1. The process of gastrulation in frog.

Gastrulation

During gastrulation, cell movements result in a massive reorganization of the embryo from a simple spherical ball of cells, the blastula, into a multi-layered organism. During gastrulation, many of the cells at or near the surface of the embryo move to a new, more interior location.

The primary germ layers (endoderm, mesoderm, and ectoderm) are formed and organized in their proper locations during gastrulation. Endoderm, the most internal germ layer, forms the lining of the gut and other internal organs. Ectoderm, the most exterior germ layer, forms skin, brain, the nervous system, and other external tissues. Mesoderm, the the middle germ layer, forms muscle, the skeletal system, and the circulatory system.

This fate map diagram of a Xenopus blastula shows cells whose fate is to become ectoderm in blue and green, cells whose fate is to become mesoderm in red, and cells whose fate is to become endoderm in yellow. Notice that  the cells that will become endoderm are NOT internal!    

from LIFE: The Science of Biology, Purves et al, 1998

Although the details of gastrulation differ between various groups of animals, the cellular mechanisms involved in gastrulation are common to all animals. Gastrulation involves changes in cell motility, cell shape, and cell adhesion.

Below are schematic diagrams of the major types of cell movements that occur during gastrulation.

Invagination: a sheet of cells (called an epithelial sheet) bends inward. Ingression: individual cells leave an epithelial sheet and become freely migrating mesenchyme cells. Involution: an epithelial sheet rolls inward to form an underlying layer. 

Epiboly: a sheet of cells spreads by thinning. Intercalation: rows of cells move between one another, creating an array of cells that is longer (in one or more dimensions) but thinner. Convergent Extension: rows of cells intercalate, but the intercalation is highly directional. 

4. Give a diagrammatic representation of the major hormonal changes that occur during menstrual cycle.

6. The three steps that contribute to homeostasis.

There are three components to this system:

1. The Sensor which detects the stress.2. The Control Center which receives information from the sensor and sends a

message to the Effector.3. The Effector which receives the message from the control center and produces

the response which reestablishes homeostasis.

7. The histology and function of adrenal gland.

Anatomically, the adrenal glands are located in the retroperitoneum superior to the kidneys, bilaterally. They are surrounded by an adipose capsule and renal fascia. In humans, the adrenal glands are found at the level of the 12th thoracic vertebra. Each adrenal gland has two distinct structures, the outer adrenal cortex and the inner medulla, both of which produce hormones. The cortex mainly produces cortisol, aldosterone and androgens, while the medulla chiefly produces epinephrine and norepinephrine. The combined weight of the adrenal glands in an adult human ranges from 7 to 10 grams.[1]

A CT scan in which the Adrenals are shown as the triangular-shaped organs on top of the kidneys

Cortex

The adrenal cortex is devoted to the synthesis of corticosteroid and androgen hormones. Specific cortical cells produce particular hormones including aldosterone, cortisol, and androgens such as androstenedione. Under normal unstressed conditions, the human adrenal glands produce the equivalent of 35–40 mg of cortisone acetate per day.[2] In contrast to the direct innervation of the medulla, the cortex is regulated by neuroendocrine hormones secreted from the pituitary gland which are under the control of the hypothalamus, as well as by the renin-angiotensin system.

The adrenal cortex comprises three zones, or layers. This anatomic zonation can be appreciated at the microscopic level, where each zone can be recognized and distinguished from one another based on structural and anatomic characteristics.[3] The adrenal cortex exhibits functional zonation as well: by virtue of the characteristic enzymes present in each zone, the zones produce and secrete distinct hormones.[3]

Zona glomerulosa (outer)The outermost layer, the zona glomerulosa is the main site for production of mineralocorticoids, mainly aldosterone, which is largely responsible for the long-term regulation of blood pressure. Aldosterone's effects are on the distal convoluted tubule and collecting duct of the kidney where it causes increased reabsorption of sodium and increased excretion of both potassium (by principal cells) and hydrogen ions (by intercalated cells of the collecting duct). Sodium retention is also a response of the salivary ducts, distal colon, and sweat glands to aldosterone receptor stimulation. The major stimulus to produce aldosterone is angiotensin II while ACTH from the pituitary only produces a transient effect. Angiotensin is stimulated by the juxtaglomerular cells when renal blood pressure drops below 90 mmHg.[4]

Zona fasciculataSituated between the glomerulosa and reticularis, the zona fasciculata is responsible for producing glucocorticoids, such as 11-deoxycorticosterone, corticosterone, and cortisol in humans. Cortisol is the main glucocorticoid under normal conditions and its actions include mobilization of fats, proteins, and carbohydrates, but it does not increase under starvation conditions.[4] Additionally, cortisol enhances the activity of other hormones including glucagon and catecholamines. The zona fasciculata secretes a basal level of cortisol but can also produce bursts of the hormone in response to adrenocorticotropic hormone (ACTH) from the anterior pituitary.

