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Unit 7 – Developmental Biology Class 12 Biology Notes

7.1 Gametogenesis: Spermatogenesis & Oogenesis. (2 teaching hours)

7.2 Development of frog: Fertilization & its effects, cleavage, morulation, blastulation, gastrulation, organogenesis – formation of notochord, nerve cord & coelom. (4 teaching hours)

Developmental biology

Gametogenesis:
The process of formation of gamet occurs in male + female gonads that ie testes + ovaries is called gametogenesis.

The cell of germinal epithelium undergoes 3 essential stages: i) multiplication phase ii) growth phase iii) maturation phase to give rise haploid gamet.

1. Spermatogenesis: Gametogenesis for the formation of sperm is known as spermatogenesis.
2. Oogenesis: Gametogenesis for the formation of female gamet (egg / ovum) is known as Oogenesis.

Spermatogenesis

Spermatogenesis occurs in the seminiferous tubules of testes. Each tubules contains 2 types of cell:

1. Germinal cell
2. Sertoli cell

The germinal epithelial cell are called primary germ cell. They are diploid (2n). Cell undergoes number of stages to give haploid spermatids whereas Sertoli cell provide nourishment to the developing sperm.

Importance Stages of Spermatogenesis

There are 4 importance stages of spermatogenesis:

1. Multiplication phase
2. Growth phase
3. Maturation phase
4. Spermiogenesis phase

1. Multiplication phase: The germinal epithelium cells or primary germ cell of seminiferous tubule divide by mitotic division to produce large no. of diploid cell (2n) called spermatogonia.
2. Growth phase: In this phase spermatogonia increase in size by fourth time by absorbing nutrients from the Sertoli cell. The size is increase and become large round diploid structure called primary spermatocytes.
3. Maturation Phase: Each primary spermatocytes which is diploid undergoes first meiotic division (meiosis) to give rise two haploid secondary spermatocytes. This in turn quick second meiotic division (equational) division to produce 4 haploid spermatides.

The spermatides are unspecialized cell and remain connected by cytoplasm to the sertoli cells. They get separated only during spermiogenesis.

Spermiogenesis

It is a transformation of unspecialized spermatides into the spermatozoo (sperm). The spermatides get transfer into sperm by loosing the great amount of cytoplasm, condensation of nucleous into head region and by forming flugellated tail.

The mature spermatozoon sperm finally get detached and is released in the lumen of the seminiferous tubules by the process called spermiation.

[Diagram: Sequence of event in Spermatogenesis showing 2n Spermatogonia -> multiplication phase -> 2n Primary Spermatocytes -> first meiotic division -> n Secondary spermatocytes -> n Spermatids -> Spermiogenesis -> Sperm]

Structure of Sperm

The human spermatozoon is an elongated, microscopic, flagellated + motile cell. It consist of following part:

1. Head: It is slightly oval + contains large nucleus (DNA). The nucleus contain haploid number of chromosome. Over the anterior half of nucleus there is a cap like structure called acrosome (modified Golgi body). Acrosome contain enzyme (lysin) necessary for fertilization.
2. Neck: It has a pair of centrioles: proximal + distal centriole which is present at right angle to each other.
3. Middle piece: It contains mitochondria (8-10) which is spirally arranged. It also contains microtubules of the centrioles.
4. Tail piece or flagellum: It is the posterior + long thread like part of the sperm. It is made up of microtubules/axial filament which is arranged in two bundles ie 9+2 arrangement. It is a modified flagellum that provides motility to the sperm.

[Diagram of Sperm: Acrosome (Hyaluronidase), Head, Centriole, Neck, Mitochondria, Tail]

Oogenesis (Gr. oon - egg; genesis - origin)

Oogenesis refers to the formation of mature female gamet. Stages during oogenesis are listed below:

1. Multiplication phase: The cells of the germinal epithelium of the ovary undergoes a no. of mitotic division to increase its no. These cell are called oogonia and contains diploid no. of chromosome.
2. Growth phase: In this phase oogonia increase in size and undergo growth phase to form the primary oocytes (2n).
3. Maturation phase: The primary oocytes undergoes the first meiotic division to produce two haploid cells. Since there is unequal division of cytoplasm one of the cell (the secondary oocyte) is large and receives more cytoplasm whereas other one is very small called 1st polar body.

   In the 2nd meiotic division the secondary oocytes divides to form ovum (large one) and a smaller polar body. The first polar body also divides to form two small polar bodies. As a result one ovum and 3 polar bodies (nutritional bodies) is produced by one oogonium.

