BIO 133 Animal Embryology Lecture Notes PDF

BIO 133 LECTURE Year 03 DEVELOPMENTAL BIOLOGY Term 01 KEY TERMS AND CONCEPTS IN ANIMAL...

BIO 133 LECTURE Year 03 DEVELOPMENTAL BIOLOGY Term 01 KEY TERMS AND CONCEPTS IN ANIMAL The time of fertilization represents the starting point in the life EMBRYOLOGY history, or ontogeny of the individual BASIC TERMS AND CONCEPTS IN ANIMAL EMBRYLOGY Ontogeny – an individual’s life history: entire life span Development – process by which a complex multicellular Phylogeny – explains the origin of species organism arises from a single cell Embryology is therefore regarded as the period starting with - Involves: (overlap with each other) fertilization that ends with metamorphosis (amphibia), hatching - Increase in cell number (reptiles and birds), birth (mammals) - Differentiation - Pattern formation CELL DIVISION AND GROWTH - Morphogenesis Cell division and growth can occur independently - Net growth - Gradual process: complexity of embryo increases During the cleavage divisions: progressively - Increase in cell number without growth The processes involved in development are the basis of how a - Egg is divided into a series of progressively smaller cells single fertilized cell, called the zygote, becomes a multicellular organism Later in development: Preformation – early theory suggesting that the early embryo - Cell division and growth occur together consist of a miniature version of the adult within the sperm cell - Growth may still occur without cell division: changes in and was merely nourished by the ovum (believers: spermists) cell size and deposition of materials such as bone into the extracellular matrix - Ovists: argues that the ovum contained a minute body, which was stimulated to grow by the seminal fluid Cell division – can either be mitosis or meiosis Epigenesis – laid to rest the preformation theory through a series The formation of an individual begins with the fusion of the of experiments by Lazzaro Spallanzani wherein he demonstrated gametes sperm and ovum that in normal circumstances both male and female sex products To come up with several numbers of sperm cells and egg cells, are necessary for initiation of development mitosis and meiosis are essential - Kaspar Friedrich Wolff – conceptualized the word epigenesis Growth – an increase in mass (may involve an increase in the - Stated that embryonic development occurs number of cellular components) through progressive remodeling and growth - Differential growth – all parts of the embryo do not grow - Highlights the role of genetics in the development of at the same time animals - Ex: rapid growth of the head region OVERVIEW OF ANIMAL DEVELOPMENT Two major patterns of growth in mammals: The development of most animals proceeds through a number of 1. Determinate growth – body grows to a certain point common stages: characteristic of the species and sex, and the growth 1. Fertilization ceases 2. Cleavage – form a group of blastomeres 2. Indeterminate growth – more common in ancestral 3. Gastrulation – reorganize the structure of the embryo vertebrates (e.g. fishes) and generate three germ layers (ectoderm, endoderm, - Growth continues throughout the life span, but mesoderm) at a reduced rate in later life 4. Neurulation – formation of the nervous system DIFFERENTIATION 5. Organogenesis – development of the individual organs Differentiation – process by which different cell types are EMBRYOLOGY generated Embryology – study of the embryo, which is the earliest stage in - Cells become structurally and functionally specialized by animal development synthesizing different proteins Almost all higher animals start their lives from a single cell, the - Reflects the activation and maintenance of different fertilized ovum (zygote) patterns of gene expression - Actual morphological or functional expression of the Zygote – has a dual origin from two gametes: a spermatozoon portion of the genome that remains available to a from the male parent and an ovum from the female parent particular cell or groups of cells BIO 133 LECTURE Year 03 DEVELOPMENTAL BIOLOGY Term 01 - Process by which a cell becomes specialized, and the Differences of undifferentiated vs. differentiated cells final product is called a differentiated cell Differentiation of cells can be viewed in terms of their biochemistry, functions, and morphological appearances: - Biochemical (B) – process by which a cell chooses one or few specialized synthetic pathways - Ex: synthesis of hemoglobin by erythrocytes; synthesis of specific crystalline proteins by the lens - Functional (F) – differentiation of a muscle cell by At the tissue level, differentiation can often be recognized as developing the ability to contract (contractility) characteristic morphological changes occurring in groups of cells - Ex: differentiation of a nerve cell to develop in certain locations and at certain times conductivity - Morphological (M) – skeletal muscle differentiates into Histiogenesis – process by which individual tissues take on elongated like a thread-like cell/fiber with several nuclei characteristic appearance through differentiation of their - A neuron differentiates into a stellate shape of component cells cells with 2 sets of processes, the axon and the dendrites DETERMINATION Definitions of differentiation can also vary: Cell differentiates through cell determination - Restrictive definition – limit differentiation to the Cell determination – series of molecular events in which the maturation of a cell during a single cell cycle: often the activities of certain genes are altered in ways to that cause a cell terminal cycle to progressively commit to a particular differentiation pathway, - Ex: primitive homocytoblastic stem cells -> making it a specialized cell erythrocytes - Broader definition – maturation of a cell and its - As it proceeds, it restricts an embryonic cell’s descendants over the span of several cell cycles developmental pathway so that its fate becomes more and more limited Once a cell has differentiated into its characteristic appearance and activities, it is irreversibly committed to its fate No matter how differentiation is defined, it follows several general pathways that ultimately ends with a highly specialized cell that can perform its role in the vertebrate body Cell differentiation – expression of changes in the activity of specific genes, which in turn is influenced by a variety of factors inside and outside the cell Differential gene expression – reason why each cells of the vertebrate body have distinct biochemistry, behavior/function, and structure/morphology BIO 133 LECTURE Year 03 DEVELOPMENTAL BIOLOGY Term 01 NUCLEAR EQUIVALENCE Gametogenesis – formation of gametes Theory of nuclear equivalence – the nuclei of essentially all - Takes place even as early in the embryonic stages of differentiated adult cells of an individual are genetically (but not vertebrates necessarily metabolically) identical to one another and to the - Occurs in the testes for males (spermatogenesis) and