Sexual Reproduction, Development and Growth

We will learn about reproductive system of humans,gametogenesis in humans,menstrual cycle,development of human foetus,formation of twins,health issues related to the human reproductive system and the growth in humans and animals.

Reproductive System of Humans

Humans reproductive system:


Gametogenesis in Humans

  1. Process of gamete formation is known as gametogenesis
  2. Gametogenesis that involved sperm formation is known as spermatogenesis, whereas those involved ovum formation is known as oogenesis

Necessity of gametogenesis:

  • Produces gametes and haploid chromosomal number through meiosis
  • Produces genetic variation in gametes
  • Maintains diploid chromosomal number of zygote
  • Produces genetic variation in offspring
  • During foetus development, primordial germ cells divide mitocially to form spermatogonia
  1. Each spermatogonia (2n) grows and develops into primary spermatocytes (2n)
  2. Each primary spermatocyte (2n) divides by meiosis I to form tqo secondary spermatocytes (n)
  3. Each secondary spermatocyte (n) divides by meiosis II to form two spermatids (n)
  4. Each spermatid (n) undergoes differentiation to form sperm (n)
  • The sperms formed move to epididymis to be matured
  • Head of sperm contains nucleus and acrosome. The nucleus contains chromosomes and the acrosome contains enzymes for fertilisation.
  • Midpiece of sperm contains abundant of mitochondria to produce more energy for movement
  • The tail helps sperm to move
  • During foetus development, primordial germ cells divide mitotically to form oogonia
  • Polar body formed has size much smaller than secondary oocyte or ovum
  • The three polar bodies will degenerate and do not take part in fertilisation
  • At first, primary oocyte that surrounded by follicle cells is known as primary follicle
  • At puberty, follicle stimulating hormone (FSH) secreted by pituitary gland stimulates the development of follicle in ovary
  • Follicle cells start to divide by mitosis during development of follicle. The primary follicle developed into secondary follicle and then into a mature follicle,graafian follicle
  • During the follicle development, primary oocyte undergoes meiosis I to form secondary oocyte.
  1. At foetal stage, the primordial germ cells divide repeatedly by mitosis to form many diploid oogonium (2n)
  2. Each oogonium grows ad develops to form primary oocyte (2n). Each primary oocyte is surrounded by a layer of follicle cells to form a primary follicle.
  3. Before birth, the primary oocytes undergoes an incomplete meiosis I until prophase I
  4. At puberty, one primary oocyte completes meiosis I to form a secondary oocyte (n) and a polar body (n). The haploid polar body formed also undergoes meiosis II to form another two haploid polar bodies
  5. Secondary oocyte is surrounded by follicle cells known as secondary follicle 
  6. Secondary follicle grows and matures to form graafian follicle which then moves to the surface of the ovarian wall
  7. Ovulation: The Grafiian follicle bursts and releases the secondary oocyte into Fallopian tube
  8. If fertilisation occurs, secondary oocyte completes meiosis II to form a polar body (n) and an ovum (n). All 3 polar bodies will degenerate.
Comparison between spermatogenesis and oogenesis:

Simillarities between spermatogenesis and oogenesis:

  • Occur in reproductive organs
  • Produce haploid gametes
  • Involved in fertilisation
Differences between spermatogenesis and oogenesis:
Aspects Spermatogenesis
Organ of gamete formation Testis
Gamete formed Sperm
Structure of gamete Head, midpiece and tail 
Ability to move Move with tail 
Presence of follicle cell No follicle cells surround the gamete
Number of gamete formed from one parent cell Four 
Formation of polar body None 
Size of gamete Small
Mitotic division on parent cell Occurs throughout life after puberty
Meiotic division during gamete formation Occurs continuously without a break
Differentiation Involves differentiation
Aspects Oogenesis
Organ of gamete formation Ovary
Gamete formed Ovum
Structure of gamete Spherical shape
Ability to move Assists by cilia in the Falllopian tube 
Presence of follicle cell Follicle cells surrounding the gamete
Number of gamete formed from one parent cell One
Formation of polar body One or three
Size of gamete Large
Mitotic division on parent cell Only occurs during foetal development stage
Meiotic division during gamete formation Occurs by stages reagrding to foetal,puberty and after fertilisation stage
Differentiation Does not involve differentiation

Menstrual Cycle

  1. Menstruation: Breakdown of the lining of uterine wall and its discharge through the vagina, with small amount of blood and cell
  2. Menstrual cycle:
  • A monthly cycle, consisting of follicle development,ovulation,thickening of the endometrium and menstruation
  • 28 days for each cycle

