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Animal Reproduction

All organisms produce offspring and thereby maintain their kind. The means by which organisms reproduce vary remarkably between species. Reproduction is an orchestrated process. It needs the co-ordinated preparation of many tissues in female starting from maturation of ovum, its release its transportation, fertilization and also the preparation of uterine wall to receive developing egg and its implantation.
The preparation for reproduction are cyclic in mammals. The change in structural and functional characteristics of both male and female reproductive tissue occur as a prelude to reproduce. These are mediated by hormones. It is also concerned with the asexual development of the embryo from structures other than egg and also with the regeneration of lost or damaged parts and repair of defects.


Types Of Reproduction


Asexual Reproduction

It involves only one parent and no formation of gametes.

The offspring produced is direct replica of the parent leaving no scope for variation.

This is also called as vegetative or somatic reproduction and common in plants and lower animals; occurs by budding, binary fission, multiple fission, sporulation and also the regeneration.

Protozoans, sponges and coelenterates reproduce mainly by this way.


Types Of Asexual Reproduction :


    1.Binary fission               –    Amoeba, Euglena, Vorticella
    2.Multiple fission             –    Amoeba, Plasmodium, Monocystis
    3.Plasmotomy                 –    In multinucleate protozoans e.g. opalina, Pelomyxa.
    4.Exogenous budding     –    Hydra, Syllis, Salpa
    5.Internal budding or      –    Spongilla, Few marine sponges
    by Gemmule formation
    6.Fragmentation             –    Asterias (Star fish)


Sexual Reproduction

Involves two individuals (of different sexes) as male and female to produce respective gametes.

Many lower animals have both sexes in the same individual called hermaphrodite (monoecious or bisexual), e.g., flatworms, annelids.

In higher animals sexes are separate (unisexual) with apparent sexual dimorphism i.e. male and female look different with different structures.


Types Of sexual Reproduction


Amphigony - Zygote is formed by the fusion of male and female gametes further of two types : syngamy and conjugation.

Syngamy - Complete and permanent fusion of gametes.

Conjugation

 1.  Involves temporary union of two parents of the same species which exchange their male pronuclei to form synkaryon and then separate to produce daughter individuals.

It corresponds to cross-fertilisation of higher animals.
e.g. Paramecium, other ciliates

2. Parthenogenesis (Virginal Reproduction)-Development of an egg (ovum) into an individual without fertilisation by a sperm.

It is of two types : Natural and Artificial.
1.Natural Parthenogenesis—occurs regularly in the life of certain animals. It may be complete, incomplete or paedogenetic.
2. Artificial Parthenogenesis
Egg is induced to develop into a complete individual by artificial stimuli (physical or chemical).
E.g. Eggs of annelids, molluscs, echinoderms, frogs, salamanders, birds (turkey, hen) and even mammals (rabbit).


Differences between Asexual and Sexual Reproduction

S.No.

     Asexual

Sexual

1.

It Is Uniparental

Uni / Biparental

2.

Gametes Are Not Formed

Formed

3.

Reproductive Organs Are Not Needed

Needed.

4.

No Fertilization

Fertilisation Occur

5.

Offsprings Are Similar To Parent

Different

6.

New Characters Do Not Appear

Appear

7.

No Variation

Variation Present

8.

No Evolution

Evolution Takes Place

9.

Definite Process

Indefinite Process

10.

Generation Leads To Extinction

Not So


Embryonic Development

It includes a definite series of phases which are fundamentally similar in all sexually reproducing organisms and transform a one-celled zygote to a multicellular and fully formed development stage till hatching or birth. It is divided into following types.
(i) Pre natal or embryonic period : It is the period of development from the diploid one-celled zygote to a multicellular embryo. It occurs either inside the egg or mother’s womb and  extends upto hatching or birth. The study of the changes during this period is called embryology.
(ii) Post natal or post embryonic period : It is the period of development which extend from hatching or birth to death. The branch of science which deals with the study of progressive, orderly and gradual changes in structure and functioning of organism during entire life history from zygote or blastos to death, is called development biology
Phases Embryonic Development
Embryonic development involves following dynamic changes and identifiable process.
(i) Gametogenesis : It involve the formation of haploid sex cells or gametes called sperms and ova from diploid primary germ cells called gametogonia present in the reproductive organs called gonads (testes and ovary). It is of two types
(a) Spermatogenesis : Formation of sperm.
(b) Oogenesis : Formation of ova
Note : (See detail in module-2 Chapter-Reproduction)
(ii) Fertilization : It involve the fusion of haploid male and female gametes to form diploid zygote. The fusion of gametic pronuclei is called Karyogamy while the mixing of two sets of chromosomes of two gametes is called amphimixis.
(iii) Cleavage : It includes the rapid mitotic division of the zygote to form a single layered hollow spherical larva called blastula and its formation is called blastulation.
(iv) Implantation : The process of attachment of the blastocyst (mammalian blastula) on the endometrium of the uterus is called implantation.
(v) Gastrulation : It includes the mass and orderly migration of the organ specific areas from the surface of blastula to their predetermined position which finally produces a 3 layered gastrula larva. It is with 3 primary layers.
(vi) Organogenesis : It includes the formation of specific organs system from three primary germ layers of gastrula and also includes the morphogenesis and differentiation.