Zona reticularisThe inner most cortical layer, the zona reticularis produces androgens, mainly dehydroepiandrosterone (DHEA) DHEA sulfate (DHEA-S), and androstenedione (the precursor to testosterone) in humans.[4]

Medulla

The adrenal medulla is the core of the adrenal gland, and is surrounded by the adrenal cortex. It secretes approximately 20% norepinephrine and 80% epinephrine.[4] The chromaffin cells of the medulla, named for their characteristic brown staining with chromic acid salts, are the body's main source of the circulating catecholamines adrenaline (epinephrine) and noradrenaline (norepinephrine). Catecholamines are derived from the amino acid tyrosine and these water-soluble hormones are the major hormones underlying the fight-or-flight response.

To carry out its part of this response, the adrenal medulla receives input from the sympathetic nervous system through preganglionic fibers originating in the thoracic spinal cord from T5–T11.[5] Because it is innervated by preganglionic nerve fibers, the adrenal medulla can be considered as a specialized sympathetic ganglion.[5] Unlike other sympathetic ganglia, however, the adrenal medulla lacks distinct synapses and releases its secretions directly into the blood.

Cortisol also promotes epinephrine synthesis in the medulla. Produced in the cortex, cortisol reaches the adrenal medulla and at high levels, the hormone can promote the upregulation of

phenylethanolamine N -methyltransferase (PNMT), thereby increasing epinephrine synthesis and secretion.[3]

8. The process of conduction of nerve impulse.

NERVE CONDUCTION

None of our perceptions, thoughts, or memories would be possible without nerve conduction, the process by which nerve impulses are propagated along our neurons.

Nerve conduction is an electrochemical process, which means that it uses electricity made with chemical molecules. In other words, the electricity in the brain is not produced by electrons flowing the way they do through a household electrical wire. Instead, the brain’s electricity is caused by the movements of electrically charged molecules through the neurons’ membranes.

The membrane of a neuron, like that of any other cell, contains tiny holes known as channels. It is through these channels that charged molecules pass through the neural membrane.

But unlike the channels in other cells, the channels in neurons are so specialized that they can coordinate the movements of these charges across the membrane so as to conduct nerve impulses. The following diagram shows in simplified form the sequence of events by which a nerve impulse is conducted (click on step numbers 2 and 3 to see the corresponding steps).

Scientists know a great deal about the charged molecules that generate nerve impulses and the sequence of their movements.

But conduction of a nerve impulse down a single neuron would serve no purpose were it not for the other major component of neuronal communication: the synaptic transmission that lets the impulse pass from one neuron to the next.

SYNAPTIC TRANSMISSION

The brain’s great computational abilities are derived from the communication among its billions of nerve cells. But the process of neural conduction that lets a nerve impulse propagate down a neuron would serve no purpose if it were not coupled with another mechanism: the synaptic transmission that lets the impulse pass from one neuron to the next.

At the synapse between two neurons, they do not actually touch each other. They therefore need to secrete chemical messengers that travel from one neuron to the other to regenerate the nerve impulse.

9. Various phases of oogenesis that form a mature ovum from primordial germ cells.

Oogenesis

Oogenesis is a complicated process as apart from the meiotic division enormous amount of growth and differentiation of egg cytoplasm takes place. During oogenesis the cells of germinal epithelium detach from the surface epithelium and enter the cortex of the ovary. These germinal cells are diploid and are called primordial germ cells. They pass through three stages:

Phase of Multiplication: The primordial germ cells undergo repeated mitotic divisions to form egg mother cell or oogonium.

Phase of Growth: The period of growth in the female gamete is very prolonged and tremendous growth occurs as the egg contributes greater part in the embryonic development. The progressive growth increases in nuclear as well as cytoplasmic substances of egg mother cells now called as primary oocytes in two stages:

o previtellogenesis growth period in which no synthesis and accumulation of food reserve material, the yolk take place, but an increase in the volume of nucleus and cytoplasm of primary oocyte occurs. The nucleus remains in a prolonged meiotic prophase. During this period various genetical events such as synapsis, duplication, chiasmatic formation and crossing over takes place between the homologus chromosomes. There is qualitative and quantitative increase in the amount of cytoplasm. The mitochondria increase in number, the network of endoplasmic reticulum with ribosomes become more complicated, the Golgi bodies manufacture cortical granules, besides their normal function.

o vitellogenous growth period is the process of formation and deposition of yolk called vitellogenesis. This is either synthesized by the cytoplasm itself or may be formed outside and then taken by oocyte called as exogenous yolk.