in nucleus of the zygote from which they descended the ovaries for females (oogenesis) - All somatic cells in an adult have the same set of genes, STRUCTURE OF THE FEMALE REPRODUCTIVE ORGANS but the cells express different subsets of genes: differential gene expression - Underlies the cloning of organisms MORPHOGENESIS AND PATTERN FORMATION Once the cells have differentiated to what they are fated to become and restricted in their respective forms and functions, these cells must become progressively organized, through shaping of the intricate pattern of tissues to become organs characteristic of a multicellular animal Morphogenesis – development of form - Proceeds through the process of pattern formation Ovary – female gonad Pattern formation – series of steps requiring signaling between cells, changes in shapes of certain cells, precise cell migrations, - Paired organs, almond-shaped, located on either side of interactions with the extracellular matrix, and even apoptosis the uterus and are positioned very near the open, funnel-shaped ends of the uterine tubes In many systems, pattern formation (laying down of the - Fimbriae – numerous, small, finger-like projections in morphogenetic blueprint) is distinguished from morphogenesis the opening of the uterine tube (realization of the plan) - Infundibulum – location of the opening of the uterine tube REPRODUCTIVE ORGANS AND THE SEXUAL - Involved in directing the ovulated egg into the CYCLE tube Segments of the fallopian tube: INTRODUCTION - Infundibulum Sexual reproduction – involves the production of two types of - Ampulla gametes: the sperms and the eggs - Isthmus - Typically, two separate parents (dioecious) are needed - Intramural segments - Male parent contributes the sperm and female parent contributes the egg The lining epithelium of the uterine tubes contain a mixture of ciliated cells that assists in gamete transport and secretory cells Hermaphroditism – a single individual (monoecious) produces the that produce a fluid supporting early development of the embryo sperms and eggs Smooth muscle layers – found throughout the uterine tubes - Sexual reproduction still exists in certain groups of animals (e.g. tapeworms) - Provide the basis for peristaltic contractions - Allow the fertilized or unfertilized egg to be Benefits of sexual reproduction transported to the uterus - Genetic variability – during meiosis Oviduct – reproductive tract - Also happens when the haploid male gamete fuses with the haploid female gamete during - Divided generally into regions such as the fallopian (or fertilization uterine) tube, uterus, cervix, and vagina - Increases the fitness (reproductive success) of Uterus – pear-shaped organ the offspring - Diploidy: backup copy of genes in case one copy gets - Main functions are to receive and maintain the embryo damaged by mutation during pregnancy and to expel the fetus at the - Permits beneficial mutations from each parent to come termination of pregnancy together in offspring that can reproduce and spread - Endometrium – mucosal lining of the uterus these mutations through the population - Receiving, implanting, and nourishing the - Removes harmful mutations from a population embryo (if pregnancy is positive) BIO 133 LECTURE Year 03 DEVELOPMENTAL BIOLOGY Term 01 - Structure and appearance change daily Clitoris – partially enwrapped by the labia minora where they throughout the menstrual cycle meet anteriorly - Myometrium – expel the fetus at term - Layer of smooth muscles of the uterus - A small erectile organ which is the homologue of the penis of the male Cervix – lower outlet of the uterus STRUCTURE OF THE MALE REPRODUCTIVE ORGANS - Its mucosal lining is different from the lining of the endometrium - Has on its surface a variety of irregular crypts - Cervical epithelium – produces a glycoprotein-rich cervical mucus, the composition of which varies considerably throughout the menstrual cycle - Changing cervical mucus makes it easier or more difficult for spermatozoa Vagina – last segment of the oviduct in mammals - Channel for sexual intercourse - Birth canal - Surface lining is non-keratinized stratifies squamous epithelium - Contains deposits of glycogen: vary in amount throughout the menstrual cycle Testes – paired male gonads where spermatogenesis occurs in - Breakdown products contribute to the humans and other vertebrates low acidity (pH 4.3) of vaginal fluids - Low pH prevents infectious agents Scrotum – part of the skin in the groin region that encloses the (bacteriostatic function) from testes entering the upper genital tract through the cervix that might spread Seminiferous tubules (15) – highly convoluted tubules where spermatogenesis takes place to the peritoneal cavity through the open ends of the uterine tubes - 360 meters in human testes, account for the daily production of 95 million spermatozoa Rete testis – where sperm cells go from the seminiferous tubules - Irregular network of slender anastomosing ducts Efferent ductules/ductuli efferentes – where spermatozoa are collected from the rete testis Head of the epididymis – from efferent ductules - Highly convoluted and compact Body of the epididymis – from the head of the epididymis - Lesser compact The anatomical position of the uterus is between the urinary bladder (ventral) and the rectum (dorsal) Tail of epididymis – from the body of the epididymis External genitalia – complex structures grouped about the - Single less convoluted segment vaginal orifice (opening) Ductus deferens/vas deferens (13) – from the tail of epididymis - Constitute collectively the vulva - Straighter tube Pelvis – lower part of the human female trunk Inguinal canal – found on both sides of the body Labia majora – outermost structures that are a pair of fat- - Where the vas deferens (1) reenter the pelvic cavity containing folds of skin Inguinal hernia or “luslos” – occur in the inguinal canals Labia minora – within the cleft between the labia majora that is a second, smaller pair of skin folds that are highly vascular and - Inguinal canal is a weak opening: straining abdominal devoid of the fat muscles tears the tissues in this site and cause some intestinal loops to bulge into the scrotum BIO 133 LECTURE Year 03 DEVELOPMENTAL BIOLOGY Term 01 FEMALE CYCLES AND ITS HORMONES Ejaculatory duct (ED) – common tube of the ampulla of the vas Sexual cycles – when a female begins to undergo regular monthly deferens and the seminal vesicle cycles at the time of puberty - Joins with the urethral part in the prostate gland called - Controlled by the hypothalamus the prostatic urethra (PU) - Form a common tube, the membranous Hypothalamus – releases GnRH (gonadotropin releasing urethra (MU) that continues into the spongy hormone/factor) urethra of the penis Release of GnRH is known to be stimulated by internal and environmental stimuli - Internal stimuli: levels of sex hormones in the blood - Environmental stimuli: olfactory or smell signals Once the GnRH is released by the hypothalamus, it is transported to the anterior lobe