Fertilization and Fetal Development

FERTILIZATION AND FETAL DEVELOPMENT

Definition: It is the process during which a haploid male gamete (sperm) unites with a haploid female gamete (oocyte) to form a single cell (ZYGOTE). Is called fertilization.

The development of fetus divided in to 3 periods.

1.     Pre-embryonic period (0 to 2 weeks)

2.     Embryonic period (3-8 weeks )

3.     Fetal period (9th week to birth of the baby)

1.  Pre- embryonic period (0-2 Weeks)

•            During coitus, sperm is released by male partner into the vagina of the female partner is called as insemination.

·           The motile sperms swim and pass the cervix to enter into the uterus and finally to reach the ovum released by the ovary in the ampullary isthmic junction.

·           Fertilization takes place in the ampulla-isthmic junction. Chemical signals from oocyte attract the sperms.

·           The sperm after reaches the ovary in the ampullary isthmic junction comes in contact with the zona-pellucida layer of the ovum and block the entry of the additional sperms thus only one sperm fertilizes the ovum.

·           The secretions of acrosome help the sperm to enter into the ovum through zonapellucida and the plasma membrane and thus secondary oocyte completes meiosis II and results in the formation of a second polar body and haploid ovum.

·           The haploid nucleus of the sperm and ovum fuse together to form a zygote which develops into new individual.

Proces of Fusion of Gametes

Definition: The process of union of sperm and ovum is called as fertilisation.

Process of Fertilization to Implantation

•            Zygote is genetically unique, a diploid cell (46 chromosomes) resulting from the fusion of two haploid gametes; a fertilized ovum.

•            Male has two sex chromosomes X and Y hence male produces 50% of sperms carrying X and 50% carrying Y, while female has two X chromosomes.

•            After fusion of the male and female gametes the zygote would carry either XX or XY depending on whether the sperm carrying X or Y fertilized the ovum.

•            The zygote carrying XX would develop into a female baby and XY would form a male.

•            So, it is the father whose gamete decides the sex.

Sex determination

Stages of Clevation

Structure of Blastocyst

•            After fertilization cleavage of zygote takes place. It consists of repeated mitotic divisions of the zygot which results in rapid increase in the number of the cells. These smaller embryonic cells are called blastomeres. This normally occurs in the uterine tube.

•            The embryo with 8 to 16 blastomeres is called a morula. The Morula reaches the uterine cavity at this stage. Spherical Morula is formed about 3 days after fertilization.

•            The morula divides further as it moves further into the uterus and transforms into blastocyst.

•            The blastomeres in the blastocyst are arranged into an outer layer called trophoblast and inner mass of cells attached to trophoblast is called as inner cell mass.

•            By 7th day, Trophoblast differentiated into 2 layers: Cytotrophoblast, inner layer, mononucleated mitotically active cells. Syncytiotrophoblast (outer multinucleated mass, with indistinct cell boundary).

•            By 8th day the blastocyst is superficially embedded in the compact layer of the endometrium.

•            By the 10th day the blastocyst is completely buried in the uterine lining, known as “Implantation” or “embedding" “some women have small amount of bleeding" during the time of implantation which is known as “Implantation Bleeding”.

Implantation of fertilized ovum in uterus

•            The implantation of the fertilized ovum of embedding is known as “Nidation or Nesting".

•            Uteroplacental circulation is established by 11th or 12th day. implantation is completed by the 11th or 12th day. Implantation it can be detected by:

•            Ultrasonography.

•            HCG (human chorionic gonadotrophin which is secreted by the syncytiotrophoblast) at about the end of 2nd week

•            By the 13th day Proliferation of Cytotrophoblast cells produce extension inside the Syncytiotrophoblast to form primary chorionic villi.

•            The chorionic villi and uterine tissue together form a structural and functional organic structure between developing embryo and tissues of the mother called as placenta.

•            After  implantation,  the  inner  cell  mass is  differentiated  into  outer  layer  called ectoderm  and  an  inner  layer  called endoderm with a middlelayer is mesoderm  Three layers give rise to all organs in adults. As shown in the picture above the cells are res ponsible for those organs.

•            If the implantation occur outside it is called as Ectopic pregnancy. 95 to 97% of ectopic pregnancies occurs in the uterine tube. Most are in the ampulla & isthmus. ( see the figure for types of ectopic pregnancy)

•            Placenta previa: placenta attach to the lower uterine segment.

2 .Embryonic period (3-8 weeks )

3^rd week

Heart Tube fuses

Cardiac muscle contraction begins

Eye & ear cells are present

Neural tube starts closing

4^th week

Optic vesicle appears, two pharyngeal arches appear.

A primitive S-shaped tubal heart is beating and peristalsis.

The rhythmic flow propelling fluids throughout the body begins at this stage,

The neural tube determines the form of the embryo.

5^th Week

Valve & septa appear in the heart.

The digestive epithelium layer begins to differentiate into the future locations of the liver, lung, stomach and pancreas.

Liver cells form in the digestive system.

Forebrain, midbrain and hindbrain forms.

Lymphatic & thyroid start to develop.

Limb buds.

First thin layer of skin.

The baby yawns.

6^th Week

Further development of nervous system, heart.

Innervation, the distribution of nerves, begins in the lower limb buds.

7^th Week

A four chambered heart and a sense of smell.

Primitive germ cells arrive at the genital area and will respond to genetic instructions to develop into either female or male genitals.

8^th Week

Spontaneous Involuntary Movement.

Brain is connected to tiny muscles and nerves and enables the embryo to make spontaneous movements.

Testes or ovaries are distinguishable.

3. Fetal Period (9th week to birth of the baby) 9-12 Weeks

9-12 Weeks

•            Brain continues to develop, liver enlarges, blood cell formation begins.

•            Sex can be determined by genitals.

•            10th week.

•            The fetus passes urine.

•            The Fetal Heart Rate can be heared by Doppler.

13-16 Weeks (Month 4)

•            Sucking starts

•            Hard palate is fused

•            Kidney structure developed

•            Bones are distinct joint cavities are apparent

•            Meconium is present in gut

17-20 weeks (Month 5)

•            Eyeballs and eyebrows present.

•            Lanugo( Silk like hair)present.

•            Quickening occurs (first fetal movement).

•            Approximate crown rump length at the end of 20 weeks is 19 cm.

21-30 weeks

•            Eyes are opened.

•            Finger and toe nails are complete.

•            Skin is wrinkled and red. Fatty layer under the skin is formed.

•            lanugo prominent.

•            Testes descent in the scrotum for male babies.

•            Approximate crown rump length at the end of 30 weeks is 28 cm.

•            Vernix ( white creamy substance) present.

30-38 weeks

•            Constant weight gain.       

•            Lanugo disappears from face.    

•            Fat  accumulates  under  the       skin

•            (hypodermis).

•            Plantar creases visible.     

•            Ear cartilage soft.    

•            Approximate crown rump length at the end of 38 weeks is 36 cm.

•            At birth, weight - 2.5-3.5 kg.