The role is to carry the paternal gamete materials into the egg cell so that after fertilization, the genetic makeup of the zygote is the mixture of the two maternal and paternal gamete materials.
HORMONAL CONTROL OF SPERM PRODUCTION
  1. When the level of testosterone is low, the hypothalamus secretes a releasing hormone (called gonadotrophin releasing hormone or GnRH) in the blood.
  2. This peptide hormone flows in the blood directly to the pituitary a pre-sized organ hanging from the base of the brain where it stimulates the two peptide hormone, lutenising hormone (LH) and follicle stimulating hormone (FSH).
  3. These hormones (called gonadotrophins, since they stimulate gonalds) then move through blood stream and activate cells in the testis. LH triggers the intestinal cells to produce and secrete testosterone.
  4. FSH cause supporting cells (sertoli cells) to enhance formation of sperms.
  5. Soon the sperm count rise. Mean while, testosterone circulate in the blood stream at higher levels and the interconnected loop feeds back on itself. High testosterone levels signal the hypothalamus to produce less releasing hormone.
  6. This inturn suppress the release of LH and FSH and without them less testosterone and fewer sperms are manufactured. In addition testosterone causes supporting cells in the testes to release the peptide hormone inhibin, which helps to inhibit FSH production.
When testosterone level drops too low again the hypothalamus is once more activated and the whole cycle starts again.
EcoleBooks | BIOLOGY A LEVEL(FORM SIX) NOTES - REPRODUCTION 2
ADAPTATIONS OF THE SPERMATOZOANS.
The adaptations of the spermatozoans to its function include the following:-
  1. It has an acrosome that contains enzymes for digesting the egg cell membrane.
  2. It has numerous mitochondria that produce energy necessary for propelling the spermatozoans towards the egg cell.
  3. It has flagellum for propelling the spermatozoans for proper binding on the egg cells.
  4. Ability to sense the chemical attractants secreted egg cell so that its movement is directed toward source of chemicals.
  5. Ability to recognize and hence bind itself into the receptor sites on the surface of egg cells.
  6. Light nuclei and head piece following their changes, this enables it to move faster towards the egg cell.
OOGENESIS
  • Oogenesis begins soon after fertilization as primordial germ cell travel from the york sac to the gonalds, where they to proliferate mitotically.
  • The gem cells multiply from only a few thousands to almost 7 million.
  • They become oocytes once they enter the stages of meiosis several months after birth, now called primordial germ cells surrounded by fellicle cells from the somatic line. The oocytes are then arrested in the first meiotic phase until puberty.
  • At puberty between 4 to 10 follicles begin to develop although only 1 – 2 are actually released.
  • Surrounding each oocyote is a zona pellucida membrane granule and the cell layer.
  • Each oocyte finishes its first meiotic division creating a secondary oocyte and polar body which serves no further functions.
  • It begins the next meiotic cycle and is arrested in its second metaphase, at which point it is released from the ovary in ovulation.
  • It will not finish the meiosis cycle until it encounters the stimuli of a sperm.
SUMMARY:
Oogenesis
At birth.
  1. Diploid cells in the ovary divide by mitosis from oogonia.
  2. Oogonia undergo meiosis I division to form primary oocyte steps at prophase I.
  3. These remain in the follicles.
At puberty.
  1. Primary oocyte completes the 1st complete meiotic division to form polar bodies and secondary oocyte.
  2. Secondary oocytes undergo 2nd meiotic division and steps at metaphase II.
  3. Secondary oocyte is shed from ovary, if fertilized it complete its 2nd meiotic division to form ovum and polar bodies.
  • Formation of the ovum involves substantial increase in cell volume as well as the acquisition of organelles that adapt the egg for reception of the sperm nucleus and support of the early embryo.
  • In the fetal ovary of mammals the oogonia undergoes meiotic divisions until the birth of the foetus, but the process involves the destruction of the majority of the developing ova by the seventh month of gestation reduces the number of oocytes from millions to a few hundred around the time of birth, the mitotic divisions ceases altogether and the fast female contains its full complement of potential ova.
Weak of gestation
Stages
No of germ cells
¾
Primordial cells in the endoderm of the yolk sac.
5 – 6
Premeiotic cells oogonia.
10,000
8
Propagation by mitosis.
500,000
8 -20
Mitosis, meiosis atresia maximum at week 20.
6 – 700,000
20 -40
Reduction of oocyte, 80% of germ cells are lost.

