JOURNEY OF AN EMBRYO

• A process where male spermatogonia develop into mature spermatozoa.

• Occur in testes and epididymis in mammals and takes approximately 64 days.

SPERMATOGENESIS

• Step 1 – Spermatocytosis• Step 2 – Meiosis in Sertoli cells• Step 3 – Spermiogenesis

STEP IN SPREMATOGENESIS

• Monospermy – fusion of a single sperm and egg nuclei

• Polyspermy – excess of adhesion sites that leads to fusion of a single engg with more than 1 sperms.

MONOSPERMY & POLYSPERMY

OOGENESIS

FERTILIZATION

EVENTS THAT LEADS TO FERTILIZATION

General steps of early embryonic development

Embryology

Morula, 8 cell stage

1 - morula, 2 - blastula

1 - blastula, 2 - gastrula with blastopore; orange - ectoderm, red - endoderm.

There are 4 stages of embryonic development:

Cleavage Patterning Differentiation Growth

Cleavage• Mitosis and cytokinesis of the zygote, an unusually large cell,

produces an increasing number of smaller cells• the genes of the zygote are not expressed at first. • The early activities of cleavage are controlled by the

mother's genome; that is, by mRNAs and proteins she deposited in the unfertilized egg.

• In humans, the switch-over occurs after 4—8 cells have been produced; in frogs not until thousands of cells have been produced.

• Cleavage ends with the formation of a blastula.

Patterning

During this phase, the cells produced by cleavage organize themselves in layers and masses, a process called gastrulation. The pattern of the future animal appears:

front and rear (the anterior-posterior axis) back side and belly side (its dorsal-ventral

axis) left and right sides.

Gastrulation forms three major "germ layers": ectoderm Mesodermendoderm

By gastrulation, the genes of the zygote genome are being expressed

Differentiation

• Is the process by which cells or other parts of organisms become different from one another and different from what they were previously

• In time, the cells of the embryo differentiate to form the specialized structures and functions that they will have in the adult.

• They form neurons, blood cells, skin cells, muscle cells, etc.

• These are organized into tissues, the tissues into organs, the organs into systems.

Growth

• Is an increase of size and mass, the enlargement of a tissues or organism

• After all the systems are formed, most animals go through a period of growth.

• Growth occurs by the formation of new cells and more extracellular matrix.

CLEAVAGE PRODUCES A BALL OF CELLS FROM ZYGOTE

• cleavage is the division of cells in the early embryo

• The zygotes of many species undergo rapid cell cycles with no significant growth, producing a cluster of cells the same size as the original zygote.

MEANING

PATTERN OF CLEAVAGE

1. Amount and distribution of yolks in their eggs• Isolechital• Mesolechital• Telolechital• Centrolechital2. Polarity of eggs• Animal pole• Vegetal pole

Types of cleavage:

Types of cleavage

HOLOBLASTIC• In the absence of a large concentration of yolk. In

holoblastic eggs the first cleavage always occurs along the vegetal-animal axis of the egg, the second cleavage is perpendicular to the first.

MEROBLASTIC• In the presence of a large amount of yolk in the

fertilized egg cell, the cell can undergo partial, or meroblastic, cleavage.

Cleavage patterns followed by holoblastic and meroblastic eggs

Holoblastic Meroblastic

•Bilateral (tunicates, amphibians)

Discoidal (fish, birds,reptile)

Radial (sea urchin, amphioxus) •Superficial (insects)

Rotational (mammals)

Spiral (annelids, mollusks)

Bilateral• The first cleavage results in bisection of the zygote into

left and right halves. The following cleavage planes are centered on this axis and result in the two halves being mirror images of one another. In bilateral holoblastic cleavage, the divisions of the blastomeres are complete and separate.

Radial • Radial cleavage is characteristic of the deuterostomes,

which include some vertebrates and echinoderms, in which the spindle axes are parallel or at right angles to the polar axis of the oocyte.

HOLOBLASTIC

Rotational• Mammals display rotational cleavage, and an isolecithal

distribution of yolk (sparsely and evenly distributed). Because the cells have only a small amount of yolk, they require immediate implantation onto the uterine wall in order to receive nutrients. Rotational cleavage involves a normal first division along the meridional axis, giving rise to two daughter cells.

Spiral• In spiral cleavage, the cleavage planes are oriented

obliquely to the polar axis of the oocyte. At the third cleavage the halves are oblique to the polar axis and typically produce an upper quartet of smaller cells that come to be set between the furrows of the lower quartet.

Meroblastic cleavage

Discoidal• In discoida cleavage, the cleavage furrows do not penetrate the

yolk. The embryo forms a disc of cells, called a blastodis, on top of the yolk. Discoidal cleavage is commonly found in birds, reptiles, and fish which have telolecithal egg cells (egg cells with the yolk concentrated at one end).

Superficial• In superficial cleavage, mitosis occurs but not cytokinesis, resulting

in a polynuclear cell. With the yolk positioned in the center of the egg cell, the nuclei migrate to the periphery of the egg, and the plasma membrane grows inward, partitioning the nuclei into individual cells. Superficial cleavage occurs in arthropods which have centrolecithal egg cells (egg cells with the yolk located in the center of the cell).

Spiral cleavage

GASTRULATION

- a phase early in the development of most animal embryo, during which the morphology of the embryo is dramatically restructured by cell migration.

GASTRULATION

- a phase early in the development of most animal embryos, during which the morphology of the embryo is dramatically restructured by cell migration.

• The purpose of gastrulation is :- to position the three embryonic germ layers, the endoderm, ectoderm and mesoderm.

