Amount and distribution of yolk and types of egg

Amount and distribution of yolk and types of egg

1. Amount and distribution of yolk and types of egg:

2. INTRODUCTION: Amount and distribution of yolk or reserve food in a fully grown oocyte (egg)- varies greatly in different animal groups Based on this variation in yolk (vitellin) the animal eggs classified into following: (A) Egg Types Based on Amount of Yolk: 1- Microlecithal or oligolecithal eggs small-sized eggs, contain a very small amount of yolk found in certain marine invertebrates For example- hydra and sea urchin in various chordates such as Amphioxus and other cephalochordates, tunicates and marsupial and eutherian mammals

3. Kent (1969) described microlecithal eggs as alecithal eggs - eggs without yolk this term is not appropriate as there are no egg without yolk (2) Mesolecithal eggs: contain moderate amount of yolk found in annelid worms molluscs Petromyzontia (Petromyzon) Dipnoi (lung-fishes) Amphibians

4. (3) Megalecithal, macrolecithal or polylecithal eggs contain enormous amount of yolk found in; Insects Myxinoidea (Myxine) Chondrichthyes (elasmobranch fishes) Reptiles Birds and Monotremata (prototherian mammals)

5. (B) Egg types based on distribution of Yolk: 1- Homolecithal or isolecithal eggs; Eggs with evenly distributed yolk in cytoplasm. The amount o egg is so little that it is found scattered almost uniformly throughout cytoplasm. Microlecithal eggs are usually homolecithal

6. 2- Telotecithal eggs; polarised distribution of yolk in ooplasm Found in mesolecithal and macrolecithal eggs yolk, due to its gravity, concentrated more in one hemisphere than in the other Because of this uneven distribution, they have ( i ) vegetal pole- where the concentration of Yolk is greatest, Lower hemisphere and Light in colour. (ii) animal pole- where the concentration of yolk is smallest, Uper hemisphere and dark in colour. in macrolecithal eggs; Yolk occupies most of animal pole

7. the active cytoplasm and germinal vesicle (nucleus) remain confined to a small cap. It may be moderately telolecithal like inAmphibia , Petromyzontia and Dipnoi Or may be highly telolecithal like in cartilaginous and boriy fishes, reptiles and birds

8. 3- Centrolecithal eggs concentrated in the centre of the egg active cytoplasm forms a thin peripheral layer around the yolk e.g insects and some hydrozoa

9. Functions of yolk reserve material i.e. yolk, lipid, glycogen granules used to provide two purposes ( i ) energy supply (ii) products required for elaboration of the embryonic body important influ e nc e on size of egg differentiation or ooplasm patterns of cleavages orderly movements (morphogenetic movements) of blastomeres during gastrulation type of development - indirect with larval forms or direct with juvenile stages

10. Functions of Ovarian tissue (accessory cells) during growth of oocyte; In species lacking lampbrush stage, oocytes receive information and nutrient reserves from another type o surrounding cells called accessory cells. Act as selective barrier between vascular system and oocyte transporting yolk precursors into oocyte cytoplasm Synthesize accessory egg membrane Two types Nurse cells and follicle cells

11. Nurse cells Only found in invertebrates like coelenterates molluscs and insects . They derived rom same oogonium that give rise to oocyte. During oogonial divisions in ovary ,a differential division separates cells destined to become oocytes and their sister cells develop into nurse cells. Nurse cells produce large amounts of maternal mRNAs and proteins that are transported to, and towards the end of oogenesis, “dumped” into the oocyte -when nurse cells contract and pour their content into the oocyte- .

12. Follicle cells A follicle that has two layers of follicular cells is called a primary follicle. These layers surrounding the oocyte. Eventually these cells become known as 'granulosa' cells. The granulosa cells will secrete progesterone after ovulation. Involved in oocyte maturation and release steroidogenesis

13. Maturation of Egg: (Completion of reduction division) Involves completion of meiotic division i.e karyokinesis and cytokinesis Germinal vesicle breakdown marks end of vegetative growth phase of oogenesis and initiation miotic metaphase. First Meiotic division: Stages: Prolonged Meiotic Prophase 1 ; Genetic events - synapsis, duplication, chiasmata formation, crossing over between homologous chromosomes Reaches Diakinesis- nuclear envelope breaks , intermingled with cytoplasmic contents.