       3. Importance of menstrual cycle:

  • Prepares the uterine lining for the implantation of an embryo
  • Controls the development of ovum and ovulation
The hormones involved in controlling menstrual cycle :
Hormone Functions Effects
Follicle stimulating hormone (FSH)

• Stimulates follicle development in the ovaries

• Stimulates oestrogen secretion by the follicles

Control the changes in the ovaries
Luteinening hormone

• Stimulates ovulation

• Causes the formation of the corpus luteum

• Promotes the secretion of progesterone

Control the changes in the ovaries

• Repair and revival of of endometrium after menstruation 

• Stimulates FSH and LH secretion before ovulation

Control the changes in the uterus

• Stimulates the endometrium to become thick,folded and highly vasculated for embryo development

• Inhibits FSH and LH secretion

Control the changes in the uterus
Events of the menstrual cycle:
Time (days) In the ovary In the uterus
0-5 Follicle develops Menstruation
6-13 Follicle matures Endometrium repaired, thickens
14 Ovulation Endometrium continues to thicken
15-24 Corpus luteum develops Tissue and blood vessels develop in the endometrium
25-28 Corpus luteum degenerates as no fertilisation occur Endometrium will begin to disintegrate after 28th day

Premenstrual syndrome (PMS):
  • Headache
  • Breast swelling and pain
  • Stomach pain
  • Muscle or joint pain
  • Feeling tired
  • Sleep problems
  • Bloating
  • Food cravings
  • Constipation
  • Trouble with concentration or memory
  • Anxiety
  • Irritable
  • Tension
  • Depression
  • Crying spells
  • Mood swings
Menopause syndrome:
  • Occurs between 46 till 54 years old
  • Ovary is less sensitive to the stimulation of FSH and LH
  • No follicle developed and no ovum formed
  • low production of progesterone and oestrogen
  • Hot flashes
  • Sleep problems
  • Urinary problems
  • Vaginal problems and infections
  • Osteoporosis
  • Irregular periods
  • Night sweats
  • Weight gains
  • Memory problems
  • Mood swings
  • Depression
  • Anxiety

Development of Human Foetus

  • Early development of a human zygote:


  1. Definition: The union of sperm nucleus and ovum nucleus to form a zygote
  2. Fertilisation occurs at the Fallopian tube
  3. Ovum is surrounded by millions of sperms, but ony one will successfully penetrate the ovum wall
  4. When the penetration is successful, a barrier known as the fertilisation membrane is formed to prevent the penetration of other sperms
  • Early embryo development: 
  1. 36 hours after fertilisation: the zygote undergoes repeated mitosis as it travels along the Fallopian tube towards the uterus
  2. The zygote continuosly undergoes mitosis forming embryo with two cells,four cells and eight cells and resulting in the formation of a solid mass known as morula
  3. On the 5th day after fertilisation, the morula develops into a ball of 100 cells with a cluster of inner cells known as blatocyst. Only one part of blatocyst will develop into foetus
  4. Implantation: The blatocyst is implanted onto the endometrium, 7 days after fertilisation. The inner cell masss develops into an embryo. Embryo will absorb the nutrients directly form the endometrium tissue
  5. 8 weeks after fertilisation, a foetus is formed and obtains nutrients through placenta
  6. At 38 weeks after fertilisation, the foetus is well developed into full term, waiting for birth to take place

Role of human chorionic gonadotrotin (HCG) during pregnancy process:

  1. If fertilisation is occured, zygote formed divides into embryo
  • The embryo begins to produce human chorionic gonadotrotin (HCG) hormone for maintaining the function of corpus luteum
  • The corpus luteum continue to function by secreting progesterone and oestrogen in the first two months of pregnancy
  • The function of corpus luteum will be taken over by placenta at the 4th month of pregnancy

Functions of placenta and umbilical cord in the development of foetus:

  1. During foetus development, blatocyst forms chorionic villi into endometrium to obtain nutrients and oxygen from maternal blood
  2. Chorionic villi will form placenta in the 4th week of pregnancy
  3. Placenta is the site of substances exchange between foetal blood and maternal blood
  4. Useful substances such as nutrients,oxygen and antibodies are transported from maternal blood to foetal blood through placenta and umbilical cord
  5. While, waste substances such as urea and carbon dioxide are transported from foetal blood to maternal blood through placenta and umbilical cord
  6. Umbilical cord has umbilical arteries and umbilical vein

Necessity of having seperate foetal and maternal circulatory system:

  1. Protects foetus's fine blood vessels from being damaged by high maternal blood pressure 
  2. Prevents agglutination if blood group of foetus is incompatible to maternal blood group
  3. Filters certain pathogens and harmful substances from entering foetal blood

