Introduction to Human Embryology PDF

Introduction to HUMAN EMBRYOLOGY Fikre Bayu (MSc) study hard 1  Introduction WHAT IS EMBRYOLOGY ? Literally, embryology means the study of embryos; however, the term generally refers to prenatal development of embryos and fetuses It is, therefore, the study of...

Introduction to HUMAN EMBRYOLOGY Fikre Bayu (MSc) study hard 1  Introduction WHAT IS EMBRYOLOGY ? Literally, embryology means the study of embryos; however, the term generally refers to prenatal development of embryos and fetuses It is, therefore, the study of the developmental events that occur during the prenatal period study hard 2  General Embryology Embryology is the study of prenatal development, beginning with gametogenesis up to the formation of full- term infant. study hard 3 Cont.…  EMBRYOLOGY The study of the development of the embryo & the fetus from fertilization to the time of birth. A single fertilized cell divides by mitosis to produce all of the cells in the body. Progression from the fertilized ovum to a full term infant involves a complex sequence characterized by cell growth, differentiation and organization. study hard 5  GENERAL EMBROLOGY The human development starts after the fusion of the female egg cell (secondary oocyte) with the sperm cell that contain haploid number of chromosome. the ovum is released from the follicle of the ovary by the process of ovulation. Then it enters the uterine tube, where fertilization normally takes place in its ampulla within 12-24 hours after ovulation. study hard 6. So embryology is the study of prenatal development  beginning with the formation of the male and female germ cells (gametogenesis) ,containing the union of sperm and egg in the female genital tract to the formation of full-term infant ready to make its way in the outside world. study hard 7 Periods of development Development ☞ Growth (↑ in mass of tissues) ☞ Differentiation (↑ in complexity)  Most development completed by the age of 25  Divided in to 1. Prenatal period – before birth 2. Postnatal period – after birth study hard 8  Prenatal Period  Subdivided in to three 1. Pre-embryonic period, blastula phase or period of blastogenesis 2. Embryonic period (phase),period of embryogenesis 3. Fetal period (phase), period of fetogenesis study hard 9 Pre-embryonic period  From fertilization/zygote formation to the implantation of blastocyst.  Ends by the formation of germ layers and appears flattened disc shaped mass of cells Embryonic period o 3rd -8th weeks o all major organ systems appear. Fetal Period: o Includes the remaining weeks of development prior to birth o The fetus continues to grow o Its organs increase in complexity study hard 10  Postnatal period Neonatal (Birth – 28 days) Infancy (neonate – 1st yr) childhood (13 mo – 12 yrs) adolescence (11 – 19 yrs) puberty (12 – 15 ♀ , 13 – 16 ♂ yrs) adulthood (18 – 21 yrs) study hard 11  Embryologic terminology  Oocyte – female germ cell produced by the ovaries.  Sperm - male germ cell produced by the testes.  Zygote –refers to a cell results from the union of an oocyte and a sperm during fertilization - is the beginning of new human being study hard 12  … Cleavage – a series of mitotic cell divisions of the zygote that result in the formation of blastomeres Morula – 16 (12-32) blastomeres or a solid mass of 12 to 32 blastomeres, formed by cleavage of a zygote Implantation − The process during which the blastocyst attaches to the endometrium study hard 13  …  Embryo – developing human during its early stages of development(3rd -8th wk). Gastrula − transformation of a blastocyst into a three-layered or trilaminar embryonic disc (third week) Neurula − the early embryo when the neural tube is developing from the neural plate (by 3rd & 4th wks)  Conceptus – entire products of conception from fertilization onward and its membranes or the embryo and its adjacent parts study hard 14  …  Fetus- the developing human during 9th week to birth  Primordium – the first discernible indication of an organ.  Trimester – a period consists of three calendar months.  Fertilization age – the age of embryo from the first day of the last normal menstrual period is gestational age( 2weeks longer than fertilization age).  Abortion – expulsion of embryo or fetus from the uterus before it is viable study hard 15 study hard 16 Descriptive terms in embryology study hard 17  OVERVIEW OF MALE AND FEMALE REPRODUCTIVE ORGANS study hard 18  Male Reproductive System Components of male reproductive system includes: Primary sex organ System of ducts Accessory sex glands External genitalia study hard 19 Male reproductive structuresTestis Epididymis Ductus/vas deferens Accessory glands study hard 20  Male reproductive structures Testis # = two Is the male sex gland It lies in the scrotum suspended by the spermatic cord It is enclosed by a serous sac, the tunica vaginalis derived from the peritoneum Each testis is divided into 250 compartments (lobules), containing – seminiferous tubules :produce sperms – interstitial cells :produce testosterone study hard 21 Testis Seminiferous tubules study hard 22 Seminiferous tubules contain 2 types of cells  Spermatogonia (stem cells)  Give rise to sperm cells through the process of spermatogenesis  Sertoli cells (supporting cells) – Support, nutrition, and protection – Produce proteins, which are necessary for spermiogenesis – Phagocytosis: During spermiogenesis, excess cytoplasm shed as residual bodies is phagocytosed and digested by Sertoli cell lysosomes study hard 23 