Fertilization and Early Embryonic Development in Human

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Implantation and Establishment of Pregnancy in Human

Fertilization in Human

The process of union of sperm (n) and ovum (n) is called fertilization. As a result of fertilization, a zygote (2n) is formed. The possibility of fertilization is more, if ovulation occurs at the time of intercourse.

Time of Fertilization
The life of the ovum discharged from the Graafian follicle of the ovary is 6-7 hours. The life of sperms discharged in the female genital passage is 72 hours but their power of fertilization remains upto 48 hours. So fertilization takes place if ovulation occurs by this time.

Place of Fertilization
Fertilization takes place at the ampullary isthmic junction of the fallopian tube.

Mechanism of Fertilization

Fertilization occurs by the following sequences:
1. Gametes:
The male gamete is called sperm or spermatozoa. A mature spermatozoon is made of four parts: Head, Neck, Body, and Tail. The anterior portion of the head of sperm contains an acrosome, which is derived from the Golgi body. After the acrosomal part nucleus is seen. Dense chromatin constructs the nucleus.

Ovum is much larger in size than the size of sperm. Ovum contains yolk, cytoplasm, nucleus, and various other cytoplasmic organelles. The ovum released from the ovaries is mainly a secondary oocyte. There is a fluid-filled space on the outer side of the plasma membrane of the ovum, which is called perivitelline space. A very distinct noncellular glycoprotein layer is seen over the perivitelline space, called zona pellucida. An irregular layer of granulosa cells is present in the outer regions of zona pellucida, known as corona radiata.

2. Arrival of Sperm:
About 200-300 million sperm are discharged with semen in the vagina of the female during copulation. This process is known as insemination.

3. Arrival of Secondary Oocyte:
After release from the mature Graafian follicle of the ovary, the secondary oocyte is received by the nearby fallopian funnel and then sent to the fallopian tube by the movement of fimbriae and their cilia.

4. Capacitation of Sperm:
Just (at once) released sperms are not able to fertilize the ovum. After residing over 5-6 hours long time in the female genital tract sperm bears the capacity to fertilize. Now sperm moves or travels up from the vaginal passage of the female towards the uterus. Most of them are destroyed in the passage. But only a few thousand find their way into the opening of the fallopian tube. Out of them only one called ‘Fit sperm’ is able to fertilize the ovum.

Primarily, contractions of the uterus and fallopian tubes help in the sperm movement but later on they move by their own ability. During a residence in the epididymis plasma membrane of the acrosome gets solidified by the deposition of glycoprotein and cholesterol. During capacitation this glycoprotein and cholesterol of the plasma membrane are released, that causes modification in the membrane which results in easy secretion of acrosomal enzymes. Thus the receptor sites on the acrosome are exposed and sperm becomes active to penetrate the egg. This phenomenon of sperm activation in mammals is known as capacitation. It takes about 5 to 6 hours.

5. Contact of Ovum and Sperm:
Contact of ovum and sperm takes place at the ampullary isthmic junction of the fallopian tube. The secondary oocyte can be fertilized only within 24 hours after its release from the ovary. And the life of sperm discharged in the female genital passage is 72 hours but their power of fertilization remains upto 48 hours.

6. Penetration:
The ovum discharged from the Graafian follicle of the ovary has a layer of granular cells with hyaluronic acid all around it. This is called corona radiata. The acrosome of sperm secretes hyaluronidase enzyme which neutralizes hyaluronic acid. The sperm then penetrates the corona radiata and touches the zona pellucida of ovum. Zona pellucida layers contain specific sperm receptors. These sperm receptors are one type of glycoprotein, which is called ZP3 (Zona protein).

At this time, a chemical substance called sperm lysin and an active enzyme called acrosin secreted from sperm digest a limited area of zona pellucida. Through this area, the sperm enters the ovum and lies in perivitelline space. After that microvilli of the vitelline membrane of the ovum covers the head of the sperm. As a result, the sperm membrane and vitelline membrane are united and the head of the sperm enters the ooplasm (cytoplasm of ovum). The remaining part of the sperm remains in the perivitelline space and is destroyed. As soon as the head of the sperm enters the ooplasm, a fertilization membrane is formed around the ovum. This membrane prevents the entry of other sperms into the ovum.

7. Formation of Male and Female Pronucleus:
In the ooplasm, the head of the sperm is converted to a male pronucleus. On the other side, the ovum undergoes a second meiotic division. It loses the second polar body and is converted to a female pronucleus.

