Transcript Cleavage
VAN 504, Lecture 07 Embryology: Gametogenesis, Fertilization, Cleavage, Gastrulation Gametogenesis The sequential stages in the differentiation and maturation of primordial germ cells into gametes in male and female animals are referred to as gametogenesis. Two Types of Gamete Formation • Spermatogenesis – the process of male gamete production in animals Oogenesis – the process of female gamete production in animals Spermatogenesis • • • • • • • • Spermatogenesis occurs inside testes Within testes spermatogenesis occurs in walls of seminiferous tubules Mature sperm are released into lumen of seminiferous tubules. Spermatigonia & Primary spermatocytes are 2n Secondary spermatocytes are 1n Spermatids and sperm are 1n The process whereby a spermatid undergoes metamorphosis into a spermatozoon is termed spermiogenesis The time required for the production of spermatozoa from type A spermatogonia may range from 40 to 60 days depending on the species. Oogenesis • Oogenesis begins with a diploid cell called an oogonium. • After growth and development, one oogonium forms one primary oocyte. • Meiosis I produces a secondary oocyte and one polar body (due to unequal division of the cytoplasm). • Meiosis II in the secondary oocyte produces an egg and polar body. Meiosis II in first polar body produces 2 polar bodies. • The egg survives, while all the polar bodies die. • Only one functional egg cell comes from this process, as the unequal division of the cytoplasm makes the egg cell big (needs extra nutrients). Oogenesis • • • • • • • Oogenesis occurs inside the ovary Within ovary oogenesis occurs in a follicle Eggs are released into oviduct Oogonium & Primary oocytes are 2n Secondary oocytes are 1n Polar bodies and eggs are 1n Release of the ovum from the follicle is referred to as ovulation Oogenesis Fertilization • Fertilization is the process of union of mature male gamete (sperm) with mature female gamete (ovum) to produce new cell of life which is called (zygote) through chain of events in the oviduct (fallopian tubes). Indeed, interruption of any event will cause fertilization failure. Folliculogenesis and ovulation • Folliculogenesis is the maturation of the ovarian follicle, describes the progression of a number of small primordial follicles into large mature follicles under the influence of FSH prior to puberty. • Ovulation By the end of the follicular phase the mature follicle will develop and rupture, excrete the oocyte with some granulosa cells into oviduct. The oocyte is now competent to undergo fertilization. • Ruptured follicle transformed into the corpus luteum, that produce large amount of progesterone that helps to prepare the uterus for implantation of fertilized egg. • In absence of fertilization corpus luteum degenerates. Image showing phases of follicle development and ovulation Ovum structure Mechanisms of ovum transport • Fimbria on terminal oviduct--acts as funnel to receive ovum • Fluids (abdominal cavity and that escaping from follicle during ovulation) serve as medium for free-floating ovum • Cilia lining oviduct and muscular contractions assists in moving ovum to site of fertilization The site of fertilization on most farm animals is (ampullary-isthmic junction) while in human is (ampulla) region. Fertile life of ovum • Estimate fertile life of ovum in different mammals is shown below Species Fertile life Cow 8-12 hours Sheep 16-24 hours Swine 8-10 hours Human 6-24 hours Mating process Is the period when female receptive and will stand for mating with males during estrous phase of estrous cycle, the main purpose of mating is deposition of fertile semen into female genital tract after ejaculation, characteristics of ejaculate in different mammals is shown below species Cow Sheep Swine Human Site of semen deposition Vagina Vagina Cervix and uterus Vagina Volume of ejaculate (ml) Sperm concentration (billion/ml) No. of sperm reaching site of fertilization 4 1 4200-27500 1 2 600-5000 125 0.200 Few 3.5 0.120 Few • Deposition of semen into female tract must be closely synchronized with ovulation to ensure the incidence of fertilization • Time of ovulation in different mammals shown in table below species Cow Sheep Goat Swine Human Time of ovulation 10-12 hours after end of estrous phase Late of estrous phase Few hours after end of estrous phase Mid-estrous phase Day(14) after initiation of