Male reproductive structures … study hard 24 Intratesticular genital ducts They are; Straight tubules (tubuli recti) Rete testis, and Efferent ductules These ducts carry spermatozoa and liquid from the seminiferous tubules to the duct of the epididymis study hard 25  Male excretory genital ducts Epididymis # = two : L = 4–5m Ductus deferens A much coiled tube for storage and maturation of sperms Adding two layers around the sperm (glycoprotein coat and seminal protein coat) study hard 26 Ductus/Vas deferens 45-50cm long Ductus deferens It is a thick muscular tube that conveys mature sperm from the epididymis to the ejaculatory duct  Ejaculatory duct = Union of ampulla of ductus deferens & ducts of the seminal vesicle From there on, the sperm enters the prostate gland and prostatic urethra study hard 27 Male Accessory Sex Glands study hard 28 Accessory male reproductive glands  Seminal vesicles  Prostate  Cowper’s glands They do not involve in the production of sperm cells but release secretions that form the major part of the semen study hard 29 Male reproductive structures … The accessory glands – Seminal vesicle: Secretes 60% of seminal fluid which contain fructose (for nutrition) prostaglandins, as well as enzymes and other proteins – Prostate gland: Secretes 30% of seminal fluid which contain citrate,fibrinolysin. – Cowper’s gland (bulbo- urethral gland): Secretes an alkaline fluid which neutralize the acidity of the urethra study hard 30 Female Reproductive Structures  Primary sex organ (ovary)  Fallopian tube  Uterus  Vagina  External genitalia study hard 31 Uterus  The uterus is a hollow muscular organ, in which the fertilized ovum is normally embedded, grows and nourished until birth.  The cavity of the uterus and the vagina form the pathway through which the foetus passes at the end of pregnancy.  This pathway is called the birth canal.  In multiparous woman it is thick, muscular and larger in size.  The uterus has a shape of an inverted “pear”. study hard 32  Uterus … Parts - Fundus, body, isthmus and cervix 1.Fundus Is the part lying above the level of openings of the uterine tubes  The area where the tubes join the uterus is called cornu (uterine horn) 2. Body Forms 2/3 of the uterus and extends from 3. Cervix the tubal openings cylindrical toone inferior a constricted third portion The most fixed part extends between the isthmus and the vaginal opening of the uterus  It is divided into two by the upper wall of the vaginal A.Supravaginal part – part above the vagina B.Vaginal part (portion vaginalis)- communicates with the vagina through the external study hard astium or os 33 Uterus … The walls of the body of the uterus consist of three layers Perimetrium -thin external layer Myometrium- thick smooth muscle layer Endometrium- thin internal layer study hard 34 Uterus …  an outer connective tissue layer, Perimetrium which is adventitia in some areas, but largely a serosa.  Firmly attached to the myometrium Myometrium  The thick highly vascular smooth muscle layer Endometrium  A mucosa, the endometrium, lined by simple columnar epithelium  Its covering simple columnar epithelium has both ciliated and secretory cells, the latter forming numerous tubular study hard 35 uterine glands Endometrium The endometrium can be subdivided into two zones Basal layer  Adjacent to the myometrium  Contains highly cellular lamina propria and the deep basal ends of uterine glands  has its own blood supply and is not sloughed off during menstruation (remains relatively unchanged) study hard 36  Endometrium Functional layer  Contains less cellular lamina propria  Richer in ground substance, most of the length of the glands  The functional layer undergoes profound changes during the menstrual cycles – Disintegrate and are shed during menstruation and after parturition (delivery of a baby) study hard 37  Endometrium During the luteal phase of the menstrual cycle, two layers of the functional layer can be distinguished microscopically I. A thin, compact layer consisting of densely packed, connective tissue around the necks of the uterine glands II. A thick, spongy layer composed of edematous connective tissue containing the dilated, tortuous uterine glands study hard 38  Uterine tubes /Fallopian tubes/oviduct The uterine tubes transport the oocytes from the ovaries to the uterus and spermatozoa in the opposite direction. It is in the uterine tubes that fertilization takes place and early cleavage starts. These tubes are about 10-12 cm in length, extending from the uterus to uterine ends of the ovaries. It is lined with secretory epithelium for nourishment of ovum Extent – Length = ~10 -12 cm : Diameter = 1-4mm study hard 39 study hard 40  Utrine tubes … Function – Carry oocytes – Sperms (ampulla of the uterine tube ) – Conveys the cleaving zygote to the uterine cavity – The ova are propelled to the uterus along this tube, partly by peristalsis and partly by ciliary action. study hard 41 study hard 42  Uterine tubes … Parts - 4 1. Infundibulum 2. Ampulla 3. Isthmus 4. Intramural (uterine) part Infundibulum Length = 1cm) ᶲ = 3mm Is a funnel-shaped part close to the ovaries It contains the abdominal or pelvic opening surrounded by thin irregular finger like processes, the fimbriae The fimbriae catch a discharged ovum and passes it to the tube study hard 43 Uterine tubes … Ampulla Length = 5cm ᶲ = 4mm Is the thin walled, longest part that has a widest diameter It follows a slightly tortuous course b/n infundibulum & isthmus Isthmus Length = 3cm) ᶲ = 2 mm