Embryogenesis Histology Lecture 1 by Professor Tatiana Bororinkina, First Moscow State Medical University

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!