* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Embryology
Survey
Document related concepts
Transcript
Introduction & Gametogenesis INTRODUCTION Embryology : Study of prenatal stages of development from the fertilization of the ovum to the birth of the new individual. Why and How Gross Form, Correlation Embryology Errors in development Structural Molecular (Clinical aspects) Cellular and Molecular phenomena Evolutionary mirror Sexual Reproduction Vast potential for variation Basis for evolution ◦ Genes and gene pools ◦ Population genetics However… Requires special cells : gametes ◦ Spermatozoa ◦ Ova (Oöcytes) Overview : ◦ Reproductive system – male and female ◦ Two types of cell division Descriptive terms of Position, Direction and Planes Cell and Cell Division Some familiarity with cell biology especially with reference to the nucleus is presumed here. Some important concepts : Chromatin : Coloured (when stained!) material in the nucleus, comprising nucleic acids and proteins. Chromosomes : supercoiled DNA with proteins, arranged as ‘coloured sticks’. Chromosomes are visible as discrete structures only during cell division. Each chromosome has two ‘arms’, short and long, with a small enlargement, the centromere, between the two. All nucleated cells in the human body, with the exception of the final stages of gametogenesis, have 23 pairs of chromosomes in their nuclei. Two members of a pair are known as homologous chromosomes. In each pair, one chromosome is paternal, one maternal. One pair of chromosomes (#23, X + Y) is designated sex chromosomes. Males have X + Y, females have X + X. A cell with 23 pairs of chromosomes is described as diploid; one with 23 single chromosomes is termed haploid. The genetic material in the nucleus of a cell undergoes ‘replication’ before cell division. This phase in a cell’s life is designated the ‘S’ phase (‘s’ for synthesis). After the S phase there is a quiescent period followed by cell division. Cell and Cell Division All body cells that can divide, with the exception of a stage of gametogenesis, divide by ‘mitosis’. Mitotic cell division produces two daughter cells which are genetically identical with the parent cell. During gametogenesis, there is a stage when a two-stage division produces four haploid daughter cells. This is meiosis or meiotic division, also called a reduction division. However, there is more to meiosis than mere reduction to the haploid state. The following slides present the phenomena associated with chromosomes during cell division. Details of the stages of cell division do not feature in these. The focus is on nuclear events, cytoplasmic details are not shown. A few more terms and concepts will be introduced in the course of the description. Mitosis The chromosomes have replicated prior to division, but not yet seen as discrete structures. The nuclear envelope is intact. A B Mitosis In each pair, there is one maternal chromosome and one paternal. A chromosome, when fully condensed may be shown as in ‘A’. It has two identical halves called chromatids, shown at ‘B’. The most important point to be understood here is that each chromosome has replicated. A chromosome in a nondividing cell is a chromatid, if at all we could see it! Only two chromosome pairs are shown in the nucleus in these pictures. One pair is shown in pink, one in blue. They are still being condensed. Mitosis During metaphase, the chromosomes are arranged along a plane in the middle of the cell (‘equatorial plane’). A ‘spindle’ of microtubules (mitotic spindle) attaches the chromosomes to the centrosomes. Shortening of the microtubules separates the chromatids. The chromatids (chromosomes of the daughter cells) migrate towards the poles of the dividing cell. Mitosis The cytoplasm divides to create two roughly equal cells. Note how the chromosomes ‘disappear’. Meiosis The picture at top left shows the cell resting after the replication of chromosomes. Once again we see two representative pairs of chromosomes (bottom). In each pair, paternal and maternal chromosomes are shown in different colours. Meiosis is a two-stage division. The two stages are often called meiosis I and meiosis II. We are looking at meiosis I. Meiosis Homologous chromosomes are arranged next to each other lengthwise. Their corresponding arms cross at a number of points. These points of crossing are called chiasmata. At these points of crossing, homologous chromosomes exchange segments. When this exchange is complete, the two homologous chromosomes cease to be what they were. Each one has segments from chromosomes. Maternal : Paternal : both parental The possibilities of chiasmata formation are virtually unlimited. Every cell undergoing meiosis therefore, can give rise to a unique combination of resultant chromosomes. Meiosis Separation of chromosomes now follows a different pattern. Chromatids do NOT separate. One entire replicated chromosome from each pair moves to one pole of the cell. At the end of meiosis I, the two resultant cells are thus haploid, containing one member from each pair. It is worth repeating that the chromosomes of these cells are NOT identical with those of any of the parents. Each of these cells has genes coming from both parents. Considering that the same process has been followed in every meiotic division in the parents of this individual, there has been a great mixing of grandparental genes as well! Meiosis The cells begin the second division soon, without an S phase. Recall that each of these cells is haploid and the chromosomes are replicated. In meiosis II, the chromatids of these chromosomes separate. On the right, we see the beginning of the formation of four cells. Meiosis At the end of meiosis therefore, four unique, haploid cells are formed. MITOSIS MEIOSIS GAMETOGENESIS Oogenesis Spermatogenesis PRIMORDIAL GERM CELLS Gametogenesis • In both sexes, a pool of cells destined for gametogenesis is set aside during embryonic life. • During gametogenesis, these cells divide by mitosis first, to retain a pool of cells. • Essentially, some of the products of these mitotic divisions must