Gametogenesis , Fertilisation , Implantation and Early Development of Embryo.pdf.pdf

Gametogenesis fertilisation implantation and early development in details

1. Gametogenesis,Fertilisation ,
Implantation and Early Development of
Embryo
By Dr Anubhuti Dey
Guide
Dr K Junnare
( HOU & Professor )

2. Gametogenesis
Spermatogenesis - The Process
of sperm production
Understanding the male gamete formation

3. Overview Of Spermatogenesis
What is Spermatogenesis ?
• Spermatogenesis is the process by which male gametes
(sperm) are produced in the testes.
• Occurs within the seminiferous tubules of the testes.
• Begins at puberty and continues throughout life.
• Results in four haploid sperm cells from one diploid
precursor cell.

4. • Spermatogonia (Stem Cells): The starting point of
spermatogenesis.
• Spermatocyte Formation: Primary spermatocytes
undergo meiosis.
• Spermatid Development: Spermatids are formed from
secondary spermatocytes.
• Sperm Maturation: Spermatids differentiate into
mature sperm (spermatozoa).
Phases Of Spermatogenesis

5. Spermatogonial Cell and
Mitotic Division
• Spermatogenesis starts with spermatogonial stem
cells.
• These stem cells undergo mitotic division to produce
more spermatogonia (stem cells) and primary
spermatocytes.
• Primary spermatocytes are diploid (2n), meaning they
contain two sets of chromosomes.

6. Meiosis in Spermatogenesis
• Meiosis I: Primary spermatocytes divide to
form two secondary spermatocytes, each
with half the chromosome number (haploid,
n).
• Meiosis II: Secondary spermatocytes divide
to form spermatids (haploid, n).

7. Spermatids undergo spermiogenesis, a morphological
transformation:
• Formation of acrosome (cap over the nucleus).
• Development of flagellum (tail) for motility.
• Condensation of the nucleus.
• Shedding of excess cytoplasm.
• Result: Mature spermatozoa
Spermiogenesis: Transformation of
Spermatids into Sperm

8. • Spermatids become spermatozoa in the seminiferous
tubules.
• Sperm undergo further maturation in the epididymis
(increased motility and functionality).
• Mature sperm are stored in the epididymis until
ejaculation.
• Upon ejaculation, sperm travel through the vas deferens,
urethra, and exit the body.
Sperm Maturation & Release
Pathway of Sperm

9. Oogenesis
The Process of Egg formation
Understanding the female gamete formation

10. Overview of Oogenesis
Definition:
Oogenesis is the process by which female gametes (eggs or ova) are
produced in the ovaries.
Location:
Oogenesis occurs in the ovaries, within specialized structures called
follicles.

The process begins during fetal development, but eggs remain arrested
in their early stages until puberty.

Oogenesis results in the production of one mature egg (ovum) from a
primary oocyte. Unlike spermatogenesis, only one viable egg is
produced per cycle.

11. 1. Oogonium (Stem Cell) Stage:
• Oogonia (stem cells) are present during fetal
development and divide by mitosis to produce primary
oocytes.
2. Primary Oocyte Formation & Meiosis I:
• The primary oocytes are formed before birth and enter
Meiosis I, but the process is halted at prophase I.
• Females are born with all their primary oocytes, and
these remain dormant until puberty.
3. Secondary Oocyte Formation & Ovulation:
• At puberty, during each menstrual cycle, a primary
oocyte completes Meiosis I, forming a secondary oocyte
and a smaller polar body.
• The secondary oocyte enters Meiosis II but is arrested at
metaphase II until fertilization.
Phases of Oogenesis

12. • Oogonia:
• The oogonia are the undifferentiated stem cells in the ovaries
that divide by mitosis during fetal development, producing
primary oocytes.
• Primary Oocytes:
• All primary oocytes are formed before birth and enter Meiosis
I, but they are arrested in prophase I. Females are born with a
finite number of primary oocytes, which do not divide further
until puberty.
• Arrest in Prophase I:
• The primary oocytes remain dormant in prophase I until
puberty. They do not resume meiosis until the hormonal signals
trigger ovulation.
Oogonium to Primary Oocyte and Meiosis I

