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NE TJ RFL E ARNE R Dev el opment al Bi ol ogy

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TABLE OF CONTENTS 5.1. BASIC CONCEPTS OF DEVELOPMENT .................................................................................................. 4 5.2.1. Production of Gametes....................................................................................................................... 11 5.2.1.1. Spermatogenesis ......................................................................................................................... 11 5.2.1.2. Capacitation................................................................................................................................. 12 5.2.1.3. Oogenesis ................................................................................................................................... 12 5.2.1.4. Ovulation ..................................................................................................................................... 14 5.2.1.5. Menstrual Cycle ........................................................................................................................... 15 5.2.2. Fertilization: Beginning a New Organism ............................................................................................ 17 5.2.2.1. Structure and maturation of Sperm .............................................................................................. 18 5.2.1.2. The egg ....................................................................................................................................... 21 5.2.1.3. Recognition of Egg and Sperm .................................................................................................... 23 5.2.1.4. Gamete Fusion ............................................................................................................................ 27 5.2.1.5. The prevention of polyspermy ...................................................................................................... 28 5.2.1.6. Preparation for Cleavage ............................................................................................................. 29 5.3. CLEAVAGE .............................................................................................................................................. 30 5.3.1. From Fertilization to Cleavage ............................................................................................................ 31 5.3.2. Cleavage is actually the result of two coordinated processes ............................................................. 32 5.3.4. Patterns of embryonic cleavage ......................................................................................................... 33 5.4. GASTRULATION ...................................................................................................................................... 35 5.4.1. Formation of germ layers in animals................................................................................................... 36 5.4.2. Gastrulation in the sea urchin embryo, a "simple" example. ............................................................... 37 5.4.3. Frog gastrulation: Similar to the sea urchin, but more complex. ......................................................... 37 5.4.4. Human gastrulation ............................................................................................................................ 38 5.5. EMBRYO SAC DEVELOPMENT AND DOUBLE FERTILIZATION IN PLANTS ........................................ 40 5.5.1. Alternation of Generations .................................................................................................................. 41 5.5.2. Flowers .............................................................................................................................................. 42 5.5.3. Pollination & Fertilization .................................................................................................................... 42 5.5.4. Gametophytes .................................................................................................................................... 43 5.5.5. Double fertilization .............................................................................................................................. 46 5.5.6. Seed and Fruit Development .............................................................................................................. 47 5.5.7. Asexual Reproduction ........................................................................................................................ 50 5.5.8. Embryogenesis .................................................................................................................................. 51 5.6. MORPHOGENESIS & ORGANOGENESIS IN ANIMALS ......................................................................... 54

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5.6.1. Differentiation and Morphogenesis in Dictyostelium: Cell Adhesion.................................................... 55 5.6.1.1. The life cycle of dictyostelium ...................................................................................................... 55 5.6.1.2. Aggregation of Dictyostelium Cells............................................................................................... 56 5.6.1.3. Cell Adhesion Molecules in Dictyostelium .................................................................................... 56 5.6.2. Axes and pattern formation in Drosophila ........................................................................................... 57 5.6.2.1. The life cycle of Drosophila .......................................................................................................... 57 5.6.2.2. Anterior-Posterior Polarity ............................................................................................................ 58 5.6.3. Axes and pattern formation in amphibian ........................................................................................... 62 5.6.3.1. Cleavage in Amphibians .............................................................................................................. 63 5.6.3.2. Gastrulation ................................................................................................................................. 64 5.6.3.3. Dorsal/ventral axis & organizer .................................................................................................... 65 5.6.3.4. Anterior/Posterior Axis ................................................................................................................. 66 5.6.4. Axes and Pattern Formation in Chick ................................................................................................. 67 5.6.4.1. Chick Embryology ........................................................................................................................ 67 5.6.4.2. Early Cleavages and Gastrulation ................................................................................................ 67 5.6.4.3. The Development of Extraembryonic Membranes ....................................................................... 69 5.6.4.4. Axes Formation in Chick .............................................................................................................. 69 5.6.5. Biology of Caenorhabitis. elegans ...................................................................................................... 71 5.6.5.1. Vulval Induction in Caenorhabditis elegans.................................................................................. 71 5.6.5.2. Steps in vulval development ........................................................................................................ 71 5.6.6. Eye lens induction .............................................................................................................................. 73 5.6.6.1. Instructive and Permissive Interactions ........................................................................................ 74 5.6.7. Limb development .............................................................................................................................. 76 5.6.7.1. Induction of the Early Limb Bud: Fibroblast Growth Factors ......................................................... 77 5.6.7.2. Induction Of The Apical Ectodermal Ridge................................................................................... 77 5.6.8. Regeneration ..................................................................................................................................... 78 5.6.8.1. Epimorphic Regeneration of Salamander Limbs .......................................................................... 78 5.6.8.2. Proliferation of the Blastema Cells: The Requirement For Nerves ............................................... 79 5.6.8.3. Pattern formation in the regeneration blastema ........................................................................... 79 5.6.8.4. Compensatory Regeneration In The Mammalian Liver ................................................................ 79 5.6.9. Differentiation of Neurons ................................................................................................................... 80 5.6.9.1. Formation of the Neural Tube ...................................................................................................... 80 5.6.9.2. Differentiation of the Neural Tube ................................................................................................ 80 5.6.10. Post Embryonic Development .......................................................................................................... 83 5.6.11. Metamorphosis................................................................................................................................. 83 5.6.12. Environmental Regulation of Normal Development .......................................................................... 91 5.6.13. Sex Determination ............................................................................................................................ 95

