Fertilization in Humans , Development of Embryo in Humans

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Fertilization in Humans :-

The Fertilization process was first study by Lillie in Echinoderms .

Fertilization is the process in which male gamete and female gamete fuse together .

In human being Fertilization takes place Fallopian tube between the two dissimilar gametes of the same Species .

Fertilization may be external ( out side the body ) or internal ( inside the body ) but its always occurs in the fluid medium .

During Fertilization , after the fusion of male and female gametes , the fertilized egg is called Zygote .

Stages during Fertilization :-

During Fertilization , approach occur Sperm towards the Ovum due to movement of tail in fluid medium .

  • Attachment of the Sperm ( head region ) with the egg membrane at the animal pole .
  • In some fishes of fine aperture called micropyle is present in the egg membrane to which the sperm enters into the Ooplasm .
  • The excitation in the Egg membrane result into the formation of Fertilization cone at the point of contact .
  • Sperm lysin ( proteolitic enzyme ) release from the acrosome dissolves the cementing material of the egg membrane at the fertilizers cone .
  • The nuclear membrane of male pronucleus disappear , releasing the male chromosome in the cortex ( outer region of Ooplasm ) .
  • The Egg membrane is now thicker and the polarised , it is now called fertilization membrane . The Ooplasm get slightly contact due to which the fertilization membrane more visible . It prevent entry of other Sperm . If more than one is sperm enters into the Egg all the Sperm nucleus except one get degenerated .
  • The male gamete now move into the horizontal path within the cortex . This path is called fertilization path .
  • The second meiotic division completed release the second polar body ( haploid chromosome ) . Now the female chromosome form pair with male chromosome , resulting diploid chromosome number . Nuclear membrane now get developed and the nucleus in now known as Zygote nucleus .

Spermatogenesis :-

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Fig . 1 . Stages of Spermatogenesis .

Spermatogenesis is the process by which small haploid ( n ) sperm develop from germ cells in the seminiferous tubules of the testis . This process starts with the mitotic division of the stem cells located close to the basement membrane of the tubules .

  • The mitotic division of these produces two types of cells . Type A cells replenish the stem cells and type B cells differentiate into spermatocytes .
  • The primary spermatocyte devides meiotically into two secondary spermatocyte , Each secondary spermatocyte divides into two equal haploid spermatids by meiosis second .
  • The spermatids transform into sperm by the process of Spermiogenesis .
  • Thus the primary Spermatocyte gives rise to two cells , the subdivision produce four Spermatozoa ( Sperms ) .

Structure of Human Sperm :-

sperm cell
Fig . 2 . Structure of Sperm .

It is a microscopic , motile structure composed of a head , a middle piece and a tail . The whole cell body is covered by plasma membrane .

  • The Sperm head contains and elongated haploid nucleus and the anterior portion is covered by a cap like structure acrosome . Acrosome release sperm lysin ( proteolitic enzyme ) who dissolve the covering of the egg during fertilization .
  • The middle piece contains numerous mitochondria which produce energy for sperm motility needed for fertilization .
  • The tail helps the Sperm cell to swim to reach the egg cell . Tail is very long , At the tip of the tail the cytoplasm is absent .The motile gamete sperm perform swimming movement with the help of tail in the fluid medium .
  • The anterior part of the middle piece is narrow known as Neck , two Centriols are present in the Neck .
  • The axoneme is made up of microtubules ( 9 + 2 ) . Filaments mitochondria are wrapped around the axoneme .

Oogenesis :-

Difference Between Spermatogenesis and Oogenesis Figure 1
Fig . 3 . Stages of Oogenesis .

The process of formation of mature female gamete is called Oogenesis . It is initiated during the Embryonic development stage , when a Couple of million gamete mother cells ( Oogonia ) are formed within each Ovary , no more Oogonium are formed and added after birth .

  • Oogonia cell start mitotic division and enter into meiotic division first to get primary Oocyte .

The primary Oocyte grow in size and complete its meiotic first . It is an unequal division resulting in the formation of a large haploid secondary Oocyte and a tiny first polar body . The secondary Oocyte enters into meiotic second and get transform into a large Ovum / Egg and another small second polar body .

  • The secondary Oocyte froms a new membrane called Zona pellucida surrounding it . The Graafian follicle now ruptures to release the secondary Oocyte or Ovum from the Ovary by the process called Ovulation .

Development of Embryo in Humans :-

zygote development
Fig . 4 . Stages of Developing Embryo .

Cleavage ( Cleave = furrow ) :-

Cleavage is the mitotic division which occurs in the Zygote after the Fertilization . Cleave appears in the cytoplasm , As Cleavage the division of segments form in the Zygote are called blastomeres .

