1. HUMAN DEVELOPMENT Dr. HARI RAM A/L RAMAYYA
OBSTETRICIAN & GYNAECOLOGIST,
HOSPITAL TUANKU FAUZIAH,
KANGAR, PERLIS.

2. Outline
FERTILIZATION
EMBRYOGENESIS
ORGANOGENESIS

3. Embryology The study of the developmental events
that occur during the prenatal period
Begins with a single fertilized cell that
divides to produce all of the cells in the
body.

4. The Prenatal Period The first 38 weeks of human development,
which occurs between fertilization and birth.
The pre-embryonic period is the first 2 weeks
of development when the zygote becomes a
spherical, multicellular structure.
The embryonic period includes the third
through eighth weeks of development during
which all major organ systems appear.

5. The Fetal Period Includes the remaining weeks of
development prior to birth
The fetus continues to grow
Its organs increase in complexity

6. The Stages of Embryogenesis
Cleavage The zygote divides by mitosis to
form a multicellular structure called a
blastocyst.
Gastrulation The blastocyst cells form three
primary germ layers, which are the basic
cellular structures from which all body tissues
develop.
Organogenesis  The three primary germ
layers arrange themselves in ways that give
rise to all the organs within the body.

7. Cell Division by Mitosis
Mitosis is a type of cell division that produces
genetically identical cells.
During mitosis DNA replicates in the parent
cell, which divides into two new cells, each
containing an exact copy of the DNA in the
parent cell.
The only source of genetic variation in the
cells is via mutations.

8. Gametogenesis Following birth, an individual undergoes maturation.
 the body grows and develops
 the sex organs become mature
 the sex organs then begin to produce
gametes

9. Chromosomes Human somatic cells contain 23 pairs of
chromosomes for a total of 46.
22 pairs of autosomes and one pair of sex chromosomes.
Autosomes contain genetic information for most human characteristics.
A pair of similar autosomes are called homologous chromosomes.

11. DNA and Chromosomes
The DNA molecule in a single human cell is 99 cm long, so is 10 000 times longer than the cell in which it resides (< 100mm).
Since an adult human has about 1014 cells, all the DNA is one human would stretch about 1014 m, which is a thousand times the distance between the Earth and the Sun.

12. Different species have different number of chromosomes, but all members of the same species have the same number.
Humans have 46, chickens have 78, goldfish have 94, fruit flies have 8, potatoes have 48, and so on.
The number of chromosomes does not appear to be related to the number of genes or amount of DNA.

13. The chromosomes are numbered from largest to smallest.
Chromosomes come in pairs, called homologous pairs ("same shaped"). So there are two chromosome number 1s, two chromosome number 2s, etc, and humans really have 23 pairs of chromosomes.
Homologous chromosomes are a result of sexual reproduction, and the homologous pairs are the maternal (inherited from the mother) and paternal (inherited from the father) versions of the same chromosome, so they have the same sequence of genes

14. One pair of chromosomes is different in males and females
One pair of chromosomes is different in males and females. These are called the sex chromosomes, and are non-homologous in one of the sexes. In humans the sex chromosomes are homologous in females (XX) and non-homologous in males (XY). (In birds it is the other way round!) The non-sex chromosomes are sometimes called autosomes, so humans have 22 pairs of autosomes, and 1 pair of sex chromosomes.

15. Diploid Cells
A cell is said to be diploid if it contains 23 pairs of chromosomes.
2N = 46

16. The Sex Chromosomes
The pair of sex chromosomes determines whether an individual is female (XX) or male (XY).
One member of each pair of chromosomes is inherited from each parent.

17. Gametogenesis Begins with meiosis.
Produces secondary oocytes in the female.
Produces sperm in the male.

18. Meiosis
A type of cell division that starts off with a diploid parent cell and produces haploid daughter cells (sperm or eggs/ova).

20. Meiosis 1 Meiosis results in the formation of gametes (sex cells).
In meiosis I, homologous chromosomes are separated after synapsis and crossing over occurs.
In meiosis II, sister chromatids are separated in a sequence of phases that resembles mitosis.

21. Interphase
                                             
This is when the cell is not dividing, but is carrying out its normal cellular functions.
chromatin not visible
DNA, histones and centrioles all replicated
Replication of cell organelles e.g. mitochondria, occurs in the cytoplasm.

22. Prophase 1
Homologous, double-stranded chromosomes in the parent cell form pairs (synapsis).
The actual pair of homologous chromosomes is called a tetrad.
Crossing over occurs between the maternal and paternal chromosomes.