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5.7. MORPHOGENESIS AND ORGANOGENESIS IN PLANTS ...................................................................... 98 5.7.1. Overview of plant structure ................................................................................................................. 99 5.7.2. Floral Development .......................................................................................................................... 110 5.7.3 Floral Development in Antirrhinum (Snapdragon) .............................................................................. 114 5.7.4. Senescence and Programmed Cell Death........................................................................................ 117 Answer To Crtitical Thinking Questions ........................................................................................................................................... 120 Test Yourself ........................................................................................................................................................................... 126 Glossary ................................................................................................................................................................................ 136

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5.1. BASIC CONCEPTS OF DEVELOPMENT

Snapshot 1.

Before cells overtly differentiate into the many cell types of the body, they undergo a "covert" commitment to a certain fate.

This commitment is first labile but later becomes irreversible (the determination step).

2.

In autonomous specification, removal of a blastomere from an embryo causes the absence in the embryo of those tissues formed by that blastomere. This mechanism of specification produces a mosaic pattern of development. (Examples include early snail and tunicate embryos.)

3.

In autonomous specification, morphogenetic determinants are apportioned to different blastomeres during cell cleavage. (An example is the yellow crescent cytoplasm that is found in the muscle forming cells of tunicate embryos.)

4.

In conditional specification, the removal of a blastomere from the embryo can be compensated for by the other cells' changing their fates. Each cell can potentially give rise to more cell types than it normally does. This produces a regulative pattern of development wherein cell fates are determined relatively late. (Examples include frog and mammalian embryos.)

5.

In conditional specification, the fate of a cell often depends upon its neighbors ("whom it meets").

6.

In conditional specification, groups of cells can have their fates determined according to a concentration gradient of morphogen. The cells specified by such a morphogen can constitute a field.

7.

In syncytial specification, the fates of cells can be determined by gradients of morphogens within the egg cytoplasm.

8.

Different cell types can sort themselves into regions by means of cell surface molecules such as cadherins. These molecules can be critical in patterning cells into tissues and organs.

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Potency What a cell could become

Commitment Restriction in potency

Differentiation   

Changes involved in the progressive diversification of the structure and function of cells.

Acquisition of the characteristics that allow different cell types to perform their functions.

The development of specialized cell types is called differentiation (Table. 5.1).

Table 5.1. Some differentiated cell types and their major products Cell type Keratinocyte

Differentiated cell product Keratin

Erythrocyte

Hemoglobin

Specialized function of cell Protection against abrasion, desiccation Transport of oxygen

Lens cell

Crystallins

Transmission of light

B lymphocyte

Immunoglobulins

Antibody synthesis

T lymphocyte

Cell-surf ace antigens

Destruction of f oreign cells; regulation of immune response

Melanocyte

Melanin

Pigment production

Pancreatic islet cells

Insulin

Leydig cell

Testosterone

Regulation of carbohydrate metabolism Male sexual characteristics

Chondrocyte

Chondroitin sulf ate; type II collagen

Tendons and ligaments

Osteoblast

Bone matrix

Skeletal support

Myocyte

Muscle actin and myosin

Contraction

Hepatocyte

Serum albumin; numerous enzymes

Neurons

Neurotransmitters

Tubule cell of hen oviduct Follicle cell of insect ovary

Ovalbumin

Production of serum proteins and numerous enzymatic f unctions Transmission of electrical impulses Egg white proteins f or nutrition and protection of embryo Eggshell proteins f or protection of embryo

Chorion proteins

 These overt changes in cellular biochemistry and function are preceded by a process involving the commitment of the cell to a certain fate.  At this point, even though the cell or tissue does not appear phenotypically different from its uncommitted state, its developmental fate has become restricted. The process of commitment can be divided into two stages. The first stage is a labile phase called specification.  The fate of a cell or a tissue is said to be specified when it is capable of differentiating autonomously when placed in a neutral environment such as a petri dish or test tube. At this stage, the commitment is still capable of being reversed. The second stage of commitment is determination.

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Specification Three basic modes of commitment have been described. (Table 5.2)

I Autonomous Specification  

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The first is called autonomous specification.

In this case, if a particular blastomere is removed from an embryo early in its development that isolated blastomere will produce the same cells that it would have made if it were still part of the embryo (Figure 5.1).