of the pituitary gland (adenohypophysis) by way of blood vessels known as the hypothalamohypophyseal portal system and stimulates it to secrete its hormones Two hormones released by the adenohypophysis: Penis – male external genitalia with three rodlike masses of - Follicle-stimulating hormone (FSH) erectile tissue held by dense connective tissue and covered by - Luteinizing hormone (LH) freely movable skin Prolactin – another hormone released by the adenohypophysis Corpora cavernosa – paired dorsal tissues - Role is important during and after pregnancy Corpora spongiosum – single erectile tissue - Located in the glans penis External urethral opening – terminal end of the urethra - Through which urine and semen pass out of the male Ovulation – the time a mature ovum is released from the ovary human body for possible fertilization by the spermatozoa - Although there is a higher level of FSH in the follicular phase than LH, both increases their peak just prior to ovulation - Just after ovulation, they decline to levels like in the start of the cycle BIO 133 LECTURE Year 03 DEVELOPMENTAL BIOLOGY Term 01 The rise and fall of the estrogen levels are correlated with the growth and decline of the follicular cells Only one “lucky” preovulatory follicle will be targeted by FSH and LH, the rest will degenerate After ovulation, the estrogen will be secreted by the corpus luteum Three distinct phases of the menstrual cycle: FSH – stimulates the development of follicle cells around the ova (primary oocyte stage) of about 5 to 12 in numbers at the start - Menstruation (bleeding phase) of the ovarian cycle - Proliferative phase - Secretory phase LH – stimulate the follicle cells for further growth and maturation Menstruation – time when bleeding occurs in females - Allows the release of the ripe egg during the ovulation period - Takes about 5 days (in average) - After ovulation, it maintains the corpus luteum which - Happens when the ovulated egg is not fertilized by a releases important hormones for maintaining pregnancy sperm cell - If there is no pregnancy, the corpus luteum Proliferative phase – after bleeding stops, the dominant hormone undergoes atresia or degeneration or is estrogen and it stimulates mitosis of the endometrial cells of regression the uterus and also promotes the start of the growth of the Follicular phase – first half of the ovarian cycle uterine glands Luteal phase – other half of the cycle - Progesterone, though at lesser levels than estrogen, also helps in building back the lining of the uterus and - Occurs once the mature egg is ovulated or liberated making it more vascularized prior to ovulation from the ovary - Prepares the endometrial lining for possible implantation of an embryo should there be Transformation of the ovum from the developing form -> mature successful fertilization follicle -> ovulation -> corpus luteum -> atresia Secretory phase – the endometrial lining is already thick enough, Two important steroid hormones are produced by follicle cells: with more blood vessels and uterine glands, all set to receive an embryo for implantation - 17β-estradiol/estrogen - Progesterone - Levels of estrogen are now lower than progesterone - Once an ovum is ovulated, all the follicle cells These hormones are key players in the third level of hormonal around the ovum is damaged and estrogen is control not released Third level of hormonal control – occurs in the ovaries and during - Corpus luteum – will be the one to secrete the pregnancy in the placental tissues estrogen and progesterone which will then maintain the strength and integrity of the A constant level of estrogen is seen from day 0 to about 10 days endometrial lining of the uterus of the cycle, after which there is an upward surge to the 14th day (ovulation) If there is no fertilization of the ovulated ovum, the corpus luteum regresses into corpus albicans until it just leaves a scar on the Estrogen then slowly declines to a level not lower than in the first ovarian tissues half of the cycle then peaks again then declines again prior to menstruation if there is no pregnancy Once atresia of the follicles happen, there is lower levels of estrogen and progesterone, the integrity of the endometrium is weakened until it is “destroyed” again during the menstruation phase (human) chorionic gonadotropin – when there is fertilization and successful implantation of the embryo - Produced by the placental tissue to maintain corpus luteum for it to secrete more estrogen and progesterone Inhibin – stops or slows down further growth of follicle cells before ovulation - Released by the ovary BIO 133 LECTURE Year 03 DEVELOPMENTAL BIOLOGY Term 01 - Along with estrogen, are brought to the hypothalamus - Reduces the activity of the hypothalamo-hypophyseal to stop releasing GnRH axis - Negative feedback mechanism ESTROUS CYCLE Ovarian cycle – the changes in the ovary and the ova that happen in a regular cycle once reproductive capacity and/or capability id Menstrual cycles are only seen in humans and some other reached by the organism primates. Most other mammals have estrous cycles - In mammals Estrous cycles – females are ruled by sexual periodicity Menstrual cycle – occurs only in primate mammals - Females have a period of strong mating impulse and is therefore, sexually receptive - Females are characterized by menstruation - What animal breeders call the “period of heat” - Bloody discharge of the uterine lining, mucus, - Biologists call it estrus and cellular debris at periodic intervals - Among male mammals, breeders call this the - Includes both ovarian and menstrual cycles “rutting” season Menarche – first appearance of menstruation in humans The same reproductive organs and hormones are involved in the - Around 12 to 14 years old, or maybe earlier estrous cycle of these mammals Menopause – during late forties, or maybe earlier of later One difference: no bleeding occurs MALE CYCLE AND ITS HORMONES - Uterus reabsorbs the thickened lining of the endometrium if conception or fertilization of the ova The first and second levels of hormonal actions in males are very does not occur much similar with the females GAMETOGENESIS When a boy reaches the age 10 years old, at the first level of hormonal action, the hypothalamus releases GnRH to stimulate GAMETOGENESIS the adenohypophysis (second level) to release FSH and LH Four phases of gametogenesis: Paired testes – target organs of the FSH and LH 1. The origin of the primordial germ cells and their FSH – stimulates the Sertoli cells or sustentacular cells to secrete migration to the gonads androgen-binding protein (ABP) and other signaling molecules - Gametes are derived from the primordial germ that are necessary for spermatogenesis cells, which are formed in the epiblast during the second week LH – stimulates the interstitial or Leydig cells to secrete - Move through the primitive streak testosterone during gastrulation and migrate to the wall of the yolk sac Leydig cells – located in small clusters among the seminiferous - During the 