23. Prophase
                                
chromosomes condense and become visible – this prevents tangling with other chromosomes.
Due to DNA replication during interphase, each chromosome consists of two identical sister chromatids connected at the centromere
centrioles move to opposite poles of cell
nucleolus disappears
phase ends with the breakdown of the nuclear membrane

24. Metaphase 1
The homologous pairs of chromosomes line up above and along the equator of the cell.
Forms a double line of chromosomes.
Alignment is random with respect to maternal or paternal origin.

25. Metaphase
                                
spindle fibres (microtubules) connect centrioles to chromosomes
chromosomes align along equator of cell and attaches to a spindle fibre by its centromere.

26. Anaphase 1
Pairs of homologous chromosomes separate and are pulled to the opposite ends of the cell.

27. Anaphase centromeres split, allowing chromatids to separate
                                
centromeres split, allowing chromatids to separate
chromatids move towards poles, centromeres first, pulled by kinesin (motor) proteins walking along microtubules (the track)
Numerous mitochondria around the spindle provide energy for movement

28. Telophase 1 & Cytokinesis
Nuclear division finishes and the nuclear envelopes re-form
The cytoplasm divides
Two new haploid cells are produced

29. Telophase spindle fibres disperse
                                
spindle fibres disperse
nuclear membranes from around each set of chromatids
nucleoli form

30. Cytokinesis
                                           
In animal cells a ring of actin filaments forms round the equator of the cell, and then tightens to form a cleavage furrow, which splits the cell in two.
In plant cells vesicles move to the equator, line up and fuse to form two membranes called the cell plate. A new cell wall is laid down between the membranes, which fuses with the existing cell wall.

32. Prophase II Resembles the prophase stage of mitosis.
In each of the two new cells, the nuclear membrane breaks down, and the chromosomes collect together.
Crossing over does not occur in this phase.

33. Metaphase II
The double-stranded chromosomes form a single line in the middle of the cell.
Spindle fibers extend from the centrioles at the poles to the centromere of each double-stranded chromosome.

34. Anaphase II
The sister chromatids of each double stranded chromosome are pulled apart at the centromere.
Each chromatid (single strand) is pulled to the opposite pole of the cell.

36. Telophase II & Cytokinesis
The single-stranded chromosomes arrive at opposite ends of the cell.
A cleavage furrow forms and the cytoplasm in both cells divides, producing a total of four haploid daughter cells.
These daughter cells mature into sperm
in males or oocytes in females.

37. Oogenesis
In females, the sex cell produced is called the secondary oocyte.
This cell will have 22 autosomes and one X chromosome.

38. Oogenesis
The parent cells that produce oocytes are called oogonia and they reside in the ovaries.
Oogonia are diploid cells.
All the oogonia start the process of meiosis and form primary oocytes prior to birth.
They are arrested in Prophase I and remain this way until the female reaches puberty.
Each month usually only one becomes a secondary oocyte.

39. Oogenesis
Only the secondary oocyte has the potential to be fertilized.
The secondary oocyte is ovulated
The corona radiata and the zona pellucida
form protective layers around the secondary oocyte.

40. Oogenesis
If the secondary oocyte is not fertilized, it degenerates about 24 hours after ovulation, still arrested in metaphase II.
If the secondary oocyte is fertilized, it first finishes the process of meiosis. Two new cells are produced, and as before, the division of the cytoplasm is unequal.

41. The cell that receives very little cytoplasm becomes another polar body and eventually degenerates.
The cell that receives the majority of the cytoplasm becomes an ovum which can be fertilized.

42. Oogenesis
Typically, only one secondary oocyte is expelled (ovulated) from one of the two ovaries each month.
The left and right ovaries alternate ovulation each month.

43. Spermatogenesis
The parent or stem cells that produce sperm are called spermatogonia.
Spermatogonia are diploid cells that reside in the the testes.
Each one first divides by mitosis to make an exact copy of itself called a primary spermatocyte.

44. Spermatogenesis
Primary spermatocytes then undergo meiosis and produce haploid cells called spermatids.
Spermatids contain 23 chromosomes, but they still must undergo further changes to form a sperm cell.
In spermiogenesis, spermatids lose much of their cytoplasm and grow a long tail called a flagellum.

45. Spermatogenesis
The newly formed sperm cells are haploid cells that exhibit a distinctive head, a midpiece, and a tail.
From a single spermatocyte, four new sperm are formed.
All sperm have 22 autosomes and either an X chromosome, or a Y chromosome.

46. Fertilization
Two sex cells fuse to form a new cell containing genetic material derived from both parents.
Restores the diploid number of chromosomes.
Determines the sex of the organism.
Initiates cleavage.
Occurs in the widest part of the uterine tube (the ampulla).

47. Fertilization
Millions of sperm cells are deposited in the female reproductive tract during intercourse.
Only a few hundred have a chance at fertilization.
Only the first sperm to enter the secondary oocyte is able to fertilize it.
The remaining sperm are prevented from penetrating the oocyte.