Figure 5.1: Autonomous specification (mosaic development). (A-C) Differentiation of trochoblast (ciliated) cells of the mollusc Patella. (A) 16-cell stage seen from the side; the presumptive trochoblast cells are shaded. (B) 48-cell stage. (C) Ciliated larval stage, seen from the animal pole. (D-G) Differentiation of a Patella trochoblast cell isolated from the 16-cell stage and cultured in vitro. (E, F)Results of the first and second divisions in culture. (G) Ciliated products of (F). Even in isolated culture, the cells divide and become ciliated at the correct time. visit: http://study.biotecnika.org for colored picture.

Moreover, the embryo from which that cell is taken will lack those cells (and only those cells) that would have been produced by the missing blastomere.

Autonomous specification gives rise to a pattern of development referred to as mosaic development, since the embryo appears to be constructed like a tile mosaic of independent self-differentiating parts.

Invertebrate embryos, especially those of molluscs, annelids, and tunicates, often use autonomous specification to determine the fate of their cells.

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In these embryos, morphogenetic determinants (certain proteins or messenger RNAs) are placed in different regions of the egg cytoplasm and are apportioned to the different cells as the embryo divides.

These morphogenetic determinants specify the cell type.

II Conditional Specification  

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A second mode of commitment involves interactions with neighboring cells.

In this type of specification, each cell originally has the ability to become many different cell types. However, the interactions of the cell with other cells restrict the fate of one or both of the participants. This mode of commitment is sometimes called conditional specification, because the fate of a cell depends upon the conditions in which the cell finds itself.

If a blastomere is removed from an early embryo that uses conditional specification, the remaining embryonic cells alter their fates so that the roles of the missing cells can be taken over.

This ability of the embryonic cells to change their fates to compensate for the missing parts is called regulation (Figure 5.2).

The isolated blastomere can also give rise to a wide variety of cell types (and sometimes generates cell types that the cell would normally not have made if it were part of the embryo).

Thus, conditional specification gives rise to a pattern of embryogenesis called regulative development.

Figure 5.2. Conditional specification. (A) What a cell becomes Regulative development is seen in depends upon its posi tion in the embryo. Its fate is determined most vertebrate embryos, and it is by interactions with neighboring cells. (B) If cells are removed obviously critical in the development from the embryo, the remaining cells can regulate and compensate for the missing part. visit: http://study.biotecnika.org of identical twins. for colored picture In the formation of such twins, the cleavage-stage cells of a single embryo divide into two groups, and each group of cells produces a fully developed individual.

III Syncytial Specification  

This type of a specification is a hybrid of the autonomous and conditional that occurs in insects.

This method involves the action of morphogen gradients within the syncytium.

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As there are no cell boundaries in the syncytium, these morphogens can influence nuclei in a concentration-dependent manner Table 5.2. Modes of cell type specification and their characteristics

I.

Autonomous specification Characteristic of most invertebrates.

Specif ication by differential acquisition of certain cytoplasmic molecules present in the egg.

Invariant cleavages produce the same lineages in each embryo of the species. Blastomere f ates are generally invariant.

Cell type specif ication precedes any large-scale embryonic cell migration.

Produces “mosaic” (“determinative”) development: cells cannot change f ate if a blastomere is lost.

II.

Conditional specification Characteristic of all vertebrates and f ew invertebrates.

Specif ication by interactions between cells. Relative positions are important.

Variable cleavages produce no invariant f ate assignments to cells.

Massive cell rearrangements and migrations precede or accompany specif ication.

Capacity f or “regulative” development: allows cells to acquire dif f erent f unctions .

III. Syncytial specification Characteristic of most insect classes.

Specif ication of body regions by interactions between cytoplasmic regions prior to cellularization of the blastoderm.

Variable cleavage produces no rigid cell f ates f or particular nuclei.

Af ter cellularization, conditional specif ication is most often seen.

Genomic Equivalence 

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Genomic equivalence means that the genomes of all or most of the somatic cells within a multicellular organism are essentially the same, i.e., have the same amount of DNA and the same nucleotide sequence Thus, the differentiation of cells does not (usually) involve changes in the nucleotide sequence of DNA Genomic equivalence can be demonstrated via various cloning experiments including the growing of a whole plant from an isolated cell (other than a zygote) and the cloning of an animal via the injection of a somatic-cell genome into a zygote (or unfertilized egg) whose own genome has been destroyed.

Stem Cells  

Cells that divide to maintain a self-renewing population, and contribute daughters that will differentiate.

Can get progressive commitment.

Cell Fate 

What a cell will become (if left alone).

Cell Lineage 

The cell lineage of an organism is the pattern of cell divisions during its Development.

Morphogen or Morphogenetic Gradients

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a molecule forms a gradient and regulates Cell fate in a Concentration dependent manner.

Cytoplasmic Determinants 

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Cell lineage can impact substantially on specialization fates, what then determines early on within an embryo that one lineage will give rise to cells contributing to one kind of tissue whereas another lineage will give rise to cells contributing to a different kind of tissue (when, in fact, all cells ultimately can trace their lineages back to the original zygote).

