Most of this if not all is copied from Biology 3 Edition, by Brooker et al An an

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Most of this if not all is copied from Biology 3 Edition, by Brooker et al An animal starts its life as a single cell, which will turn into a multicellular organism. There are four essential processes that are needed to make a multicellular organism: Cell Proliferation, which produces more cells, Cell specialization, which produces cells with different characteristics, Cell interactions, which coordinates the behavior of one cell with that of its neighbors and finally cell movement, which rearranges the cells to form structured tissues and organs Embryonic development is the process by which a fertilized egg (that is, a zygote) is transformed into an animal with distinct physiological systems and body parts. The biological information that controls embryonic development resides in both the organism's DNA and in the cytoplasm of the egg. As you have probably observed, the adult forms of animals vary immensely, however, embryonic development follows a similar pattern in most animals. Most modern animals including humans are triploblasts, which means they have three distinct germ layers, and development in these animals can be divided into five general events: fertilization, cleavage, gastrulation, neurulation, and organogenesis. Many species also go through an additional event called metamorphosis, which is a transition from a feeding larval form to an adult. Metamorphosis occurs after organogenesis Read through 3" slide FERTILIZATION The events in fertilization are similar in all triploblasts Fertilization is the union of two gametes, such as an egg cell with a sperm cell, to form a zygote. We will be describing fertilization in sea urchins a well-studied model organism. In our discussion we will talk about the acrosomal reaction and the cortical reaction. The acrosomal reaction is an event in fertilization in which enzymes released from a sperm's acrosome break down the outer layers of an egg cell, which allows the entry of the of the sperm cell's nucleus into the egg cell. The cortical reaction is a n event in fertilization i

Explanation / Answer

Embryonic development of fertilized zygote into a mature organism through organogenesis:

The initiatil fertilization porcess takes place between haploid sperm cell and haploid ovum result in formation of diploid zygote finally it is going to be implanted into the wall of uterus to become a mature embryo. The initially entry of sperm cessl into the ovum through acrosome reaction followed by hyalurinidase enzymatic action on the ovum.

Sperm cells produce from meiotic division of the germ cells inside the epididymis of male testes. Later, sperm cells travel through Vas deferens finally enters the ampulla of ductus deferens through abdominal cavity. This is path of movement is followed by movement of mature sperm cells into the seminal vesicle (fructose rich semen produced) and then to prostatic urethra via ejaculatory ducts. Prostate gland secretes acid i.e. mucoid secretion to make sperm cells slightly acidic followed by passage of sperm cells through membranous urethra. Here bulbourethral glands secrete alkaline fluids finally; semen surrounded the sperm cells to nourish and ejaculated at the time of intercourse into the female vaginal orifice through penile urethra.

Inside the female uterus, ampulla of fallopian tube, fertilization initiates due to acrosomal reaction takes place due to secretion of hyalurinidase fluids at the penetration of sperm cell into the secondary oocyte using acrosome. Sperm cells uses whip like flagella movement to penetrate into the ovum (chemotaxis) zona pellucida layer finally sperm cells uses hyalurinidase enzyme to penetrate inside the egg through corona radiata.

The fusion of plasma membranes of egg and sperm during the process of fertilization triggers a series of changes in the egg, which blocks polyspermy. Fusion of plasma membranes of sperm and egg cells triggers depolarization of the membrane. Now, this membrane causes “fast block to polyspermy,” which temporarily blocks the sperm penetration. This occurs prior to the cortical reaction.

The initial barrier which broken down by the sperm cell in the ovum is corona radiate by the hyalurinidase enzyme followed by digestion of zona pellucida by secreting digestive enzymes from acrosome to contact directly cell membrane. Depolarization of oocyte riggers the exocytosis and thus releases the enzymes that cause hardening of zona pellucida. The secreted materials clip off the sperm binding receptors and form the fertilization envelop, which act as a “slow block polyspermy.”