menstrual cycle Mechanisms of sperm transport • Pass into cervix by own movement • As the sperm enter the cervix, orient themselves into current of thin mucus during estrus • Myometrium moves them through the uterus • Moved through oviduct by cilia of oviduct and uterine contractions and a thin fluid secreted by glandular cells. Fertile life of sperm • Estimate fertile life of sperm in different mammals is shown in table below Species Fertile life Cow 24-48 hours Sheep 30-48 hours Swine 24-48 hours Human 28-48 hours Fertilization events 1. Sperm capacitation: Freshly ejaculated sperm are unable to fertilize an egg. Rather, they must first undergo a series of changes known as capacitation. Capacitation is associated with removal of adherent seminal plasma proteins, reorganization of plasma membrane lipids and proteins. Acrosome reaction The acrosome reaction involves breakdown and fusion of outer acrosome membrane with the plasma membrane of the sperm. This results in formation of vesicles and release of enzymes needed for sperm to penetrate the cumulus oophorus and corona radiata as well as zona pellucida . During the penetration process the acrosome is lost with only the inner acrosomal membrane remaining around the sperm head. Sperm penetration • Sperm cell penetrate cumulus oophorus by the enzyme (hyaluronidase) released during acrosome reaction. • Sperm cell penetrate corona radiata by the enzyme (corona-penetrating enzyme) also released during acrosome reaction. • Sperm cell penetrate zona pellucida by the acrosin (trypsinlike enzyme) also released during acrosome reaction. • Then membrane of sperm fuses with the vitelline membrane of the egg, the egg cytoplasm around the area of contact surrounds the sperm head and incorporating it into the egg • The nucleus of sperm is then release into the egg cytoplasm without the tail Consequences of fertilization • In most mammals after releasing the sperm nucleus into egg cytoplasm it stimulates the diffusion of cortical granules into the previtelline space, the erection of a barrier to prevent fertilization by more than one sperm will occur, this process is called zona reaction and vitelline block. • Then the male and female pronuclei are formed and unite (syngamy) to establish the diploid one cell zygote. • Time of formation of zygote and time required to entry the fertile ovum into uterus in different mammals is shown in table below Species Cow Sheep Goat Swine Human Time of formation of zygote (hours) Entry to uterus (days) 0-24 3-4 0-38 2-4 0-30 4 0-15 2-4 0-24 3 Post fertilization events • The first of many changes following fertilization is to become multicellular, and the one-cell embryo rapidly cleaves into 2, 4, 8 and more cells. • It then starts to do some interesting things like develop a discrete inside and an outside. • Finally, the embryos of many species start to secrete hormones that ensure their survival - a process called maternal recognition of pregnancy. The Zygote • The zygotea single cell formed at fertilisation. • Structure. --diploid nucleus from both parents. --cytoplasm is maternal. --surrounded by zona pellucida • Cleavagemitotic division at 12hours, 2days and 3days in mouse, pig and dog. • Rate about one division/day. • Zygote period lasts from fertilisation to hatching of the blastocyst. • Nutrition/embryotroph --mammalian zygotes is provided by uterine secretions/ --histiotroph and the zygotes own reserve. --avian zygotes feed on the yolk. Cleavage • Cleavage is the first phase of embryonic development Functions of cleavage: Multicellular for differentiation The zygote is partitioned into blastomeres. Each blastomere contains different regions of the undivided cytoplasm and thus different cytoplasmic determinants. Mammalian Cleavage • • • • • in oviduct slow cell divisions asynchronous cell divisions accompanied by gene expression produces a blastocyst – inner cell mass - primordial embryo – trophoblast - primordial placenta component Cleavage • Cleavage is a rapid series of mitotic divisions that occur just after fertilization. • There are two critical reasons why cleavage is so important: 1. Generation of a large number of cells that can undergo differentiation and gastrulation to form organs. • 2. Increase in the nucleus / cytoplasmic ratio. Eggs need a lot of cytoplasm to support embryogenesis. It is difficult or impossible for one nucleus to support a huge cytoplasm, and oocytes are one of the largest cells that exist. One small nucleus just cannot transcribe enough RNA to meet the needs of the huge cytoplasm. Cleavage differs from normal mitoses in 2 respects 1. Blastomeres do not grow in size between successive cell divisions as they do in most cells. This leads to a rapid increase in the nucleus / cytoplasmic ratio. Cells undergoing cleavage. 