Is the part between the widest part, ampulla, and the uterus Is narrower and thicker than the ampulla study hard 44 Uterine tubes … Intramural part Length = 1cm ᶲ = 1 mm Is the part with in the wall of the uterus  It is the narrowest part of the tube Its opening into the uterine cavity study hard 45 ovaries: Is the female sex gland It lies in the ovarian fossa in the side wall of the pelvis Its function is production of ova study hard 46  It secretes two hormones: a. Estrogen (sex hormone): responsible for appearing of 2nd female sex characters. This hormone is secreted from Graffian follicle b. Progesterone (pregnancy hormone): responsible for maintenance of pregnancy by increase the thickness and vascularity of uterine endometrium. This hormone is secreted from corpus luteum after ovulation process study hard 47 study hard 48  ovaries … Color and texture Before the first ovulation the surface is smooth and pink After ovulation, become grey because of the scars Dimensions Vertical = ~3cm Transverse = ~1.5cm Thickness = ~ 1cm Weight = ~ 10-15 gms  The size varies with age and the phases of ovarian cycle but generally they become study hard 49 smaller after menopause Female reproductive cycle  Beginning at puberty, females undergo reproductive cycles (sexual cycles), involving activities of the hypothalamus of the brain, pituitary gland, ovaries, uterus, uterine tubes, vagina, and mammary glands.  These monthly cycles prepare the reproductive system for pregnancy.  Ovarian cycle  Uterine cycle study hard 50  The Ovarian Cycle  The monthly series of events associated with the maturation of an egg is called the ovarian cycle  It has two consecutive phases  The follicular phase is the period of follicle growth, lasting from the 1st to the 14th day of the cycle  The luteal phase is the period of corpus luteum activity, days 14-28 study hard 51. study hard 52  The Ovarian Cycle… The typical ovarian cycle repeats at intervals of 28 days, with ovulation occurring midcycle However, cycles as long as 40 days or as short as 21 days are fairly common In such cases, the length of follicular phase and timing of ovulation vary, but the luteal phase remains constant: It is 14 days between the time of ovulation and the end of the cycle study hard 53  The Follicular Phase Maturation of a primordial follicle to the mature state occupies the first half of the cycle and involves several events – Growth and differentiation of primary oocyte – Proliferation of follicular cells – Formation of zona pellucida – Development of the theca folliculi study hard 54  Ovarian Follicles An ovarian follicle consists of an oocyte surrounded by one or more layers of epithelial cells. 1. PRIMORDIAL FOLLICLE The follicles that are formed during fetal life primordial follicles consist of a primary oocyte enveloped by a single layer of the flattened follicular cells. found in the superficial areas of the cortex. study hard 55 2. Primary follicle Unilaminar primary follicle – Beginning in puberty with the release of follicle- stimulating hormone (FSH), a small group of primordial follicles each month begins a process of follicular growth – This involves growth of the oocyte, proliferation and changes in the follicular cells (form a simple cuboidal epithelium) Multilayered primary follicle – The follicular cells continue to proliferate, forming a stratified follicular epithelium, the granulosa – Follicle cells are now termed granulosa cells and the follicle is a multilayered primary follicle – Between the oocyte and granulosa cells, a layer of glycoprotein called the zona pellucida develops. study hard 56 study hard 57 3.Secondary or antral follicles – As the follicles grow the oocyte increases in size and numbers of granulosa cells also increase, – Small spaces develop within the granulosa layer as the cells secrete follicular fluid – This fluid accumulates, the spaces gradually coalesce, and the granulosa cells reorganize themselves around a larger cavity, the antrum – During the reorganization of the granulosa layer to form the antrum, some cells form a small hillock, the cumulus oophorus, surrounding the oocyte and protruding into the antrum study hard 58  … The granulosa cells around the oocyte make up the corona radiata and accompany the oocyte when it leaves the ovary The stromal cells immediately around the follicle differentiate to form the follicular theca – This layer differentiates into two: Theca interna: well-vascularized endocrine tissue Theca externa: more fibrous outer layer containing smooth muscle and fibroblasts study hard 59 study hard 60  … 4. Mature or graafian follicle – During each menstrual cycle, usually one follicle grows much more than the others and becomes the dominant follicle, while many of the other follicles eventually enter atresia – The dominant follicle undergo ovulation – The antrum increases greatly in size by accumulating follicular fluid and the oocyte adheres to the wall of the follicle through the cumulus oophorus of granulosa cells study hard 61 study hard 62 study hard 63  Ovulation At ovulation the large mature secondary oocyte escapes from the ovary and is caught by the dilated end of the uterine tube. Ovulation normally occurs midway through the menstrual cycle, ie, around the fourteenth day of a typical 28-day cycle. In humans usually only one oocyte is liberated during each cycle, but sometimes either no oocyte or two or more simultaneous oocytes may be expelled. In the hours before ovulation the ovarian follicle, under the influence of FSH and LH, undergoes a sudden growth, producing a cystic swelling on the surface of the ovary A small avascular spot, the stigma, soon appears on this swelling study hard 64  1. Fallopian tube 2. Fimbriae 3. Ovary 4. Follicle 5. Stigma Graafian follicle bulge on the ovary surface. The beginning formation of a stigma (whitish region free of study hard blood 65 vessels).  … The ovulated secondary oocyte adheres loosely to the ovary surface and its drawn into the opening of the uterine tube where fertilization may occur. If not fertilized within about 24 hours, the secondary oocyte begins to degenerate. Some women experience a cramp of pain in the lower abdomen when ovulation occurs which is known as middle pain (mittelschmerz) This episode is caused by the intense stretching of the ovarian wall during ovulation study hard 66  The expelled secondary oocyte Relation of fimbriae and ovary. Fimbriae collect the oocyte and sweep it into the uterine tube study hard 67 Corpus luteum study hard 68  Corpus Luteum After ovulation, the granulosa cells and theca interna of the ovulated follicle reorganize to form a larger temporary endocrine gland, the corpus luteum (L., yellowish body), in the ovarian cortex. Once formed, the corpus luteum begins to secrete progesterone and some estrogen study hard 69  … The fate of the corpus luteum depends on whether a pregnancy occurs. Corpus luteum of menstruation If pregnancy does not occur, the corpus luteum starts degenerating in about 10-12 days and its hormonal output ends. The corpus luteum that persists for part of only one menstrual cycle is called a corpus luteum of menstruation. In this case all that ultimately remains is a scar called the corpus albicans (white body) study hard 70  … Corpus luteum of pregnancy If the oocyte is fertilized, degeneration of the corpus luteum is prevented by human chorionic gonadotropin (hCG), a hormone secreted by the trophoblast of the developing embryo. HCG targets the corpus luteum, maintaining it and promoting further growth of this endocrine gland and stimulating secretion of progesterone to maintain the uterine mucosa. This corpus luteum of pregnancy becomes very large and is maintained by HCG for 4–5 months, by which time the placenta itself produces progesterone (and estrogens) at levels adequate to maintain the uterine mucosa. It then degenerates and is replaced by a large corpus albicans. Removal of the corpus luteum of pregnancy before the fourth month usually leads to abortion. study hard 71 Follicle development and changes within the ovary: A diagram of a sectioned ovary shows the different stages of follicle maturation, ovulation, and corpus luteum formation and degeneration study hard 72  Duration of Ovarian Cycle Ovarian cycle persist Throughout the reproductive life of women and terminate at menopause No ovarian cycle During pregnancy At menopause – Menopause, the permanent cessation of menstruation, usually between the ages of 48 and 55. study hard 73 study hard 74  Hormones and the Ovarian Cycle A gonadotropin-releasing hormone (GRH ) is synthesized by neurosecretory cells in the hypothalamus and is carried by the hypophysial portal system to the anterior lobe of the pituitary gland GRH stimulates the release of two hormones: follicle-stimulating hormone (FSH) and luteinizing hormone (LH) produced by pituitary gland that act on the ovaries. FSH stimulates the development of ovarian follicles and the production of estrogen by the follicular cells LH serves as the "trigger" for ovulation (release of secondary oocyte) and stimulates the follicular cells and corpus luteum to produce progesterone These ovarian hormones also induce growth of the endometrium study hard 75 Hormonal Control of Ovarian Function study hard 76  Female reproductive cycles The fig. illustrates the interrelations of the hypothalamus of the brain, pituitary gland, ovaries, and endometrium study hard 77  The Uterine (Menstrual) Cycle The monthly changes in the internal layer of the uterus constitute the endometrial/uterine cycle, commonly referred to as the menstrual cycle or period The endometrium responds in a consistent manner to the fluctuating concentrations of gonadotropic (FSH and LH) and ovarian hormones (estrogen and progestrone) The average menstrual cycle is 28 days (In 90%, +5) study hard 78  The Uterine (Menstrual) Cycle… These endometrial changes are coordinated with the phases of the ovarian cycle. During this menstrual cycle, the uterine endometrium passes through three stages, the follicular or proliferative phase, the secretory or progestational phase, and the menstrual phase. study hard 79 study hard 80  The Uterine (Menstrual) Cycle… Days 1 - 5 Menstrual phase Usually lasts 4-5 day The functional layer of the uterine wall is sloughed off and discarded with the menstrual flow-menses (monthly bleeding)  Blood + part of endometrium After menstruation, the eroded endometrium is thin At the beginning of this stage, gonado- tropins are beginning to rise a bit and ovarian hormones are at their lowest normal levels. Then FSH levels begin to rise study hard 81  The Uterine (Menstrual) Cycle… The detached tissue and blood pass out through the vagina as the menstrual flow By day 5, the growing ovarian follicles are starting to produce more estrogen study hard 82  The Uterine (Menstrual) Cycle… Days 6-14: Proliferative/follicular phase In this phase the endometrium rebuilds itself Under the influence of rising blood levels of estrogen, the basal layer of the endometrium