1. HUMAN EMBRYOLOGY (First eight weeks of early human development)
Recommended reading:
Keith Moore, Vid Persaud
The developing human: Clinically oriented embryology. 8th ed. Elsevier, 2008: 1-91,

2. Progenesis – includes formation of spermatozoa and oocytes - male and female germ cells
Spermatogenesis – spermatozoa development - is maintained in the seminiferous tubules of testes
Oogenesis – oocyte production - is maintained within the ovarian follicles

4. MORPHOLOGY OF SPERMATOZOON AND OOCYTE

5. FERTILIZATION
Fusion of a spermatozoon with oocyte resulting in formation of one-cell embryo – zygote
Fertilization in most of the cases occurs within the ampullary region of the uterine tube on days of ovarian-menstrual cycle (after the ovulation – oocyte release from the ovary) and insemination with sperm
Fertilization is accounted as the first day of embryogenesis

6. MAIN EVENTS DURING FERTILIZATION
Capacitation
Acrosomal reaction
Penetration of corona radiata and zona pellucida
Fusion of the oocyte and sperm cell membranes
Cortical and zona pellucida reactions
Resumption of 2nd meiotic division
Metabolic activation of the egg
Restoration of diploid number of chromosomes

8. Fertilization

9. The main results of fertilization:
Restoration of the diploid number of chromosomes
Determination of the sex of embryo
Initiation of a cleavage

10. CLEAVAGE of the zygote (first week of development)
Zygote → blastomeres → 8-cell stage (compaction) →16-cell stage (morula, or mullberry) → blastocyst ↓
inner cell mass → embryoblast
outer cell mass → trophoblast
Cleavage means enlargement of total cell number with simultaneous reduction of their size resulting from the abbreviated mitotic cycle

11. Cleavage of the zygote and formation of the blastocyst

14. Summary of the ovarian cycle, fertilization and human development during the first week

15. IMPLANTATION
Adhesion – attachment of blastocyst to the surface of endometrium – day 5
Invasion – introduction of blastocyst inside the uterine wall resulting from its lytic digestion – days 6-12
Uteroplacental circulation – starts day 10, when maternal blood from eroded uterine vessels provide nutritional support to the developing embryo

17. Early stages of implantation: A – 6 days – blasocyst attachment; B – 7 days – invading blastocyst

18. BILAMINAR EMBRYONIC DISC FORMATION: SECOND WEEK OF DEVELOPMENT
Embryoblast differentiates into bilaminar embryonic disc:
Epiblast
Hypoblast (local thickening of which forms prechordal plate, indicating future cranial region of the embryo)
Trophoblast differentiates into:
Cytotrophoblast
Syncytiotrophoblast, which erodes blood vessels of endometrium and establish haematotrophic circulation
Cavities formation:
Amniotic cavity
Umbilical vesicle (yolk sac)

19. Implanted blastocysts: A – 10 days; B – 12 days

20. End of the second week (day 14): formation of chorionic sac and chorionic villi

21. THIRD WEEK OF DEVELOPMENT: GASTRULATION – formation three of germ layers
Formation of the primitive streak with primitive node and primitive pit
Migration of cells from epiblast
Epiblast gives birth to three primary tissues:
- ectoderm
- mesoderm
- endoderm
Resulting from cellular migration and cavities formation embryo is becoming three laminar, surrounded with two cavities – umbilical vesicle (yolk sac) and amniotic cavity

22. Origin of embryonic tissues

23. Dorsal view of a 16-day embryo

24. Third week: further events
Notochord formation
Development of somites (each of it including dermatome, sclerotome and myotome)
Neural tube and neural crest formation (neurulation)
Development of primordial cardiovascular system
Development of intraembryonic coelom
Development of secondary and tertiary chorionic villi

25. 18 day: formation of notochord and neural groove

26. Embryos of days 19 to 21: formation of somites

27. Embryonic days 19-21: neurulation

28. Day 21: primordial cardiovascular system and tertiary chorionic villi

29. Organogenesis: fourth to eight weeks
During the fourth week a flat trilaminar embryonic disc is folding with the formation of a cylindrical shape embryo
Folding of the ends of the embryo in the median plane results in formation of cranial and caudal regions moving ventrally as the embryo elongates
Folding of the embryo in the horizontal plane produces right and left lateral folds

30. Folding of embryos during days 21-22-26-28

31. Derivatives of three germ layers: endoderm, ectoderm and mesoderm

32. ENDODERMAL GERM LAYER DERIVATIVES
Epithelial lining of
- gastrointestinal tract
- respiratory tract
- urinary bladder
Parenchyma of
- thyroid, parathyroid glands
- liver and pancreas
Epithelial lining of the tympanic cavity and auditory tube

33. ECTODERMAL GERM LAYER DERIVATIVES:
central nervous system
peripheral nervous system
sensory epithelium of ear, nose and eye
epidermis (including hair and nails)
pituitary, mammary and sweat glands
oral cavity mucosa, enamel of the teeth

34. MESODERMAL GERM LAYER DERIVATES
splanchnic, or visceral mesoderm layer
intermediate mesoderm
somatic, or parietal mesoderm layer
dorsal mesoderm, which forms somites
Each somit consists of three components:
- sclerotome
- dermatome
- myotome

35. Human embryo 28 days with somites

36. Number of somites correlated to approximate age in days
Approximate age No. of somites
(days)

37. EXTRAEMBRYONIC ORGANS
Placenta
- embryonic part – chorionic membrane (transformed trophoblast of the blastocyst)
- maternal part – decidual membrane (pregnancy-transformed functional layer of endometrium)
Umbilical cord (formerly connecting stalk)
Umbilical vesicle (yolk sac)
Allantois
Amnion

39. Sagittal section through gravid uterus at 4 weeks

44. 56 days old embryo with fetal membranes