The answer is positional information, such that cells found in one part of the embryo contain or receive different information from cells that are found in different parts of the embryo.

Positional information can be traced back to the zygote itself, which contains a heterogeneous cytoplasm such that one part of the cytoplasm does not resemble the cytoplasm found in another part of that cell.

We call these cytoplasmic differences that give rise to differently differentiated cell lineages, cytoplasmic determinants.

Cytoplasmic determinants include proteins as well as mRNAs found in the cytoplasm of the zygote.

Genomic Imprinting     

Genomic imprinting is an epigenetic phenomenon which, in most cases, is believed to occur in gametogenesis.

Genomic imprinting occurs when both maternal and paternal alleles are present, but one allele will be expressed while the other remains inactive.

It is not completely evident why genes are imprinted.

The most prominent assumption is that this process is necessary for development and may somehow regulate growth in the embryo and neonate.

Evidence for this suggestion came from experiments with androgenotes (embryos with two paternal genomes) and gynogenotes (embryos with two maternal genomes), which were produced by nuclear transplantation. These zygotes were formed, but neither type was able to undergo further development.

Induction and Competence  

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Organs are complex structures composed of numerous types of tissues.

In the vertebrate eye, for example, light is transmitted through the transparent corneal tissue and focused by the lens tissue (the diameter of which is controlled by muscle tissue), eventually impinging on the tissue of the neural retina. The precise arrangement of tissues in this organ cannot be disturbed without impairing its function.

Such coordination in the construction of organs is accomplished by one group of cells changing the behavior of an adjacent set of cells, thereby causing them to change their shape, mitotic rate, or fate.

This kind of interaction at close range between two or more cells or tissues of different history and properties is called proximate interaction, or induction.

There are at least two components to every inductive interaction. The first component is the inducer: the tissue that produces a signal (or signals) that changes the cellular behavior of the other tissue. The second component, the tissue being induced, is the responder.

Not all tissues can respond to the signal being produced by the inducer.

This ability to respond to a specific inductive signal is called competence. It is not a passive state, but an actively acquired condition.

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5.2. GAMETOGENESIS, FERTILIZATION AND EARLY DEVELOPMENT

Snapshot 1.

Fertilization accomplishes two separate activities: sex (the combining of genes derived from two parents) and reproduction (the creation of a new organism).

2.

The events of conception usually include: (1) contact and recognition between sperm and egg; regulation of sperm entry into the egg; (3) fusion of genetic material from the two gametes; and (4) activation of egg metabolism to start development.

3.

The sperm head consists of a haploid nucleus and an acrosome. The acrosome is derived from the Golgi apparatus and contains enzymes needed to digest extracellular coats surrounding the egg. The neck of the sperm contains the mitochondria and the centriole that generates the microtubules of the flagellum. Energy for flagellar motion comes from mitochondrial ATP and a dynein ATPase in the flagellum.

4.

The egg contains a haploid nucleus, and an enlarged cytoplasm storing ribosomes, mRNAs, and nutritive proteins. Other mRNAs and proteins, used as morphogenetic factors, are also stored in the egg. Cortical granules lie beneath the egg's plasma membrane. Many eggs also contain protective agents needed for survival in their particular environment.

5.

Surrounding the egg plasma membrane is an extracellular layer often used in sperm recognition. In most animals, this extracellular layer is the vitelline envelope. In mammals, it is the much thicker zona pellucida.

6.

In sea urchins, the acrosome reaction is initiated by compounds in the egg jelly. The acrosomal vesicle undergoes exocytosis to release its enzymes. Globular actin polymerizes to extend the acrosomal process. Bindin on the acrosomal process is recognized by a protein complex on the sea urchin egg surface.

7.

In mammals, sperm must be capacitated in the female reproductive tract before they are capable of fertilizing the egg.

8.

Mammalian sperm bind to the zona pellucida before undergoing the acrosome reaction. In the mouse, this binding is mediated

by ZP3 (zona protein 3) and one or many sperm proteins that recognize it. The mammalian acrosome reaction is initiated on the zona pellucida, and the acrosomal enzymes are concentrated there.

9.

Fusion between sperm and egg is mediated by protein molecules whose hydrophobic groups can merge the sperm and egg plasma membranes. In sea urchins, bindin may mediate gamete fusion. In mammals, fertilin proteins in the sperm bind to integrins in the egg and allow the membranes to fuse.

10. There are often two blocks to polyspermy. The fast block is electrical and is mediated by sodium ions: the egg membrane resting potential rises, and sperm can no longer fuse with the egg. The slow block is physical and is mediated by calcium ions.

A wave of calcium ions propagates from the point of sperm entry, causing the cortical granules to fuse with the egg cell membrane. The released contents of the granules cause the vitelline membrane to rise and to harden into the fertilization envelope.

11. In mammals, blocks to polyspermy include the modification of the zona proteins by the contents of the cortical granules.