4th week, they migrate from the tubules in the testes primitive streak to the developing gonads The human pituitary secretes LH in humans in pulses roughly at where they arrived by the 5th week 2. The multiplication of the germ cells in the gonads 90-minute intervals especially in the night through mitosis The local effect of the testosterone is to maintain - Germ cells increase in number spermatogenesis. But since it is transported by blood, it reaches 3. Reduction of the chromosome number by half during other tissues and organs, including the brain of the male body meiosis 4. The final stages of maturation and differentiation of the gametes into spermatozoa and ova: cytodifferentiation ABP – maintains the levels of testosterone in the seminiferous tubules Inhibin – mediates the regulation of the release of FSH and LH BIO 133 LECTURE Year 03 DEVELOPMENTAL BIOLOGY Term 01 5. And in the reproductive life of a woman, there are about 400-500 oocytes that can be ovulated and be fertilized Spermatogonia: 1. Mitosis begin in the embryonic testes and continue to divide throughout the human male’s life 2. The seminiferous tubules of the tetes are lined with a germinative population of spermatogonia - Source of continuous mitosis or proliferation throughout the males life 3. At puberty, there is a wave of mitosis in subpopulations of spermatogonia 4. The progeny of these divisions enter meiosis as synchronous groups at the onset of puberty - Continuous spermatogenesis through male’s At the very start, the events are similar: mitotic proliferation life (spermatogonium & oogonium) The spermatogonium and oogonium will undergo growth and differentiation to transform them into the primary spermatocyte and primary oocyte respectively Meiosis I to transform the primary spermatocyte into two equal sized secondary spermatocytes, the primary oocyte will divide into two unequal products, representing the secondary oocyte and first polar body First polar body – contains less cytoplasm than the secondary oocyte Secondary spermatocytes undergo meiosis II and the products are four equally sized spermatids. The secondary oocyte will only undergo meiosis II if there is fertilization. If there is fertilization, meiosis II occurs which transforms the secondary oocyte to form the ootid and second polar body (unequal sizes). The first polar body will also undergo meiosis II Before meiosis, the cell is 2n and 4c End product of spermatogenesis are highly differentiated cells: spermatozoa Meiosis I – marked by a highly prolonged prophase Spermiogenesis – differentiation of the spermatids into the Prophase I – process of pairing or synapsis of homologous spermatozoa chromosomes would occur Spermatogenesis – yields four spermatozoa - Frequent crossing over results - Exchange of segments between members of Oogenesis – product is one functional egg cell homologous chromosomes Oogonia: Tetrads – two homologous chromosomes 1. Intense mitosis in embryonic ovary from the 2nd through - Line up in the equatorial plate for metaphase I the 5th month of pregnancy - Intense proliferation of oogonia Anaphase I – homologous chromosomes separate to opposite 2. At this time, the increase is from a few thousand to poles nearly 7 million, which then stops here Telophase I – nuclear membrane has been reintegrated - Mitosis has a time limit 3. From 7 million, this number drastically drops to about Cytokinesis – complete division 1M-2M by birth time - Due to atresia and degeneration Products of meiosis I: two cells that are haploid but with 4. Then further drops to about 400,000 by 10 years old chromosomes with 2 chromatids (2c, 1n) and by age 50, the number has declined to almost zero. Decline is due to atresia Meiosis II – second phase of meiosis - Continuous atresia and degeneration BIO 133 LECTURE Year 03 DEVELOPMENTAL BIOLOGY Term 01 - Similar to any mitotic division but resulting cells are Tertiary follicle – mature follicle haploid - No replication of DNA occurring prior to prophase II Mammals are different from other vertebrates because they cannot replenish the oocytes that were generated during birth Metaphase II – the chromosomes line up along the equatorial plate Folliculogenesis – follicular development Anaphase II – sister chromatids separate from each other Follicle – oocyte surrounded by follicle cells Telophase II – there are four cells that are forming Oogonium becomes surrounded by follicle cells, transforming it into the primary oocyte where it enters meiosis I: 4C because it Cytokinesis II – four haploid cells are eventually formed will have 2 sister chromatids attached by the centromere Resulting daughter cells are fully haploid and described as 1n and Further growth would involve the increase in the number of 1c follicle cells but still in the primary oocyte phase Genetic makeup of the chromosomes has already been modified Eventually, the growth will involve formation of the antrum which due to crossing over at prophase I is initially crescent shape and contains fluid: eventually increases in size Different phases of prolonged prophase I: Cumulus oophorus – hillock of cells that contains the oocyte Primary oocyte undergoes the first meiotic division to form the secondary oocyte and polar body I composing the tertiary follicle Secondary oocyte – stage of the egg that is ovulated Ovulated egg will only undergo meiosis II if it is fertilized in order to transform it into the ootid and the second polar body With the sperm inside, the fertilized ovum becomes diploid Leptotene – replicated chromosomes condense When the oogonium starts meiosis, it is now referred to as a Zygotene – synapsis begins primary oocyte, but it remains in an arrested prophase I (diplotene) stage and remain dormant for 40 years: risk of - Pairing of the homologous chromosomes chromosomal abnormalities increase with age and are more vulnerable to damage Pachytene – the bivalent has formed and crossing over has occurred Diplotene – the synaptonemal complex will start to dissociate Diakinesis – nuclear membrane would start to fragment in preparation for entering the next stage Follicle cells are also called the granulosa cells Ovulated oocyte is surrounded by the corona radiata, which is several layers of follicle cells Granulosa cells that remain after ovulation transforms into the corpus luteum At puberty, a pool of growing follicles is established and continuously maintained from the supply of primordial follicles Each month, there are 15-20 follicles that are selected from this pool. Some of these would die whereas others would begin to accumulate fluid in the antrum BIO 133 LECTURE Year 03 DEVELOPMENTAL BIOLOGY Term 01 Fluids continue to accumulate such that immediately prior to Theca interna – capable of steroid secretion ovulation, the follicles are quite swollen and are called mature vesicular follicles or the Graafian follicle Theca externa – gradual merges with the ovarian stroma tissue The granulosa cells/follicle cells change from flat to cuboidal and With each ovarian cycle, a number of