48. Cleavage
Shortly after fertilization, the zygote begins to undergo a series of divisions.
Divisions increase the number of cells in the pre-embryo, but the pre-embryo remains the same size.
During each succeeding division, the cells are smaller and smaller.

49. Cleavage
Before the 8-cell stage, cells are not tightly bound together, but after the third cleavage division, the cells become tightly compacted into a ball called a morula.

50. Implantation
By the end of the first week after fertilization, the blastocyst enters the lumen of the uterus.
The zona pellucida around the blastocyst begins to break down as the blastocyst prepares to invade the endometrium.
Implantation is the process by which the blastocyst burrows into and embeds within the endometrium.

51. Amnion
Eventually encloses the entire embryo in a fluid-filled sac called the amniotic cavity to prevent desiccation.
The amniotic membrane is specialized to secrete the amniotic fluid that bathes the embryo

52. Chorion
The outermost extraembryonic membrane, is formed from rapidly growing cells.
These cells blend with the functional layer of the endometrium and eventually form the placenta.

53. The Placenta
Functions in exchange of nutrients, waste products, and respiratory gases between the maternal and fetal bloodstreams.
Transmission of maternal antibodies to the developing embryo or fetus.
Production of hormones to maintain and build the uterine lining.

54. Gastrulation
Occurs during the third week of development immediately after implantation.
One of the most critical periods in the development of the embryo.
Cells of the epiblast migrate and form the three primary germ layers which are the cells from which all body tissues develop.
The three primary germ layers are called ectoderm, mesoderm, and endoderm.

55. Organogenesis
Once the three primary germ layers have formed, and the embryo has undergone folding, organogenesis begins.
The upper and lower limbs attain their adult shapes, and the rudimentary forms of most organ systems have developed by week 8.
By the end of the embryonic period, the embryo is slightly longer than 2.5 centimeters (1 inch), and yet it already has the outward appearance of a human.

56. WEEK 7
The embryo begins to move spontaneously. The jaw forms, including teeth buds in the gums. Soon the eyelids will seal to protect the embryo's developing light sensitive eyes, and will reopen at about the seventh month. At week 8, the embryo is a little more than an inch long and is now called a fetus, Latin for "young one" or "offspring." Everything is now present that will be found in a fully developed adult.

57. A thin membrane separates the embryo's blood in the villi from the mother's blood that flows through the space surrounding the villi (intervillous space). This arrangement allows materials to be exchanged between the blood of the mother and that of the embryo.
The embryo floats in fluid (amniotic fluid), which is contained in a sac (amniotic sac). The amniotic fluid provides a space in which the embryo can grow freely. The fluid also helps protect the embryo from injury. The amniotic sac is strong and resilient.

58. MONTH 9
Toward the end of this month, the baby is ready for birth. The average duration of pregnancy is 280 days from the first day of the mother's last menstrual period. By this time the infant normally weighs between 6 and 9 pounds and his heart is pumping about 250 gallons of blood a day. He is fully capable of life outside the womb.

59. Tracking Fetal Development
Fetal Organs
First Month (Embryo)
Vital organs are forming and the brain and beginning of the spine are evident.
Fifth Week
Heart begins to beat and circulate blood; arm and led buds emerge; brain, spinal cord, and nervous system are established.
Sixth Week
Digestive system is forming and arms and legs begin to grow.
Seventh Week
The umbilical cord joins the embryo to the placenta; long bones and internal organs are developing.
Second Month (Fetus)
Human face, arms, legs, fingers, toes, elbows, knees, eyelids and bone cells are forming.
Twelfth Week
Sex is distinguishable; fingers and toes are moving; teeth buds are present and the kidney and bladder form. Baby is 2-4" long and weighs an once or two.
Sixteenth Week
Baby moves and kicks, sleeps and wakes, swallows; hair forms, digestion becomes active; fetus is pick in color and has a large.

60. 20 Weeks
Spurt in baby's growth; internal organs are maturing; hair, eyebrows and lashes are present; baby increases storage of iron. Baby is 8-12" long and weighs 1/2 pound.
24 Weeks
Baby's skin is wrinkled; covered by lanugo and vernix; and baby has an audible heartbeat.
28 Weeks
Most rapid growth; red and wrinkled; eyelids can open and close; baby storing large amounts of calcium and iron; fetus has a chance of surviving if born. Baby is 15" long and weighs 3 pounds.
32 Weeks
Weight gain and rapid growth; settles in favorite position; valuable fat increases.
36 Weeks
Baby gains 1/2 pound per week; bones of head are soft and flexible; baby has developed immunities. Baby is 18" long and weighs 6 pounds.
Birth Weeks
Organs developed; respiratory system is mature.