1. Embryonic Development
Ch 28

2. Human Development Vocabulary
Pregnancy- an event that occurs from fertilization to birth
Conceptus- developing offspring
Gestation period- time from last menstrual period until birth (~280 days)
Embryo- from fertilization through week 8
Fetus- from week 9 through birth

3. Stages of Development Pre-Embryonic Development Fertilization Cleavage
Gastrulation
Organogenesis
Fetal Development

5. Fertilization Facts! Oocyte is viable for 12-24 hrs after ovulation
Sperm are able to fertilize oocyte for up to 48 hrs
In order for fertilization to occur, coitus must take place 48 hrs before-24 hrs after ovulation
Fertilization usually occurs when oocyte is 1/3 way down fallopian tube

6. Fertilization and Preembryonic Development

7. Fertilization
Corona radiata
Zona pellucida

8. Ovum and Sperm

9. sperm head

10. Fertilization

11. Fertilization
first polar body
metaphase of second meiotic division

12. Fertilization Mitotic spindle with chromatids 1st polar body
Pellucid zone
Perivitelline space
Cell membrane of the sperm
Kinocilium
Nucleus (compact) of the sperm
Proximal centrosome of the sperm

13. Fertilization 1st polar body Nucleus of the sperm
Proximal centrosome of the sperm
2nd polar body (being formed)
Remainder of the mitotic spindle

14. Fertilization polar bodies Sperm pronucleus Oocyte pronucleus
four hours after impregnation. A nucleic membrane forms around both pronuclei. (In the pronuclei themselves a doubling of the DNA occurs at the decondensing chromosomes.)
Sperm pronucleus
Oocyte pronucleus
Centrosome brought in by the spermatozoon

15. Approach of Pronuclei Sperm pronucleus Oocyte pronucleus
The paternal and the maternal pronuclei move towards each other with the help of microtubules, which begin to be formed immediately after impregnation, i.e., by the penetration of the spermatozoon. They grow in a star-like pattern out of the paternal centrosome directly beside the forming paternal pronucleus (= formation of an aster made of dozens of microtubules). The microtubular proteins themselves arise from the cytoplasma of the oocyte.
Sperm pronucleus
Oocyte pronucleus
Paternal centrosome
"Inner bodies"
Maternal astral microtubule

16. Approach of Pronuclei
While the microtubules of the aster pull the pronuclei together in the center of the oocyte, the synthesis of the DNA is taking place in the pronuclei. This duplication takes roughly 12 hours. The pronuclei grow in size in this time.

17. Formation of the Zygote
After the two pronuclei have come as close together as they can, no merging of them takes place, i.e., a fitting together of the chromosomes of the two pronuclei within a single nucleic membrane does not happen. It is much more accurate to say that the nucleic membranes of both pronuclei dissolve and the chromosomes of both align themselves on the spindle apparatus at the equator. The zygote, the first cell of a new organism with an individual genome (2n4C) is created by the alignment of the maternal chromosomes together with the paternal ones on a common spindle apparatus.
Nucleic membranes of the pronuclei dissolve
Microtubules of the mitotic spindle

18. Formation of the Zygote
The mitotic spindle divides the chromosomes that have just been brought together into the two first cells of the embryo. This proceeding towards the two-cell stage occurs on average between 22 and 26 hours after fertilization

19. Cleavage

20. Cleavage

21. Implantation

22. Implantation Blastocyst floats for 2–3 days
Implantation begins 6–7 days after ovulation
Trophoblast adheres to a site with the proper receptors and chemical signals
Inflammatory-like response occurs in the endometrium

23. Endometrium Uterine endometrial epithelium Inner cell mass Trophoblast
Blastocyst cavity
Lumen of uterus
(a)
Figure 28.5a

24. Implantation Trophoblasts proliferate and form two distinct layers
Cytotrophoblast (cellular trophoblast): inner layer of cells
Syncytiotrophoblast: cells in the outer layer lose their plasma membranes, invade and digest the endometrium