13. • Completion of Meiosis I:
• During each menstrual cycle, a few primary oocytes resume meiosis
and complete Meiosis I.
• The result of Meiosis I is one secondary oocyte and one small polar
body (which typically degenerates).
• Arrest at Meiosis II:
• The secondary oocyte enters Meiosis II but is arrested at metaphase II
until fertilization occurs. If fertilization does not happen, the secondary
oocyte will degenerate.
• Ovulation:
• The mature follicle containing the secondary oocyte ruptures during
ovulation, releasing the oocyte into the fallopian tube.
• If fertilization occurs, Meiosis II is completed, producing a mature ovum
and another polar body. If no fertilization occurs, the secondary oocyte
will not complete Meiosis II and will disintegrate.
Meiosis II & Ovulation

14. Structure Of Mature Ovum
•Largest cell in the body

•Consists of cytoplasm and a nucleus with its nucleolus in eccentric position

•Contains 23 chromosomes (23 X )

•Sorrounded by a cell membrane called a vitelline membrane

•Outer transparent mucoprotein envelope is called zona pellucida

•Tiny channels in zona pellucida are for the transport of the materials from the granulosa cells to the
oocyte

•Space between the vitelline membrane and zona pellucida is called perivitelline space which
accomodates the polar bodies

•Oocyte after its escape from the follicle, retains a covering of granulosa cells known as corona radiata
derived from the cumulus oophorus

15. Causes of Ovulation-
Combined FSH/LH midcycle surge is responsible for the final stage of maturation, rupture of the follicle
and expulsion of the oocyte
LH surge-
•Sustained peak levels of oestrogen for 24-36 hours in the late follicular phase cause LH surge from
anterior pitutary.
•Ovulation occurs apprx 16-24 hours after LH surge.
•LH stimulates completion of reduction division of the oocyte,initiates leutinisation of the granulosa
cells, synthesis of progestrone and prostaglandins
FSH rise
•Preovulatory rise of progestrone facilitates the positive feed back action of estrogen to induce FSH surge
FSH surge causes increase in plasminogen activator which converts plasminogen into plasmin , which in
turn causes lysis of the wall of the follicle

16. Fertilisation
The Union of Sperm and Egg
Beginning of Life

17. Overview of Fertilisation
Definition:
Fertilization is the process by which a sperm cell from a male fuses
with an egg (ovum) from a female to form a zygote, the first stage of
development for a new organism.
• Location:
Fertilization typically occurs in the fallopian tube of the female
reproductive system.

Fertilization happens after ovulation when a mature egg is released
from the ovary and is available to meet the sperm.

The result is the formation of a zygote, a single-cell organism that
contains a full set of chromosomes (diploid)

18. Stages of Fertilisation
1. Sperm Capacitation:
• Before fertilization can occur, sperm undergo capacitation, a process that occurs
in the female reproductive tract. This involves biochemical changes that enable
sperm to penetrate the egg’s outer layers.
2. Sperm Penetration of the Egg:
• The sperm travels through the cervix, into the uterus, and towards the fallopian
tube.
• The sperm must penetrate the corona radiata (the outer layer of the egg) and the
zona pellucida (the glycoprotein layer surrounding the egg).
3. Fusion of Sperm and Egg Membranes:
• Once a sperm binds to the zona pellucida, it releases enzymes that help it
penetrate and fuse with the egg membrane, allowing the sperm’s nucleus to enter
the egg.

20. The Events Following Fertilisation
• Cortical Reaction:
• After the sperm enters the egg, the egg undergoes a cortical reaction to prevent any
other sperm from fertilizing the egg. This involves the release of cortical granules that
alter the egg’s surface, making it impermeable to additional sperm.

• Completion of Meiosis II:
• The egg, which was arrested in metaphase II of meiosis, completes Meiosis II after
fertilization, resulting in the formation of the ovum and another polar body.
• Zygote Formation:
• The sperm’s nucleus fuses with the egg’s nucleus to form a single diploid cell called the
zygote, which contains 46 chromosomes (23 from each parent).

22. Implantation and Early Embryonic Development
Cleavage Process and Formation of Blastula

23. Cleavage - Definition and Overview
• Definition:
Cleavage refers to the series of rapid mitotic divisions that a zygote undergoes
immediately after fertilization. These divisions increase the number of cells (called
blastomeres) without increasing the overall size of the embryo. This process
transforms the zygote into a multicellular structure called the morula.
• Key Features of Cleavage:
• No growth in size: The embryo does not grow during cleavage, as the total volume
remains constant.
• Rapid mitotic division: Cleavage is a process of repeated cell division, but the cells
produced (blastomeres) are smaller with each division.
• Embryonic Stage: The early cleavage stages are essential in setting the foundation
for later developmental stages, including the formation of the blastocyst and further
differentiation.