Phase of Maturation: The primary oocyte completes first meiotic division generally called as first maturation division. This is an equal nuclear division and an unequal cytoplasmic division, so it result in one large cell called as secondary oocyte and other small cell called first polar body or polocyte. The secondary oocyte again undergoes second maturation division resulting in one secondary polocyte and a large sized ootid. The primary polocyte may or may not divide to form secondary polocytes. These and the other secondary polocyte later disintegrate. The ootid undergoes further differentiation to form the mature ovum as functional female gamete.

12. The role of various hormones in the physiology of male and female reproduction.

Hormonal Control of the Male Reproductive System

In males, the FSH and LH secreted in response to GnRH are both required for normal spermatogenesis. Each acts on a distinct type of cell in the testis . FSH promotes the activity of Sertoli cells. Within the seminiferous tubules, these cells nourish developing sperm. LH regulates Leydig cells, cells located in the interstitial space between the seminiferous tubules. In response to LH, Leydig cells secrete testosterone and other androgens, which promote spermatogenesis in the tubules. Both androgen secretion and spermatogenesis occur continuously from puberty onward. Two negative-feedback mechanisms control sex hormone production in males. Testosterone regulates blood levels ofGnRH, FSH, and LH through

inhibitory effects on the hypothalamus and anterior pituitary. In addition, inhibin, a hormone that in males is produced by Sertoli cells, acts on the anterior pituitary gland to reduce FSH secretion. Together, these negative-feedback circuits maintain androgen production at optimal levels.

Ovarian Hormones

Oestrogen:

Stimulates the development of the endometrium (lining of the uterus) and it associated blood supply.

During the first half of the cycle there is positive feedback through increased sensitivity of the follicle cells to FSH (Up-regulation of receptors on the follicular cell plasma membrane).

During the second half of the cycle (high oestrogen) there is negative feedback on FSH and LH.

Progesterone:

maintains the lining of the endometrium

negative feedback on FSH and LH.

Q-2 Describe in short

1) Spermiogenesis

Spermiogenesis (spermatohistogenesis)

The differentiation of the spermatids into sperm cells is called spermiogenesis. It corresponds to the final part of spermatogenesis and comprises the following individual processes that partially proceed at the same time:

Nuclear condensation: thickening and reduction of the nuclear size, condensation of the nuclear contents into the smallest space.

Acrosome formation: Forming a cap (acrosome) containing enzymes that play an important role in the penetration through the pellucid zone of the oocyte.

Flagellum formation: generation of the sperm cell tail. Cytoplasma reduction: elimination of all unnecessary cytoplasm.

Classification of Eggs and Types of Cleavage

Classification by yolk content

Isolecithal - little yolk, evenly distributed

Mesolecithal - moderate yolk

Telolecithal - large amounts of yolk

Yolk content affects how cells divide

Holoblastic cleavage - complete division

Meroblastic cleavage - incomplete division

3)Homeostasis

The tendency of an organism or a cell to regulate its internal conditions, usually by a system of feedback controls, so as to stabilize health and functioning, regardless of the outside changing conditions.

1. stimulus2. stimulus effect on body3.* receptor: receives specific signal4.* afferent transmission: sends signal5.* integration center: processes signal6.* efferent transmission: sends signal7.* effector: effects action8. response9. response effect on body

4)Name four types of neuroglial cells and their functions

The types of neuroglia cells Three main groups of neuroglia cells have been identified: (1) astrocytes, subdivided into fibrous and protoplasmic types; (2) oligodendrocytes, subdivided into interfascicular and perineuronal types; and sometimes (3) microglia. Fibrous astrocytes are prevalent among myelinated nerve fibres in the white matter of the central nervous system. Organelles seen in the somata of neurons are also seen in astrocytes, but they appear to be much sparser. These cells are characterized by the presence of numerous fibrils in their cytoplasm. The main processes exit the cell in a radial direction (hence the name astrocyte, meaning "star-shaped cell"), forming expansions and end feet at the surfaces of vascular capillaries. Unlike fibrous astrocytes, protoplasmic astrocytes occur in the gray matter of the central nervous system. They have fewer fibrils within their cytoplasm, and cytoplasmic organelles are sparse, so that the somata are shaped by surrounding neurons and fibres. The processes of protoplasmic astrocytes also make contact with capillaries. Oligodendrocytes have few cytoplasmic fibrils but a well-developed Golgi apparatus. They can be distinguished from astrocytes by the greater density of both cytoplasm and nucleus, the absence of fibrils and glycogen in the cytoplasm, and large numbers of microtubules in the processes. Interfascicular oligodendrocytes are aligned in rows between the nerve fibres of the white matter of the central nervous system. In gray matter perineuronal oligodendrocytes are located in close approximation with the somata of neurons. In the peripheral nervous system, neuroglia that are equivalent to oligodendrocytes are called Schwann cells. Microglial cells are small, crenate cells with dark cytoplasm and a dark nucleus. It is uncertain whether they are merely damaged neuroglial cells or occur as a separate group in living tissue.