generates a new functional layer study hard 83  The Uterine (Menstrual) Cycle… As the new functional layer thickens, its glands enlarge and its spiral arteries increase in number There is a two- to three fold increase in the thickness of the endometrium study hard 84  The Uterine (Menstrual) Cycle… Normally, the cervical mucus is thick and sticky, but rising estrogen levels cause it to thin and become crystalline, forming channels that facilitate the passage of sperm into the uterus Ovulation occurs in the ovary at the end of this stage (day 14) in response to the sudden release of LH from the anterior pituitary LH converts the follicle to a corpus luteum study hard 85  The Uterine (Menstrual) Cycle… Days 15-28: Secretory /luteal Phase In this phase, the endometrium prepares for implantation of the embryo Rising levels of progesterone from the corpus luteum act on endometrium causing the spiral arteries to elaborate and coil more tightly. study hard 86  The Uterine (Menstrual) Cycle… The uterine glands enlarge, coil, and begin secreting nutritious glycoproteins into the uterine cavity All events of the secretory phase are promoted by progesterone. Increasing progesterone levels also cause the cervical mucus to become viscous again, forming the cervical plug, which blocks sperm entry and plays an important role in keeping the uterus “private” in the event an embryo has begun to implant study hard 87  The Uterine (Menstrual) Cycle… If fertilization has not occurred, Progesterone levels fall, depriving the endometrium of hormonal support, and the spiral arteries kink and go into spasms Fragmentation continues causing the functional layer to slough off The mentrual cycle starts again on this first day of menstrual flow study hard 88  Ischemic Phase- occurs when the oocyte is not fertilized. Ischemia (reduced blood supply) occurs as the spiral arteries constrict, giving the endometrium a pale appearance. Pregnancy Phase- if pregnancy occurs, the menstrual cycles cease and the endometrium passes into a pregnancy phase. With the termination of pregnancy, the ovarian and menstrual cycles resume after a variable period (usually 6 to 10 weeks if the woman is not breast- feeding her baby). If pregnancy does not occur, the reproductive cycles normally continue until menopause. study hard 89  The Uterine (Menstrual) Cycle… Notice that the menstrual and proliferative phases overlap the follicular stage and ovulation in the ovarian cycle study hard 90  The Uterine (Menstrual) Cycle… The uterine secretory phase corresponds to the ovarian luteal phase study hard 91  Anovulatory menstruation & artificial induction of ovulation Some women fail to ovulate because of a low concentration of gonadotropins. In these cases, administration of an agent to stimulate gonadotropin release and hence ovulation can be employed. Although such drugs are effective, they often produce multiple ovulations, so that the risk of multiple pregnancies is 10 times higher in these women than in the general population. study hard 92  The Beginning of Human Development  The development of human being begins with fertilization  Fertilization is a process by which the spermatozoon from male and oocyte from the female unite to give rise to a new organism, the zygote  The zygote divides many times & progressively transformed into a multicellular human being through  Cell division  Cell migration  Programmed cell death (apoptosis)  Cell Differentiation  Preparation for fertilization/pregnancy involves 2 main programs of events:  Cell Growth 1. Gametogenesis  formation of the male and female gametes in the gonads (ovary or testis) 2. Cyclic changes in the female genital tract  ovarian & uterine cycles study hard 94  Gametogenesis Types of body cells A human being is formed of trillions of cells; these cells are of two types: Somatic cells - which are present in the whole tissues of the body Gametes - which are the sperms and ova study hard 95  Chromosomes Human somatic cells contain 23 pairs of chromosomes for a total of 46; 22 pairs of autosomes and one pair of sex chromosomes. Diploid Cells A cell is said to be diploid if it contains 23 pairs of chromosomes. 2N = 46 One chromosome of each pair is derived from the maternal gamete, the oocyte, and one from the paternal gamete, the sperm. Autosomes contain genetic information for most human characteristics. Homologous and pair chromosomes: sex chromosomes determines whether an individual is female (XX) or male (XY). One member of each pair of chromosomes is inherited from each parent. study hard 96  Gametogenesis The process by which gametes (ova & spermatozoa) is formed: Oogenesis: The production of ovum Spermatogenesis: The production of spermatozoa.  