8. Fusion of Male and Female Pronucleus:
After the second meiotic division of the ovum and the formation of the female pronucleus, the male pronucleus comes nearer to it. At this time, asters are formed from the pronucleus. The centrosome of sperm appears between the two pronuclei. The centriole divides into two which are arranged in the opposite poles. Then the male and female pronucleus each with 23 chromosomes unite together. Mixing up of the chromosomes of a sperm and ovum is termed Amphimixis or Karyogamy. As a result, a zygote with 46 chromosomes is formed. With the formation of the zygote, pregnancy begins.

Embryo Development

Zygote divides rapidly by mitotic cell division. The process of zygote division is termed cleavage or segmentation. Repeated division of the zygote produces a little ball-like cell mass which is called a morula. Morula is a primordial embryo. Morula reaches the uterus from the fallopian tube and again divides to form the blastula, which is one layered hollow ball-like cell structure. The cell layers of the blastula are called blastoderm and the central cavity is called the blastocoel. Blastula is then transformed into gastrula by the invagination method.

Morula
Repeated division of the zygote forms a cell mass structure that comprises 16 cells called a morula. Morula is an embryo at an early stage of embryonic development or it is called proembryo.

Characteristics of Morula

• It is a hollow ball-like structure of 16 cells.
• It originates from the fallopian tube and enters the uterus remaining in the same condition.
• Morula is formed by the cleavage of the zygote.
• Centrally located cells of the morula give rise to the embryonic part and peripheral cells give rise to the placental part.

Blastula
Morula is divided further and modified into a one-layer hollow sphere of cells, which is called a blastula.

Characteristics of Blastula

• Primarily the clear enclosure of the morula is intact/unaffected in the blastula.
• A common feature of a vertebrate blastula is that it consists of a layer of blastomeres, this is known as blastoderm. Blastoderm surrounds the blastocoel. The mammalian blastula is referred to as a blastocyst.
• When the level of fluid present in the blastocyst increases then the clear enclosure ruptures, which causes rapid growth of the blastula.
• Blastula or blastocyst is implanted into the endometrium of the uterus.
• The human blastocyst comprises 70-100 cells.
• The blastocyst contains an embryoblast (inner cell mass) that will eventually give rise to the definitive structures of the fetus, and the trophoblast, which goes on to form the extra-embryonic tissues.

Gastrula
The undifferentiated cells of blastida get reorganized into a trilaminar (three-layered) structure known as gastrula. These three germ layers are known as ectoderm, mesoderm, and endoderm.

Characteristics of Gastrula

• Gastrula possesses three layers such as ectoderm, mesoderm, and endoderm.
• Gastrulation involves cell movement, that helps to attain a new shape and morphology of the embryo. This movement is called morphogenetic movement.
• Gastrulation is followed by organogenesis when individual organs develop within the newly formed germ layers.
• Each layer gives rise to specific tissues and organs in the developing embryo.
• Skin, nervous system, eyes, ears, etc. are formed from ectoderm layers.
• Notochord, heart, muscle, kidney, and gonads are formed from mesoderm.
• The alimentary system, respiratory system, and excretory system are formed from endoderm layers.

Differences between Morula and Blastula:

Morula	Blastula
1. Morula is formed from zygotic division.	1. Blastula is formed from morula, by its division.
2. Morula looks like a solid ball-like cell mass.	2. It is a hollow ball of one layered cell.
3. No cavity is formed in the morula.	3. The fluid-filled cavity is called the blastocoel.
4. It is formed of similar cells.	4. It is formed of an outer nutritive layer, a trophoblast, and an inner cell mass.

Differences between Blastula and Gastrula:

Blastula	Gastrula
1. Blastula is formed from morula.	1. Gastrula is formed from blastula.
2. Blastula is one layered cell mass.	2. Gastrula is three layered cell mass.
3. Blastula is formed by rapid mitotic division.	3. Formed by slow mitotic division.
4. The cell layer of the blastula is called blastoderm.	4. The cell layers of the gastrula are called ectoderm, mesoderm, and endoderm.

Implantation

The attachment of the blastocyst to the endometrium of the uterine wall which occurs 7-8 days after fertilization is called implantation.

Site of implantation
Implantation occurs in the dorsal wall of the uterus.