Fate of germ layer

Ectoderm

Mesoderm

Endoderm

Epidermis and skin derivatives

Dermis

Gut

Cutaneous gland

Muscular tissue

Glands of stomach and intestine

Nervous system (Brain + spinal cord)

Connective tissue

Tongue

Motor and optic nerve

Endoskeleton

Lung, trachea and bronchi

Eye (Retina, lens and cornea)

Vascular system (heart and blood vessel)

Urinary bladder

Conjuctiva, ciliary and iridial muscle

Kidney

Primordial germ cells

Nasal epithelium

Gonads (Reproductive system)

Gills

Internal ear (membranous labyrinth)

Urinary and genital ducts

Liver

Lateral line sense organ

Coelom and coelomic epithelium

Pancreas

Stomodaeum (mouth)

Choroid and sclerotic coat of eye

Thyroid gland

Salivary gland

Adrenal cortex

Parathyroid gland

Enamel of teeth

Spleen

Thymus

Proctodaeum

Notochord

Middle ear

Pituitary gland

Parietal and visceral peritoneum

Eustachian tube

Pineal body

 

Mesoderm (Mid gut)

Adrenal medulla

 

Lining of vagina and urethra

Hypothalamus

 

Prostate gland

Significance of gastrulation
(a) Three primary germ, layers are formed.
(b) It marks the beginning of morphogenesis and differentiation.
(c) Metabolic activities of the cells are increased due to great morphogenetic activities of the blastomeres.


Some Important Events In The Human Development

Time from fertilization

Stage/organs

Event

24 hours

Cleavage

Embryo is at two-cell stage

3 days

Morula

The morula reaches uterus

7 days

Blastocyst

Implantaion of blastocyst begins

2.5 weeks

Notochord

Notochord formed, differentiation of tissues that will give rise to heart, blood cells formed in yolk sac and chorion.

3.5 weeks

Organ system

Neural tube formed, primordial eye and ear vesicle, pharyngeal pouches formed, liver bud differentiates, respiratory system and thyroid gland begin to develop, heart tube bends and begins to beat, blood vessels are formed.

4 weeks

Limb buds

Development and appearance of limb buds, brain forms three primary vesicles.

2 months

Muscles and gonads

Muscles differentiate, embryo capable of movement, gonads distinguishable as testes or ovaries, ossification of bones begins, cerebral cortex is differentiated, blood vessels assume final position.

3 months

Sex differentiation

By external examination sex can be determined, notochord degenerates, lymph glands develop.

4 months

Face

Face begins to look human, eye, ear and nose look ‘normal’, cerebral lobes differentiate.

6-9 months (third trimester)

Lanugo  (hairs) body

growth

Lanugo appear but are shed later, tremendous growth of body occurs, neurons become myelinated

 

266 days

Birth (parturition)

Baby is born, neonate arrives in outer world.


Types and functions of extra embryonic membrane

S.No.

Name of membrane

Characteristics and functions

Remarks

(1)

Yolk sac

(1) Formed by inner endoderm and outer mesoderm  (= splanchnopleura)

(2) Digestive function (= extra embryonic duct)

(3) Absorbs dissolved yolk and supplicate it to developing embryo.

Vestigeal in humans.


Well developed in reptiles, bird and prototherians.

(2)

Amnion

(1) Formed by inner ectoderm and outer mesoderm (somatopleur) above the embryo.

(2) Between the embryo and amnion there is a cavity called amniotic cavity filled with amniotic fluid secreted by amnion and embryo.

(3) Amniotic fluid act as shock absorber and prevent desiccation of embryo.

(4) Ex-foliated embryonic (= foetal) cells are used for (a) pre-natal sex determination (b) congenital defects (c) inborn metabolic disorders. This technique is called amniocentesis.



Well developed in all amniotes

(3)

Allantois

Develops as fold of splanchnopleur; developed from gut of embryo.

(1) In placentals, it combines with chorion to form allanto-chorion placenta. (reduced in human)

(2) Acts as extra-embryonic kidney in reptiles, birds and prototherians.

(4)

Chorion

Outermost fold of somatopleur and surrounds the embryo

(1) Takes part in forming the true chorio allantoic placenta

(2) Acts as extra-embryonic lung in reptiles, birds and prototherians.

  • No natural death in organisms showing binary fission e.g. Amoeba, so are called immortal.
  • Protandry : When male organs mature earlier than female organs in the bisexual organisms so ensuring cross-fertilization e.g. Scypha, Herdmania.
  • Protogyny : When female organs mature earlier than male organs in the bisexual organisms to ensure cross-fertilization e.g. Scypha, Herdmania.