Sperm can no longer bind to the zona.

12. Inositol 1, 4, 5-triphosphate (IP3) is believed to be responsible for releasing calcium ions from storage in the endoplasmic reticulum. DAG (diacylglycerol) is thought to initiate the rise in egg pH. The free calcium ions, supported by the alkalization of the egg, activate egg metabolism, protein synthesis, and DNA synthesis.

13. The male pronucleus and the female pronucleus migrate toward each other, replicating DNA as they move.

14. Microtubular changes cause cytoplasmic movements. These rearrangements of cytoplasm can be critical in specifying which portions of the egg are going to develop into which organs.

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Gamete  

Haploid cells which, in animals, consist of the male produced sperm and the female produced egg (ovum).

Note that in animals and other multi-celled organisms, it is generally the case that gametes do not undergo mitosis.

5.2.1. Production of Gametes 5.2.1.1. Spermatogenesis  

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Spermatogenesis is the production of sperm from the primordial germ cells.

At puberty, the testes begin to secrete greatly increased amounts of the steroid hormone testosterone. This hormone has a multitude of effects.

In addition to stimulating development of many secondary sex characteristics, it triggers growth of the testes, maturation of seminiferous tubules, and commencement of spermatogenesis.

Under the influence of testosterone, Sertoli cells differentiate into a system of seminiferous tubules.

The dormant primordial germ cells (PGCs) resume development, divide several times by mitosis, and then differentiate into spermatogonia.

These spermatogonia are located immediately under the basement membrane surrounding the seminiferous tubules, where they occupy pockets between Sertoli cells (Figure. 5.3).

Each spermatogonium is connected to the adjacent Sertoli cells by specialized membrane junctions.

In addition, Sertoli cells are joined to each other by dense bands of intercellular membrane junctions that surround each sertoli cell and thus isolate spermatogonia from the tubule lumen.

Cells that will undergo spermatogenesis arise by mitosis from the spermatogonia. These cells are gradually translocated between the Sertoli cells from the basal to the luminal side of the seminiferous epithelium while spermatogenesis takes place (Figure 5.3).

During this migratory phase, primary spermatocytes pass without interruption through both meiotic divisions, producing first two secondary spermatocytes and then four spermatids.

Figure 5.3. Schematic section through the wall of the seminiferous tubule. Spermatogonium just under the outer surface of the tubule wall (basal side) undergoes mitosis to produce daughter cells, which may either continue to divide by mitosis (thus renewing the spermatogonial stem cell population) or commence meiosis as primary spermatocytes.

As spermatogenesis and spermiogenesis occur, the di fferentiating cell is translocated between adjacent Sertoli cells to the tubule lumen. Daughter spermatocytes and spermatids rem ain linked by cytoplasmic bridges.

The entire clone of spermatogonia derived from each primordial germ cell is linked by cytoplasmic bridges.

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The spermatids undergo dramatic changes that convert them into mature sperm while they complete their migration to the lumen. This process of sperm cell differentiation is called spermiogenesis.

Spermatogenesis takes place continuously Dev Bio facts from puberty to death. Gametes are It takes around 10 weeks for the sperm to reach maturi ty. produced in synchronous waves in each local Once the sperm is inside the woman's body it can live up area of the germinal epithelium, although the to 5 days. However, when it is outside the body, it can process is not synchronized throughout the only survive from few minutes to several hours. seminiferous tubules.

In many different mammals, the clone of spermatogonia, derived from each spermatogonial stem cell, populates a local area of the seminiferous tubules and displays synchronous spermatogenesis. That may be the case in humans as well.

About four waves of synchronously differentiating cells can be observed in a given region of the human tubule epithelium at any time.

Ultra structural studies provide evidence that these waves of differentiating cells remain synchronized because of incomplete cytokinesis throughout the series of mitotic and meiotic divisions between the division of a spermatogonium and formation of spermatids.

Instead of fully separating, daughter cells produced by these divisions remain connected by slender cytoplasmic bridges (Figure 5.3) that could allow passage of small signaling molecules or metabolites.

In the human male, each cycle of spermatogenesis takes about 64 days. Spermatogonial mitosis occupies about 16 days, the first meiotic division takes about 8 days, the second meiotic division takes about 16 days, and spermiogenesis requires about 24 days.

5.2.1.2. Capacitation    

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During its journey from the seminiferous tubules to the ampulla of the oviduct, a sperm cell undergoes a process of functional maturation that prepares it to fertilize an oocyte.

Sperm produced in the seminiferous tubules are stored in the lower part of the epididymis, a 40-foot long highly coiled duct connected to the vas deferens near its origin in the testis.

During ejaculation, sperm are propelled through the vas deferens and urethra and are mixed with nourishing secretions from the seminal vesicles, prostate, and bulbourethral glands.

As many as 300 million spermatozoa may be deposited in the vagina by a single ejaculation, but only a few hundred succeed in navigating through the cervix, uterus, and oviduct and into the expanded ampullar region.