5)Foetus

A fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.

In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.

6)Cleavage

 The series of mitotic cell divisions by which a single fertilized egg cell becomes a many-celled blastula. Each division produces cells half the size of the parent cell.

Classification by yolk content

Isolecithal - little yolk, evenly distributed

Mesolecithal - moderate yolk

Telolecithal - large amounts of yolk

Yolk content affects how cells divide

Holoblastic cleavage - complete division

Meroblastic cleavage - incomplete division

7) Vitellogenesis

Vitellogenesis (also known as yolk deposition) is the process of yolk formation via nutrients being deposited in the oocyte, or female germ cell involved in reproduction. It starts when the fat body stimulates the release of juvenile hormones and produces vitellogenin protein. It occurs in all animal groups lower than the mammals. In cockroaches, for example, vitellogenesis can be stimulated by injection of juvenile hormone into immature females and mature males.In mosquitoes infected with Plasmodium, vitellogenesis may be manipulated by the parasites to reduce fecundity (Hurd, 2003).

8)Principles of experimental embryology

Descriptive embryology and evolutionary embryology both had their roots in anatomy. At the end of the nineteenth century, however, the new biological science of physiology made inroads into embryological research. The questions of “what?” became questions of “how?” A new generation of embryologists felt that embryology should not merely be a guide to the study of anatomy and evolution, but should answer the question, “How does an egg become an adult?” Embryologists were to study the mechanisms of organ formation (morphogenesis) and differentiation. This new program was called Entwicklungsmechanik, often translated as “causal embryology,” “physiological embryology,” or “developmental mechanics.” Its goals were to find the molecules and processes that caused the visible changes in embryos. Experimentation was to supplement observation in the study of embryos, and embryologists were expected to discover the properties of the embryo by seeing how the embryonic cells responded to perturbations and disruptions. Wilhelm Roux (1894), one of the founders of this branch of embryology, saw it as a grand undertaking:

We must not hide from ourselves the fact that the causal investigation of organisms is one of the most difficult, if not the most difficult, problem which the human intellect has attempted to solve … since every new cause ascertained only gives rise to fresh questions regarding the cause of this cause.

In this chapter, we will discuss three of the major research programs in experimental embryology. The first concerns how forces outside the embryo influence its development. The second concerns how forces within the embryo cause the differentiation of its cells. The third looks at how the cells order themselves into tissues and organs.

9)Cortical reactions

The cortical reaction occurs in fertilisation when a sperm cell unites with the egg's plasma membrane, (zona reaction).This reaction leads to a modification of the zona pellucida that blocks polyspermy; enzymes released by cortical granules digest sperm receptor proteins ZP2 and ZP3 so that they can no longer bind sperm, in mammals.

The cortical reaction is exocytosis of the egg's cortical granules. Cortical granules are secretory vesicles that reside just below the egg's plasma membrane. When the fertilizing

sperm contacts the egg plasma membrane, it causes calcium to be released from storage sites in the egg, raising the intracellular free calcium concentration. This triggers fusion of the cortical granule membranes with the egg plasma membrane, liberating the contents of the granules to the extracellular space. Fusion begins near the site of sperm contact, and then as the wave of calcium release sweeps around the egg, a wave of cortical granule fusion results. The contents of the granules vary with the species, and are not fully understood.

In the well-studied sea urchin model system, the granule contents modify a protein coat on the outside of the plasma membrane (the vitelline layer) so that it is released from the membrane. The released cortical granule proteins exert a colloid osmotic pressure causing water to enter the space between the plasma membrane and the vitelline layer, and the vitelline layer expands away from the egg surface. This is easily visible through a microscope and is known as "elevation of the fertilisation envelope". Some of the former granule contents adhere to the fertilisation envelope, and it is extensively modified and cross-linked. As the fertilisation envelope elevates, non-fertilizing sperm are lifted away from the egg plasma membrane, and as they are not able to pass through the fertilisation envelope, they are prevented from entering the egg. Therefore, the cortical reaction prevents polyspermic fertilisation, a lethal event. Another cortical granule component, polysaccharide-rich hyalin, remains adherent to the outer surface of the plasma membrane, and becomes part of the hyaline layer.

This is considered the slow block to multiple fertilisation at an animal egg cell.

10)Pancrease

Definition

The pancreas is an organ important in digestion and blood sugar regulation. It is considered to be part of the gastrointestinal system. The pancreas produces digestive enzymes to be released into the small intestine to aid in reducing food particles to basic elements that can be absorbed by the intestine and used by the body. It has another very different function in that it forms insulin, glucagon and other hormones to be sent into the bloodstream to regulate blood sugar levels and other activities throughout the body.

