The initial week of embryonic development is a vital period commencing with fertilization, leading to the creation of the zygote and early cell divisions. It's noteworthy that, throughout this week, the developing embryo remains in the pre-implantation stage, journeying from the fallopian tube toward the uterus. Key events such as fertilization, cleavage, and the formation of the blastocyst are crucial for the embryo's early development. These events lay the foundation for subsequent processes in the following weeks. The successful implantation of the blastocyst into the uterus marks the transition from the first week to the second week of embryonic development.

1. Dr / Ahmed Salah Ashour(Ph.D.)
Associate professor of human anatomy
Dr.Ahmedashour@gmu.ac.ae
USMLE Clinical Anatomy

2. FIRST WEEK

3. ILOs
• Describe capacitation
• Explain acrosome reaction
• Enumerate fertilization

4. The initial week of embryonic development is a vital
period commencing with fertilization, leading to the
creation of the zygote and early cell divisions. It's
noteworthy that, throughout this week, the developing
embryo remains in the pre-implantation stage,
journeying from the fallopian tube toward the uterus.
Key events such as fertilization, cleavage, and the
formation of the blastocyst are crucial for the embryo's
early development.
These events lay the foundation for subsequent
processes in the following weeks. The successful
implantation of the blastocyst into the uterus marks the
transition from the first week to the second week of
embryonic development.

6. Journey of sperms towards the ovum

7. • Ejaculation:
Sperm are released from the male reproductive system during ejaculation contained
within seminal fluid, which includes alkaline secretions from the prostate gland that
helps protect sperm from the acidic environment of the vagina.

8. • Travel through the Female Reproductive Tract:
Once ejaculated into the vagina, sperm begin their journey through the female
reproductive tract. They must travel through the cervix, uterus, and fallopian tubes
to reach the ovum (egg).
sperms

9. • Chemotaxis and Navigation:
Sperm navigate through the female reproductive tract using various mechanisms,
including chemotaxis, which involves following chemical signals released by the
egg or surrounding cells. This helps guide sperm toward the ovum.

10. • Competition and Selection:
While millions of sperm are ejaculated
during sexual intercourse, only a fraction
of them (100-200 sperms) will reach the
vicinity of the egg.
Additionally, factors such as sperm
motility, morphology, and health play
crucial roles in determining which sperm
are most likely to successfully fertilize
the egg.
Ultimately, only ONE healthy, motile
sperm is capable of penetrating the egg.

11. Sperms that do not fertilize an egg
typically have a relatively short
lifespan within the female
reproductive tract, they typically
undergo a process called spermatozoa
degeneration or spermatozoa
phagocytosis.
In this process, non-fertilizing sperm
cells are broken down and engulfed by
phagocytes or washed out as discharge
from female genital tract.

12. Capacitation

13. • Definition: is a process that occurs within the female reproductive tract,
specifically in the fallopian tubes, where sperm undergo biochemical and
physiological changes to become capable of fertilizing an egg.
• Site : It occurs in the uterus & the uterine tube .
• Duration : It lasts for 7 hours.

14. • Aim : Removal of a glycoprotein layer covering the acrosomal cap of the
sperms.
• Result: This increases the activity of the sperms.
Glycoprotein

15. Acrosome reaction

16. It is the development of multiple perforations in the acrosome of the sperms.
• Site: Uterine tube.

17. • Time : When sperms encounter the ovum.
• Aim : Release of the enzymes that are stored in the acrosome.
• Result : Facilitation of fertilization.

18. Fertilization

19. • Definition:
Fertilization is the fusion between a single sperm & ovum to form a zygote. It could
be considered the actual beginning of intrauterine life.
• Site:
It occurs in the ampulla, the widest part of the uterine tube.
• Duration:
The process of fertilization usually completes within 12-24 hours after ovulation.

20. • Phases:
1- Penetration of the corona radiata.
This is helped by the action of acrosomal enzymes & tail motility of the sperms.