A mammalian embryo is totipotent until 16-celled stage finally become trophoblast. Normally, a zygote is totipotent because it can develop into a embryo with blastomere stage. Mammalian embryos after 8-celled blastomeres (in case of rats) are going to stop being totipotent. In case of sea urchins, the embryo is totipotent until 2 to 8 -celled stage. It is not possible to acquire totipotency after this stage. As the embryo progresses of developing into adult with cell differentiation, 8-celled blastomere is going to lack "the property of totipotent"

Later developmental stages associated with embryo with blastula undergoes metamorphosis and produces germinal layer formation with ectoderm, mesoderm and endoderm to produce differential organs but the cellular components of the every cell after development are same as they contain similar characteristics as modern vertebrates in the evolution.

Fertilization is going to produce diploid zygote finally become embryo and the implantation of embryonic tissue to the uterine wall is mainly based on the egg polarity. The development of embryo into adult organism follows the following steps

Cleavage:

Fertilization: In mammals’ haploid gametes mixes with the haploid ovum result in diploid zygote (2n). This process is referred as fertilization. The zygote implants to the uterine wall and become mature embryo. During the organogenesis, a single celled zygote undergoes the following maturity process.

Cleavage is the process of formation of 8-celled

Balstocoel ---> it is the inner fluid filled cavity formed after zygote and morula stage during embryogenesis enables to form cavities such as thoracic, abdomenal and cranial region.

Archenteron ---> This is the cavity formed during invagination of infolded cell at the time of gastrulation.

Blastula: The formation of the ball of cells is due to the repeated cell division by multiple steps of cleavage. Finally, a mass of cells with a cavity filled with fluid yolk, and the blastocoel formed at the center. Little yolk, at this blastula stage zygote undergo two types of further cleavages called holoblastic cleavage and meroblastic cleavage and finally produces initial two primitive germinal layers which will become finally embryo. The outer later finally metamorphosized to placenta called as tropoblast. Now this stage referred as blastocyst. Three germinal layers will be produced during the gastrulation period in order to produce internal organs of the mature organism by the process called organogenesis. They are as follows A. Ectoderm: produces nervous system and skin, epidermis.

Mesoderm: produces circulatory system, connective tissue, skeletal system, muscular system. Endoderm: produces digestive system and respiratory system, internal organs.

Gastrulation: Formation of gastrula stage from blastula with three germ layers in which cell potency of dividing embryonic stem cells is varied according to the induction of organ or organ system

Organogenesis:

Somites are derived from paraxial mesoderm; somites are divided into three main regions and somite formation represents the age of embryo

Myotome: adult structures transformed from this skeletal muscle of back, limbs, and trunk

Dermatome of somitea is going to transform into dermis and connective tissues

Sclerotome of Somites is going to transform into vertebrae and ribs

Notochord & neural crest formation also developed from mesoderm but it from axial (chordamesoderm) but not developed from paraxial mesoderm. Brain parts are such as medulla, Pons, cerebellum are developed from germ layer, ectoderm.

The oropharygeal membrane of developing face is a barrier found in the embryo that separates brain and heart. These primitive pharyngeal structures are found in the primitive mouth that is covered by oro-pharyngeal membrane and it is a shallow depressed structure found in the embryonic ectoderm surface. The pharynx is going to connect the stomodeum with esopharyngeal membrane.

Morphogens are molecules mainly useful to promote positional changes during embryonic development through the process of morphogeneis

The HIV and othe diseases are nomrally acquired through mother's placental immune cell (innate & adaptive) migration into the fetus

Karl von Baer has illustrated that most conserved stage of developmental process is mid-embryonic stage or organogenesis stage associated with pharyngula stage followed by cleavage & formation of germinal layers during gastrulation to produce differential organs in the organism.

The average gestation period of humans is 38 weeks from the fertilization of ovum. Gestation period can be divided into three trimesters of three months each, these are called critical periods. Critical period is defined as “a definite period of embryonic and foetal development during which formation of tissues and organs can occur.”

First trimester is the period of organogenesis, during which the rudiments of all the organs develop. All the major organs of an adult are present in rudimentary form in an 8 weeks zygote, and it is called a “foetus.” Heartbeat starts by the 4th week and can be detected at 8-10 weeks.

During the second trimester, the foetus grows to 30 cm long and the pregnancy become obvious. The mother can feel the foetal movements during this stage.

During the third trimester, the activity decrease as the foetus grows to 3-4 kg weight and fills the available space. Development of all the major organs completes in this stage.

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