Description

The pancreas is a pear-shaped organ about 6 in (15 cm) long located in the middle and back portion of the abdomen. It is connected to the first part of the small intestine, the duodenum, and lies behind the stomach. The pancreas is made up of glandular tissue, or cells that form substances to be secreted outside of the organ. Glandular tissues can be categorized as endocrine (secreting directly into the bloodstream or lymph) or exocrine (secreting into another organ). The pancreas is both an exocrine and an endocrine organ.

11)Thyroid gland

The thyroid gland covers the windpipe from three sides. The hormones of the Thyroid gland, T3 & T4, help the body to produce & regulate adrenaline, ephinephrine, and dopamine; all three of which are active in brain chemistry. Other hormones from this gland also help regulate metabolism. Without a functional thyroid, the body would not be able to break down proteins, and it would not be able to process carbohydrates and vitamins. For this reason, glandular problems can lead to uncontrollable weight gain. For many people, these irregularities can be controlled through medication, as well as an attention to their diet. However, there is one other controlling factor. The gland cannot produce hormones on its own. It needs the assistance of the pituitary gland, which creates thyroid stimulating hormone (TSH). As a result, a nonfunctional pituitary gland will eventually lead to thyroid-gland-related issues. TSH will either trigger the production of thyroxine and triiodothronine. If TSH is not present at the right levels, too much or too little of either hormone will be made.

12)Epigenesis

EPIGENESIS refers to the influence of the environment on the expression of the genetic code. Many genes require specific environmental circumstances in order to be expressed; many genes are never expressed. The genetic "program" refers to the potential for initiating

and orchestrating specific physiological processes that MAY ultimately manifest themselves in a trait such as behavior.

13)Preformation

In the history of biology, preformationism (or preformism) is the idea that organisms develop from miniature versions of themselves.

Instead of assembly from parts, preformationists believe that the form of living things exist, in real terms, prior to their development.[1] It suggests that all organisms were created at the same time, and that succeeding generations grow from homunculi, or animalcules, that have existed since the beginning of creation.

Epigenesis [2] , then, in this context, is the denial of preformationism: the idea that, in some sense, the form of living things comes into existence. As opposed to "strict" preformationism, it is the notion that "each embryo or organism is gradually produced from an undifferentiated mass by a series of steps and stages during which new parts are added." (Magner 2002, p. 154) [3] This word is still used, on the other hand, in a more modern sense, to refer to those aspects of the generation of form during ontogeny that are not strictly genetic, or, in other words, epigenetic.

The historical ideas of preformationism and epigenesis, and the rivalry between them, are obviated by our contemporary understanding of the genetic code and its molecular basis together with developmental biology and epigenetics.

14)Morphogenetic Movements

Morphogenetic Movements

Gastrulation is complicated! Because of this, it is helpful to break the movements of gastrulation down into their component events wherever possible. In general, sheets of cells can engage in only a limited number of morphogenetic movements. This "morphogenetic repertoire" is helpful to keep in mind when we are presented with what seems to be an incomprehensible change in the shape of the embryo. Through careful observation and experimental manipulation that will be discussed in this section, gastrulation can be analyzed in convenient organisms such as amphibians. On this screen and the next, the various major morphogenetic movements that occur during gastrulation in diverse organisms are schematically represented. Some of these movements are only performed by epithelial cells, while others can be performed by both bona fide epithelial cells and by deeper, non-epithelial cells that nevertheless behave as integrated sheets of cells. The latter are poorly understood, but are common in amphibians as well as in higher vertebrates.

Click on a cell movement below to get a fuller description of what is going on.

Invagination Ingression Involution

Invagination

During invagination, an epithelial sheet bends inward to form an inpocketing. One way to think of this in three dimensions is to imagine that you are poking a partially deflated beach ball inward with your finger. The resulting bulge or tube is an invagination. If the apical side of the epithelium forms the lumen (central empty space) of the tube, then the movement is termed invagination. If the lumen is formed by basal surfaces, then the movement is termed an evagination.Return to top of page.

Ingression

During ingression, cells leave an epithellial sheet by transforming from well-behaved epithellial cells into freely migrating mesenchyme cells. To do so, they must presumably alter their cellular architecture, alter their program of motility, and alter their adhesive relationship(s) to the surrounding cells. Neural crest cell are an example of a mesenchymal cell type that emigrates out of an epithelium (do you kno which one?).Return to top of page.

Involution

During involution, a tissue sheet rolls inward to form an underlying layer via bulk movement of tissue. One helpful image here is of a tank tread or conveyor belt. As material moves in from the edges of the sheet, material originally at the sites of inward rolling (shown in blue here) is free to move further up underneath the exterior tissue. Return to top of page.

15)Meroblastic Cleavage

In eggs with more yolk, cleavage cannot cut through the large mass of yolk, and so the egg does not completely divide with each cleavage. This is known as meroblastic cleavage (from the Greek “meros,” meaning “part”). In meroblastic cleavage, what ultimately happens is that a clump of dividing cells sits atop the undivided yolk.