21. 2- Penetration of the zona pellucida
Results from the action of acrosomal enzymes which cause lysis of the zona
pellucida.
Zona pellucida

22. Once a sperm penetrates the zona pellucida & touches the ovum membrane, a
reaction occurs, resulting in hardening of the zona pellucida making them
impermeable to other sperms (Zonal reaction).
Corona radiata
zona pellucida
ovum membrane

23. 3- Fusion of the plasma cell membranes of the ovum & sperm
The nucleus of the sperm enter the cytoplasm of the ovum.

24. • Results of fertilization:
ü Restoration of the diploid number
of the chromosomes (46).
ü Variation of the human species
through mixing of the maternal &
paternal chromosomes.
ü Determination of sex of the
embryo: An x-bearing sperm
produces a female embryo & a y-
bearing sperm produces a male
embryo.

25. ü Corpus luteum enlarges & remains
active for the first half of gestation i.e.
corpus luteum of pregnancy.
ü High levels of estrogen &
progesterone produced by the corpus
luteum.
ü Stopping of the menstrual cycle.
ü The endometrium continues to grow.

26. Failure of fertilization (FOF)
Refers to the inability of a sperm to successfully fuse
with an egg.
Etiology:
• Sperm-related issues of (FOF)
• Egg-related issues of (FOF)
• Fertilization process issues
• Incompatibility between sperm and egg
Clinical Insight

27. Clinical Insight
Sperm-related issues of (FOF)
Low sperm count: A low concentration
of sperm in the semen may reduce the
chances of successful fertilization.
Poor sperm motility: If sperm cannot
move effectively, they may struggle to
reach and penetrate the egg.
Abnormal sperm morphology: Sperm
with structural abnormalities may have
difficulty penetrating the egg.

28. The swimming speed of human sperm
in fluid is approximately 5 mm/min, so
sperm could swim through the cervical
canal in a matter of 30 minutes.

30. Clinical Insight
Egg-related issues of (FOF) :
Quality of the egg: The quality of the egg is
crucial for fertilization. As women age, the
quality of their eggs tends to decline, making
fertilization less likely.
Polycystic ovary syndrome (PCOS): Women
with PCOS may have difficulty releasing
eggs regularly, affecting the chances of
fertilization.

31. Clinical Insight
Fertilization process issues:
Failure of sperm penetration: Even if
sperm reach the egg, they must penetrate
its outer layer (zona pellucida) to fertilize
it. If this process fails, fertilization will
not occur.
Defective sperm-egg interaction:
Successful fertilization also depends on
proper recognition and interaction
between sperm and egg. Any disruption
in this process can lead to failure.

32. Clinical Insight
Incompatibility between sperm and egg:
Immunological factors: Sometimes, the immune system may produce antibodies
that interfere with the sperm's ability to fertilize the egg.

33. Cleavage of the zygote

34. • Definition:
Is repeated mitotic cell divisions of the zygote (within its zona pellucida), resulting
in a rapid increase in the number of cells (The resulting cells, are called
blastomeres, become smaller in size with each cleavage division).
• Site :
Uterine tube.
zona pellucida
Blastomeres

35. The zygote divides to form 2 cells, 4 cells & 8 cells in the 2nd day.
When there are 16 blastomeres by the 3rd day, the mass is called a morula.

36. Sperm Ovum
1 cell stage 2 cell stage
4 cell stage
Blastomere
Zona
8 cell stage
16 cell stage - MORULA
Blastomere
Zona

37. During embryonic development, blastomeres undergo a series of cleavage divisions.
As these divisions progress, the blastomeres continue to divide rapidly, forming a
compact mass of cells known as a morula.
MORULA
Blastomeres
Zona pellucida Blastomere

38. The morula reaches the uterine cavity by the 4th day after fertilization.
Due to passage of uterine fluid through the zona pellucida, morula became
balstocyst.

39. Sperm Ovum
1 cell stage 2 cell stage
4 cell stage
Blastomere
Zona
8 cell stage
16 cell stage - MORULA
Blastomere
Zona
blasto-cycst
embryoblast
trophoblast
blastocele

41. Balsto-cyst

42. • Definition:
is an early stage of embryonic
development in mammals, including
humans.
• Time:
It typically forms about five to six
days after fertilization.
• Structure:
The blastocyst is a hollow sphere
consisting of three main parts.