24. Stages of Cleavage:
1. First Cleavage (2-cell stage):
The zygote divides into two blastomeres, each containing an
identical set of genetic material.
2. Second Cleavage (4-cell stage):
The two blastomeres each divide, resulting in four cells. At this
stage, the cells are still small and the embryo does not grow in
size.
3. Morula Stage (16-32 cell stage):
Cleavage continues, and the cells compact tightly together,
forming the morula (a solid ball of cells). This process continues
until the cells become compacted enough to transition into the
next stage—the blastocyst.

25. Cellular Compaction:
• As cleavage continues, the individual blastomeres begin to tightly
adhere to each other, undergoing compaction. This prepares them for
the next stage, where the outer cells of the embryo will form the
trophoblast (future placenta), and the inner cells will form the inner cell
mass (future embryo).

26. Implantation of Blastocyst
The process by which the developing mass
get imbeded within the uterine wall
Time -
Implantion occur at 6th day after
fertilization and its completed about 11th
day
Site of implantation:
Posterior wall of the uterine body in the functional layer of the endometrium during
the secretory phase of the cycle..
Implantation in the lower segment leads to placenta praevia.

27. Stages of Implantation
Apposition and Adhesion:
Upon hatching, the blastocyst attaches to the endometrial lining of the uterus in
a process called apposition, followed by adhesion, where the trophoblast
interacts with the uterine epithelium.
• Invasion by Trophoblast:
The trophoblast differentiates into cytotrophoblast (inner layer) and
syncytiotrophoblast (outer multinucleated layer) and begins to invade the
endometrial tissue, establishing the early placenta and maternal blood supply.

28. Complications of Implantation
Ectopie Pregnancy
It means implantation outside the uterus.
80 % of ectopic pregnancies occurs
in the ampulla of uterine tube.
Most common are in the ampulla &
isthmus.
Hydatidiform mole formation
Abnormal
Sites Of Implantation
Abnormal Implantation sites:
1. Abdominal (1.4%)
2. Ampullary region
(80%),
3. Tubal (12%),
4. Interstitial (0.2%),
5.Placenta previa (0.2%),
6. Ovarian implantation (0.2%)

29. Complications in Pregnancy
Placenta accreta: Excessive invasion can lead to deficient development of the decidua
with abnormally firm attachment of the placenta directly onto the myometrium
Placenta increta: Extension of the placenta into the myometrium
Placenta percreta: Invasion through the myometrium to the uterine serosa and even
into adjacent organs.
These disorders are associated with maternal illness and death, primarily due to
hemorrhage.
Preeclampsia: Due to shallow invasion of cytotrophoblasts & more endovscular
invasion

30. Inner Cell Mass Differentiation:
The inner cell mass (ICM) within the blastocyst differentiates
into two layers:
• Epiblast: The upper layer that will give rise to all three germ
layers (ectoderm, mesoderm, and endoderm).
• Hypoblast: The lower layer that contributes to extra-
embryonic structures such as the yolk sac.
• Formation of the Bilaminar Disc:
These two layers form a flat structure called the bilaminar disc,
which will eventually form the body of the embryo.
Formation of Bilaminar Disc

31. Formation of Trilaminar Disc and Gastrulation
Gastrulation:
Around day 15-16, the bilaminar disc undergoes gastrulation, leading
to the formation of three primary germ layers:
• Ectoderm: Forms the nervous system, skin, and sensory organs.
• Mesoderm: Forms muscles, bones, heart, and circulatory system.
• Endoderm: Forms internal organs like the gut, lungs, and liver.
• Primitive Streak:
The primitive streak forms along the epiblast, marking the site where
cells will begin to migrate and differentiate into the three germ layers.

33. Formation Of Chorionic Villi
The essential functional elements of the placenta are very small finger like processes or villi

•These villi are surrounded by maternal blood

•In the subustance of the villi, there are capillaries through which fetal blood circulates

•Exchanges between maternal and fetal circulations take place through the tissues forming the
walls of the villi

•The villi are formed as offshoots from the surface of the trophoblast

•As the trophoblast along with the underlying extra-embryonic mesoderm constitutes chorion,
the villi are known as Chorionic Villi.