ovarian follicles would begin proliferate to become that stratified theca folliculi to develop, but only one develops into a mature follicle Theca folliculi – is differentiated into the theca interna and theca When the follicle is mature, there is a surge in LH, which induces externa the preovulatory phase Meiosis I is completed, resulting in the production of the The granulosa cells and the oocyte would secrete a layer of secondary oocyte and 1st polar body glycoproteins on the surface of the oocyte called the zona pellucida 1st polar body – found lying between the zona pellucida and the membrane of the secondary oocyte The cell enters meiosis II and arrests at metaphase II hours before ovulation - Metaphase II is completed when the egg is fertilized - If no fertilization, it degenerates approximately 24 hours after ovulation The egg and corona radiata at ovulation Follicle maturation and ovulation Timing of pregnancy: Oocytes - ~2 million at birth - Embryologists: - Fertilization age: moment of fertilization is DO - ~40,000 at puberty - Division of pregnancy corresponding to - ~400 ovulated over lifetime development: - 0-3 weeks – early development Luteinizing hormone surge (from pituitary gland) causes changes - 3-8 weeks – embryonic period in tissues and within the follicle: (organogenesis) - Swelling within follicle due to increased hyaluronan - 8 weeks – fetal period - Matrix metalloproteinases degrade surrounding tissue - Total gestation time = 38 weeks causing rupture of the follicle - Clinicians: menstrual age: last menses is divided into trimesters Egg and surrounding cells (corona radiata) ejected into - Menstrual age: last menses is DO peritoneum during ovulation - Division of pregnancy into trimesters - Total gestation time: 40 weeks Corona radiata – provides bulk to facilitate capture of egg As the follicle continue to grow, the cells of the theca folliculi would organize into an inner layer of secretory cells (theca interna) and an outer layer (theca externa) There would be small fingerlike processes of the follicular cells that would extend across the zona pellucida and interdigitate with the microvilli of the plasma membrane of the oocytes - Transport of materials from the follicular cells and the oocyte At maturity, the mature follicle may be 25 mm or more in size. BIO 133 LECTURE Year 03 DEVELOPMENTAL BIOLOGY Term 01 SPERMATOGENESIS Spermiogenesis Acrosome – extends over half of the nuclear surface and contains enzymes to assist in the penetration of the egg and its Testes – composed of seminiferous tubules surrounding layers during fertilization The different stages of sperm development take place and are Spermatid – product of the second meiotic division (haploid) found in each tubule - Spherical At birth, the germ cells in the male infant can be recognized in the sex cords (??) as large pale cells surrounded by supporting Events during spermiogenesis: cells which are derived from the surface epithelium to the testis 1. Formation of the acrosome Sertoli cells/sustentacular cells – support the development of the - Formation of the acrosomal vesicle and Golgi spermatogenic cells apparatus at the apical end of the nucleus - Golgi apparatus at the apical end forms the The primordial germ cells would give rise to the spermatogonial proacrosomal granules which fuse with the stem cells which undergo mitosis acrosomal vesicle 2. Condensation of the nucleus At regular intervals, cells would emerge from this stem cell - Occurs as a result of the decrease in size and population to form a type A spermatogonia significant loss in fluid Production of Type A spermatogonia would mark the initiation of - Chromatin condenses and become granular spermatogenesis and eventually nucleus becomes compacted 3. Formation of the head, neck, middle piece, and tail Type A cells undergo a limited number of mitotic divisions to form - Centrioles become more conspicuous on the clones of cells which after the last cell division would produce other end of the nucleus type B spermatogonia - Becomes the anchorage of the developing flagellum Type B spermatogonia – divide to form the primary - Microtubules of the distal centriole will become spermatocytes which will now start the meiotic division process continuous with the microtubules of the developing flagellum Spermatogenesis is regulated by LH production by the pituitary - The mitochondria begin to form a spiral gland investment around the proximal part of the LH – binds to receptors in the Leydig cells and stimulate flagellum testosterone production - Becomes part of the middle piece 4. Shedding of most of the cytoplasm as residual bodies Testosterone – binds to Sertoli cells (androgen-binding protein) phagocytosed by the Sertoli cells to promote spermatogenesis - The rest of the differentiation involves the cytoplasm FSH – binding to Sertoli cells stimulates testicular fluid production - Becomes aggregated into a remnant: and synthesis of the intracellular androgen-receptor proteins residual body - Residual body is detached from the differentiating spermatozoon and phagocytosed by Sertoli cells At the end of spermiogenesis, the result would be a sperm cell which consists of a head that contains the nucleus and acrosome, middle piece that contains the centriole, proximal part of flagellum, mitochondrial helix, and the tail BIO 133 LECTURE Year 03 DEVELOPMENTAL BIOLOGY Term 01 The spermatozoa formed in the seminiferous tubules immature - Glycoprotein coat and seminal plasma proteins morphologically because they are nonmotile and incapable of are removed from the plasma membrane that fertilizing an egg overlies the acrosome of the spermatozoon Physiological or biochemical maturations has to occur when the Acrosome reaction – occurs after binding with the zona pellucida spermatozoa travel to the epididymis via the currents of the - Induced by zona proteins seminiferous tubules: - Culminates in the release of enzymes needed to - Glycoprotein substances are added to their surfaces penetrate the zona pellucida - Eventually be removed when the spermatozoa - Acrosine & trypsin gets into the female genital tract: capacitation Steps/components of fertilization - Enables successful fertilization - Initial membrane contact between egg and sperm In humans, the time required for a spermatogonia to develop into - Entry of the sperm cell into the egg a spermatozoon is 74 days. 300 million sperm cells are produced - Prevention of polyspermy daily - Once sperm has entered, barriers should be put up FERTILIZATION - Metabolic activation of the egg FERTILIZATION - Completion of meiosis by the egg - Secondary oocyte is still in the arrested Fertilization – fusion of the male and female gametes that occurs metaphase II stage in the ampullary region of the uterus, which is the widest part of - Formation of the male and female pronuclei the tube and close to the ovary - Fusion of the pronuclei Spermatozoa may remain viable in the female reproductive tract Fertilization is a multi-step process whereby multiple sperm bind