Process of Implantation
1. A fertilized ovum starts dividing rapidly to form morula (16-cell stage) and from it blastocyst (100-cell stage) occurs. The blastocyst has an inner cell mass called a trophoblast surrounded by zona pellucida. This trophoblast cell layer has great sticking properties to the epithelial cells of the fallopian tube.

2. Within the next 3-4 days blastocyst is transported into the cavity of the uterus. In that cavity blastocyst first floats for some time and after that zona pellucida disappears and the trophoblast cell layer is exposed.

3. Due to its high sticking property trophoblast cell layer come in contact with the hormonally prepared endometrium of the uterus. Trophoblast has an inner layer called cytotrophoblast and an outer layer syncytiotrophoblast from which later the placenta will develop.

4. Now, syncytiotrophoblast secretes proteolytic enzymes that digest and liquefy the endometrium cells, so that blastocyst erodes and burrows into the endometrium resulting in implantation.

5. The blastocyst goes deeper into the uterus until the whole of it lies within the endometrium.

Embryo and Foetus

From conception till the end of the 8th week (2nd month) the organism found in the uterus is known as an embryo. From the 9th week of pregnancy till birth, the embryo transformed into a fully mature baby is called foetus.

Embryo Formation
Through various stages, the morula transforms into an embryo (Morula → Blastula → Gastrula → Embryo).

Embryonic Disc Formation
As we know that human blastocyst consists of a trophoblast and inner cell mass. The stem cells which have the capacity to give rise to organs and all tissues are present in the inner cell mass. After 8 days of fertilization, the cells of the inner cell mass differentiate into two layers- a hypoblast and an epiblast. The hypoblast or primitive endoderm is a columnar cell layer and the epiblast or primitive ectoderm is a cuboidal cell layer. The hypoblast and epiblast cells combinedly form a two-layered embryonic disc. The three germ layers which constitute the embryonic disc are Endoderm, Mesoderm, and Ectoderm.

Endoderm and Ectoderm Formation
Some inner cells of the blastocyst lie on its free surface, which constitutes the first germ layer or endoderm. After a rapid increase in cell number a complete second layer is formed inside the original outer layer of the blastocyst. At that time embryo just looks like a tube enclosing another tube of a smaller diameter. Endoderm when internally bounded by the inner tube with a lumen, it is known as the primitive gut. At the late stage of development, the primitive gut differentiates into two portions:

• One of them is present in the body of the embryo which constitutes the gut tract and
• another one is known as the yolk sac, which communicates with the gut of the embryo.

After endoderm formation, the remaining cells of the inner cell mass form a cuboidal cell layer, which is called ectoderm.

Amniotic Cavity Formation
Between the epiblast and trophoblast, a fluid-filled space appears which is called an amniotic cavity and the fluid is called amniotic fluid. Amniogenic cells derived from the trophoblast constitute the roof of the amniotic cavity and ectoderm formed the floor of the cavity.

Mesoderm Formation
After endoderm establishment as a distinct cell layer, mesoderm formation occurs. This mesoderm is called extra embryonic mesoderm as it lies outside the embryonic disc. As the extraembryonic mesoderm lies in between the trophoblast and flattened endodermal cell lining the yolk sac, it separates the two. It does not give rise to any tissues of the embryo itself. This extra-embryonic mesoderm is differentiated into two parts the parietal or somatopleuric extra-embryonic mesoderm and the visceral or inner splanchnopleuric extra-embryonic mesoderm. Both these layers enclose the extraembryonic coelom.

Secondary Yolk Sac Formation
After the appearance of extraembryonic mesoderm and extra embryonic coelom the yolk sac is now called the Secondary yolk sac as it becomes smaller than before, due to changes in the nature of cell lining. Outer splanchnopleuric extra embryonic mesoderm and inner endodermal cells both make the secondary yolk sac. It is a source of blood cells as well as functions as a shock absorber and helps to prevent desiccation of the embryo.

After that near the margin of the disc, a circular area is differentiated and termed a prochordal plate which determines the central axis of the embryo. After prochordal formation, a few ectodermal cells of the disc begin to proliferate and formed a primitive streak which is an elevation of that bulges into the amniotic cavity. The proliferative cells in the region of the primitive streak pass sideways and form the intra-embryonic membrane, which mainly constitutes the third germ layer.

Chorion and Amnion Formation
During this time two (chorion and amnion) embryonic membrane is formed. Chorion is formed from the parietal or somatopleuric extra embryonic mesoderm inside and the trophoblast outside. Whereas Amnion is formed from the amniogenic cells inside and somatopleuric extra embryonic mesoderm outside. The chorion becomes the main embryonic part of the placenta. Amnion mainly surrounds the embryo.