14. Rapture of nuclear membrane helps chromosomes for subsequent divisions and also important because substances like rRNA, tRNA, mRNA, Riboproteins are chemicals which initiate the Microtubule formation which are accumulated in nucleoplasm during growth phase of cytoplasm, released and mixed with cytoplasm. After the breakdown of the nuclear membrane, the chromosomes which have become greatly contracted and concentrated towards the centre of unbroken germinal vesicle, are carried to the periphery of animal pole of the oocyte. Here, centriolcs form a spindle of microtubules which takes up a position perpendicular to the surface of the primary oocyte.

15. Typically, the microtubules of the meiotic apparatus become attached to a narrow region around the animal pole which remains predetermined for the expulsion of polar bodies. Each synapsed homologous chromosome or tetrad is ultimately separated into two component chromosomes. Now chromosomes as tetrads Metaphase 1 ; alignment of homologous contracted chromosomes at the equatorial plate Anaphase 1; chromosomes carried to the periphery of the animal pole of oocyte Outer centriole of spindle with haploid set of chromosomes each This bulge pinched off from the rest of the oocyte

16. Unequal cytokinesis Unequal distribution of cytoplasm Hence, formation of primary polocyte or polar body- small, haploid And large sized cell- secondary oocyte or ootid which have same size as primary oocyte. Equal nuclear division results but unequal cytoplasmic division occur

17. Second Meiotic Division : Equal nuclear division Unequal cytoplasmic division Hence produced; Haploid secondary polocyte Haploid large sized ovum Primary polocyte undergo second meiotic division , forms two secondary polocyte Out of 4 meiotic products, only ovum as female gamete and have capacity to develop into adult animal. Polocytes - no food reserve, later die off

18. Completion of Meiosis: eggs do not complete meiosis before released from ovary ready for fertilization at various stages as vegetative growth completed depending on species, may be completed within the ovary, in the oviduct or outside body or after fertilization Example in the sea urchin, germinal vesicle breakdown and polar body formation are completed within the ovary before eggs are shed. In related species such as starfish, eggs are shed into the sea before germinal vesicle breakdown, while in many worms germinal vesicle breakdown is only by after sperm entry.

19. Reduction divisions in an oocyte. A—Oocyte immediately before the meiotic divisions; B, C, D-—first meiotic division; B—First polar body separated, preparation for second meiotic division; F, G—Second meiotic division and, simultaneously division of first polar body; H-—Meiosis completed

20. I. Causes of meiotic arrest during oogenesis: . Point to ponder? Why meiosis delayed until after oocyte differentiation in the female unlike the male?

21. Answer to this query: According to Berrill and Karp (1978); tetraploid amount of DNA utilized as template for RNA synthesis rather being rcstrictcd to one-fourth of this amount. Another factor may relate to the DNA/ cytoplasmic ratio significant relationship between the size of a cell and its DNA content. higher premeiotic DNA content of the germinal vesicle facilitates the greater cytoplasmic volume.

22. 2. Significance of unequal cytokinesis during oogenesis: asymmetrical fractioning of the primary oocyte into three small-sized, haploid polocytes one large-sized, yolky haploid egg has great embryological significance. If the equal cytoplasmic divisions of primary oocyte might have been occurred, yolk would have been distributed equally to the four daughter cells which might prove quite insufficient for the developing embryos. Hence unequal cytokinesis allows one cell out of four oogenetic meiotic products to contain most of the cytoplasm and reserve food material which may be quite sufficient for the developing embryo.