35. Neurulation -
Neurulation is the process that follows the formation of the three germ layers and
marks the beginning of the central nervous system (CNS) development. It begins
around day 18-20 in humans.
• Steps of Neurulation:
1. Neural Plate Formation: The ectoderm above the notochord thickens to form
the neural plate.
2. Neural Groove Formation: The lateral edges of the neural plate fold upwards,
forming the neural groove.
3. Neural Tube Formation: The neural folds fuse, forming the neural tube, which
will later differentiate into the brain and spinal cord.

37. Organogenesis -
Development of Somites -
• Somite Formation:
The mesoderm gives rise to somites, which are segmental blocks of tissue formed on either
side of the neural tube. Somites form from paraaxial mesoderm and appear during the third
week of embryonic development.
• Somite Derivatives:
Somites differentiate into:
1. Sclerotome: Forms bones and joints of the axial skeleton.
2. Dermatome: Forms the dermis of the skin.
3. Myotome: Forms the skeletal muscles.
Somites play a crucial role in establishing body segmentation and contribute to the
development of the vertebral column, muscles, and skin.

38. Formation of Heart and Cardiovascular System
• Heart Development:
The heart begins to form from mesodermal precursor cells called cardiogenic
mesoderm during the third week of development.
• Early Heart Tube Formation:
The heart tube is formed by the fusion of bilateral heart fields in the mesoderm. By the
end of the third week, this tube begins to undergo folding and looping, laying the
foundation for the heart chambers.
• Vascular Development:
The first blood vessels also form during this period. The vasculogenesis process creates
the primary vascular network, which is later remodeled into arteries and veins.
• Heartbeat:
The heart starts to beat by around day 22-23.

39. Formation of Gastro-Intestinal Tract
(endoderm derivative )
Endodermal Development:
The endoderm gives rise to the gastrointestinal tract and several associated organs.
During the fourth week of development, the gut tube forms.
• Gut Tube Differentiation:
The gut tube develops into:
1. Foregut: Forms the pharynx, esophagus, stomach, and part of the duodenum.
2. Midgut: Forms the small intestine and part of the large intestine.
3. Hindgut: Forms the rectum and part of the colon.
• Organ Formation:
The endoderm also gives rise to accessory organs such as the liver, pancreas, and
lungs.

40. Formation of Heart Formation Of GI Tract

41. Limb Bud Development
• Limb Bud Formation:
Limb buds appear around day 26-28 in humans and are derived from the lateral plate mesoderm.
• Limb Patterning:
The upper and lower limb buds develop in response to signals from the zone of polarizing activity
(ZPA) and apical ectodermal ridge (AER), which regulate the patterning of the limb.
• Development of Limb Structures:
As the limb buds elongate, the mesoderm differentiates into muscle, bone, and cartilage, while the
ectoderm forms the outer epidermis of the limbs.
• Joint Formation:
Joint development occurs when mesodermal cells undergo differentiation to form articular cartilage
and other joint structures.

42. Formation of the Kidneys and Excreatory System
• Kidney Development:
The kidneys develop from the intermediate mesoderm and consist of three stages:
1. Pronephros: The earliest stage, which is transient and non-functional.
2. Mesonephros: Functions temporarily before the definitive kidneys form.
3. Metanephros: The definitive kidneys, which are fully functional by the end of the first
trimester.
• Ureteric Buds and Nephron Formation:
The metanephros begins to develop from the ureteric buds, which form the renal pelvis,
ureters, and collecting ducts. Nephrons are formed as the mesodermal tissue
differentiates.

43. Development of neural crest
• Neural Crest Cells:
Neural crest cells, derived from the ectoderm, migrate from the neural tube during
neurulation and differentiate into a variety of structures.
• Derivatives of Neural Crest Cells:
• Peripheral Nervous System (PNS): Including neurons, Schwann cells, and ganglia.
• Facial Structures: Bones and cartilage of the face.
• Pigment Cells: Melanocytes.
• Adrenal Medulla: The inner part of the adrenal glands.

44. Early Vascularisation and Blood Formation
Content:
• Vasculogenesis and Angiogenesis:
The initial blood vessels form from angioblasts, which are derived from mesodermal
cells. Vasculogenesis creates the early blood vessel network, which then undergoes
angiogenesis to form more blood vessels.
• Hematopoiesis:
Early blood cell formation, or hematopoiesis, occurs in the yolk sac before the liver
and bone marrow take over later in development.
• Heart Function and Circulatory System:
The heart starts to pump blood around day 22, establishing the circulation of
nutrients and oxygen to developing tissues.