Germinal Layers and Their Fate

During the gastrulation phase, the three germ layers are formed. Like Ectoderm, Mesoderm and Endoderm. Each germ layer gives rise to specific tissues and organs of the foetus. The organs derived from the three layers are
Ectodermal Derivatives: The organs derived from these layers are-

• Skin, hair, nail, sebaceous gland, arrector pili muscle of skin, chromatophores pigment of the skin.
• Teeth enamel, salivary gland, mucous membrane part of the palate, nasal cavity, paranasal sinuses.
• The nervous system mainly neurons, neuroglia, pia matter, and arachnoid matter.
• External and internal ear and tympanic membrane.
• The pituitary gland, pineal gland, and medulla of the adrenal gland.
• Mammary gland, Labia majora, Labia minora, male urethra.

Mesodermal Derivatives:

• All types of muscles except the iris muscle.
• Adipose tissue, cartilage, and bone.
• Kidney, ureters, trigone of the urinary bladder.
• Connective tissue, ligament, tendon.
• Coelomic epithelium.
• Heart and blood vessels, lymphatics, blood cells, and spleen.
• The cortex of the adrenal gland, reproductive system (except prostate), notochord.
• Sclera, choroid, ciliary body, iris.
• Dentine of teeth.
• Dura matter, microglia.

Endodermal Derivatives:

• Epithelium of mouth, palate, tongue, tonsil, oesophagus, stomach, large intestine, etc.
• Langerhans, epithelium of urinary bladder (except trigon).
• Epithelium of thyroid, parathyroid, and thymus gland.
• Epithelium of larynx, trachea, bronchi, and lungs.
• Epithelium of eustachian tube, middle ear, inner ear, tympanic membrane.
• Lower part of vagina, vestibuli, labia minora (inner surface).
• Epithelium of gall bladder, liver, gastric gland, etc.

Foetal Membrane and Extra Embryonic Membrane

The growing embryo develops four membranes. Those membranes are as follows:
1. Chorion:
It covers the embryo and protects it. Chorion is made up of trophoblast outside and somatopleuric extra embryonic mesoderm inside. This layer takes part in placenta formation.

2. Amnion:
It is the next part after the chorion. Amnion is made up of somatopleuric extra embryonic mesoderm outside and trophoblast inside. Amniotic cavity present here, filled with amniotic fluid. Amniotic fluid protects the embryo from shocks.

3. Allantois:
It is made up of endoderm inside and splanchnopleuric extra embryonic mesoderm outside. It is a Sac-like structure that originates from the embryonic gut near the yolk sac.

4. Yolk Sac:
The primary Yolk sac mainly consists of the endoderm inside and the splanchnopleuric mesoderm outside. The definite function of the human yolk sac is not identified.

Different Stages of Development and their Characteristics:

Stages of Development	Characteristics
1. Zygote	Unicellular fertilized ovum.
2. Morula	Multicellular hollow ball-like phase.
3. Blastula	One layered cell with blastocoel cavity.
4. Gastrula	Three-layered phase.
5. 1st phase of the embryo (3 weeks)	Small pea-like alive embryo.
6. 5th week embryo	The embryo develops further, and the heart and blood vessels appear.
7. 8th week embryo	Now it looks like a human embryo, arms, and legs come out.
8. Mature embryo (child)	After 40 weeks fully developed foetus appears and birth takes place.

Sex Determination of the Baby

The sex of the baby is determined by genes. Genes are the microscopic DNA particles of the chromosomes, which are carried from one generation to another. These carry hereditary characteristics along them. In human females the sex chromosome pattern is XX and in males it is XY. It indicates that all the haploid female gametes have X type sex chromosome, whereas the haploid male gamete is either X or Y. That means about 50% of sperms carry the X chromosome while the other 50% carry the Y chromosome.

After fertilization, the zygote carries either XX or XY chromosome, it depends upon paternal sperms carrying either X or Y which fertilize the ovum. The baby will be female if the zygote carries the XX chromosome or the baby will be male if the zygote carries the XY chromosome. For this reason, it is said that the sex of the baby is determined by the father. The Human Y chromosome is smaller than the X chromosome. The sperms containing the Y chromosomes are lighter and able to swim faster in the female genital tract thus able to reach rapidly into the ovum.