  • The blastomeres remain adhered in which each other .
  • During cleavage there is no increase in size of Embryo .The number of blastomeres increase but the size of blastomeres remains constant .
  • The increase in size of blastomeres occurs only after the completion of Cleavage .
  • Cleavage is important for organogenesis because it increase the number of cells . the differentiation in the cells may occurs before or after the cleavage .

Morula :-

The Cleavage increase the number of blastomeres forming solid group of blastomeres the embryo of this stage is called Morula . There is no space or cavity within the Morula . Number of cells in Morula stage is about 8 to 16 .

Blastula :-

Futher division in the blastomeres , results into transformation of Morula into Blastula . Blastula is always a single walled structure , made up of one or more layers of blastomeres . The Blastula usually possess cavity is called blastocoel , filled with the fluid which has been secreted by the blastomere .

  • The blastocoel is the first cavity space which has appears in the Embryo . Therefore also known as first Embryonic cavity .
  • Blastocyst is the characteristic of Mammal , after Morula stage most of the cell get arranged at the Periphery to form single layer trophoblast . The rest of the cells form of group get attached to one pole within the trophoblast , this pole is called Embryonic Pole . The space between inner cell mass and trophoblast is called blastocoel .
  • Blastocyst is not a real blastula but as it contains a single walled surrounding space it is compared to the blastula . Example – Eutheria . ( Viviparous mammal including Human ).
  • The Morula continues to divide and transform into the blastocyst , it moves further into the uterus . The blastomeres in the blastocyst are arranged into an outer layer called trophoblast and inner group of cells attached to trophoblast called the inner cell mass .
  • The trophoblast layer gets attached to the Endometrium and the inner cell mass gets differentiated as the Embryo .
  • After attachment of trophoblast , the uterine cell divide rapidly and covers the blastocyst as a result the blastocyst becomes embedded in the the Endometrium of the uterus . It is called Implantation and it leads to pregnancy .
  • After implantation finger like projections appears on the trophoblast called Chorionic Villi which are surrounded by the uterine tissue and maternal .

Also read :- Developement of embryo in flowering plants

Placenta :-

The Chorionic Villi and uterine tissue become inner digiteted with each other and jointly form a structural and functional unit between developing Embryo or foetus and maternal body called Placenta .

  • The Placenta faciliate the supply of O₂ and nutrients to the Embryo and also removal of CO₂ / waste materials produced by the Embryo .
  • The Placenta is connected to the Embryo through an umbilical cord which helps in the transport of substances to and form of Embryo .
  • Placenta also acts as an Endocrine gland and produces several hormones like human chorionic gonadotropin ( hCG ) , human placental lactogen ( hpl ) , oestrogens , progesterones and relaxin hormones . hCG hpl and relaxin hormones are produced in women only during pregnancy . In addition , during pregnancy the levels of other hormones like oestrogen , progesterones cortisol , prolactin ,Thyroxine etc . are increased in the Maternal blood . Increased production of the hormones is essential for supporting the Foetal growth , metabolic changes in the Mother and maintenance of pregnancy .

Three germinal layers in Human Embryo :-

Immediately after Implantation , the inner cell mass differentiates into an outer layer called Ectoderm and an inner layer called Endosperm . A Mesoderm soon appears between the Ectoderm and Endoderm .

  • These three layers gives rise to all tissues ( Organs ) in adult . It needs to be mentioned here that the inner cell mass contains certain cells called Stem cells which have the potency to give rise to all the tissues and Organs .
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Fig . 5 . Three germinal layers and it’s derivatives .

Change in Embryo during Pregnancy :-

  • The human Pregnancy last a month . In human beings , after one month of Pregnancy the Embryo heart is formed .The first sign of growing Foetus many be noticed by listening to the heart sound carefully through the Stethoscope .
  • By the end of second month of Pregnancy , the Foetus develops limbs and digits .
  • By the end of 12 weeks ( first trimester ) , most of the major Organ Systems are formed .
  • The first movement of the Foetus and appearance of hairs on the head are usually observed during the fifth month .
  • By the end of about 24 weeks ( end of second trimester ) , the body is covered with fine hairs , eye lids separate and eyelashes are formed .
  • By the end of nine months of Pregnancy , the Foetus is fully developed and is ready for go delivery .

Parturition :-

The average duration of human Pregnancy is about 9 months which is called the Gestation period .Vigorous contraction of the Uterus at the end of Pregnancy causes delivery of the Foetus . This process of delivery of the Foetus ( Child birth ) is called Parturition .

Lactation :-

The mammary glands of the Female undergo differentiation during Pregnancy and starts producing milk towards the end of Pregnancy by the process called Lactation .This helps the Mother in feeding the new born baby .

  • The milk produced during the initial few days of Lactation is called Colostrum , which contains several antibodies absolutely essential to develop resistance for the new born babies .
  • Breast feeding during the initial period of infant growth is recommended by Doctors for bringing up a healthy baby .

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