In birds, reptiles, most fishes, some amphibians, cephalopod mollusks, and monotreme mammals (all of which have telolecithal eggs), so much yolk is present that cleavage can occur only in a narrow disk at the extreme animal end of the egg. As the embryo grows, blood vessels penetrate into the yolk. The yolk is gradually broken down and absorbed, but it never divides. In insects with centrolecithal eggs, cleavage occurs in a ring surrounding the central, uncleaved yolk. As the embryo develops, the yolk is gradually broken down and absorbed.

A zebrafish (Phylum Chordata) has telolecithal eggs and meroblastic cleavage.Two cleavage divisions have occurred in this egg. The animal pole (left) has

divided into four cells, but the vegetal pole (right) has not divided at all.

16)Contraception

Definition: The intentional prevention of conception through the use of various devices, sexual practices, chemicals, drugs, or surgical procedures. This means that something (or some behavior) becomes a contraceptive if its purpose is to prevent a woman from becoming pregnant. There are several types of contraceptives that have been officially labeled as such because they have shown reliability in preventing conception from occurring

Reversible Methods of Birth Control

Intrauterine Contraception

Copper T intrauterine device (IUD) Levonorgestrel intrauterine system (IUS)

Hormonal Methods

Implant Injection or "shot" Combined oral contraceptives Progestin only pill Patch Hormonal vaginal contraceptive ring Emergency contraception

Barrier Methods

Male condom

Female condom

18)Saltatory Conduction

Saltatory conduction (from the Latin saltare, to hop or leap) is the propagation of action potentials along myelinated axons from one node of Ranvier to the next node, increasing the conduction velocity of action potentials without needing to increase the diameter of an axon.

Mechanism

Because the cytoplasm of the axon is electrically conductive, and because the myelin inhibits charge leakage through the membrane, depolarization at one node of Ranvier is sufficient to elevate the voltage at a neighboring node. Thus, the voltage at the first node of Ranvier extends spatially to the next node of Ranvier. At each successive node, the membrane potential of the axon is thereby brought to the threshold potential to initiate an action potential. Ions need only to cross the axon membrane to propagate the action potential at the nodes, but not anywhere under the myelin along the axon. Thus in myelinated axons, action potentials do not propagate continuously as waves, but instead recur at successive nodes, and in effect "hop" along the axon, by which process they travel faster than they would otherwise. (The action potential only moves in one direction, because the sodium channels at the previous node of Ranvier are inactivated, and cannot regenerate another action potential, even when depolarized.) In summary, the charge will passively depolarize the adjacent node of Ranvier to threshold, triggering an action potential in this region and subsequently depolarizing the next node, and so on. This phenomenon was discovered by Ichiji Tasaki[1][2] and Andrew Huxley[3] and their colleagues.

Energy efficiency

Apart from increasing the speed of the nerve impulse, the myelin sheath helps in reducing energy expenditure as the area of depolarization and hence the amount of sodium/potassium ions that need to be pumped to bring the concentration back to normal, is decreased.

Distribution

Saltatory conduction had been found exclusively in the myelinated nerve fibers of vertebrates, but was later discovered in a pair of medial myelinated giant fibers of Fenneropenaeus chinensis and Marsupenaeus japonicus,[4][5][6] as well as a median giant fiber of an earthworm.[7] Saltatory conduction has also been found in the small- and medium-sized myelinated fibers of Penaeus shrimp.

19)Infertility

Infertility is defined as a couple not being able to conceive while having frequent, unprotected sex for at least a year. There are many causes of infertility in both men and women, as well as possible treatment options to allow for successful conception. When a couple suspects they may be infertile, the first thing to do is to see a doctor. Most likely, the couple will be referred to a reproductive endocrinologist, who specializes in infertility. The RE will proceed to assess the couple and perform tests of their reproductive systems. When the cause of infertility has been identified, the reproductive endocrinologist will propose a course of action for treatment.

Types

The two main types of infertility are:- Primary Infertility, in which a couple has never been able to conceive.- Secondary Infertility, in which a couple has already conceived in the past and carried to full term, but is suddenly unable to conceive again.There are a few more classifications that break down the types of infertility even further. For example, there is "Combined Infertility," which is characterized by both the male and female partner having fertility disorders, thus increasing the difficulty in conceiving. About 15% of infertile couples are said to have "Unexplained Infertility," which means that neither partner has shown any abnormalities in their reproductive system, but still cannot conceive. This can be due to many reasons, such as poor quality of the woman's eggs, sub-optimal ovulation timing and the egg not reaching the fallopian tubes.

20)Father of modern embryology

Caspar Friedrich Wolff (German) is credited as the "Father of Embryology," even though he did not first conceptualize epigenesis.

Actually, Aristotle (384-322 B.C.) was one of the first to champion the theory of epigenesis. He described the two historically important models of development known as "preformation" and "epigenesis".