23. Formation of Egg Membranes In most animals' oocyte maturation is not completed until additional structures- egg envelopes or egg membranes produced outside their plasma membrane vary in different animal groups reflect the *adaptations made by the animal to ensure development of young in its particular environment. Classification according to their origin

24. 1- Primary egg membrane; Formed in the ovary and laid down between the egg plasma membrane and follicle cells . They are formed either by ovum or Follicle cells and so cooperative effort of both, Primary egg membranes are of the following types;

25. Vitelline membrane: not a membrane but a nonecllular , transparent layer of mucoprotein, forming an extracellular which is both fibrous and amorphous Thicker and stronger than the underlying fine plasma membrane of egg. Has tensile and elastic properties Given different names in different animals for example; in amphibians and birds it is in close contact with the ooplasmic surface until the egg is fertilized, at which time it separates from this surface, forming a tough fertilization memmbrane . In fish called chorion ,in reptiles and mammals- zona pellucida It remains intact through several stages of cleavage where it appears to help hold the dividing cells together-

26. B- Zona radiata The primary envelope of shark, some bony fishes, amphibians and some reptiles have striated appearance and is called zona radiata It represents the degraded microvilli of the growing oocyte C- Jelly envelope In echinoderms and many other eggs of marine invertebrates, the primary egg envelope is much thicker structure of jelly coat Perivitelline space ; at a later stage a space filled with fluid may appear between the egg plasma membrane and primary egg membrane

27. Micropyle a small opening, a micropyle found in the primary egg membrane The location always bears a definite relation to the animal-vegetal polarity of the egg, being situated either at the presumptive animal pole , (e.g., Echinoidea , Pisces) or at the presumptive vegetal pole (e.g., certain Mollusca) Sperm usually makes its entry inside egg during fertilization through it arise through inhibition of the growth of egg membrane either because of the attachment to the ovarian wall (vegetal pole) or because a follicle cell establishes contact with the egg surface (animal pole)

28. Secondary egg membranes; secreted outside the primary egg membrane by a layer of follicle cell that surrounds the oocyte chitinous shell surrounding the egg in insects, ascidians and cyclostomes and is called chorion In teleost fishes, chorion is formed by oocyte and contains proteins and polysaccharides In many insects, the surface of chorion shows a complicated sculpture, which is typical of each species. Not fourd in amphibians, reptiles, birds and even in mammalian eggs

29. in mammals, Graafian follicle ruptures and releases the mature ovum, the egg carries with it, for some distance down the fallopian tube, a layer of follicle cells columnar in shape arranged radially in a single layer, called corona radiata , around the ovum outside the zona pellucida The corona radiata does not form a true membrane, since, its cells are peeled off as the egg descends the oviduct

30. 3 Tertiary egg membranes secreted by the cells or the oviduct as egg travels down the duct toward the cloaca involved albumen layers, jelly coats, shell membranes and the shell itself In oviparous sharks and rays (elasmobranchs), the egg surrounded by albumen and hard horny capsule of a complicated shape The shell of capsule is drawn out into long twisted horns to entangle the eggs among sea weeds horny egg capsule is secreted by shell glands the oviduct

31. ( b)As the egg of the amphibian spirals down the oviduct, three uniform layers of albumen (jelly) are deposited around it The inner layer is a thin one and most viscous; the outer , wide and more fluid; the middle l ayer, intermediate When the amphibian egg is deposited in water, the jelly absorbs water and swells These jellies hold the egg together in masses or strands it protect the eggs from infection insulate the eggs make them unappetising to predators since they are tasteless, and anchor the eggs to twigs or plants

32. the jelly coats apparently also help to make the frog's egg fertilizable As no egg taken directly from the body cavity has been induced to develop into embryo (c) In reptiles, birds and chick Five tertiary membranes make up the envelopes of the egg External to vitelline membrane are added the white of egg (albumen) two shell membranes and a porous shell

33. Significance of egg membranes The extraneous envelopes of eggs of different oviparous vertebrates basically provide protection to the contents of eggs of developing embryos from different ecological hazards variable i.e pH, temperature variations, radiations, pollutions, danger of desiccation etc from méchanical injuries; secondarily, prevent self-fertilization provide buoyancy to the eggs as in amphibians and other chordates Even in viviparous mammals, the extraneous coat such as zonapelliicida is found

34. (l) to check polyspermy i.e. fertilization by more than one spermatozoon; (2) as a means of preventing egg fusion (3) as necessary mean of maintaining normal cleavage of the egg following fertilization Likewise, the primary egg membranes may be protective Osmoregulative aid may even be of great morphogenetic significance