2. Cleavage occurs very rapidly, and mitosis and cytokinesis in each round of cell division are complete within an hour. Typical somatic cells divide much more slowly (several hours to days) and even the fastest cancer cells divide much slower than occurs in a zygote during cleavage. • Cleavage differs in different types of eggs. The presence of large amounts of yolk alters the cleavage pattern, leading to incomplete cleavage that characterizes birds and reptiles. Eggs are classified by how much yolk is present 1. Isolecithal eggs (iso = equal) have a small amount of yolk that is equally distributed in the cytoplasm (most mammals have isolecithal eggs). 2. Mesolecithal eggs (meso = middle) have a moderate amount of yolk, and the yolk is present mainly in the vegetal hemisphere (amphibians have mesolecithal eggs). 3. Telolecithal eggs (telo = end) have a large amount of yolk that fills the cytoplasm, except for a small area near the animal pole (fish, reptiles, and birds). 4. Centrolecithal eggs have a lot of yolk that is concentrated within the center of the cell (insects and arthropods). The pattern of cleavage of the zygote depends upon the pattern of yolk distribution 1. Holoblastic cleavage: occurs in isolecithal eggs (mammals, sea urchins). The entire egg is cleaved during each division. 2. Meroblastic cleavage occurs when eggs have a lot of yolk. The egg does not divide completely at each division. Two types: a. Discoidal cleavage is limited to a small disc of cytoplasm at the animal pole. All of the yolk filled cytoplasm fails to cleave (characteristic of telolecithal eggs such as birds). b. Superficial cleavage is limited to a thin surface area of cytoplasm that covers the entire egg. The inside of the egg at is filled with yolk fails to cleave (centrolecith al eggs such as insects). Mammalian eggs have rotational cleavage that is holoblastic • The mammalian egg is a little slow. It begins to cleave in the oviduct and continues until it implants in the wall of the uterus (1 cleavage / 24 hr). • Asynchronous cleavage: mammalian embryos are unusual in that they have asynchronous cleavage. Not all blastomeres divide at the same time. • The first cleavage is meridional, and the second cleavage is rotational. The 2 blastomeres divide in different planes (one is equatorial and one is meridional. Mammalian embryos undergo compaction at the 8 cell stage • At first, the blastomeres of mammalian embryos have a loose arrangement, and touch only at the basal surfaces. • After compaction, blastomeres adhere tightly, maximizing the area of contact. • During compaction, each blastomere undergoes polarization. Tight junctions develop on the outer surface, allowing proteins to specialize. Cells take up fluids from the uterine environment and secrete into the blastocoel. • Gap junctions form on the outer cells to aid in intercellular communication A blastocoel develops as cleavage proceeds to the 32-64 cell stage • After compaction at the 8-16 cell stage, there are 2 types of blastomeres. Outside blastomeres are tightly joined and number about 9-14. They surround 2-7 inside blastomeres that are loosely joined. • Cavitation: the outside blastomeres start to take up fluid from the uterus and pump it into the center, creating the blastocoel. The blastocyst is the hallmark of early embryonic development in mammals. Trophoblast: a structure consisting of outside blastomeres, this contributes to forming the placenta Inner cell mass: this gives rise to the embryo, and develops from the inside blastomeres • The next step in development of telolecithal eggs is formation of the upper and lower blastoderm. • Epiblast: (epi = upon) this is the upper layer and it forms the embryo proper. Hypoblast: (hypo = under) this is the bottom layer that will form the extraembryonic endoderm that surrounds the yolk. • Blastocoel: lies between the 2 layers. • Subgerminal space: lies between the hypoblast and yolk Timing of cleavage divisions • • • Normal