In the ampulla of the oviduct, sperm survive and retain their capacity to fertilize an oocyte for 1 to 3 days.

Capacitation, the final step of sperm maturation, consists mainly of changes in the acrosome that prepares it to release the enzymes required to penetrate the zona pellucida, a shell of glycoprotein surrounding the oocyte.

Capacitation takes place within the female genital tract and is thought to require contact with secretions of the oviduct.

Spermatozoa used in in-vitro fertilization (IVF) procedures are artificially capacitated.

5.2.1.3. Oogenesis   

By 12 weeks of development, oogonia in the genital ridges enter the first meiotic prophase and then almost immediately become dormant.

The nucleus of each of these dormant primary oocytes, containing the partially condensed prophase chromosomes, becomes very large and watery and is referred to as a germinal vesicle.

The swollen condition of the germinal vesicle is thought to protect the oocyte’s DNA during the long period of meiotic arrest.

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A single-layered, squamous capsule of epithelial follicle cells derived from the somatic support cells tightly encloses each primary oocyte.

This capsule and its enclosed primary oocyte constitute a primordial follicle (Figure 5.4). By 5 months, the number of primordial follicles in the ovaries peaks at about 7 million.

Most of these follicles subsequently degenerate. By birth only 700,000 to 2 million remain, and by puberty, only about 400,000.

After reaching puberty, also called menarche in females, and until the woman enters menopause several decades later, monthly cycles in the secretion of hypothalamic, pituitary, and ovarian hormones control a menstrual cycle, which results each month inthe production of a female gamete and a uterus primed to receive a fertilized embryo. Specifically, this 28-day cycle consists of: o The monthly maturation of (usually) a single oocyte and its enclosing follicle o The concurrent proliferation of the uterine endometrium o The process of ovulation by which the oocyte is released from the ovary o The continued development of the follicle into an endocrine corpus luteum o The sloughing of the uterine endometrium and involution of the corpus luteum (unless a fertilized ovum implants in the uterus and begins to develop) The menstrual cycle is considered to begin with menstruation (also called the menses), the shedding of the degenerated uterine endometrium from the previous cycle.

On about the 5th day of the cycle (the 5th day after the beginning of menstruation), an increase in secretion by the hypothalamus of the brain of Dev Bio facts a small peptide hormone, gonadotropinreleasing hormone (GnRH), stimulates the About 500 million sperm mature every day in a normal pituitary gland to increase its secretion of two male adult. There is an equal number of male and female sperm. The female sperm is slower, but stronger, while gonadotropic hormones (gonadotropins): the male sperm is faster, but weaker. follicle-stimulating hormone (FSH) and luteinizing hormone (LH).

The rising levels of pituitary gonadotropins regulate later phases of folliculogenesis in the ovary and the proliferative phase in the uterine endometrium.

Before a particular cycle, and independent of pituitary gonadotropins, the follicular epithelium of a small group of primordial follicles thickens, converting the single-layered follicular epithelium from a layer of squamous cells to cuboidal cells (Figure 5.4). These follicles are now called primary follicles.

The follicle cells and the oocyte jointly secrete a thin layer of a cellular material, composed of only a few types of glycoprotein, onto the surface of the oocyte.

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Although this layer, the zona pellucida, appears to form a complete physical barrier between the follicle cells and oocyte, actually it is penetrated by thin extensions of follicle cells that are connected to the oocyte cell membrane by intercellular junctions.

These extensions and their intercellular junctions remain intact until just before ovulation, and they probably convey both developmental signals and metabolic support to the oocyte.

The follicular epithelium of five to twelve of these primary follicles then proliferates to form a multilayered capsule of follicle cells around the oocyte (Figure 5.4).

The follicles are now called growing follicles. At this point, some of the growing follicles cease to develop and eventually degenerate, whereas a few continue to enlarge in response to rising levels of FSH, mainly by taking up fluid and developing a central fluid-filled cavity called the antrum. These follicles are called antral or vesicular follicles.

At the same time, the connective tissue of the ovarian stroma surrounding each of these follicles differentiates into two layers, an inner layer called the theca interna and an outer layer called the theca externa. These two layers become vascularized, in contrast to the follicle cells, which do not.

Eventually, one of the growing follicles gains primacy and continues to enlarge by absorbing fluid, whereas the remainder of the follicles recruited during the cycle degenerate (undergo atresia).

The oocyte, surrounded by a small mass of follicle cells called the cumulus oophorus, increasingly projects into the expanding antrum but remains connected to the layer of follicle cells that lines the antral cavity and underlies the basement Dev Bio facts membrane of the follicle. This layer is called the membrana granulosa.

The largest cell in the female human body is the ovum. At The large, swollen follicle is now called a birth human ovaries contain from 150,000 to 500,000 cells which can potentially develop into eggs, 300 to 400 mature vesicular follicle or mature graafian of which will actually mature into ovum. follicle (Figure 5.4). At this point, the oocyte still has not resumed meiosis.