43. 1- Blastocele:
This refers to the fluid-filled cavity
within the blastocyst.
It is essential for maintaining the
structure and integrity of the blastocyst
as it undergoes further development and
prepares for implantation.
blasto-cycst
embryoblast
trophoblast
blastocele

44. 2- Inner cell mass [embryoblast]:
The cluster of cells located inside the
blastocyst named as embryoblast, which
will develop into the embryo itself.
blasto-cycst
embryoblast
trophoblast
blastocele

45. 3- Outer cell mass [Trophoblast]:
The outer layer of cells that will
eventually give rise to the placenta and
other supporting tissues.
It also plays a crucial role in
implantation.
blasto-cycst
embryoblast
trophoblast
blastocele

46. Endometrium
At the 7th day, the trophoblast will further
differentiate into:
1. Inner Cytotrophoblasts:
Inner cellular layer. Cells are separated
with cell membranes.
2. Outer Syncytiotrophoblasts:
Outer multinucleated layer with no cellular
membranes. Syncytiotrophoblasts has the
ability to erode into the endometrium
allowing implantation.

47. Sperm Ovum
1 cell stage 2 cell stage
4 cell stage
Blastomere
Zona
8 cell stage
16 cell stage - MORULA
blasto-cycst
embryoblast
Trophoblast
blastocele
Cyto
Syncytio

48. Implantation

49. By the end of 6th day, Once the blastocyst reaches the uterus, it undergoes
apposition, where it comes into close proximity with the uterine lining, where it
attaches to the endometrial epithelium.
Blastocyst
Endometrium
Uterine cavity

50. Trophoblast
Embryoblast
After adhesion, the syncytiotrophoblasts begin to invade the endometrial lining.

51. Embryoblast
Blastocyst
Syncytio
Trophoblast
Cyto

52. Selectivity
Not all embryos progress to the blastocyst stage, so selecting embryos at this stage
allows for better identification of those with the highest developmental potential.
This can improve the chances of a successful pregnancy.
Clinical Insight

53. Formative Quiz

54. Q1 A 32-year-old woman presents to her obstetrician's office with concerns about
her inability to conceive despite actively trying for over a year. She reports regular
menstrual cycles and no significant medical history. Upon further examination, her
partner's semen analysis reveals normal parameters. After further investigation, it is
determined that the woman has a history of pelvic inflammatory disease (PID) from
a previous untreated Chlamydia trachomatis infection. A hysterosalpingogram
(HSG) confirms bilateral tubal occlusion.
Which of the following events is most critical for reproduction and is likely
compromised in this patient?
A. Ovulation
B. Fertilization
C. Implantation
D. Embryo development

55. Q2 A 32-year-old woman and her husband have been trying to conceive for the past
12 months without success. They present to your clinic seeking assistance. On
further evaluation, the woman's menstrual cycles are regular, and she has no history
of pelvic inflammatory disease or previous surgeries. The husband's semen analysis
shows normal sperm count, motility, and morphology. The woman undergoes
hormonal evaluation, which reveals normal levels of follicle-stimulating hormone
(FSH), luteinizing hormone (LH), and estradiol. A hysterosalpingogram (HSG) is
performed and demonstrates patent fallopian tubes. Despite these findings, the
couple has been unable to achieve pregnancy. Which of the following steps in
fertilization is most likely impaired in this couple's case?
A) Capacitation of sperm
B) Sperm penetration through the zona pellucida
C) Binding of sperm to the zona pellucida
D) Fusion of sperm and egg plasma membranes
E) Formation of the fertilization cone

56. Q3 A 28-year-old woman presents to her gynecologist with concerns about her
fertility. She reports regular menstrual cycles occurring every 28-30 days. She and
her partner have been actively trying to conceive for the past 6 months without
success. She is curious about the timeline of conception and asks, "How long does it
typically take for an egg to get fertilized after ovulation?" Which of the following
statements best answers her question?
A) Fertilization usually occurs within 6-8 hours after ovulation.
B) Fertilization typically takes place within 24-48 hours after ovulation.
C) It may take approximately 1-2 days for fertilization to occur after ovulation.
D) Fertilization generally occurs between 36-72 hours after ovulation.
E) The process of fertilization usually completes within 12-24 hours after ovulation.