25. Endometrial stroma with blood vessels and glands Syncytiotrophoblast
Cytotrophoblast
Inner cell mass
(future embryo)
Lumen of uterus
(c)
Figure 28.5c

26. Implantation
The implanted blastocyst is covered over by endometrial cells
Implantation is completed by the twelfth day after ovulation

29. syncyticotrophoblast
cytotrophoblast
Amniotic cavity
hypoblast
Yolk sac
epiblast

30. The Formation of Primary Germ Layers

31. Gastrulation Primary Germ Layers Ectoderm Endoderm Mesoderm
Gastrulation rearranges the embryo into a triploblastic gastrula.
Primary Germ Layers
Ectoderm
Endoderm
Mesoderm

32. Fates of the Primary Germ Layers
Ectoderm
hair, nails, epidermis, brain, nerves
Mesoderm
notochord (in chordates), dermis, blood vessels, heart, bones, cartilage, muscle
Endoderm
internal lining of the gut and respiratory pathways, liver, pancreas

33. Gastrulation In mammals, embryo develops from inner cell mass
A thickening of cells forms the primitive streak
In the middle of the primative streak, cells sink inward forming the primative groove
Cells entering the sides of the primative groove become mesoderm and endoderm
Cells that stay on outside become ectoderm

34. Gastrulation

35. Implantation

38. Extraembryonic Membranes
Amnion: epiblast cells form a transparent sac filled with amniotic fluid
Provides a buoyant environment that protects the embryo
Helps maintain a constant homeostatic temperature
Allows freedom of movement and prevents parts from fusing together
Amniotic fluid comes from maternal blood, and later, fetal urine
amnion

39. Extraembryonic Membranes
Yolk sac: a sac that hangs from the ventral surface of the embryo
Forms part of the digestive tube
Source of the earliest blood cells and blood vessels
Yolk sac

40. Extraembryonic Membranes
Allantois: a small outpocketing at the caudal end of the yolk sac
Structural base for the umbilical cord
Becomes part of the urinary bladder
Chorion: helps form the placenta
Encloses the embryonic body and all other membranes
Allantosis
Chorion

41. Embryo Pronuclei fuse Male & female pronuclei approach 2-cell stage
1st cleavage
30 hrs after fertilization

42. Embryo
8-cell stage
2 days
16-cell stage
2.5 days
Morula
2.5-3 days

43. Embryo
Blastocyst
4-5 days
4 weeks
5-6 mm
3-4 weeks
Neurulation
4-5 mm

44. Embryo
7 weeks
14-17 mm
6 weeks
10-11 mm
7-8 weeks
30 mm

45. Phylum Chordata
Subphylum Vertebrata

46. Neurulation
A: Embryonic disc accomplished gastrulation - ectoderm thickens
B: Neural plate forms neural folds and neural groove

47. Neurulation C: Neural folds close
D: Neural tube detached from surface ectoderm

48. Embryonic Folding
The rapid growth of the neural tube is thought to cause the embryo to fold and bring the developing primitive heart caudal to the brain plate. Early in the third week of development, the germ disk has the appearance of a flat oval disk and is composed of two layers: the epiblast and the hypoplast. The first faces the amniotic cavity and the latter faces the yolk sac. A primitive groove, ending caudally with the primitive pit surrounded by a node, first appears at approximately 16 days of development and extends half the length of the embryo.
The primitive groove serves as a conduit for epiblast cells that detach from the edge of the groove and migrate inwards toward the hypoblast and replace it to form the endoderm. After the endoderm is formed, cells from the epiblast continue to migrate inwards to infiltrate the space between the epiblast and the endoderm to form the intraembryonic mesoderm. After this process is complete, the epiblast is termed the ectoderm

49. Differentiation of primary germ layers into tissues and organs.
Organogenesis
Differentiation of primary germ layers into tissues and organs.

51. Development of Reproductive Organs