Gametogenesis is divided into four phases: 1. Extra-gonadal origin of primordial germ cells (gametes are derived from primordial germ cells ,PGCS) 2. Proliferation of germ cells by mitosis 3. Meiosis 4. Structural and functional maturation of the ova and spermatozoa study hard 97  Gametogenesis … During early embryonic development, primordial germ cells (PGCs) from the dorsal endoderm of the yolk sac migrate along the hindgut to the gonadal ridge. They multiply by mitosis and once they have reached the gonadal ridge they are called gametogonia. Begin meiosis. =Produces secondary oocytes in the female. =Produces sperm in the male. study hard 98 study hard 99  Primordial germ cells Yolk sac Primordial The germ cells cells,later through differentiate amoeboid into movement, mature gametes move towards i.e. the spermatogonia gonads where (male) theyor arrive oogonia at about 5th week (female) study hard 100 Mitosis CELL DIVISION Mitosis is the process whereby one cell divides, giving rise to two daughter cells that are genetically identical to the parent cell. Each daughter cell receives the complete complement of 46 chromosomes. Before a cell enters mitosis, each chromosome replicates its deoxyribonucleic acid (DNA). Four main steps are found i.e prophase ,metaphase ,anaphase and telophase. study hard 101 study hard 102 Mitotic Stages study hard 103  Meiosis A type of cell division. Starts with a diploid parent cell Produces haploid daughter cells (sperm or eggs/ova). involves two division called meiosis I and meiosis II but only a single cycle of DNA replication. in both steps four steps are found i.e prophase ,metaphase ,anaphase and telophase. two cells at the end of Meiosis I and four haploid cells at the end of meiosis II.. study hard 104  Meiotic events can be grouped under the following phases: Meiosis I Meiosis II Prophase I Prophase II Metaphase I Metaphase II Anaphase I Anaphase II Telophase I Telophase II study hard 105  Meiosis I Prophase I: During this stage chromosomes start pairing together and this process of association is called synapsis. Such paired chromosomes are called homologous chromosomes. Crossing over occurs between non-sister chromatids of the homologous chromosomes. Crossing over is the exchange of genetic material between two homologous chromosomes. study hard 106  … These X-shaped structures, are called chiasmata. Then there will be terminalisation of chiasmata. the meiotic spindle is assembled to prepare the homologous chromosomes for separation. study hard 107  Tetrads Form in Prophase I Homologous chromosomes Join to form a TETRAD (each with sister chromatids) Called Synapsis study hard 108 study hard 109 Metaphase I: The chromosomes align on the equatorial plate. The microtubules from the opposite poles of the spindle attach to the pair of homologous chromosomes. Anaphase I: The homologous chromosomes separate, while sister chromatids remain associated at their centromeres. study hard 110  METAPHASE I ANAPHASE I study hard 111 Telophase I: The nuclear membrane and nucleolus reappear and cytokinesis follows. The stage between the two meiotic divisions is called interkinesis and is generally short lived. Interkinesis is followed by prophase II. study hard 112  Meiosis II Meiosis II is initiated immediately after cytokinesis In contrast to meiosis I, meiosis II resembles a normal mitosis. Prophase II: The nuclear membrane disappears by the end of prophase II. The chromosomes again become compact. Metaphase II: At this stage the chromosomes align at the equator and the microtubules from opposite poles of the spindle get attached to the kinetochores of sister chromatids. study hard 113  METAPHASE II PROPHASE II study hard 114 Anaphase II: It begins with the simultaneous splitting of the centromere of each chromosome (which was holding the sister chromatids together), allowing them to move toward opposite poles of the cell Telophase II: Meiosis ends with telophase II, in which the two groups of chromosomes once again get enclosed by a nuclear envelope; cytokinesis follows resulting in the formation of tetrad of cells i.e., four haploid daughter cells. study hard 115  ANAPHASE II TELOPHASE II study hard 116  Meiosis II: Reducing Chromosome Number Prophase II Metaphase II Telophase II Anaphase II 4 Identical haploid cells study hard 117  Significance of meiosis:- to provide consistancy of chromosomes number by reduction division for recombination of genetic material and allow random assortment of maternal and paternal chromosomes study hard 118 In meiosis I: – homologous chromosomes are separated after synapsis – crossing over occurs. – Disjunction without separation of cetromeres occur. – From dipoid to haploid study hard 119  In meiosis II: – Crossing over doesn’t occur. – sister chromatids are separated with centromere separation. – sequence of phases resembles mitosis. – It is same as mitosis except the chromosome number it receives is haploid. study hard 120 study hard 121  Mitosis Meiosis Number of divisions 1 2 Number of daughter cells 2 4 Genetically identical? Yes No Chromosome # Same as parent Half of parent Where Somatic cells Germ cells When Throughout life At sexual maturity Sexual Role Growth and repair reproduction study hard 122 Gametogenesis study hard 123  Oogenesis Process of forming mature ova in the ovary. the sequence of events by which oogonia are transformed into mature/secondary/ oocytes in the ovary This maturation process begins before birth and is completed after puberty and continues to menopause Involves 1. Prenatal maturation 2. Postnatal maturation study hard 124  Prenatal maturation 6th wks: migration of the germ cells from their extragonadal site to the gonadal ridge where they proliferate by mitosis In the gonads these cells divide and transform into oogonia 8-10th wks: about 6,000 oogonia populate the developing ovary. 