for several days to the corona radiata, but only a single sperm usually fertilizes the egg Only 1% of the sperms deposited in the vagina enters the cervix where they may survive for many hours The movement of the sperm from the cervix to the uterine tube occurs by muscular contraction of the uterus and uterine tube. Very little may be because of the own propulsion of the sperms The trip from the cervix to the oviduct can occur as rapidly as 30 minutes to as slow as 6 days After reaching the isthmus, the sperm becomes less motile and cease their migration At ovulation the sperms become motile again because of chemoattractants produced by cumulus cells surrounding the egg and they swim to the ampulla where fertilization occurs Acrosome reaction – sperm bind to zona pellucida proteins in the Chemoattractants – produced by the cumulus cells that form the zona pellucida corona radiata around the egg - Initiated the release of enzymes from the sperm Sperms are not able to fertilize the oocyte immediately upon allowing it to burrow through the zona pellucida arrival in the female genital tract and have to undergo capacitation and acrosome reaction Zona reaction – binding of the sperm and egg plasma membranes initiates Ca+ influx into the egg and release of cortical granules Capacitation – removal of glycoproteins in the sperm as they pass from the egg that blocks other sperm from fertilizing the egg the female genital tract - First block to polyspermy - Period of conditioning in the female reproductive tract - In humans, can last for approximately 7 hours - Speeding into the ampulla is not an advantage: sperm is unable to fertilize the egg without capacitation - Occurs in the uterine tube and involves epithelial interactions between the sperm and mucosal surface of the tube BIO 133 LECTURE Year 03 DEVELOPMENTAL BIOLOGY Term 01 Fertilization in sea urchin - Contents of the granules are enzymes and macromolecules - Trigger the cortical reaction - Cortical reaction results in the lifting of the vitelline layer from the plasma membrane, increasing the distance between the two - Eventually, the layer would harden into a protective fertilization envelope - Make certain enzymes cut of the sperm-binding receptors, discarding other sperms attached to the egg surface The egg contains the jelly coat and sperm-binding receptors that - One of the mechanisms for are found on the egg membrane prevention of polyspermy Cortical granules are found aligned on the cortical layer of the Cortical granule enzymes – digest zona pellucida proteins so cell other sperm can no longer bind First process is contact: Hyaluronic acid & other proteoglycans – also released that become hydrated and swell, thus pushing the other sperm away - Sperm establishes contact with the egg jelly coat - Triggers the exocytosis of the sperm’s acrosome - Calcium ions from the sea water move into the acrosome - Responsible for the swelling of the acrosome , disorganization of the acrosomal membrane and eventually the release of hydrolytic enzymes from the acrosome - Hydrolytic enzymes – digests or clears the path for the sperm from the jelly coat to the vitelline envelope - Allows deeper penetration of sperm to reach the vitelline layer Second process is the acrosomal reaction: - Jelly coat has been digested - Actin filaments coming out from the acrosome: acrosomal processes - Protrude out from the sperm head and penetrate the jelly coat even deeper - Proteins (bindin) on the acrosomal process bind with the receptors (species-specific) found on the egg plasma membrane - Formation of the fertilization cone - Plasma membrane of the egg is covered with microvilli or finger-like processes and several of this can elongate to surround the egg - Once membrane of both the sperm and egg fuse, a fertilization cone is formed Three phases of oocyte penetration/fertilization - Draws the sperm into the egg by 1. Penetration of the corona radiata contraction - Corona radiata – cells of the cumulus oophorus Next step is the cortical reaction: that join the secondary oocyte during ovulation 2. Penetration of the zona pellucida - Just beneath the egg plasma membrane is the cortex 3. Fusion of the sperm and oocyte cell membrane - Contains numerous vesicles called the cortical - Sperm loses its own plasma membrane granules - In a matter of seconds, as soon as the sperm binds to Penetration of the corona radiata the egg membrane, the contents of the cortical granules - 200-300 million sperms deposited in the female genital are released into the space between the plasma tract membrane and vitelline layer - Only 300-500 reach the fertilization site BIO 133 LECTURE Year 03 DEVELOPMENTAL BIOLOGY Term 01 - Only one fertilizes the egg 2. Zona pellucida alters its structure and composition - It is thought that the others aid the fertilizing sperm in to prevent sperm binding and penetration penetrating the barrier protecting the female gamete - Prevent polyspermy: penetration of more than one - Capacitated sperms pass freely through the corona sperm into the oocyte radiata - Very immediate reactions - Capacitation involves interaction between epithelium of the uterine tube and the sperm: Prevention of polyspermy glycoprotein coat that covers the sperm is - Fast-block – reaction of cortical granules and zona removed pellucida Penetration of the zona pellucida - Slow-block – formation of the very thick fertilization envelope - Zona pellucida – glycoprotein shell surrounding the egg and maintains sperm binding and induces the acrosome Resumption of the second meiotic division: reaction - Completion of meiosis II right after penetration of the - Both binding and acrosome reaction are mediated by sperm into the oocyte the ligand ZP3, a zona protein - Products are the second polar body with hardly any - Release of acrosomal enzymes (Acrosin) allows sperm cytoplasm and the definitive oocyte to penetrate the zona thereby coming in contact with - The chromosomes (22+X) arrange themselves into a the egg cell membrane vesicular nucleus called the female pronucleus - Permeability of the zona pellucida changes when the head of the sperm comes in contact with the oocyte Metabolic activation of the egg: surface - Results in the release of the lysosomal - The activating factor is probably carried by the enzymes from the cortical granules lining the spermatozoon plasma membrane of the oocyte - Activation encompasses the initial cellular and molecular - Alters the properties of the zona events associated with early embryogenesis: pellucida to prevent sperm - Increase in aerobic respiration penetration and inactivate species- - Maternal enzymes and proteins become active specific receptor sites for (which were already present in the egg prior spermatozoa on the zona surface to fertilization) - Other spermatozoa have been found embedded in the - More proteins are synthesized in preparation zona but only one penetrates the oocyte for cleavage - Egg nucleus is stimulated to complete meiosis Fusion of the oocyte and sperm cell membranes - Initial