1. Embryogenesis begins with fertilization of an egg cell by a sperm cell, forming a zygote. The zygote then undergoes cleavage, dividing into multiple cells while maintaining the same size. 2. After the 7th cleavage division forming 128 cells, it is called a blastula. Cells then migrate inward during gastrulation, forming the endoderm, mesoderm and ectoderm germ layers. 3. The endoderm forms the inner lining of the digestive system. The mesoderm forms muscles, bone, blood and other tissues between the endoderm and ectoderm. The ectoderm forms the outer layer including the skin and nervous system.

1. Presented To: Dr. Riaz Hussain Presented By: 14-Arid-2022
PMAS FAISAL SHAHZAD SOMROO

2. “Embryogenesis is the process by which the embryo forms and develops”
In mammals, the term refers chiefly to early stages of prenatal
development, whereas the terms fetus and fetal development describe
later stages….

3.  Embryogenesis Starts with the fertilization oh the egg cell OVUM by a
sperm cell Spermatozoon
 Once fertilized, the ovum Is known as ZYGOTE ( a single diploid cell).
 Depending on the specie fertilization can be :
a. Inside body
b. Outside of the body

4.  The egg cell is always asymmetric, having an "animal pole" (future
ectoderm and mesoderm) and a "vegetal pole" (future endoderm).
 It is also covered with different protective envelopes, with different
layers.
1. The first envelope - the one in contact with the membrane of the egg
- is made of glycoproteins and is known as the vitelline membrane
(zona pellucida in mammals).
2. Different cellular and acellular envelopes englobing the vitelline
membrane.

5. “Cell division with no significant growth, producing a cluster of cells that is the same size as the original zygote, is
called cleavage”
I. Firstly cells divides into 2 cells
II. Then these two cells into 4 cells
III. Then these four cells into 8 cells
Forming polar bodies and a middle line.
There are two types of cleavage formation:
 Holoblastic (total)
 Meroblastic (partial)
z.p( Zona striata) p.gl (Polar bodies) a (Two-cell stage) b (Four-
cell stage) c (Eight-cell stage)

6.  Holoblastic cleavage:
Occurs in animals with little yolk in their eggs, such as humans
and other mammals who receive nourishment as embryos from the mother, via
the placenta or milk.
 Meroblastic cleavage:
Occurs in animals whose eggs have more yolk; i.e. birds and
reptiles. Because cleavage is impeded in the vegetal pole, there is a very uneven
distribution and size of cells, being more numerous and smaller at the animal pole
of the zygote

7. “At sixteen cell division
stage of cleavage is
known as MORULA”
1. Morula 2. Blastula

8. “After the 7th cleavage has produced 128 cells, the embryo is called a blastula”
Blastocoel:
Surrounds a fluid-filled or yolk-filled cavity (the blastocoel).
Blastocyst:
Form a structure called the blastocyst, characterized by an inner cell mass
that is distinct from the surrounding blastula.
The blastocyst must not be confused with the blastula; even though they are
similar in structure, their cells have different fates.

10. Trophoblast:
 They form the ectoderm of the chorion and play an
important part in the development of the placenta.
 On the deep surface of the inner cell mass, a layer of
flattened cells, called the endoderm, is differentiated
and quickly assumes the form of a small sac, called
the yolk sac.
 The embryonic ectoderm, derived from the inner cell
mass and lying in apposition with the endoderm

12. “During gastrulation cells migrate to the interior of the blastula, subsequently
forming two (in diploblastic animals) or three (triploblastic) germ layers. The
embryo during this process is called a gastrula”
 The germ layers are referred to as the ectoderm, mesoderm and endoderm.
 In diploblastic animals only the ectoderm and the endoderm are present
 Specific organs and tissues are derived from the cell layers.

15. “A germ layer, is a primary layer of cells that form during embryogenesis”
The three germ layers in vertebrates are particularly pronounced:
 Ectoderm
 Mesoderm
 Endoderm

16. “Cells migrating inward along the archenteron form the inner layer of the gastrula,
which develops into the endoderm”
 The endoderm consists at first of flattened cells, which subsequently become
columnar.
 It forms the epithelial lining of the whole of the digestive tube except part of the
mouth and pharynx and the terminal part of the rectum (which are lined by
involutions of the ectoderm).
 It also forms the lining cells of all the glands which open into the digestive tube,
including those of the liver and pancreas; the epithelium of the auditory tube and
tympanic cavity; the trachea, bronchi, and air cells of the lungs; the urinary bladder
and part of the urethra; and the follicle lining of the thyroid gland and thymus.
 The endoderm forms: the stomach, the colon, the liver, the pancreas, the urinary
bladder, the epithelial parts of trachea, the lungs, the pharynx, the thyroid, the
parathyroid, and the intestines

17. “Some of the cells migrating inward contribute to the mesoderm, an additional layer
between the endoderm and the ectoderm”
Several components : intermediate mesoderm, paraxial mesoderm, lateral plate
mesoderm, and chorda-mesoderm.
I. The chorda-mesoderm develops into the notochord.
II. The intermediate mesoderm develops into kidneys and gonads
III. The paraxial mesoderm develops into cartilage, skeletal muscle, and dermis.
IV. The lateral plate mesoderm develops into the circulatory system (including the
heart and spleen), the wall of the gut, and wall of the human body .
The mesoderm forms: Muscle (smooth and striated), bone, cartilage, connective
tissue, adipose tissue, circulatory system, lymphatic system, dermis, genitourinary
system, serous membranes, and notochord.

18. “The ectoderm generates the outer layer of the embryo, and it forms from the
embryo's epiblast”
 The ectoderm develops into the surface ectoderm, neural crest, and the
neural tube.
 The surface ectoderm develops into: epidermis, hair, nails, lens of the eye,
sebaceous glands, cornea, tooth enamel, the epithelium of the mouth and
nose.
 The neural crest of the ectoderm develops into: peripheral nervous system,
adrenal medulla, melanocytes, facial cartilage, dentin of teeth.
 The neural tube of the ectoderm develops into: brain, spinal cord, posterior
pituitary, motor neurons, retina.