According to "preformation" theory, an embryo or miniature individual preexists in either the mother's egg or the father's semen and begins to grow when properly stimulated.

Aristotle actually favored the theory of epigenesis, which assumes that the embryo begins as an undifferentiated mass and that new parts are added during development.

Aristotle thought that the female parent contributed only unorganized matter to the embryo. He argued that semen from the male parent provided the "form," or "soul", that guided development and that the first part of the new organism to be formed was the heart.

Differentiate between

1) Spermatogenesis and oogenesis

1. Spermatogenesis takes place in the testes of males, oogenesis takes place in the ovary of the females.

2. Spermatogenesis produces small, motile spermatozoa whereas in oogenesis the ovum is spherical, not motile and is much larger with more food reserves and cytoplasm.

3. Spermatogenesis involves a metamorphosis stage called spermiogenesis, in oogenesis there is no metamorphosis stage.

4. In spermatogenesis, 4 gametes are produced from each meiotic division, whereas in oogenesis there is only 1 gamete produced from each division as the unequal cytokinesis leads to the formation of polar bodies.

5. Spermatogenesis occurs in males (human males) continuously from puberty to death whereas oogenesis starts in females at puberty and then occurs on a monthly basis until the menopause. It takes 70 days for sperm to be produced in males.

6. In oogenesis, the development of the oocyte is arrested at prophase I of meiosis until puberty, and then stops at metaphase II of meiosis until fertilisation, where the meiotic division is finally completed. In males meiosis occurs continuously

7. The hormones in the 2 processes have different effects - in oogenesis, FSH is responsible for the choice of the primary oocyte and causes the cells of the membrana granulosa to proliferate to form the theca interna which secretes oestrogen, and the theca externa. LH in females stimulates ovulation and the maturation of the Graafian follicle. In males, on the other hand, FSH increases the activity of the Sertoli cells which are involved in spermiogenesis (the metamorphosis stage of spermatogenesis). LH in males stimulates the Leydig cells to secrete testosterone.

2) Animal Pole and Vegetal Pole

The part of egg which has more concentration of yolk is the vegetal pole and the part with less concentration of yolk is called the animal pole.

1. Discoblastula and coeloblastua

A coeloblastula usually arisesby holoblastic radial cleavage as a hollow ball of cells, its wall usually one cell thick.

Meroblastic cleavage generally results in the arrangement of the blastomeres as a plate of cells over the animal pole; such an embryo is a discoblastula.

Fig. Discoblastula

Fig. Coeloblastula

3)Spermatogenesis and Spermeogenesis

Spermatogenesis

Spermatogenesis is a serial event that finally produces millions of completely matured fast swimming sperms from primary sperm cells. Each primary cell undergoes different stages and finally becomes a complete sperm cell with a wagging tail and a piercing acrosome. Spermatocytogenesis, Spermatidogenesis, Spermiogenesis, and Spermiation are the four main steps of spermatogenesis. Spermatocytogenesis starts from the diploid spermatogonium cells, and those become primary spermatocytes at the end of this stage after undergoing through mitosis. Spermatidogenesis is the second stage of the main process where primary spermatocytes produced from the previous step become secondary spermatocytes after going through meiosis – 1. The second phase of this step produces haploid spermatids through meiosis – 2 from secondary spermatocytes. Spermiogenesis is a very crucial stage of spermatogenesis where the facilitation takes place, and it proceeds to the final stage of spermiation. Finally, the well-developed and fully functional sperms are produced inside the male reproductive system. The initial stages of spermatogenesis occur in the testes and then the spermatids progress to the epididymis for spermiogenesis. In brief, the genetic composition of the primary sperm cells change from diploid to haploid status during spermatogenesis, and it is a process that takes place in stages. The number of cells increase due to mitosis and meiosis occur during the process.

Spermiogenesis

Spermiogenesis is one of the very crucial steps in spermatogenesis, and it is the time when sperms are facilitated by organelles, and form the characteristic structure of each sperm. The resulted spermatids from the previous stage are more or less circular in shape, and each contains the genetic materials with centrioles, mitochondria, and Golgi bodies. The arrangement of those organelles are arranged in such a way that the sperm would be able to penetrate all the obstacles could be conquered. The acrosome is formed at one end of the cell by secreting enzymes from the Golgi bodies and the mitochondria are concentrated at the other end of the cell forming the mid piece. The Golgi complex then covers the condensed genetic materials and the acrosome. Tail formation is the next phase of spermiogenesis, and one of the centrioles is extended to become the tail of the sperm. It is interesting to know that the tail is oriented towards the lumen of the seminiferous tubule. During this stage, the genetic materials do not undergo changes but becomes condensed and protected. The shape of the cell is changed into more like an arrow with a long tail and a defined head.