Types of Natural parthenogenesis :

  • Arrehenotoky : Males are parthenogenetically developed e.g. honey bee (Apis indica), parasitic wasp (Habrobracon), mite (Tetrarhynchus), etc.
  • Thelytoky : Only females are parthenogenetically developed e.g. lacerta saxicola armeniaca only (Caucasian rock lizard).
  • Amphitoky : Parthenogenesis may result into any sex e.g. Aphis.

Historical background of Reproduction :

  • Aristotle : Father of Embryology described chick embryology in “de Generatione Animalum”.
  • Von Bear : Father of Modern Embryology. Discovered ovum.
  • De Graf (1672) : Discovered Graafian follicle in human ovary.
  • Leeuwenhock (1677) : Discovered sperms and named them animalcules. He believed in Preformation theory  as individual is preformed in animalcule, so was called Animalculist.
  • William Harvey : Proposed “Omne viovum ex ovo” all life formed from egg.
  • W. Roux : Father of Experimental embryology.
  • Newport : Fertilization of Frog’s egg.
  • Polyembryony : many youngs formed from same zygote e.g.
  • Identical or monozygotic twins in human beings.
  • Four  youngs from same zygote in Armadillo.
  • Eight youngs in a parasitic insect-Platygaster.
  • More than 1,000 youngs in an insect-Litomastrix.    
  • Neoteny or Paedogenesis : When larva develops the gonads and reproductive power without undergoing metamorphosis. It is found in certain urodels e.g. Necturus (mud puppy). Ambystoma (tiger salamander). Triturus, etc.
  • Strobilation : Asexual multiplication by transverse fission and is found in Scyphistoma of Aurelia,  and also found in Taenia.
  • Metagenesis : Alternation of diploid sexual and diploid asexual generation in the life cycle of Obelia.
  • Zygote is called the first cell of next generation.
  • Syngamy : Another term for fertilization.
  • Segmentation : Another term for cleavage.
  • Insemination : Ejaculation.
  • Fertilizin is a glycoproteinous or mycopolysaccharide molecule, while antifertilizin is a protein a substance of acidic amino acids on the surface of head of sperm.
  • Cone of reception (Fertilization Cone) : A conical outgrowth given by the egg of frog to receive the sperm. Not found in human egg.
  • Amphimixis : Mixing up of maternal and paternal chromosomal sets.

Types of Fertilization :

  • External fertilization. In this, the gametes fuse outside the female body and is found in most of bony fishes (e.g. Labeo) amphibians (e.g. frog), all echinoderms (e.g. starfish) and lower chordates (e.g. Herdmania).
  • Internal fertilization. In this, the fusion of gametes in some part of female genital tract and generally near the ostium. It is found in all terrestrial animals which may be oviparous (all birds, prototherians), ovo-viviparous (rattle-snake) or viviparous (all marsupials and eutherians).
  • Self fertilization (Endogamy). In this, two fusing gametes are derived from different parents (Uniparental) e.g. Taenia Fasciola (sheep liver fluke).
  • Cross fertilization (Exogamy). In this, two fusing gametes are derived from different parents (biparental). It is found in all unisexual animals and some bisexual animals e.g. Pheretima (earthworm-due to protandry), Scypha (Scycon - due to protogyny).
  • Isogamy : When two fusing gametes are morphologically and physiologically similar e.g. Monocystis.
  • Anisogamy : When two fusing gametes are morphologically or physiologically different e.g. frog, human beings etc.
  • Monospermic fertilization : When only one sperm enters and fuses with ovum. It is found in most of animals.
  • Polyspermic fertilization : When many sperms penetrate the ovum and may be pathological polyspermy (due to over-ripening of egg) or physiological polyspermy (natural energy of many sperms). But only one sperm fuses with ovum.

Types of blastulae :

  • Coeloblastula : A hollow blastula in which blastocoel is surrounded by either single layered (e.g. echinoderms, Amphioxus) or many layered blastoderm (e.g. frog).
  • Stereoblastula : Solid blastula with no blastocoel e.g. in annelids and molluscs.
  • Discoblastula : The blastula is a multilayered flat disc at the animal pole lying on the top of well-developed yolk. It is found in reptiles, birds, prototherians and fishes.
  • Blastocyst. In this, the blastula is as a cyst with 2 types of cells : an outer epithelium - like layer of trophoblast or nutritire cells ; and an inner mass of formative cells collectively called embryonal knob. Embryonal knob is called precursor of the embryo, while trophoblast forms protective and nutritive extra-embryonic membranes.
  • Superficial blastula or Periblastula. In this, the blastocoel is filled with yolk and is surrounded by a peripheral layer of cells. It is found in insects.

Types of Implantation :

  • Central or superficial implantation : Embryo is attached only to surface of uterus and most of it projects in the uterine cavity e.g. ungulates, carnivores and lower primates.
  • Concentric implantation : Embryo is embedded in a groove of uterine cavity and is covered by mucosal folds of uterine epithelium e.g. most of rodents.
  • Interstitial type : Embryo is buried in the uterine epithelium which completely surrounds it e.g. higher primates, pig and a few rodents.

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