57. Q4 A 28-year-old woman is concerned about the timing of intercourse for
conception. She asks her healthcare provider about the lifespan of sperm in the
female reproductive tract. Which of the following statements accurately reflects the
lifespan of sperm?
A) Sperm can survive up to 48 hours in the female reproductive tract.
B) Sperm can remain viable for up to 72 hours in the female reproductive tract.
C) Sperm can survive for approximately 96 hours in the female reproductive tract.
D) Sperm can only survive for about 24 hours in the female reproductive tract.
E) Sperm lifespan varies greatly and can range from 12 to 120 hours depending on
various factors.

58. Q5 Mrs. Anderson, a 34-year-old woman, and Mr. Anderson, a 36-year-old man,
have been trying to conceive for the past 18 months without success. They present
to a fertility clinic seeking assistance. Mrs. Anderson has regular menstrual cycles,
and both partners have no significant medical history. Initial evaluations reveal
normal hormonal levels, regular ovulation, and no apparent male factor infertility.
The couple undergoes in vitro fertilization (IVF) as the next step. The couple
undergoes IVF, where Mrs. Anderson undergoes ovarian stimulation, egg retrieval,
and fertilization with her husband's sperm in the laboratory setting. However,
despite seemingly normal conditions, fertilization fails to occur. The embryologist
reports a lack of fertilized eggs for implantation. What is the primary step in the
process of fertilization that has likely failed in this case?
a) Ovarian stimulation
b) Egg retrieval
c) Sperm preparation
d) Fusion of sperm and egg

59. Q6 After fertilization of the ovum, implanta- tion occurs in the endometrium. At this
time the developing placenta begins to produce a hormone necessary for embryonic
viability.
Which of the following best describes this hormone’s action?
(A) Increases the production of milk by the mammary glands
(B) Increases the threshold for uterine contrac- tion
(C) Initiates parturition at the end of preg- nancy
(D) Stimulates the corpus luteum to produce estriol and progesterone
(E) Stimulates the placenta to produce estriol and progesterone

60. Q1 Fertilization
Q2 Sperm penetration through the zona pellucida
Q3 The process of fertilization usually completes within 12-24 hours after
ovulation.
Q4 Sperm can remain viable for up to 72 hours in the female reproductive tract.
Q5 Fusion of sperm and egg
Q6 Stimulates the corpus luteum to produce estriol and progesterone

61. List of Texts and Recommended Readings
• Last's Anatomy, Regional and Applied. Chummy S. Sinnatamby. 12th edition 2011, ISBN:13 - 978 0 7020 3394 0
(Available in ClinicalKey: https://www.clinicalkey.com/#!/browse/book/3-s2.0- C2009060533X)
• Estomih Mtui, Gregory Gruener and Peter Dockery. Fitzgerald's Clinical Neuroanatomy and Neuroscience. 7th
edition; 2016, ISBN: 13 - 978-0-7020- 6727-3 (Available in ClinicalKey:
https://www.clinicalkey.com/#!/browse/book/3-s2.0- C20130134113
• Drake, Richard L. Gray's Anatomy for Students, Third Edition, Elsevier Saunders 2015. ISBN-13: 978-0702051319
(Available in ClinicalKey: https://www.clinicalkey.com/#!/browse/book/3- s2.0-C20110061707).
• Sobotta Atlas of Human Anatomy. F. Paulsen. Vol.1, 15th Edition; 2013, ISBN: 9780702052514 (Available in
ClinicalKey: https://www.clinicalkey.com/#!/content/book/3- s2.0-B9780702052514500067)
• Sobotta Atlas of Human Anatomy. F. Paulsen. Vol.2, 15th Edition; 2013, ISBN:13 - 978-0-7020-5252-1 (Available in
ClinicalKey: https://www.clinicalkey.com/#!/browse/book/3- s2.0-C20130046919)

62. Recap