The document summarizes key stages of human fertilization and embryonic development. It describes how fertilization occurs through the fusion of sperm and egg cells. It then outlines the early cleavage and differentiation stages, including the formation of the morula, blastula and gastrula. It discusses the development of the three germ layers and how organs begin to form. Later stages covered include placenta and fetal development, as well as the roles of the umbilical cord, amniotic sac and other structures. Common complications like teratogens and types of twins are also summarized.

1. General Anatomy
Lecture # 24
Fertilization

2. Fertilization

3.  At the time of coitus 200-600 million sperms are deposited into
the female vagina in the vicinity of cervix.
 Following ejaculation, semen undergoes clotting by the action of
enzymes present in the prostatic secretion.
 After about 5 minutes clot liquefies and spermatozoa become
actively motile: the tails perform undulating movements which
propels them forwards. The speed varies with the environment
but on the average it is 2 -3 mm/min.
 Prostaglandinds reverse the peristaltic contractions of the uterus
and uterine tubes thus helping the ascent of spermatozoa to the
site of fertilization.
 Out of millions of spermatozoa only 300-500 reach the site of
fertilization and it is only one which is able to fertilize.

4.  A freshly deposited sperm is not able to fertilize the
secondary oocyte until it is conditioned in the female
genital tract for sometimes lasting about 7 hours.
 During this time glycoprotein coat and seminal
plasma proteins are removed from the plasma
membrane that overlies the acrosomal region of the
spermatozoa.
 This process whereby freshly ejaculated sperms
develop the capacity to fertilize a secondary oocyte is
called capacitation of sperm.

5. Fertilization:
the fusion of the sperm cell
nucleus with the egg cell
nucleus to produce a zygote
(fertilized egg)

6. Fertilization:
 External
 Occurs outside of the body of the
female
 Increased number of eggs produced to
insure the survival of the species

7. Fertilization:
 Internal
 Occurs inside the body of the female
 Fewer number of eggs are produced

8. Fertilization:
 fertilization in
mammals
occurs in the
oviduct
 The ova is
viable for
approximately
24 hours after
ovulation

9. REPRODUCTION AND
EMBRYONIC DEVELOPMENT
 THE PROCESS OF FERTILIZATION
 THE SPERM APPROACHES THE EGG
 THE SPERM’S ACROSOMAL ENZYMES DIGEST THE
EGG’S JELLY COAT
 PROTEINS ON THE SPERM HEAD BIND TO EGG
RECEPTORS
 THE PLASMA MEMBRANES OF SPERM AND EGG
FUSE
 THE SPERM NUCLEUS ENTERS THE EGG
CYTOPLASM
 A FERTILIZATION ENVELOPE FORMS

11. Implantation
 After approximately
a week, the
developing embryo
is implanted into the
uterus
 Embryo:
conception to 8
weeks

12. Embryonic Development

14. Embryo:
 a multicellular organism in the early
stages of development
2 four cell stage embryos
Eight cell stage embryo

15. Embryo:
The beginning developmental processes
are always the same in all animals:
1) cleavage
2) growth
3) differentiation

16. Embryo:
 after fertilization
the diploid
ZYGOTE
undergoes
cleavage
divisions in the
oviduct

17. Cleavage
the first series of cell divisions by
mitosis after fertilization
Cell division is rapid, new cells do not
take time for the growth phase G1
cell growth does not occur so cells
decrease in size with each
cleavage division

18. REPRODUCTION AND
EMBRYONIC DEVELOPMENT
 CLEAVAGE PRODUCES A BALL OF CELLS FROM THE
ZYGOTE
 CLEAVAGE
 RAPID SUCCESSION OF CELL DIVISIONS THAT PRODUCES A
BALL OF CELLS – A MULTICELLULAR EMBRYO – FROM THE
ZYGOTE

19. REPRODUCTION AND
EMBRYONIC DEVELOPMENT
 CLEAVAGE PRODUCES A BALL OF
CELLS FROM THE ZYGOTE
 BLASTOCOEL
 FLUID-FILLED CAVITY IN THE
CENTER OF THE EMBRYO
 BLASTULA
 HOLLOW BALL OF CELLS THAT
RESULTS WHEN CLEAVAGE IS
FINISHED

20. Cleavage divisions

21.  Morula forms (solid ball of cells)
 Blastula forms (hollow ball of cells)
 Cells begin to grow before dividing

23. Differentation
 Gastrulation: one side of the
blastula invaginates (indents)
forming a gastrula
 Three cell layers form.
 SECOND MAJOR
PHASE OF EMBRYONIC
DEVELOPMENT; ADDS MORE
CELLS TO THE EMBRYO;
SORTS
THE CELLS INTO DISTINCT
CELL
LAYERS

24. Differentation

25. Differentiation
 The changing of unspecialized
embryonic cells into the specialized
cells, tissues and organs of a
multicellular animal

26. REPRODUCTION AND
EMBRYONIC DEVELOPMENT
 GASTRULATION PRODUCES A THREE-
LAYERED EMBRYO
 GASTRULA
 THREE-LAYERED STAGE OF THE EMBRYO
 THREE LAYERS ARE LABELED
 ECTODERM
 ENDODERM
 MESODERM