adhesion of the sperm to the oocyte mediated by the integrin of the oocyte and their ligands disentegrins on the sperm - Chemical communication via integrin and disintegrin - After adhesion, the plasma membranes of the sperm and the egg fuse - Plasma membrane covering the acrosome disappears because it releases the hyaluronidase (hyaluronic acid) - What is left is fused with the membrane of the oocyte A: arrested metaphase II stage - Further binding and adhesion through B: sperm has entered the oocyte which is now finishing its second binding of integrin and disintegrin meiotic division; formation of female pronucleus Reaction of the egg when the sperm enters the oocyte C: male and female pronucleus 1. Cortical and zona reactions D & E: chromosomes become arranged in the spindle fibril and 2. Resumption of the second meiotic division eventually split longitudinally and move to the opposite poles 3. Metabolic activation of the egg F: first cleavage division Cortical and zona reactions - Release of the cortical granules which contain lysosomal enzymes: 1. Oocyte membrane becomes impenetrable to other sperms BIO 133 LECTURE Year 03 DEVELOPMENTAL BIOLOGY Term 01 Flagella, tail, and mitochondria are left outside or if they come inside, they eventually detach in the cytoplasm of the egg cell and degenerate Centrioles of the sperm persist with the nucleus and guide the sperm in moving toward the female pronucleus by forming the sperm asters Fusion of the male and female pronucleus - Both pronuclei would grow further as they move closer to each other - When both pronuclei come in contact with each other, their membranes would fuse, making the maternal and paternal chromosomes enclosed in a single membrane - Called the pronuclear fusion - Haploid maternal and paternal chromosomes replicate and synthesize their DNA in preparation for the first cleavage division - Interphase of the cell cycle - Meiosis II complete By the time the chromosomes are aligned in the metaphase plate, - Formation of male and female pronuclei the process of fertilization is complete and the fertilized egg is - Spermatozoon moves forward until it lies very now called a zygote close to the female pronucleus - The tail detaches and degenerates - Morphologically indistinguishable from each other - Decondensation of male chromosomes - Fusion of pronuclei - Eventually come into contact and lose their nuclear envelope - Each pronucleus must replicate its DNA - Immediately after, the chromosomes organized at the equatorial plate in preparation for the first cleavage - Longitudinal split - As sister chromatids move to the opposite pole, a deep furrow appears on the surface of Fertilization stimulates the egg to finish the second meiotic the cell, gradually dividing the cytoplasm into division. It restores the normal diploid number of chromosomes two cells: cleavage furrow by bringing together the haploid sperm and haploid egg - Continues to deepen until two cells are completely divided The sex of the future embryo is determined by the chromosomal - Zygote complement of the spermatozoon (Y chromosome = male) Formation of the male and female pronuclei Mixing of the maternal and paternal chromosomes: combination of the reassortment of traits of the parents leading to genetic Chromosomal materials of the egg after the second meiotic variation division become surrounded by a pronuclear membrane which is said to be derived from the endoplasmic reticulum. With the Fertilization activates the egg metabolically, causing it to proceed formation of the pronuclear membrane, the female pronucleus to the next stage of embryonic development: cleavage has formed. CLEAVAGE, BLASTULATION, AND For the sperm nucleus, once the head of the sperm has entered GASTRULATION the egg cytoplasm, the permeability of the nuclear membrane is increase, leading to its disintegration. This makes the sperm’s CLEAVAGE AND BLASTULATION chromatin to spread out in the egg’s cytoplasm: Decondensation Cleavage – rapid cell division resulting in a single fertilized egg A new nuclear membrane forms around the chromatin material into a multicellular embryo known as the blastula or blastodisc and is called the male pronucleus - Involves intense cellular assembly BIO 133 LECTURE Year 03 DEVELOPMENTAL BIOLOGY Term 01 - Heightened activity for DNA and protein - Spiral cleavage – cleavages are rotated from the first synthesis, then packaging them into division chromosomes - Occurs such that the resulting daughter cells - New plasma membranes must be available for are not located exactly on top of one another; all the daughter cells that results from the instead, they are located at a slight angle cytokinesis of parent cells - In protostomes such as annelids, Mollusca, and some invertebrates Cell division may be slowed down by moderate amount of yolk - Protostomes – primitive invertebrates resulting in formation of unequal size of cells - Blastopore is developed into a mouth The egg As soon as a sperm enters, opposite to the entry of the sperm, - Animal pole – consisting the upper hemisphere of the microtubular structures pull and rotate the cortex and core of the egg (dark stained area) vegetal pole 30 degrees to the animal pole - Vegetal pole – consisting the lower hemisphere of the egg (light stained area) Gray crescent – lighter zone produced by the rotation or - Unfertilized eggs – unsegmented and enclosed tightly movement of the cortex and vegetal pole by transparent jelly coats for protection - The first cleavage division bisects the gray crescent Yolk – proteins, phospholipid, and fats that are food for the - Marks the future dorsal side of the embryo as well as developing embryo the other axes Classification of eggs based on amount of yolk - Megalecithal – huge amount of yolk present - Ex: birds, reptiles, bony fish - Mesolecithal – moderate amount of yolk present - Ex: amphibians - Microlecithal – very little amount of yolk present - Ex: amphioxus, mammals Classification of eggs based on distribution of yolk - Isolecithal – yolk is evenly distributed all throughout the egg - Ex: sea urchins, amphioxus, humans - Telolecithal – yolk is distributed in a gradient - Ex: reptiles, fish, birds, amphibians - Centrolecithal – yolk is concentrated in the center of the egg Stages and parts of cleavage in Frog’s embryo - Ex: mostly arthropods Cleavage types: - Holoblastic – total cleavage formation - Entire egg is divided thus resulting a successive blastomeres - Equal – microlecithal eggs, equal sizes of divided cells - Unequal – mesolecithal eggs, ventral region of the egg is larger than on the dorsal region Morula – at about 16-cell to 32-cell stage - Meroblastic – there is a small area on the animal pole - A solid ball of blastomeres that division only occurs during - Results in a blastodisc Blastula stage – several hundreds of blastomeres after more cleavage events happen Cleavage patterns: Avian blastula - Radial cleavage – symmetrical cleavages at first division - Occurs such that the resulting daughter cells - Blastodisc, germinal