5.2.1.4. Ovulation       

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On about day 13 or 14 of the menstrual cycle (at the end of the proliferative phase of the uterine endometrium), levels of FSH and LH suddenly rise very sharply.

This ovulatory surge in pituitary gonadotropins stimulates the primary oocyte of the remaining mature graafian follicle to resume meiosis.

This response can be observed visually about 15 hours after the beginning of the ovulatory surge, when the membrane of the swollen germinal vesicle (nucleus) of the oocyte breaks down.

By 20 hours, the chromosomes are lined up in metaphase. Cell division to form the secondary oocyte and first polar body rapidly ensues.

The secondary oocyte promptly begins the second meiotic division but, about 3 hours before ovulation, is arrested at the second meiotic metaphase.

As the germinal vesicle breaks down, the cumulus cells surrounding the oocyte lose their cell-to-cell connections and disaggregate.

As a result, the oocyte and a mass of loose cumulus cells detach into the antral cavity. Over the next few hours, the cumulus cells secrete an abundant extracellular matrix, consisting mainly of hyaluronic acid, which causes the cumulus cell mass to expand severalfold.

This process of cumulus expansion may play a role in several processes, including the regulation of meiotic progress and ovulation.

In addition, the mass of matrix and entrapped cumulus cells that accompanies the ovulated oocyte may play roles in the transport of the oocyte in the oviduct, in fertilization, and in the early development of the zygote.

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The process of ovulation (the expulsion of the secondary oocyte from the follicle) has been likened to an inflammatory response.

The cascade of events that culminates in ovulation is thought to be initiated by the secretion of histamine and prostaglandins, well-known inflammatory mediators.

Within a few hours after the ovulatory surge of FSH and LH, the follicle becomes more vascularized and is visibly pink and edematous in comparison with non-responding follicles.

The follicle is displaced to the surface of the ovary, where it forms a bulge (Figure 5.4). As ovulation approaches, the projecting wall of the follicle begins to thin, resulting in formation of a small, nippleshaped protrusion called the stigma.

Finally, a combination of tension produced by smooth muscle cells in the follicle wall plus the release of collagen-degrading enzymes and other factors by fibroblasts in the region causes the follicle to rupture.

Rupture of the follicle is not explosive: the oocyte, accompanied by a large number of investing cumulus cells bound in hyaluronic acid matrix, is slowly extruded onto the surface of the ovary.

Ovulation occurs about 38 hours after the beginning of the ovulatory surge of FSH and LH. The sticky mass formed by the oocyte and cumulus is actively scraped off the surface of the ovary by the fimbriated mouth of the Figure 5.5. The ovulated oocyte clings to the oviduct (Figure.5.5). surface of the ovary by the gelatinous cumulus oophorus and is actively scraped off by the The cumulus oocyte complex is then moved fimbriated oviduct mouth. After ovulation, the into the ampulla of the oviduct by the membrana granulosa layer of the ruptured synchronized beating of cilia on the oviduct follicle proliferates to form the endocrine wall. corpus luteum.

Within the ampulla, the oocyte may remain viable for as long as 24 hours before it loses its capacity to be fertilized.

After ovulation, membrane granulosa cells of the ruptured follicular wall begin to proliferate and give rise to the luteal cells of the corpus luteum (Figure.5.4 and Figure.5.5).

Dev Bio facts As described later, the corpus luteum is an The average lifespan of an egg once it is released from the endocrine structure that secretes steroid ovary is 1 2- 2 4 h after which it disintegrates out of the body with the menstrual flow. The female human body is hormones to maintain the uterine capable of giving birth to 35 children in one lifetime. endometrium in a condition ready to receive an embryo.

If an embryo does not implant in the uterus, the corpus luteum degenerates after about 14 days and is converted to a scar like structure called the corpus albicans.

5.2.1.5. Menstrual Cycle   

Beginning on about day 5 of the menstrual cycle, the thecal and follicle cells of responding follicles secrete steroids called estrogens (Figure 5.6).

These hormones in turn cause the endometrial lining of the uterus to proliferate and undergo remodeling.

This proliferative phase begins at about day 5 of the cycle and is complete by day 14.

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After ovulation occurs, thecal cells in the wall of the corpus luteum continue to secrete estrogens, and luteal cells that differentiate from remaining follicle cells also begin to secrete high levels of a related steroid hormone, progesterone.

Luteal progesterone stimulates the uterine endometrial layer to thicken further and to form convoluted glands and increased vasculature.

Unless an embryo implants in the uterine lining, this secretory phase of endometrial differentiation lasts about 13 days.

At that point (near the end of the menstrual cycle), the corpus luteum shrinks and levels of progesterone fall. The thickened endometrium, which is dependent on progesterone, degenerates and begins to slough.

The 4- to 5-day menstrual phase, during which the endometrium is sloughed (along with about 35 mL of blood and the unfertilized oocyte), is by convention considered the start of the next cycle.