[Diagram: Sequence of event in oogenesis showing 2n oogonia -> multiplication phase -> 2n primary oocyte -> Growth phase -> n secondary oocyte + 1st polar body -> Maturation phase -> Ovum + 3 polar bodies]

Structure of Egg of frog

[Diagram: Fertilized egg of frog showing Animal pole, Nucleus, Vitelline membrane, Plasma membrane, Grey crescent, Previtelline fluid, Yolk half (phospholipid + lecithin), Vegetal pole, Pigmented half (melanin)]

Egg of frog is non-motile: Egg of frog is non-motile, spherical + large sized. It is externally covered by double membrane outer is vitelline and inner layer is plasma membrane. The vitelline membrane secrets glue like substance called albumin that swells up and binds eggs by forming sperm-mass of egg. Albumin ie blue in color. Frog egg as it contains melanin pigments that prevents from UV radiation from sun.

Egg of frog is about 1.8 mm in diameter and is mesolecithal and telolecithal. It is divisible into 2 parts: the upper pigmented position is called animal pole with polar body outside and the lower the yolk contain non-pigmented area or vegetal pole or vegetal hemisphere. The vegetal pole faces downward and remains submerged in water.

Classification based on amount of Yolk:

1. Microlecithal Egg: The amount of albumen is more than the yolk eg: Human.
2. Mesolecithal Egg: The amount of albumen + yolk is equal then it is called mesolecithal egg. Eg: Amphibians.
3. Macrolecithal Egg: The amount of albumen is less than the amount of yolk. Eg: Birds, Reptiles.

On the basis of Position of Yolk:

The egg is of 2 types:

1. Telolecithal egg: The yolk is present on the vegetal pole or on any one hemisphere. Ex: Amphibian egg.
2. Centrolecithal egg: The yolk is present in centro and albumen surrounds the yolk. Eg: Human.

Development of Frog (Embryology)

It consist of 2 stages:

1. Early embryonic stage: It include various step such as:
   • a) Copulation
   • b) Fertilization
   • c) Cleavage
   • d) Morula
   • e) Blastulation
   • f) Gastrulation
2. Post embryonic stage (Organogenesis)

a) Copulation:

The raining season is the breeding season of frog (July to September). Male frog produces the croaking sound + attracts on her arrival. Male goes near by mount on her back + holds with the help of amplexus pad / nuptial pad. During this process female lay egg in large number on the surface of water (egg sperm). Soon after this male shed sperm on this eggs + this is called insemination. This copulation called pseudocopulation.

b) Fertilization:

The process of fusion of male + female gamet + formation of zygote is called fertilization. In frog fertilization is external + takes place in water.

[Diagram of fertilization: sperm entry, sperm path, 1st polar body, grey crescent, previtelline line, VP zygote]

After fertilization, the zygote undergoes a series of change by mitosis cell division + forms embryo. The development of zygote take place as:

c) Cleavage:

It starts after 30 min to 2 hr of fertilization. It is the rapid mitotic division of zygote in which number of cell increases but the size of embryo remains constant.

[Diagrams of Cleavage: 2-cell (Holoblastic unequal cleavage), 4-cell (4 Blastomere equal in shape + size), 8-cell (4 micromere, 4 megamere), 16-cell stage, 32-cells stage]

The process of fertilization is completed in the following stages:

• Sperm swim in water + comes in contact with the swelling part of the ovum is called cone of reception.
• The cone of reception stimulate acrosome of sperm. Thus it releases proteolytic enzyme sperm (lysin) which dissolve the wall of ovum + middle piece enter inside the ovum by leaving the tail outside.
• During penetration the ovum undergoes 2nd maturation division + form 2nd polar body.
• As the head and middle piece enters into the ovum at that time cytoplasmic fluid oozes out in between the vitelline membrane to form fertilization membrane and fluid is called previtelline fluid.
• This fluid prevent further entry of other sperm into ovum.
• Inside the ovum male nucleous comes in contact with the female nucleus to form zygote (2n).
• Grey cresent is form just behind the cone of reception it determines the future archeteron.

Cleavage in frog

A fertilize zygote undergoes series of mitotic division known as cleavage. In frog cleavage is hollow blastula. It occurs as follow:

• The first division of cleavage is meridian which passes from VP to AP resulting in the formation of 2 equal cell called holoblastic.
• The second cleavage is also vertical but on right angle to first one + forms 4 equals cells called blastomere.
• The third cleavage is horizontal + unequal just above the middle line or equatorial line so that 8 unequal cell are formed, the upper 4 cell are smaller and called micromere where as lower 4 cell are larger called megamere.
• The 4th cleavage vertically + forms 16 unequal cells out of them upper 8 are micromere and lower 8 are megamere.
• The 5th cleavage is horizontal to forms 32 cells in which upper 16 are micromere + lower 16 are megamere.