12th wks: they differentiate into primary oocyte. study hard 125  … 5th months : maximum number of primary oocyte colonize the ovary are 7,000,000. At birth there are about 1,000,000 primary oocytes. After birth, by puberty there are only 400,000 oocytes. Majority of oogonia (as well as primary oocytes) degenerate The oocyte becomes enclosed by somatic cells of the ovary forming a unit called follicle study hard 126  … Primary oocytes begin the first meiotic division before birth … By 7th month Most of the serviving primary oocytes (at prophase of meiosis I.) But instead of proceeding into metaphase, they enter diplotene stage of meiosis I (Diplotene stage-resting stage during prophase) The primary oocytes remain dormant in the ovarian follicles until puberty study hard 127  Postnatal Maturation As a follicle matures, the primary oocyte increases in size and, shortly before ovulation, completes the first meiotic division to give rise to a secondary oocyte and the 1st polar body Unlike the corresponding stage of spermatogenesis, however, the division of cytoplasm is unequal The secondary oocyte receives the bulk of the cytoplasm, large compared to ordinary cell The polar body is a small ( recieves atiny bit of the cytoplasm), nonfunctional cell that soon degenerates. study hard 128  Oogenesis … Only the secondary oocyte has the potential to be ovulated and fertilized. The corona radiata and the zona pellucida( a layer of glycoproteins on the surface of the oocyte) form protective layers around the secondary oocyte At ovulation, the nucleus of the secondary oocyte begins the second meiotic division, but progresses only to metaphase, when division is arrested study hard 129  Oogenesis … If the secondary oocyte is not fertilized, it degenerates about 24 hours after ovulation, as it is still arrested in Metaphase II. If the secondary oocyte is fertilized, it first finishes the process of meiosis i.e Anaphase II and Telophase II. Two new cells are produced, and as before, the division of the cytoplasm is unequal. study hard 130  Oogenesis … The cell that receives very little cytoplasm becomes another polar body and eventually degenerates. The cell that receives the majority of the cytoplasm becomes an ovum which can be fertilized. Typically, only one secondary oocyte is expelled (ovulated) from one of the two ovaries each month. The left and right ovaries alternate ovulation each month. only approximately 400 secondary oocytes are expelled at ovulation during the reproductive period study hard 131  ♂ Primordial germ cells ♀ Mitosis Spermatogonia Oogonia 10 Oocyte 10 Spermatocyte Meiosis I 20 Oocyte 20 Spermatocytes & polar body Meiosis II Ovum Spermatids & polar bodies Spermiogenesis Spermatozoa Unequal division of cytoplasm produces oocytes and polar bodies study hard 132 study hard 133 Ovulation study hard 134  The ovum  The ovum resembles any ordinary cell in its structure.  It is large oval cell which varies from (117-142) μm in diameters.  Coverings : a. 2 membranes :an inner thin delicate one called vitelline membrane and an outer thick transparent membrane called zona pellucida b. Corona radiata: 2 or 3 layers of cells which surround the zona pellucida when the study hard ovum is shed from follicle 135 The ovum  Significance of egg membranes: a. Provide the protection to the contents of egg b. Prevent polyspermia i.e. Fertilization by more than one sperm c. Maintain the normal study cleavage hard of the egg 136  Spermatogenesis Maturation of sperm cells refers to the entire sequence of events by which primary germ cells (spermatogonia) are transformed into mature sperms (spermatozoa) It begins at puberty and continues to old age The primordial germ cells give rise to spermatogonia The process of spermatogenesis involves - Spermatocytosis - Spermiogenesis study hard 137  Spermatogenesis  Spermatogonia, which have been dormant in the seminiferous tubules of the testes since the fetal period, begin to increase in number at puberty  After several mitotic divisions, the spermatogonia grow and undergo changes producingstudysuccessive hard generations of cells 138 Spermatogenesis …  The newly formed cells can follow one of two paths:  They can either continue dividing as stem cells, also called type A spermatogonia Or  They can differentiate during progressive mitotic cycles to become type B spermatogonia study hard 139 Spermatogenesis … Type B spermatogonia are progenitor cells that will differentiate into primary spermatocytes The primary spermatocyte has 46 chromosomes Soon after their formation, these cells enter the prophase of the first meiotic division Because this prophase takes about 22 days, the majority of spermatocytes seen in sections will be in this phase study hard 140  Spermatogenesis …  Each primary spermatocyte subsequently undergoes a reduction division (1st meiotic division) to form two haploid secondary spermatocytes  They are approximately half the size of primary spermatocytes  Subsequently, the secondary spermatocytes undergo a 2nd meiotic division to form four haploid spermatids, which are almost half the size of secondary spermatocytes  The meiotic process therefore results in the formation of cells with a haploid number of chromosomes  With fertilization, the normal study hard diploid number is again attained 141 Spermatogenesis … study hard 142  Spermiogenesis the process of transformation of spermatids into sperms It is the final stage of production of spermatozoids By the process of spermiogenesis, the spermatids change their physical structure to become mature spermatozoa (sperm) Spermiogenesis includes 1. Formation of the acrosome 2. Condensation of the nucleus 3. Development of the flagellum 4. Loss of much of the cytoplasm study hard 143 study hard 144  Spermiogenesis can be divided into 3 phases 1. The Golgi Phase 2. The Acrosomal Phase 3. The Maturation Phase Golgi phase The cytoplasm contains a prominent golgi apparatus near the nucleus Small