Formation of Twins

Types of twins Identical twins 
Fertilisation 1 ovum fertilised by 1 sperm
Zygote formation Formed from 1 zygote
Placenta Share one placenta but have their own umbilical cord
Embryo division Divide into two
Umbilical cords 2
Gender Same sex
Genetic constitution Identical
Physical characteristics Identical
Fingerprints Unique (different)
Types of twins Siamese twins
Fertilisation 1 ovum fertilised by 1 sperm
Zygote formation Formed from 1 zygote
Placenta Share one placenta but have their own umbilical cors
Embryo division Incomplete division
Umbilical cords 2
Gender Same sex
Genetic constitution Identical
Physical characteristics Identical
Fingerprints Unique (different)
Types of twins Fraternal twins
Fertilisation 2 ovum fertilised by 2 sperm
Zygote formation Formed from 2 different zygote
Placenta Each has their own placenta and umbilical cords
Embryo division Not divide into two
Umbilical cords 2
Gender Same/different sex
Genetic constitution Different
Physical characteristics Different
Fingerprints Unique (different)

Health Issues Related to the Human Reproductive System

Infertility in male and female:
Causes of interfility Description
Less sperm count Probability of fertilisation to occur is low
Abnormal sructure of sperm Sperms do not function and fertilisation does not occur
Blockage of sperm duct No sperm found in semen
Inactive sperm Sperm unable to move to Fallopian tube for fertilisation
Infection in testis No sperm formed or abnormal sperm formed
Impotence Sexual reproduction does not occur
  • Female:
Causes of infertility Description
No ovum formed Fertilisation does not occur
No ovulation No ovum is released and fertilisation does not occur
Blockage of Fallopian tube

Sperm could not fertilise with ovum

Uterus problems (abnormal structures,tumuor) Implantation difficult or does not occur
Irregular menstrual cycle Production of ovum and ovulation is affected

Overcoming infertility in male:

  1. Sperm bank
  • Donor's sperms are collected and stored frozen in liquid nitrogen at a temperature of -196 degree celcius
  • The sperms chosen by the couple will be injected into the woman's Fallopian tube during ovulation

       2. Artificial insemination (AI)

  • Due to the small amount of sperm, this technique will accumulate sperms over a certain period of time to achieve a large quantity
  • Sperms are injected into the Fallopian tube during ovulation

Infertility treatment:

  1. In vitro fertilisation (IVF)
  • A mature ovum is removed form the ovary and is fertilised by the sperm in a culture plate in the laboratory
  • Fertilisations occurs after 5-6 hours
  • The fertilised ovum is re-inserted into the uterus for the embryo implantation process
  • The baby born is known as a test tube baby

Growth in Humans and Animals

  1. Growth in multicellular organism such as humans and animals is an irreversible process which involve the increase in quantitive parameters such as size (height or length),body mass,volume and number of cells.
  2. Parameters for the growth of humans: body size,height and length
  3. Parameters for the growth of animals: body mass,fresh mass and dry mass

Growth in insects:

  1. Organism with exoskeleton such as insects,crabs and prawns undergo growth by either complete metamorphosis or incomplete metamorphosis
  2. In development of exoskeletal organisms, metamorphosis is a developement process from larval stage to adult stage
  3. During development stage, organism undergoes a moulting process known as ecdysis
  4. In complete metamorphosis, the growth of organisms involves four stages starts from egg to larva,larva to pupa and pupa to adult.
  5. In incomplete metamorphosis, the growth of organisms only involves three stages starts from egg to larva and from larva to adult
Sigmoid growth curve of an organism:
Phase Explanation
A (lag phase)
  • Slow growth rate
  • Little or no cell division
B (exponential phase)
  • Growth rate at the fastest
  • Cell division and cell elongation are active
  • The size of organisms increase rapidly
C (slow growth phase)
  • Steady growth rate
  • Limited by growth factor such as food shortage
D (stationary phase)
  • Form a plateau
  • The growth rate is zero
  • The organism reached maturity
  • Cell division occurs to replace the dead cells
E (death phase)
  • The organism undergoes ageing and death
Growth curve of insects:

  • Insects have a hard exoskeleton made of chitin
  • In order to growth in size, insects must undergo a moulting process known as ecdysis
  • The horizontal lines show zero growth periods known as instars
  • During ecdysis:
  1.    I: An insect breathes in a lot of air to expand the body
  2. II: The old exoskeleton breaks
  3. III: The insect expands by breathing in more air, before the new exoskeleton hardens
  4. IV: Ecdysis is repeated several times until the adult phase