Download lect-1-embryology-part-b

Unit 5
Chapter 28
Pregnancy and human development
Learning Objectives
At the end of the discussion students should be
able to:
1. Summarize all the stages of human
development.
2. Define the specific embryological terms.
3. Explain the embryonic development till the
fetus.
4. Enlist the derivatives of embryonic germ layers
Lecture outlines
Human development from egg to zygote.
1. Ovulation
2. Fertilization
3. Sperm transport and capacitation
4. Effects of capacitation
5. Acrosomal reaction
6. Completion of meiosis and fertilization
7. Formation of zygote.
Lecture outlines
Embryonic development involving
1.
2.
3.
4.
5.
Cleavage and blastocyst formation
Implantation
Placentation
Gastrulation
Formation of embryonic germ layers.
Embryonic germ layer derivatives
Human development
(From egg to zygote)
Oocyte female reproductive cell
Ovulation release of the oocyte(egg) from the ovary.
The oocyte is viable for 12 to 24 hours after it is cast out from
ovary.
(Sperm is viable 24 to 48 hours after ejaculation)
Human development
(From egg to zygote)
Fertilization
Fusion of the sperm with the oocyte to form the
fertilized egg the zygote.
EGG
SPERM
Sperm transport and capacitation
(From egg to zygote)
Millions of sperms enter into vaginal canal during
copulation. Only few thousand reaches to the
uterine tubes by reverse peristalsis.
Capacitation
The process by which sperm membranes become
fragile and it became able to penetrate the oocyte.
Sperm transport and capacitation
(From egg to zygote)
Effects of capacitation
1. capacitation takes 6 to 8 hours to be completed.
2. Mobility of the sperm enhanced
3. Membranes of the sperm become fragile.
Acrosomal reaction and sperm penetration
(From egg to zygote)
Acrosomal reaction involves
1.
2.
The breakdown of plasma membrane and
Acrosomal membrane.
Release of Acrosomal enzymes to digest the
holes in the zona pelluccida
Acrosomal reaction and sperm
penetration
(From egg to zygote)



Sperm head approaches the oocyte
Oocyte and sperm membranes fuse
Only one sperm is allowed to penetrate the
oocyte (monospermy)
Acrosomal reaction and sperm
penetration
(From egg to zygote)




Sperm head approaches the oocyte
An acrosomal process forms and binds to
receptors
Oocyte and sperm membranes fuse
Only one sperm is allowed to penetrate the
oocyte
Completion of Meiosis II and Fertilization
(From egg to zygote)



As sperm nucleus moves toward the oocyte
nucleus it swells to form the male pronucleus
Ovum nucleus swells to become a female
pronucleus
Membranes of the two pronuclei rupture and the
chromosomes combine and produce the diploid
zygote
Embryonic Development
Cleavage
 Mitotic divisions of zygote results in the
formation of 16 or more cells called morula

At day 3 or 4, the embryo of ~100 cells
(blastocyst) has reached the uterus


Blastocyst: fluid-filled hollow sphere
Trophoblast cells


Participate in placenta formation
Inner cell mass

Becomes the embryonic disc (embryo and three of
the embryonic membranes)
Implantation


Blastocyst floats for 2–3 days
Implantation begins 6–7 days after ovulation


The implanted blastocyst is covered over by
endometrial cells
Implantation is completed by the twelfth day
after ovulation
Placentation

Formation of the placenta from embryonic and
maternal tissues

Placenta is fully formed and functional by the end
of the third month

Placenta also secretes human placental lactogen,
human chorionic thyrotropin, and relaxin
Embryonic Development: Gastrula
to Fetus

Germ Layers



During implantation, the blastocyst starts to convert
to a gastrula
Inner cell mass develops into the embryonic disc
(subdivides into epiblast and hypoblast)
The three primary germ layers and the
extraembryonic membranes develop
Gastrulation

Occurs in week 3, in which the embryonic disc
becomes a three-layered embryo with ectoderm,
mesoderm, and endoderm

Begins with appearance of primitive streak, a
raised dorsal groove that establishes the
longitudinal axis of the embryo




The primitive tissues from which all body organs
derived
Ectoderm  nervous system and skin epidermis
Endoderm  epithelial linings of the digestive,
respiratory, and urogenital systems
Mesoderm  forms all other tissues
Summary
Key points
1.
2.
3.
Ovulation- release of the egg from the ovary.
Fertilization - Fusion of the sperm with the oocyte
to form the fertilized egg the zygote.
Capacitation - The process by which sperm
membranes become fragile and it became able to
penetrate the oocyte.
4. Acrosomal reaction - involves the breakdown
of plasma membrane and Acrosomal membrane to
allow the sperm to unite with an egg.
5. Zygote formed by union of male and female
pronuclei.
6. Cleavage - Mitotic divisions of zygote results in
the formation of 16 or more cells called morula.
7. Blastocyst: fluid-filled hollow sphere
8. Trophoblast cells - Participate in placenta
formation
9. Inner cell mass - Becomes the embryonic disc
10. Implantation – attachement of the zygote with
utrine wall.
11. Placentation - Formation of the placenta from
embryonic and maternal tissues
12. Gastrulation – formation of three germ layers.
1.
2.
3.
Ectoderm
Mesoderm
Endoderm
ANY QUESTIONS ?????????