proacrosomal vesicles accumulate in the golgi apparatus and coalesce to form a single membrane-limited acrosomal cap study hard 145  Acrosomal Phase The acrosomal vesicle spreads to cover the anterior half of the condensing nucleus and is then known as the acrosome The acrosome is a specialized type of lysosome containing several hydrolytic enzymes, including  Hyaluronidase, neuraminidase, acid phosphatase, and acrosin (a trypsin-like protease) These hydrolytic enzymes help to  Dissociate cells of the corona radiata  Digest the zona pellucida study hard 146  Maturation Phase Residual cytoplasm is shed and phagocytosed by sertoli cells The end result is the mature spermatozoon, which is then released into the lumen of the seminiferous tubule Mature sperms are free- swimming, actively motile cells consisting of Head, neck, mid-piece & tail study hard 147  … One of the centrioles grows concomitantly, forming the flagellum  Mitochondria aggregate around the proximal part of the flagellum, forming a thickened region known as the middle piece study hard 148 study hard 149  Mature Sperm  The head The Head contains 2 main parts (the nucleus and the acrosome) both have two basic functions of the sperm (genetic and activating respectively) – The nucleus occupies most of the available space of sperm head and its shape determines the shape of the head of the sperm. – It contains only its haploid complement of DNA. – The acrosome contains hydrolytic enzymes study hard 150  Mature Sperm…  Mid-piece Connected to the head with a neck (contains mitochondria)  Tail  Its function is movement of the sperm to swim with head foremost  The sperm cell is devoid of stored food (yolk) and protective envelope  It is also devoid of most cytoplasm organelles such as ribosomes and endoplasmic reticulum study hard 151  Applied Morphological abnormalities of sperms  The head and the tail may be abnormal , they may be: a. Giants b. Dwarfs c. Some times , sperms are joined in head or in tail d. No tail  Sperms with morphological abnormalities lack motility and don’t fertilize the egg. study hard 152 Comparison of female and male gamates Oocyte Sperm large microscopic less mobile Mobile Abundant cytoplasm Very little 22 autosome plus X chr. 22 autosomes plus X or Y chromosome study hard 153  Male and female gametes A, The main parts of a human sperm. B, A sperm drawn to approximately the same scale as the oocyte. C, A human secondary oocyte , surrounded by the zone pellucida and corona radiata. study hard 154  Transportation of gametes Sperm Transport Sperms are stored in the epididymis, mainly in its tail They are transported to the urethra by peristaltic contractions of the muscular coat of the ductus deferens The accessory sex glands-seminal glands (vesicles), prostate, and bulbourethral glands-produce secretions – The secretions are added to the sperm-containing fluid in the ductus deferens and urethra From 200 to 500 million sperms are deposited around the external os of the uterus and in the fornix of the vagina during sexual intercourse The sperms pass slowly through the cervical canal by movements of their tails study hard 155  … 2 to 6 ml of semen is expelled per ejaculation and there are between 50 and 130 million sperm per milliliter. The sperms move 2 to 3 mm per minute, but the speed varies with the pH of the environment. They move slowly in the acid environment of the vagina, but move more rapidly in the alkaline environment of the uterus. 5 -45 minutes to complete the journey. /2-7 hours Only approximately 200 sperms reach the fertilization site. Viability of gametes (24+48 hrs) Most sperms degenerate and are resorbed by the female genital tract. study hard 156  Sperm Transport in the male reproductive system Sagittal section of male pelvis to show the male reproductive system study hard 157 Applied Numerical abnormalities of sperms A.Azo spermia (Aspermia ): No sperms at all , is found in the semen B.Oligosperms : the number of sperms is few in the semen 20 million sperms per milliliter, or 50 million in the total specimen, are probably fertile Fewer than 10 million sperms per milliliter of semen is likely to be sterile C. Necrospermia : the sperms are found dead study hard 158  Male infertility may result from A low sperm count poor sperm motility abnormal sperms medications/drugs endocrine disorders exposure to environmental pollutants cigarette smoking obstruction of a genital duct study hard 159 Oocyte Transport The secondary oocyte is expelled at ovulation from the ovarian follicle with the escaping follicular fluid During ovulation, the fimbriated end of the uterine tube becomes closely applied to the ovary – The fingerlike processes of the tube, fimbriae, move back and forth over the ovary – The sweeping action of the fimbriae and fluid currents produced by the cilia of the mucosal cells of the fimbriae "sweep" the secondary oocyte into the funnel-shaped infundibulum of the uterine tube – The oocyte passes into the ampulla of the tube, mainly as the result of peristalsis-movements of the wall of the tube characterized by alternate contraction and relaxation-that pass toward the uterus study hard 160  The movement of the uterine tube that occurs during ovulation  Note that the infundibulum of the tube becomes closely applied to the ovary. Its fingerlike fimbriae move back and forth over the ovary and "sweep" the secondary oocyte into the infundibulum as soon as it is expelled from the ovary during ovulation thereby to transport the oocyte study hard 161