The document provides an in-depth overview of human embryology, detailing the prenatal period from fertilization to birth which spans 280 days. It outlines the stages of embryonic development, including fertilization, cleavage, implantation, and the formation of provisional organs such as the chorion, amnion, and yolk sac. Key processes such as the acrosome reaction during fertilization, the changes in the zygote, and the stages of gastrulation and implantation are also discussed.

1. Human Embryology I

2. Prenatal period or embryogenesis
 from fertilization till birth
 lasts 280 days
40 weeks
10 lunar months

3. Embryogenesis periods
 Initial period – the 1st week of development
 Embryonic period – from the 2nd week until the 8th weeks
 Fetal period – from the 8th weeks up to the birth
an embryo 
during the first two months
a fetus

from the third month until birth

4. Embryonic development stages
 Fertilization
 Cleavage
 Gastrulation
 Germ layer initial differentiation
and axial organ formation
 Histogenesis and organogenesis
 Embryogenesis is accompanied by
- provisory organ development
- implantation
- placentation

5. Initial period – the first week of development
 Fertilization
 Cleavage
 Implantation

6. Fertilization is the sequence of events by which a sperm
fuses with an ovum, forming a zygote
 occurs in the oviduct ampulla
 lasts about 24 h

7. Ovum passes into the tubular ampulla by
 beating action of cilia
 muscular contractions
(stimulated by estrogens)
 an ovum
(secondary oocyte)

8. Spermatozoa pass into the tubular ampulla by
 movements of sperm tails
 uterus muscular contractions
(stimulated by seminal plasma prostaglandins)
 chemotaxis and reotaxis

9. Spermatozoa are reduced in number during the passage
through the female reproductive tract
 due to the elimination of abnormal and poorly motile sperms
 sperm selection mainly occurs in
the cervical canal
the oviduct isthmus
and

10. Capacitation is the enzymatic removal of glycocalyx coat
from the sperm membrane over the acrosome

 It takes about 7 hours to capacitate

11. Acrosome reaction consists of structural changes
 Acrosome membrane fuses at many places with the sperm head plasmalemma
 Fused membranes rupture  multiple perforations
 Enzymes leave the acrosome through perforations

12. Membrane fusion in acrosome reaction

13. Acrosomal enzymes
 facilitate the sperm passage through the ovum envelopes
Hyaluronidase  Corona radiata
Acrosin  Zona pellucida

14. Penetration
 Sperm head is attached to the ovum surface
 Plasma membranes of the ovum and sperm fuse
and then break down
 Sperm nucleus and sperm centriole enter the ovum
cytoplasm
 Sperm plasma membrane and the tail
remain outside and degenerate

15. Cortical reaction occurs when the first sperm passes
through the zona pellucida
 Cortical granules open and release enzymes
 Enzymes modify the zona pellucida
 Zona pellucida becomes zona fertilization
 Zona fertilization is impermeable to sperms
 Cortical reaction prevents polyspermy

16. Human zygote is synkaryon
because contains two pronuclei
 Secondary oocyte completes the meiotic division 
 Ovum nucleus is the female pronucleus
 Sperm nucleus enlarges to form the male pronucleus


17. Fertilization completion
 Male and female pronuclei
- approach each other
- come in contact
- lose their nuclear membranes
 Maternal and paternal chromosomes intermingle
at the metaphase of the first division of cleavage



18. Cleavage is a process of successive rapid mitotic divisions
without growth of the daughter cells called blastomeres
 Cleavage mitotic divisions lack G1 phase
 Cell cytoplasm volume does not increase
 Blastomeres become progressively smaller
until they acquire the size of the somatic cells
 The normal nucleus to the cytoplasm
volume ratio is restored


19. Human cleavage characteristics
 Holoblastic (total)
 Unequal
 Asynchronous

20. Early blastomeres retain totipotentiality
 blastomere separation  development of identical twins

~30%

21. Cleavage begins in the oviduct and is completed in the uterus
 in the oviduct during the first 3 days

morula
 in the uterus for 3 to 4 days

blastula

22. Human morula
 consists of from 8 to 16 blastomeres
 enters the uterus as it is forming


23. Human blastula is called blastocyst
 Trophoblast

chorion
part of the placenta
 Embryoblast

embryo proper
other provisional organs
 Blastocele with fluid
Blastocyst is surrounded by the zona pellucida that prevents early implantation

24. Duplication of the embryoblast results in twinning
~70%


25. Implantation is the blastocyst invasion into the endometrium
 begins on the 5th or 6th day after fertilization
 Zona pellucida
- disappears
 Trophoblast
- attaches itself to the uterine epithelium

26. Beginning of implantation
 hatching 
 adhesion 

27. Trophoblast differentiation
 inner layer – cytotrophoblast
 outer layer – syncytiotrophoblast
(symplastotrophoblast)

28. Syncytiotrophoblast displays high metabolic activity
 produces and releases enzymes facilitating the blastocyst invasion

29. Syncytiotrophoblast processes invade the uterine mucosa
 - epithelium
 - stroma
 - vessels
 - glands

30. Blastocyst goes deeper and deeper until the whole of it has
buried itself in the thickness of the endometrium
 Invasion lasts about 40 hours