1-2,000,000
Birth to puberty
Further oocytes are lost by atresia.
  • Unlike the formation of sperm in which the two divisions of meiosis produce four equivalent daughter cells, the cytoplasm of the oocyte is divided unequally so that three polar bodies with reduced
cytoplasm and one oocyte are final products.
  • Egg cytoplasm contains large stores of ribonucleic acid RNA in the form of ribosomal messenger and transfer RNA. These RNA’s direct the synthesis of proteins in the early embryo and have decisive
influence on the course of development.
DEVELOPMENT OF GERM CELLS IN THE OVARY
Following the immigration of the primordial germ cells into the gonadal ridge, they proliferate are enveloped by coelomic epithelial cells and form germinal cords that though keep their connection with the

coelom epithelium.
In the genital primodium, the following processes take place;
  • A wave of proliferation begins that lasts from 15th weak to the 7th month.
Primary germ cells arise in the cortical zone via mitosis of oogonia dones, bound together in cellular bridges that happen in rapid succession.
The cell bridges are necessary for a synchronous onset of the subsequent meiosis.
  • With the onset of meiosis earliest in the prophase in the 12th week the designation of the germ cells change. They are now called primary oocyte.
  • The primary oocyte become arrested in the diplotene stage of prophase I the prophase of the 1st meiotic division.
  • Shortly before birth, all the total oocytes in female ovary have attained this stage.
The meiotic resting phase that then begins is called the dictyotene and it lasts till puberty during which each month and each month thereafter until menopause a pair of primary oocyte complete the first meiosis.
  • Only few oocytes, secondary oocyte plus one polar body though reach the 2nd meiosis and the subsequent ovulation. The remaining oocytes that mature each month become atretic.
  • The primary oocytes that remain in the ovaries stay in the dictyotene stages up to menopause. In extreme cases without ever maturing during the menstrual cycle.
  • From birth, there are thus two different structures to be distinguished that at least conceptually do not develop further synchronously.
  1. Female germ called primary oocyte and which can develop further only during and after puberty hormonal cycle is necessary.
  2. Follicular epithelium, that can develop further from the primordial follicle via several follicle stages while oocytes remain in their primary states.
  • The developmental sequence of the female germ cell is as follows:-
EcoleBooks | BIOLOGY A LEVEL(FORM SIX) NOTES - REPRODUCTION 2
The continous of the development/maturation of the oocyte begins again only a few days before ovulation.
  • The developmental sequence of a follicle goes through various follicle stages.
EcoleBooks | BIOLOGY A LEVEL(FORM SIX) NOTES - REPRODUCTION 2
  • Since the follicle can die at any moment in this development (atresia) not all reach the tertiary follicle stage.
EcoleBooks | BIOLOGY A LEVEL(FORM SIX) NOTES - REPRODUCTION 2
PRIMARY OOCYTE
  • In the first week of the cycle the maturation of the oocyte in its associated follicle depends on the progress of maturation of the surrounding follicle cell.
  • The fittest follicle with its oocyte becomes the dominant follicle in the second cycle week and later the graafian follicle.
  • Up to just days before ovulation, the maturation of the oocyte consists in its ingestion of substances growth of the yolk, they are supplied by the granulose cells. This exchange of substances is mediated through cytoplast processes of the granulose cells that are anchored through zona pellucida at the ocoyte substance.
  • The oocyte nucleus is also matured in the last days before the LH peak.
Up to this point it was arrested in the extremely elongated prophase (a dictyotene) of the first meiotic division, the corrested condition that has existed since the foetal period.
  • Through the maturation the nucleus stages in the darkness of the prophase and prepares itself for the completion of the first meiosis which is triggered by the LH peak.
With the LH peak, the following maturation steps are now triggered in and around the oocyte up to ovulation.
In the oocyte:
  • Termination of the first meiosis with ejection of the first polar body.
  • Begin of the 2nd meiosis with arrest in the metaphase.
  • Maturation of the oocyte cytoplasm by preparing molecules and structures that will be needed at the time of fertilization.
In the follicle:
  • The granulosa cells that sit just outside on the zona pellucida withdraw their processes from oocyte surface back into the pellucida zona. These processes were in charge of transferring substances to
the oocyte.
  • The periritelline space forms between the oocyte and the pellucida zona. This space is necessary for allowing division of the oocyte and for harbouring the first polar body formed in the division.
  • Loosening of the granulosa cells in the vicinity of the cumulus oophorus and proliferation of the granulosa cells.
  • Increasing the progesterone concentration in the follicle fluid via increased production in the granulosa cells.
Termination of the first meiosis
  • The spindle apparatus for dividing the chromosomes has formed and oriented itself radically in the cellular surface.
  • The first polar body will arise at the spot where the spindle apparatus is anchored on the cellular surface.
SPERMATOZOA
EGG CELL
Small in size.
Larger than a sperm.
Has a large nucleus.
Has a smaller nucleus.
Has a very small amount of cytoplasm.
Has a very large amount of a cytoplasm.
No food reserves (does not store food).
Stores large amount of food.
Has acrosome.
Has no acrosome.
No cortical granules.
Has cortical granules.
Has head, middle piece principal and end piece.
No such division.