Morula stage:

After 5th cleavage segmentation becomes irregular + micromere divides more rapidly than the megamere. As a result, solid ball of micromere + megamere is formed called morula. (Superficially resemble to a mulberry fruit).

Blastula

In this process regular division of zygote does not occurs that means micromeres in animal pole divides faster as they are smaller in size + they has albumin but megamers division slowly due to the presence of yolk.

[Diagram: V.S. of Blastula showing Micromere, Blastocoel, Megamere, AP (Animal Pole), UP (Vegetal Pole)]

So the number of micromere increases than the megamere. They slowly migrate towards vegetal pole due in which a small cavity develop at animal pole. The cavity is called blastocoel + is filled with fluid. It is dome-shaped the roof of the cavity covered by micromere + floor is occupied by megameres. So this type of blastula stage in frog is called coeloblastula ie internal cavity (fluid filled). These, this fluid filled cavity along with blastomere is called blastula + formation of blastula is called blastulation.

Presumptive area / layer in BLASTULA (Fate map)

Walther Vogt (1932) use vital staining method for the observation of blastula. After micro surgery and staining it has been revealed that:

1. The entire animal pole represent future ectoderms which can be further divisible in future epidermis + future neutral plate.
2. The grey crescent area represent presumptive mesoderm + notochord.
3. The vegetal cord represents the endoderms.

Note: Blastula is of following types:

• a) Stereoblastula: It means solid blastula ie there is no blastocoel. Example: Sandworm (Nereis).
• b) Coeloblastula: It has hollow cavity which is filled with fluid called Blastocoel. Example: Frog.
• c) Discoblastula: It's mainly seen in megalecithal egg where the shape of blastula appears as disc. Example: Birds, fish.
• d) Superficial blastula: It occurs in centrolecithal egg.

Gastrula and Gastrulation

Gastrulation is the process of formation of hollow gastrula from blastula. It involved dynamic movement and rearrangement of blastomer such moments of blastomers along specific port during gastrulation is called morphogenetic movement.

It involves following steps:

1. Epiboly: The epiboly is the overgrowth of micromeres of animal pole. The micromeres divides and redivide in large number which spread all over the macromeres of the v.p enveloping it from all the side except the yolk.
2. Imboly (Invagination of endoderm): This process is the inpushing or insinking of the outercells of the blastula forming a canal inside it. It begins with the formation of small groove that further extends and forms cavity called Archenteron (future gut). The opening of archenteron is blastopore. It is the future anus. As the archenteron enters inside the blastocoen reduces and finally disappears so the blastopore is guarded by lips ie Dorsal and ventral lips.
3. Involution of chordomesoderm: (It means the form): The micromere cells migrates from both dorsal and ventral lips to the inward (toward megamere) due to migration blastopores become narrow and megamere comes out through ventral side of blastopore forming yolk plug. The upper side of blastomer are differentiated into neural plate and ectoderm. Thus the mesoderm layer starts to develop.
4. Rotation of gastrula: As we know the early embryo remains covers with vitelline membrane, the gastrula due to shifting heavy megamers rotates at right angle with in the vitelline membrane in such a way that AP become anterior + the blastopore occupies the posterio-dorsal position.

[Diagrams of Gastrulation: Epiboly, Imboly (Invagination), Gastrular pit, Neural plate, Notochord, Mesoderm, Archenteron, Yolk plug, Ventral lip, Dorsal lip, Endoderm, Blastocoel]

Formation of 3 Germs layer

After the formation of diploblastic gastrula it divides rapidly. The mesodermal part of the chordomesoderm divides further and forms 3 germ layer.

• The outer layer is called ectoderm which is made up of micromeres.
• The middle layer mesoderm which is made up of both micro + megameres.
• The inner layer is endoderms which is only made up megamer.