31. Decidual cell layer restricts the blastocyst invasion
 Invasion stimulates the decidual reaction
endometrial stromal cells

undergo transformation

decidual cells 
- large
- pale
- rich in glycogen

32. By the 10th day the blastocyst is completely embedded
in the endometrium
 Interstitional type of implantation 


33. For about two days, the endometrium defect is closed
by a closing plug
 Consisting of
- blood clot
- cellular debris



34. Implantation continues and is completed in the second week
 epithelium covers the endometrium defect
 by the 12th day of development



35. Implantation window is the period
when the uterus is receptive for blastocyst invasion
 from the 6th day till the 10th day
after ovulation

 Progesterone secreted by the corpus luteum prepares the endometrium for implantation

36. Implantation site is an endometrial region
where the blastocyst invasion occurs
 midportion of the uterine body
 on the posterior wall

37. Implantation in the lower uterine segment
 results in the placenta previa (placental presentation)

and severe bleeding

38. Ectopic (abnormal) implantation
 is implantation in other organs

39. Implantation in the uterine tube
 Decidual reaction fails to occur in the oviduct
 Tubular pregnancy ends with the oviduct rupture and bleeding

40. Embryonic period - the second week of development
 The 1st stage of gastrulation
 Formation of the provisory organs
- amniotic vesicle
- yolk sac
- chorion



41. The first stage of gastrulation
 results in
- bilaminar embryonic disk formation
 occurs
- on the 7th day of development
- concurrently with implantation

42. Only embryoblast participates in the first stage of gastrulation
 Embryoblast

 delamination

 bilaminar disk 
- upper layer – epiblast
- lower layer – hypoblast

43. Epiblast gives rise to all three germ layers
Hypoblast does not take part in the embryo body formation
 Epiblast

ectoderm
mesoderm
endoderm
 Hypoblast

is displaced to extraembryonic regions

44. Provisory organ formation
 Amniotic vesicle
 Yolk sac
- are associated with the embryonic disk

45. Epiblast forms the amniotic vesicle floor, hypoblast forms
the yolk sac roof
 Amniotic wall

amniotic epithelium
 Yolk sac wall

extraembryonic endoderm 


46. Extraembryonic mesoderm
 surrounds the amnion and yolk sac
 forms connecting stalk
 underlies the trophoblast


47. Chorion wall surrounds the chorionic cavity and forms villi
 Primary chorionic villi
- cytotrophoblast
- syncytiotrophoblast
 Secondary chorionic villi
- extraembryonic mesoderm
- cytotrophoblast
- syncytiotrophoblast

48. Trophoblastic lacunar network
 Syncytiotrophoblast produces enzymes
 Enzymes erode the endometrium, forming the lacunae
 Lacunae are filled with maternal blood from ruptured vessels
chorionic villi contact with maternal blood,
providing the mother-embryo exchange


49. Embryo associated with the amnion and yolk sac is
suspended in the chorionic cavity by the connecting stalk
 connecting stalk attaches the complex
to the inner surface of the chorionic sac


50. Second week of development is called “the period of twos”
 Embryonic disc
- epiblast
- hypoblast
 Vesicles
- amnion
- yolk sac
 Trophoblast
- cytotrophoblast
- syncytiotrophoblast

51. Human embryology is characterized by
the early development of the provisory organs
 Chorion
 Amnion
 Yolk sac

52. Chorion appears in the 2nd week
 forms the chorionic sac
 consists of villi
- primary
- secondary
- tertiary
 is in contact with maternal blood 

53. Chorion exists up to birth
 Chorion functions
- mother-embryo exchange
- nutrient production
- enzyme secretion
(to erode the endometrium)
- immune defence
- hormone release
- placenta formation


54. Human chorionic gonadotropin - hCG
 Is secreted
- by the syncytiotrophoblast
- into maternal blood
 Is excreted
- with maternal urine
 Maintains
- corpus luteum activity
- progesterone secretion

• hCG detection in the woman’s urine is a simple, rapid, and an early test of pregnancy

55. Amnion appears in the 2nd week of embryonic development
 is filled with amniotic fluid
 consists of
- amniotic epithelium
- extraembryonic mesoderm


56. Amnion exists up to the birth
 amnion grows with fetus development
and fills the chorionic cavity
 by the 8th week
- surrounds the umbilical cord
- becomes a part of the fetal bladder
or amniochorionic membranes
- underlies the fetal placenta part


57. Amnion functions
 Amnion provides
- fetus watery environment
- fetus mechanical defence (water cushion)
- fetus body temperature control
- fetus free movements
- fetus muscular-skeletal development

58. Yolk sac appears in the 2nd week of embryonic development
 consists of
- yolk sac endoderm
- extraembryonic mesoderm
 is filled with
- serous fluid
(yolk has been spent in the 1st day of development)

59. Yolk sac exists for two months
Its remnant will be a part of the umbilical cord
 Yolk sac functions
- primary hemopoiesis
- primary angiogenesis
- primary germ cell (gonoblast) localization

60. Provisory organs of identical twins
~70%

61. The End
Thank you for attention!