No microvilli.
Has microvilli.
Single layered.
Multi layered.
It has flagellated and motile.
It lacks flagellum and non motile.
It has numerous mitochondria.
Has few mitochondria.
SPERMATOGENESIS
OOGENESIS

Differentiation follow offer its meiotic division they are farmed only until the end of meiosis.
– Egg grows primarily in extend period of prophase i.e. prophase that is secondary oocyte is already matured.
It occurs in male gonads i.e. testis.
– It occurs in female gonad i.e. ovaries.
Four sperms are produced from one spermatogonium.
– Only one ovum is produced from one oogonium.
The spermatocyte sperm mother cell divides by meiotic division into four equal sized cells and all the four cells are transformed into spermatozoa to act as reproductive unit.
-The oocyte divides unequally and produces a large sized ovum and three small sized bodies or polocytes which are sexually inert only ovum acts as reproductive unit.
Spermatozoa are produced in large number.
-Ova are produced in large number.
Spermatozoa are minute yolkless and motile.
– Ova are much large, often with yolk and non-motile.
Continous production process although from puberty to old age sperm cells are being endangered, the production is subject to extreme fluctuations regalding both quality and quantity.
– Using up the oocyte generated before birth, continual decrease of the oocyte, beginning with the foetal period-exhaustion of supply at menopause.
During foetal period no meiotic division, no germ cell production.
– During focal period, entering meiosis (arrested in dictyotene stage), there is a production of entire supply of germ cells.
i) Failure to ovulate due to hormonal causes.
– Absence of sperm due to blockage of tubes between the testes and seminal vesicles.
ii)Uterus damage, pregnancy cannot be maintained (occurrence of miscarriage).
– Low sperm count.
iii)Damage to the oviduct due to tubal diseases hence oviduct is blocked.
– Production of abnormal sperm.
iv) Cervix damage due to abortion or difficult birth hence inability to produce cervical mucus for sperm to reach the egg.
– Autoimmunity: Antibodies are made by the male body which attract the sperm reducing sperm count.
v)Antibodies to sperm.
– Impotence.
Placenta as a link
Placenta as a barrier
  • O2, H2O, food, salt from the mother to the foetus.
  • Prevent blood mixing.
  • CO2 and other nitrogenous wastes from foetus to mother.
  • Prevent high maternal blood affecting foetus directly.
  • Partly antibodies from the mother to the foetus.
  • Filter out some hormones
  • Partly filter out some pathogens.
IDENTICAL TWINS
NON- IDENTICAL TWINS
They result from one zygote.
Results from two different zygotes.
Share the same placenta.
Each has its own placenta.
Enclosed in the same membrane.
Each has its own membrane.
They are of the same genetic makeup.
Genetically different.
They are of the same sex.
They may be of different sexes and can be a girl/boy.
EcoleBooks | BIOLOGY A LEVEL(FORM SIX) NOTES - REPRODUCTION 2
OESTRUS CYCLE
MENSTRUAL CYCLE
Common to lower mammals eg. Cats, dogs, caw etc.
– Is characteristics to the higher mammals of the order primate eg. Human beings, chimpanzee, gorilla, monkey etc.
In oestrus cycle, the endometrium cycle is absorbed if conception does not occur during the cycle.
-Animals with menstrual cycle shed the endometrium through menstruation.
In species with oestrus cycle, females are generally only sexually active during the oestrus cycle. This is referred to as in heat.
– Females of species with mentrual cycle can be sexually active at any time in their cycle even when they are not ab
out to ovulate.
Period of heightened sexual activity conciding with ovulation is the most prominent event.
– Menstruation, the discharge of blood and uterine lining is the most prominent event.
Occurs less frequently eg. once per year.
– Occurs more frequently than oestrus cycle eg. once every month.
EcoleBooks | BIOLOGY A LEVEL(FORM SIX) NOTES - REPRODUCTION 2
EcoleBooks | BIOLOGY A LEVEL(FORM SIX) NOTES - REPRODUCTION 2


EcoleBooks | BIOLOGY A LEVEL(FORM SIX) NOTES - REPRODUCTION 2

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