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

EMBRYOGENESIS

“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….

Definition

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 bodyb. Outside of the body

Fertilization

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.

ZYGOTE

“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 cellsII. Then these two cells into 4 cellsIII. Then these four cells into 8 cellsForming polar bodies and a middle line.

There are two types of cleavage formation: Holoblastic (total) Meroblastic (partial)

Cleavage

z.p( Zona striata) p.gl (Polar bodies) a (Two-cell stage) b (Four-cell stage) c (Eight-cell stage)

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

Continue….

“At sixteen cell division stage of cleavage is known as MORULA”

Morula

1. Morula 2. Blastula

“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.

BLASTULA

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

Formation Of Germinal Layers

“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.

Gastrula

Differentiation of Germinal Layers

“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

Germ Layer

“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

Endoderm

“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 gonadsIII. 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.

Mesoderm

“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.

Ectoderm

Overview

THANK YOU