27. REPRODUCTION AND
EMBRYONIC DEVELOPMENT
 GASTRULA LAYERS DEVELOP INTO…

28. REPRODUCTION AND
EMBRYONIC DEVELOPMENT
 ORGANS START TO FORM AFTER
GASTRULATION
 NOTOCHORD
 NEURAL TUBE
 SOMITES
 COELOM

29. REPRODUCTION AND
EMBRYONIC DEVELOPMENT
 CHANGES IN CELL SHAPE, CELL
MIGRATION, AND PROGRAMMED CELL
DEATH GIVE FORM TO THE
DEVELOPING ANIMAL
 PROGRAMMED CELL DEATH =
APOPTOSIS
 TIMELY, TIDY SUICIDE OF CELLS
 NECESSARY TO CREATE SPACE BETWEEN
FINGERS AND TOES

30. REPRODUCTION AND
EMBRYONIC DEVELOPMENT
 THE EMBRYO AND PLACENTA TAKE SHAPE DURING THE FIRST
MONTH OF PREGNANCY
 GESTATION
 PREGNANCY; THE CARRYING OF DEVELOPING YOUNG
WITHIN THE FEMALE REPRODUCTIVE TRACT
 AN OVERVIEW OF DEVELOPMENT EVENTS
 BLASTOCYST  MAMMALIAN EQUIVALENT OF A
BLASTULA
 TROPHOBLAST  OUTER LAYER THAT ALLOWS
UTERINE IMPLANTATION
 PLACENTA  COMBINATION OF TROPHOBLAST AND
MATERNAL CELLS

31. REPRODUCTION AND
EMBRYONIC DEVELOPMENT
 AN OVERVIEW OF DEVELOPMENT EVENTS

32. REPRODUCTION AND
EMBRYONIC DEVELOPMENT
 EXTRAEMBRYONIC MEMBRANES
 AMNION  PROTECTIVE FLUID
 YOLK SAC  NO YOLK IN HUMANS, BUT SAME NAME;
NOURISHMENT FOR NON-PLACENTALS
 CHORION  BECOME EMBRYOS PART OF PLACENTA
 RELEASES HUMAN CHORIONIC GONADOTROPIN (HCG)
 CAUSES CORPUS LUTERUM TO STAY; PREVENTS
MENSTRUATION
 ALLANTOIS  FUNCTIONS IN WASTE DISPOSAL; WILL
BECOME THE UMBILICAL CORD

33. REPRODUCTION AND
EMBRYONIC DEVELOPMENT
 EXTRAEMBRYONIC MEMBRANES

34. REPRODUCTION AND
EMBRYONIC DEVELOPMENT
 THE PLACENTA
 CHORIONIC VILLI  CONTAIN
EMBRYONIC BLOOD VESSELS
 NEAR MATERNAL BLOOD; NO DIRECT
MIXTURE OF BLOOD
 NUTRIENTS, GASES, WASTE PRODUCES
ARE PASSED THROUGH THE PLACENTA
 DRUGS, ALCOHOL, VIRUSES CAN PASS
THROUGH
 ANTIBODIES CAN PASS THROUGH

35. Placenta
 organ that
forms from
the embryo
and the uterus

36. Placenta
 contains blood vessels from
the mother and the
developing baby

37. Placenta
 Oxygen & nutrients diffuse from the
mother’s blood vessels into the
baby’s blood vessels
 Wastes diffuse from the baby’s blood
vessels into the mother’s blood
vessels

38. Umbilical Cord
 two arteries and a
vein Connects the
fetus to the
placenta

39. Amniotic Sac
 Contains fluid
(amniotic fluid)
that protects
fetus by giving it
a stable
environment and
absorbing shock

40. Fetus
 By the end of the 8th
week of pregnancy
the embryo is called
a fetus and all of the
major structures are
present

41. Later Stages of Fetal
Development

42. Human gestation
 the period
between
fertilization and
birth
 approximately
38-40 weeks

43. Teratogens
 Substances that may harm the
developing fetus and result in the
formation of birth defects

44. Teratogens include:
 Alcohol, certain
drugs/medications, infections,
and certain chemicals

45. How do twins form???

46. Monozygotic Twins
(Identical Twins)
 One egg is fertilized by one sperm
 Embryo splits into two during the early stages
of development
 Have identical genes and must be of the
same sex
 (Incidence: about 3 in every 1000 births)

47. Dizygotic Twins
(Fraternal Twins)
 Two eggs are ovulated and each is fertilized
by a sperm cell
 No more genetically similar than any other
sibling in the family (can be same/different
sexes)
 Maternal age, use of assisted reproductive
technologies are factors
 Incidence (6.7/1000 births in Japan to
40/1000 births in Nigeria)

48. REPRODUCTION AND
EMBRYONIC DEVELOPMENT
 NO TIME FOR FUNNY JOKES…WE HAVE TO RACE TO THE FINISH!!!! NERVOUS
SYSTEM NEXT!!!