6)Emboly and Epiboly

Epibolic morphogenetic movements (Epiboly)- The epibolic morphogenetic movements occur only in the prospective ectodermal blastomeres, which have a inherent property of flattening and forming a cohering epithelial layer. Thus, epiboly may be referred to the motility displayed by amoebocytes. Further, besides flattening, due to multiplication and rearrangement of ectodermal cells, the ectoderm expands and extends in the antero-posterior direction and eventually, envelops the inwardly migrating prospective mesodermal and endodermal blastomeres. Such type of morphogenetic movement of ectodermal cells has been observed in holoblastic eggs (e.g., Porifera, cephalochordata and Amphibia) as well as, meroblastic eggs, during the gastrulation. In the rounded blastula of Amphioxus, frog, etc., the tendency of prospective ectodermal cells to extend antero-posteriorly produces an enveloping movement in the antero-posterior direction. As a result, the prospective ectodermal blastomeres, actually engulf and surround the inwardly moving presumptive

notochordal, mesodermal blastomeres. In flattened blastulae of teleost fishes, reptiles, and birds, epibolic morphogenetic movements are concerned largely with antero-posterior extension, associated with peripheral migration and expansion of the ectodermal blastomerres.

Embolic morphogenetic movements (Emboly)- The embolic morphogenetic movements are concerned with inward migration of prospective chorda-mesodermal and endodermal blastomeres from the external surface of the blastula and their extension along antero-posterior axis of the developing embryo. The tendency of inward migration of mesodermal and endodermal cells is inherent to these cells and there is a convincing reason for such a peculiar behaviour of these cells. Balinsky (1970) has explained the reason, that why do these blastomeres move inward, not to the outward direction in following manner: "................. in the stage preceding gastrulation (the blastula stage) the cells are in the form of an epithelium (the blastoderm) which is more or less distinctly polarized. As in typical epithelium, it has a distal surface, in contact with the external environment, which is different from the proximal surface facing the inner miliue of the embryo. Now it has been noticed (Gundernatsch, 1913), epithelium as ameboid or mesenchyme cells, the direction of movement is nearly always in the proximal end of the cell that is more likely to start forming pseudopodia and embarking an ameboid locomotion, and that is why in an intact embryo the movement of both mesoderm ane endoderm is inward and not outward."

7)Protostomes and Deuterostomes

Protosomes

1. The animals in which the mouth develops first and anus is formed later are called protostomes.

2. Cleavage is spiral and determinate types.

Deuterostomes:

1. The animals in which the anus develops first in the embryo followed by development of mouth are called deuterostomes.

2. Cleavage is radial.

8)Synergistic and Antagonistic Effect

Antagonistic effect

Many cells use more than one second messenger. In heart cells, cAMP serves as a second messenger, speeding up muscle cell contraction in response to adrenaline, while cyclic guanosine monophosphate (cGMP) serves as another second messenger, slowing muscle contraction in response to acetylcholone. It is in this way that the sympathetic and parasympathetic nervous systems achieve antagonistic effect on heartbeat. Another example

of antagonistic effect is insulin, which lowers blood sugar level, and glucagons, which raises it.

Synergistic effect

Another type of hormonal interaction is known as synergistic effect. Here, two or more hormones complement each others actions and both are needed for full expression of the hormone effects. For example, the production, secretion and ejection of milk by mammary glands require the synergistic effects of estrogens, progesterone, prolactin and oxytocin.

10)Electrical and Chemical synapse

An electrical synapseAn electrical synapse is a mechanical and electrically conductive link between two abutting neuron cells that is formed at a narrow gap between the pre- and postsynaptic cells known as a gap junction. Each gap junction contains numerous gap junction channels which cross the membranes of both cells. With a lumen diameter of about 1.2 to 2.0 nm, the pore of a gap junction channel is wide enough to allow ions and even medium sized molecules like signaling molecules to flow from one cell to the next thereby connecting the two cells' cytoplasm. Thus when the voltage of one cell changes, ions may move through from one cell to the next, carrying positive charge with them and depolarizing the postsynaptic cell.Gap junction channels are composed of two hemi-channels called connexons in vertebrates, one contributed by each cell at the synapse.

A chemical syapseThe space between a chemical syapse is much larger than an electrical synapse. The release of a neurotransmitter is triggered by the arrival of a nerve impulse (or action potential) and occurs through an unusually rapid process of cellular secretion, also known as exocytosis: Within the pre-synaptic nerve terminal, vesicles containing neurotransmitter sit "docked" and ready at the synaptic membrane. The arriving action potential produces an influx of calcium ions through voltage-dependent, calcium-selective ion channels. Calcium ions then trigger a biochemical cascade which results in vesicles fusing with the presynaptic-membrane and releasing their contents to the synaptic cleft.

An electrical synapse is faster than a chemical synapse but chemical synapases are far more common.