undergo meiosis. • The details of the actual process differ in the male and the female. Male Reproductive System • • The principal organ, testis, produces male gametes or spermatozoa. A spermatozoon is cell with very little cytoplasm and nucleus forming its head, a neck with a single, long, spiral mitochondrion and a tail. Spermatozoa produced by the testis are carried by a tube, the ductus deferens to the pelvis where glands add their secretions, forming semen. Semen is ejaculated through the penis and deposited in the female reproductive system. Seminal vesicle Prostate Testis Ductus deferens Spermatogenesis In the male, spermatogenic activity begins at puberty. The spermatogenic cells, called spermatogonia undergo mitotic division, whereby some cells remain as spermatogonia and some take the path of meiosis. Each meiotic division produces four haploid cells called spermatids. Spermatids then undergo structural changes – loss of some cytoplasm, formation of a tail. Spermatogenesis is a continuous process. SPERMATOGENESIS SEMINIFEROUS TUBULE SHOWING ARRANGEMENT OF SPERMATOCYTES SPERMIOGENESIS SPERMIOGENESIS Spermatid to spermatozoon Spermatid is circurlar cell containing nucleus, golgi apparatus, centriole and mitochondria. Nucleus – head Golgi apparatus – acrosomic cap Centriole-div 2 ,one lie in the neck and other give rise to axial filament and the one in the neck forms annulus Mitochondria-sheath around middle piece Female Reproductive System • • • The uterine tube has a funnel-like end facing the ovary, called the infundibulum (meaning a funnel!). The fronds forming its border are the fimbriae. The somewhat dilated portion (*) towards the uterus is the ampulla, followed by the narrow part leading to the uterus. Due to the thickness of the uterine wall the last part of the tube is actually within the uterine wall (intramural part). The uterus has three parts – the fundus (A) at the top, the large body (B) and the cervix [‘neck’, (C)] at its junction with the vagina. Uterine tube * * A B Uterus C Ovary Vagina Oögenesis • Process begins during intrauterine life • Meiosis I incomplete, suspended until puberty • During reproductive life : – Each month several oöcytes begin maturation – Normally only one is released (others degenerate) • “Ovulation” – Meiotic divisions highly unequal – Oöcyte and ‘polar bodies’ – Meiosis completed after the entry of a sperm Oögenesis : Implications • Unequal meiotic divisions produce only one gamete with a huge amount of cytoplasm. The polar bodies are small cells, normally unfertilised. • Long suspension of division probability of abnormal separation of chromosomes – ‘nondisjunction’. • Longer suspension greater chromosomal abnormalities. chances of OOGENESIS MATURATION OF OVUM Primary Ovarian Follicle Antral Follicle SECONDARY OOCYTE AT OVULATION The Menstrual Cycle – 1 • The release of one oöcyte every month has functional rationale. • The uterus must be prepared, in anticipation of fertilisation of the oöcyte, for receiving it and sustaining development for the duration of pregnancy. This preparation involves the lining of the uterus (‘endometrium’ = epithelium + supporting tissues). If the oöcyte is not fertilised the prepared endometrium is discarded. Thus there are coordinated cyclic changes in the ovary and the uterus. All these are controlled by hormones. • The histological details of uterus, ovary and those of their hormonal control will dealt with in the reproductive block. At this stage the salient features are mentioned. • During each cycle, as the endometrium matures, it increases in thickness, blood vessels proliferate and glands develop from the lining epithelium. The Menstrual Cycle – 2 The average duration of the cycle of changes is 28 days, though cycles as short as 20 days or as long as 35 days are normal. In the average 28-day cycle, ovulation takes place around day 14. If the oöcyte is not fertilised, most of the endometrium is discarded. This is accompanied by a significant amount of blood loss through the broken blood vessels. The endometrial tissue with the blood constitutes menstrual flow which lasts 3 to 5 days. Menstrual flow is the only reliable external marker of the cyclic changes. The first day of the flow is thus taken as “Day 1” of the cycle, though this in fact marks the end of the previous cycle. It is noteworthy that the interval between ovulation and the next menstrual period is rather constant around 14 days, whereas that between day 1 and ovulation varies according to the length of the cycle in a paticular individual. COMPARISION • Fertilisation Fertilisation is the union of the male and female gametes. It is pertinent to note that the female gamete has not completed the second meiotic division at the time of release from the ovary, and should be correctly called an oöcyte. In the adjoining picture note that its is surrounded by a thick ‘wall’ called the zona pellucida (‘clear zone’). This is in turn surounded by a layer of supporting cells forming a ‘crown’, the corona radiata. At this stage the zona pellucida also includes the polar body formed at the end of meiosis I (not shown in this picture). • Fertilisation most commonly takes place in the ampulla of the uterine tube. Zona pellucida Corona radiata Acrosome Fertilisation At the tip of the sperm head is an enzyme-containing structure called acrosome. Acrosomal enzymes allow the sperm to ‘bore’ through the zona pellucida. The entry of one sperm causes a molecular reaction in the zona pellucida which prevents the entry of any other sperm. The entry of the sperm is also followed by the completion of the second meiotic division and a second polar body is formed. The ovum now has two ‘pronuclei’ male and female. These soon lose their nuclear membranes and a diploid cell is formed, called the zygote. These events are shown in the next slide where the polar bodies are also shown. Fertilisation thus has some ‘consequences’ : • • • • Restoration of diploidy Chromosomal sex determination Completion of meiosis II in the oöcyte Initiation of the first cell division (cleavage) ABNORMAL GAMETES