Fate of Germ layer

All the body parts ie body tissue, organs + organ system are developed from the 3 primary germ layer - ectoderm, endoderm + mesoderm which as follows:

1. Ectoderm: (Epic nerve pin in eye at 6:00 am)
   • Epidermis of skin
   • Nervous system (CNS)
   • Pituitary gland
   • Pineal gland
   • Internal ear
   • Adrenal medulla
   • Teeth - enamel
   • eye @ LCCR (Lens, Cornea, Conjuctiva, Retina)
2. Mesoderm: DD CT SEAN Records Blood Vessel, Bones, muscles
   • Dentine
   • Dermis of skin
   • Connective tissue
   • Sclera, spleen
   • Choroid
   • Notochord
   • Co- Coelom
   • Bone - skeletal system
   • Muscle - muscular system
   • R- Reproductive System
   • E- Excretory System
3. Endoderm: (U BAT throat has glands has respiratory + digestive system)
   • UB - Urinary bladder
   • At - Auditory tube
   • glands = Thyroid, Parathyroid, Thymus, liver, pancrease.

Post-Embryonic Development (Organogenesis)

(Gr. organon - organ, genesis - origin). The organogeny is the formation of organ during development. The process of organogeny begins soon after the formation of gastrula which takes place as follows:

1. Neural tube formation / Neurulation / Neurogenesis: (Gr. neuron - nerve).
   • Neural plate helps in the formation of neural tube.
   • The neural plate becomes thickened of the middle + forms medullary plate.
   • The lateral side is raised + is called neural fold. But some of cell neural plate does not involve in fold are called neural crest.
   • The two neural fold comes close to each other + large depressed cavity fuse ist formed + fuse together.
   • So just below the ectoderm neural canal or neural tube is formed.
   • In embryo, the neural table extends throughout the length of embryo to it dorsal surface. It has two pore.
   • The pore which is present near the head is called neuropore + near the tail region is called neurenteric pore.
   • Later the pore is closed + forms a complete tube. The anterior parts form the brain so this central canal extends from the brain of the spinal cord + remain field with CSF in adult.
   • And the neural crest forms the spinal nerves, ganglion part of autonomic nervous system + peripheral nerve system (cranial + spinal nerves).
   [Sketch of Brain]
2. Notochord formation / Notogenesis: (Gr. notos = back, chorde = string, genesis = origin)

   In this process the chordomesoderm which is present just below the neural plate is responsible for the formation of notochord.

   • This process begins side by side along with neurulation.
   • The mesodermal cell along the mid-dorsal line of the archenteron becomes differentiated + separated to form a solid mass of cells know as notochord.
   • During this process the notochord forming cells become vacuolated.
   • The vacuolated cells present on outer layer of notochord it forms meninges that covers brain + spinal cord.
   • So it extends rod like structure just below the neural tube is called notochord + formation of notochord is called notogenesis.
   • The notochord is later transformed into vertebral column in adult stage.
   [Diagram showing Notochord development stages from mesoderm]

Coelom formation

Coelom is body cavity found in animal from the phylum coelomata to chordata. The true coelom is form by the splitting of mesodermal layer which is filled coelomic or splanchnic fluid. It is form during post gastrulation development embryo, among the 3 germ layer, mesodermal layer first divides into 3 parts: epimere, mesomere and hypomere.

• The epimere again divide longitudinal into 3 layer: Dermatome, myotome + sclerotome.
• The dermatome represents dermis of skin just below it is myotome which forms muscles + sclerotome lateral convert conjunctive tissue resp.
• Middle portion is mesomere which splits partially forming central canal called nephrostome, which gives rise to excretory organs, + reproductive organ etc.
• The hypomere layer consist of endodermal layer which consist of liver + alimentary canal.
• In hypomere a small slit like structure is formed due which it is divided into 2 layers.
• Outer is called somatic layer or parietal + inner layer is called visceral or splanchnic layer. In both them there is a gap called splanocoel + it is filled with the fluid is called splanchnic fluid.
• It indicate the begning of s-coelom formation later the size of splanchnic coel increases and both of them fuse together and u-shaped structure is formed.
• As the size increases visceral layer hypodermis fuses with the gutwall + form splanchnopleural coat.
• Similarly outer piteral layer fuses with ectoderm + forms body wall.
• So the space betn gutwall + body wall is called coelom or body cavity. As it is mesodermal in horizon it is called schizocoel (true coelom).

[Diagram of Cross-section showing: Ectoderm (body wall), Neural crest, Nerve chord, Dermatome, Myotome, Sclerotome layer, Notochord, Nephrostome, Visceral layer, Parietal (somatic) layer, Splanchnocoel, Gut, Liver]

Further development

After the formation of nerve chord, Notochord, coelom, the embryonic cell divides repetly to form tadpole larval stage. It develop gills, fins etc which look like fish + spend its life in water.

Later it passes through a process called metamorphosis during which the forms of body is change + becomes frog ie is the amphibious character + develops lungs, nostrils, and web feed.