disc, or blastoderm are located exactly on top of one another - In deuterostomes such as echinoderms and chordates - Deuterostomes – blastopore is developed into an anal opening BIO 133 LECTURE Year 03 DEVELOPMENTAL BIOLOGY Term 01 Differences during the cleavage of a mammalian zygote: 1. As early as the 2-cells stage, a new gene transcription is detected, called zygotic gene transcription 2. Limited maternal control or organization of the egg or embryo 3. There is almost no yolk (alecithal) Area pellucida – cells here have less yolk and do not lie directly on the yolk mass 4. Elaborate extracellular membranes are formed very early in its development Area opaca – region of the blastodisc that is lying above the yolk 5. Some genes in the male and female may be expressed differentially during embryonic development, a Subgerminal cavity – space separating the yolk from the phenomenon called parental imprinting blastodisc At the 8-cell stage of the mammalian embryo, compaction takes Epiblast – layer of cells left when some cells of the blastodisc place: the blastomeres become tightly bound to each other detach from it (as it travels down the oviduct) and die within the subgerminal cavity The loosely attached blastomeres become a true polarized epithelium. The outer cell surfaces bear microvilli, and tight Posterior marginal zone – made of cells in the periphery of the junctions develop between adjacent cells blastodisc that meets with the yolk Polarized – apical surfaces face the outside of the embryo and basal surfaces facing the inside At the next (fourth) division, many of the cell division planes are angled such that the resulting cells remain on the surface, but some divisions take place with the division planes angled parallel to the surface, thus generating some three or four “internal” cells Primary hypoblast – transient or temporary layer formed by delaminated or detached cells Inner cell mass – internal cells Köller’s sickle – cells from the deeper region of the posterior zone Trophoblast – external cells that migrate forward and join with the cells of the initial hypoblast Gap junctions – formed among the internal cells - Forms the secondary hypoblast (or simply the hypoblast) Blastocoel – space between the epiblast and the hypoblast Mammalian blastula - Inner cell mass by delamination eventually becomes the epiblast and the hypoblast Cavitation – important process that occurs in the 32-cell stage - Leads to the formation of the blastocyst cavity (blastocoel) - Involves the build-up of fluid within the cavity - Accumulate through the sodium transport system based on Na+, K+-ATPase that develops in the outer blastomeres Right after fertilization, the zygote divides by mitosis into two daughter cells, commencing a series of mitotic divisions as it travels down to the uterus. When it is made of 16 to 32 cells, it is called a morula By the time it reaches the uterus, the embryo is now called a blastocyst Cleavage in mammals takes place at a slower pace compared to the other nonmammalian animals BIO 133 LECTURE Year 03 DEVELOPMENTAL BIOLOGY Term 01 - Regulative development – distribution of the determinants or important molecules are homogenous/same - Embryo develops as a self-regulating whole GASTRULATION Fate map of the frog At the blastocyst stage, the embryo consists of two distinct groups of cells: - Smaller group called the inner cell mass (ICM) - Outer superficial layer called the trophoblast Inner cell mass – gives rise to the future embryo - By delamination forms the outer epiblast and inner Gastrulation – migration of cells leading to the formation of a 3- hypoblast germ layered embryo, the gastrula - Outer epiblast – gives rise to the entire embryo - A highly orchestrated movement of cells that establish - Inner hypoblast – from where the extra- the basic body plan embryonic membrane develops - Cells are brought into new positions, allowing inductive Trophoblast – forms the placenta interactions Additional cell divisions take place, and the germ layers become established through a combination of processes, such as: - Many cells lose their old cell-to-cell contacts and establish new ones through cell recognition and adhesion processes involving interactions among the integrins and other plasma membrane proteins and the extracellular matrix Attachment in the uterus - Many cells undergo cytoskeletal changes - Alterations in the distribution of actin microfilaments - Changes in the internal architecture of the cells - Allow them to change shape and/or undergo specific, directional amoeboid movements Morphogenetic movements: - Invagination – inpocketing of cell sheet into the embryo - About 7 days after fertilization, the blastocyst drifts to - Ex: sea urchin endoderm an appropriate site along the uterine wall and begins to implant using the polar trophoblast (layer closer to ICM) - About 10 days after fertilization, the embryonic disc (blastodisc) resulting from cleavage has 3 layers, the epiblast and hypoblast - Blastocoel – space surrounded by the yolk sac - Involution – inturning of the cell sheet over the basal - Chorion – formed from the trophoblast surface of the outer layer - Makes the future embryo connected - Ex: amphibian mesoderm with the maternal blood vessels Kinds of development in animals based on distribution of determinants: - Mosaic development – developmental patterns are relatively rigid - Ingression – migration of individual cells into the - Distribution of important materials during embryo division is not homogenous/equal - Ex: sea urchin mesoderm, Drosophila - Course of development of the cells run neuroblasts differently BIO 133 LECTURE Year 03 DEVELOPMENTAL BIOLOGY Term 01 3. Epiboly – blastocoel collapses and is replaced by the archenteron - Formation of 3 germ layers: ectoderm, mesoderm, endoderm - Yolk plug is internalized - Completion bridge – thin layer between the - Delamination – splitting or migration of one sheet into forming gastrocoel and the diminishing two sheets blastocoel - Ex: mammalian and bird hypoblast formation - Epiboly – the expansion of one cell sheet over the other cells - Ex: ectoderm formation in amphibians, sea Frog gastrulation: urchins, and tunicates Amphibian gastrulation - The cells from the animal pole move down the embryo surface (epiboly) until they reach the region of the gray crescent and move into the anterior by invagination - Made possible by the change in shape of cells - Dorsal lip of the blastopore – spot where the cells turned inward - At the movement inside (involution) continues, the blastopore becomes ring-shaped as cells lateral, and then ventral, to its dorsal lip become involved in similar movements 1. Invagination – pharyngeal endoderm - Yolk plug – formed by the yolk-filled cells filling the - Cells begin to invaginate at the dorsal lip of the space enclosed by the lips of the blastopore blastopore - As the new cavity archenteron enlarges as the cells roll inside, the original blastocoel is pushed until it disappears - At the end of the gastru