Questions And Comments

These questions are asked by students through e-mail. Trough this site they are made available to all students. The most recent questions are at the top of the list,  so that you need not go through the whole document to reach them.

Question 8. Embryology 15^th January 2004

Primordial follicles are in meiotic arrest during prophase of Meiosis I. Is this referred to as the DICTYOTENE stage of the DIPLOTENE stage? Also what's the difference between the two?

When does the second Meiotic arrest occur? Is it after the formation of the Graafian follicle just prior to ovulation or is it before the distention of the fluid-filled antrum to form the Graafian follicle?

Answer

All primordial follicles enter meiosis I during foetal life, and, when they are in the diplotene stage, i.e. the chromosomes are paired and exchanging segments, they go into the restng phase, which is known as the dictyotene stage. They are released from this arrest as the follicles begin to undergo maturation FSH to become secondary oocytes. Therefore, the dictyotene stage is only present in human oocytes, and oocytes of other mammals. When the oocyte responding to FSH completes the first meiotic division, within a secondary follicle, it becomes a secondary oocyte. This is theoretically ready to proceed straight to meiosis II, as occurs in spermatocytes. However, they go into a second meiotic arrrest, until the follicle is mature and ruptures. The purpose of the second meiotic arrest is, therefore, to give time to the follicle to secrete enough oestrogens for endometiral stimulation. This fact is very important in assisted fertilization, when the time of retrieval of oocytes is regulated by hormones, to the requires time. The stimulus for release from the second meiotic arrest is sperm penetration of the vitelline membrane.

Question7.

When saying lysosomal contents are heterogenous are we saying that contents of ONE lysosome are heterogeneous or that contents between differentlysosomes are heterogeneous? And if it is that the contents of one lysosome are heterogenous what is thisdue to? (John Schembri)

Answer 

When saying lysosomal contents are heterogenous, we mean that the appearance of the contents in EM varies from one lysosome to another.Each lysosome is a mixed bag of hydrolytic enzymes (over 60 of them) to digest the whole range of biological substrates, but this hydrolytic enzyme kit does not vary from one lysosome to another.This means that there is no specificity of lysosomes for the substrates they digest.

The heterogeneity in the appearance of the lysosomes is due to (a) different substrates being hydrolysed; (b) different stages of hydrolysis; (c) different residual partially digested contents.

Question 6 What is the main difference between a chromatin strand and a chromosome?Thank you in advance.(Carlo Refalo)

Answer

They are both made of basically the same stuff, but organised and packaged in different ways. The chromatin strand represents a secondary coiling.DNA and associated proteins are organised first as a beaded chain of nucleosomes. These are further coiled (secondary coiling) to form chromatin strands, which are distinctly visible in high resolution electron micrographs of nuclei or of chromosomes. In nuclei they form irregular masses of heterochromatin and euchromatin.In chromosomes they are more neatly organised in discrete packages – the chromosomes.Note, however, that each chromosome is a duplicated structure, consisting of two chromatids, and each chromatid is formed of one continuous chromatin strand. 

Question 5. The chapters from World of the Cell that you recommend us to read for every chapter expand much

more on the subject. For example they describe the mechanisms involved, the enzymes and other details we did not cover during the corresponding lectures. Do we need to know these details or is it just a background for the notes you give us in order to understand them better? (Jossef Zahra)

Answer.

I have taken the trouble to write out a set of“notes” that represent the core knowledge that is expected from every student.Further details are not required for the purpose of your course.You may, therefore, safely omit those sections that contain more detail, but some background reading is important to grasp the subject better.I recommended the “World of the Cell” for general reading on the subject, but it is certainly not necessary to read through the whole of each chapter.Of course there is no upper limit to learning, and it is up to individual students to decide whether to read beyond what is required, as outlined in my web presentations.

As I had told you from the very beginning, my “lectures” are the web presentations.The discussion sessions held in place of lectures are intended to stimulate thought and a deeper understanding of the basic concepts set out in my presentations.They do not cover everything that students are expected to know.

Question 4

Could you please tell me the method by which Sodium Chloride would be absorbed in the cell?

Answer.

Sodium chloride in aqueous solution is ionised into Na+ and Cl-.This means that sodium chloride as such does not pass through membranes, but the individual Na+ and Cl- pass through their respective channels according to electrochemical gradients. In addition to the usual Na+ channels, Na+ may betransported across membranes against a electrochemical gradient by means of Na pumps, which incorporate an ATPase to hydrolyse ATP and release the energy to drive this processs. 

Question3.

The lysosomes and proteosomes are described as being electron dense - what does this mean exactly - do they contain electrons?

Answer.

This means that the structures do not allow less electrons to pass through and so cast a "darker" or denser image under the electrom microscope.Electron microscopy(EM) uses a beam of electrons, which is focussed using electromagnetic lenses, and the electron beam is the focussed on to a fluorescent screen, which makes the image visible, or on to a photographic film to obtain an electron micrograph.The light microscope has a limited resolution, limited by the wavelength of visible light.Electrons produce a very high resolution.The beam of electrons has to be focussed, as is the light beam in an optical microscope, andthis can be achieved using 

electromagnetic lenses.Tissue sections to be examined under the EM have to be stained.The EM stains commonly used are osmium tetroxide and uranyl acetate, which bind differentially to different structures in cells.Both stains contain heavy metals, which absorb electrons and do not allow them to pass through.Cell components, such as lysosome contents have a high affinity for unanyl acetate,take up more of this stain, and therefore appears “dark” or electron-dense.Other structures, which have a low affinity and take up very little uranyl acetate, would appear "pale" or electron lucent.In light microscopy we could speak of darker or paler staining of structures when stained with a dye.In EM we speak of electron dense and electron lucent with, of course a whole range of intermediate shades.

Question2.

The objectives of the lecture notes entitled "Membranes" stress thatwe must know what happens to the membrane once it is ruptured - yet thispoint is not expanded in the lecture notes - does this mean that weshall be discussing the issue as a class?

Answer. 

The objective states: "Explain how membranes can repair themselves when damaged".This is an application of the basic knowledge of thephospholipid component of membrane structure, with its hydrophobic and hydrophilic regions. When damaged the exposed fatty acid tails of the membranes, being hydrophobic, are highly unstable.The cut ends will spontaneously merge with one another to form a stable structure.This process does not require utilisation of energy.Of course repair necessitates that the broken ends are close to one another.Extensive damage to the plasma membrane will kill the cells.This is really quite a simple concept but an important one, because of its applicationin themicromanipulation of living cells. 

P.S.If you wish to discuss it further during one of the sessions, just let me know

Question 1.

Membranes (in order to remain functional) must remain just below or above the phase transition temperature? 

Answer.

The phase transition temperature (PTT) can be thought of as the “melting point” of membranes.Above the PTT the membrane is fluid, dynamic and functional; below the PTT the membrane is a more rigid structure with limited mobility of its molecules and therefore decreased function.

Note that the composition and degree of unsaturation of the fatty acid molecules largely determines the PTT.Membranes can adjust these its composition to lower or raise their PTT according to physiological requirements.