Figure 5.6. Ovarian, endometrial, and hormonal events of the menstrual cycle. Pituitary follicle-stimulating hormone (FSH) and luteinizing hormone (LH) directly control the ovarian cycle and also control production of estrogen and progesterone by responding follicles and corpus luteum of the ovary. These ovarian hormones in turn control the cycle of the uterine endometrium. visit: http://study.biotecnika.org for colored picture.

Critical thinking Questions 1.

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Why the growth phase of oogenesis is longer than that of spermatogenesis?

How are spermatogenesis and oogenesis similar and how are they different?

How do the structures of the male and female human gametes aid them in performing their functions?

If humans were photoautotrophs, how might our anatomy be different?

Why do you think four sperm cells are produced as a result of gametogenesis,but only one ovum?

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5.2.2. Fertilization: Beginning a New Organism   

Fertilization is the fusion of gametes to form a diploid zygote (Figure 5.7).

Fertilization is the means by which haploid gametes restore the diploid form.

The first function of fertilization is to transmit genes from parent to offspring, and the second is to initiate in the egg cytoplasm those reactions that permit development to proceed.

Figure 5.7. The basics of fertilization

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Generally fertilization consists of four major events 1. Contact and recognition between sperm and egg. In most cases, this ensures that the sperm and egg are of the same species.

2. Regulation of sperm entry into the egg. Only one sperm can ultimately fertilize the egg. This is usually accomplished by allowing only one sperm to enter the egg and inhibiting any others from entering.

3. Fusion of the genetic material of sperm and egg.

4. Activation of egg metabolism to start development.

Sperm Discovery  Anton van Leeuwenhoek, the Dutch Dev Bio facts microscopist who co-discovered sperm in Human sperm travels at the staggering rate of up to 4 1678, first believed them to be parasitic millimeters per minute, The average length of the animals living within the semen (hence the journey to the fallopian tubes is 175 millimeters, the term spermatozoa, meaning "sperm sperms takes up to 3 days. animals").  The first evidence suggesting the importance of sperm in reproduction came from a series of experiments performed by Lazzaro Spallanzani.

Sperm

Ovum

Follicile cells Vitelline membrane Nucleus Cytoplasm Zona pellucida

Figure 5.8.Structure of Gametes

5.2.2.1. Structure and maturation of Sperm Head  Each sperm consists of a haploid nucleus, a propulsion system to move the nucleus, and a sac of enzymes that enable the nucleus to enter the egg (Figure 5.8).  Most of the sperm's cytoplasm is eliminated during maturation, leaving only certain organelles that are modified for spermatic function.  During the course of sperm maturation, the haploid nucleus becomes very streamlined, and its DNA becomes tightly compressed.  In front of this compressed haploid nucleus lies the acrosomal vesicle, or acrosome, which is derived from the Golgi apparatus and contains enzymes that digest proteins and complex sugars; thus, it can be considered a modified secretory vesicle. These stored enzymes are used to lyse the outer coverings of the egg.  In many species, such as sea urchins, a region of globular actin molecules lies between the nucleus and the acrosomal vesicle. These proteins are used to extend a fingerlike acrosomal process from the sperm during the early stages of fertilization.

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In sea urchins and several other species, recognition between sperm and egg involves molecules on the acrosomal process.

Together, the acrosome and nucleus constitute the head of the sperm.

Figure 5.9. The modification of a germ cell to form a mammalian sperm. (A) The centriole produces a long flagellum at what will be the posterior end of the sperm, and the Golgi apparatus forms the acrosom al vesicle at the future anterior end. The mitochondria (hollow dots) collect around the flagellum near the base of the haploid nucleus and become incorporated into the mid piece of the sperm. The remaining cytoplasm is jettisoned, and the nucleus condenses.

The size of the mature sperm has been enlarged relative to the other stages. (B) Mature bull sperm.

The DNA is stained blue with DAPI; the mitochondria are stained green, and the tubulin of the flagellum is stained red. (C) Acrosome of mouse sperm, stained green by GFP. A construct whereby the GFP gene was combined to the proacrosin promoter caused GFP to accumulate in the acrosome.

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In most species, however, each sperm is able to travel long distances by whipping its flagellum.

In some species (such as the parasitic roundworm Ascaris), the sperm travel by the amoeboid motion of lamellipodial extensions of the cell membrane.

Flagella are complex structures. The major motor portion of the flagellum is called the axoneme.

It is formed by microtubules emanating from the centriole at the base of the sperm nucleus (Figures 5.9 and 5.10).

The core of the axoneme consists of two central microtubules surrounded by a row of nine doublet microtubules.

Actually, only one microtubule of each doublet is complete, having 13 protofilaments; the other is Cshaped and has only 11 protofilaments (Figure 5.10).

Although tubulin is the basis for the structure of the flagellum, other proteins are also critical for flagellar function.

The force for sperm propulsion is provided by dynein, a protein that is attached to the microtubules (Figure 5.10).