• gastrulation occurs after implantation, around days 14-16 after fertilization in human embryogenesis. (in human )

• The process gastrulation (in human) are :

- The formation of the primitive streak and Hensen's node and the ingression of cells through the primitive groove to form the endoderm and the mesoderm.

- Thus, gastrulation creates all three germ layers of the embryo: ectoderm, mesoderm, and endoderm

- Extraembryonic mesoderm forms within the hypoblast or embryonic mesoderm and migrates out to form the blood vessels of the chorion and connect the chorion to the embryo through the umbilical cord.

GATRULATION IN SEA URCHINS

• The archenteron is elongated by three mechanisms :

- First, the initial invagination is caused by a differential expansion of the inner layer made of fibropellins and outer layer made of hyalin to cause the layers to bend inward.- Second, the archenteron is formed through convergent extension.-Third, secondary mesenchyme pull the tip of the archenteron towards the animal pole.

• The process of gastrulation in amphibian is at higher density of yolk in the vegetal half of the embryo results in the blastocoel cavity being placed asymmetrically in the animal half of the embryo.

• four kinds of tissue movements that drive gastrulation in Xenopus, that are :

- At the vegetal edge of the dorsal marginal zone, cells change from a columnar shape to become a bottle cell and drive invagination.

- At this invagination, cells begin to involute into the embryo. This initial site of involution is called the dorsal lip.

- Directed cell intercalation within the dorsal mesoderm drives convergent extension. The dorsal cells become the first to migrate along the roof of the blastocoel cavity and form the anterior/posterior axis of the embryo.

- Both prior to and during the involution, the animal cap undergoes epiboly and spread toward the vegetal pole.

GASTRULATION IN FISH

GASTRULATION IN BIRD

EVENTS IN DEVELOPMENT THAT INVOLVED THE MIGRATION OF

CELLS WITHIN THE EMBRYO DEVELOPMENT

About 1th week•After fertilization,embryo reaches two-cell stage•The blastula implants into the uterus

2th week•Within 2 weeks,many thousand of cells formed(embryo)

About 5th week•A gestational sac on ultrasound•Embryo at 4 weeks after fertilization.

6th week•In the beginning of the 6th week,a small ring called yolk sac on ultrasound•At the end of the 6th week the fetal pole and perhaps cardiac activity in the embryo

Embryo at 4 weeks after fertilization.

At 7th weekA well defined fetal pole and deinite cardiac cavity

By 9th weekA baby is called fetus. At this time the heartbeat with a doptone device about 50% of the time can heard

is about five weeks old (or from the seventh week of menstrual age).

Fetus at 8 weeks after fertilization.

Fetus at 8 weeks after fertilization.

This embryo is also from an ectopic pregnancy, this one in the cornu (the part of the uterus to which the Fallopian tube is attached). The features are consistent with a developmental age of seven weeks (reckoned as the ninth week of pregnancy

At end of first trimester(12th week)Placenta formed and supply the baby with oxygen from mother’s blood supply,and ridding wastes tthrough mom’s blood system At 13th weekBaby growing very quickly Week 16th – 20th Mother may feel a fluttering that is baby’s movement(quickening)Fetus at 18 weeks after fertilization 20th week Baby is half-way fully formedBaby is quite active and moving often

21th weekBaby’s eyes still closed,movement is stronger,skin is pinkAs baby and uterus grow,they are displacing organs that reside normally in the lower abdomen and pelvis

Fetus at 18 weeks after fertilization

By 24th weekUterus having intermitent contractionsBaby weights over one and one half poundBaby is consider viable(half babies born is survive at this stage)

26th to 28th weekLungs maturedBaby starting to store part in the subcutaneous layer of skin and hair growingBaby eyes is open

32th to 33th weekBaby weight about 4 ½ pounds and about 16-17 inches along

About 34th weekBaby lung start to work well

From 36th weekBaby consider fully develop Fetus at 38 weeks after

fertilization

NEURULATIONAND

ORGANOGENESIS

The process involved in the formation of the neural plate and neural folds and closure of the folds to form the neural tube constitute neurulation.Neurulation is complete by the end of the fourth week.During neurulation, the embryo may be referred to as neurula.Neurulation in vertebrates results in the formation of the neural tube, which gives rise to both the spinal cord and the brain.Neural crest cells are also created during neurulation. Neural crest cells migrate away from the neural tube and give rise to a variety of cell types, including pigment cells and neurons.

PROCESS OF NEURULATION

Neurulation begins with the formation of a neural plate, a thickening of the ectoderm caused when cuboidal epithelial cells become columnar.

Changes in cell shape and cell adhesion cause the edges of the plate fold and rise, meeting in the midline to form a neural tube.

The cells at the tips of the neural folds come to lie between the neural tube and the overlying epidermis. These cells become the neural crest cells.

Both epidermis and neural plate are capable of giving rise to neural crest cells.

Organogeneis is the period of animal development during which the embryo is becoming a fully functional organism capable of independent survivial.Organogenesis is the process by which specific organs and structures are formed, and involves both cell movements and cell differentiation. Organogenesis requires interactions between different tissues. These are often reciprocal interactions between epithelial sheets and mesenchymal.

ORGAN PRODUCED BY THE 3 GERM LAYERS

ENDODERM

The endoderm produces tissue within the lungs, thyroid and pancreas.

MESODERM

The mesoderm aids in the production of cardiac muscle, skeletal muscle, smooth muscle, tissues within the kidneys, and red blood cells.

ECTODERM

The ectoderm produces tissues within the epidermis and aids in the formation of neurons within the brain, and melanocytes.