eukaryotic cells divide slowly, once every several hours or days. The cell cycle has G1 and G2 periods. During G1 the cell synthesizes RNA and other components for cell growth. Cleavage consists of very rapid successive mitoses. Since the egg has stored large amounts of RNA and other material, it does not need G1 or G2. However, as the number of cells increases, the nucleus / cytoplasmic ratio also increases. The rate of cell division slows because the cell now needs to synthesize its own RNA and grow between divisions. Thus, G1 and G2 are restored = midblastula transition. Unique Features of Mammalian Cleavage • Mammalian Cleavage Cleavage is series of mitotic division of the zygote into progressively smaller cellular units; blastomeres • First cleavage synchronous. • Differences in pattern of cleavage dependent upon amount of yolk meroblastic /incomplete and holoblastic/complete • Totipotent early blastomeres • 1st cleavage is meriodal • 2nd cleavageone meriodional and one equatorial/termed rotational cleavage • Second cleavage not synchronised in 2 blastomeres • Compaction occurs at 8-cell stage. --blastomeres flatten and form intercellular connections --E-CAD(cadherin:glycoprotein) on cell surface adhesion. --microvilli(actin) extend on surfaces of adjacent cells and anchor cells together. --tight junctions prevent free exchange of fluid between the inside and outside allowing accumulation of fluid inside blastomeres. --the gap junctions couple all the blastomeres of the compacted embryo and permit exchange of ions and small molecules from one cell to the next. The Morula • Morula; 16-cell stage, embryo enclosed in the zona pellucida. • Late morula, first differentiation event in mammalian development. --cells aggregate into internal inner mass cells(ICM) and external trophoblast. --at 64-cell stage ICM and trophoblast form distinct populations. • Trophoblast/trophoectoderm forms ectoderm of chorion/ placenta. • Inner mass cells are pluripotent; form embryo and partly extraembryonic membranes. Blastulation Formation of the blastocyst • --A series of rapid cell divisions produce a blastula of 64 cells with an inner cell mass(ICM) and an outer layer of trophoblasts. --trophoblast secrete fluid into the morula. Creates a cavity; the blastocoele(A) --ICM/embryo proper lies to one side • Transition from morula to blastula marked by: --rapid enlargement of blastocoele --differentiation of blastomeres into ICM and trophoblast cells(B). • Trophoblast cells induce special changes in the uterine lining at implantation. Trophoblast cells preferentially express maternal genes, and inactivate paternal genes. • • The blastula hatches from the zona pellucida and implants in the uterus. • Blastogenesis starts at 32-cell stage. Formation of hypoblast and epiblast. • Formation of hypoblast begins late in blastulation 1. Segregation(A) 2. Delamination of ICM cells.Cells expand beneath trophoblast form hypoblast(B)/extraembryonic endoderm • Hypoblast tube(blastocoele) inside tube of trophoblast(C) • Formation of hypoblast results in two-layered embryo(C) • Cells on surface ICM form epiblast(C) • • • Blastula: Hatching and Implantation Blastulation embryo arrives in uterus(4) Blastula hatches from zona pellucida and contact uterus. Blastocyst surrounded by ZP prevents premature implantation and ectopic pregnancy. • Hatching involvesA trypsin-like protease lyses of ZP --Trophoblast cells secrete proteases which degrades the endometrial wall and blastocyst embeds(5). 3 1 4 4 5 2 Gastrulation • Gastrulation rearranges the cells of a blastula into a three-layered embryo, called a gastrula, which has a primitive gut • The three layers produced by gastrulation are called embryonic germ layers – The ectoderm forms the outer layer – The endoderm lines the digestive tract – The mesoderm partly fills the space between the endoderm and ectoderm Organogenesis • During organogenesis, various regions of the germ layers develop into rudimentary organs • Early in vertebrate organogenesis, the notochord forms from mesoderm, and the neural plate forms from ectoderm • The neural plate soon curves inward, forming the neural tube • The neural tube will become the central nervous system (brain and spinal cord) • Neural crest cells develop along the neural tube of vertebrates and form various parts of the embryo (nerves, parts of teeth, skull bones, and so on) • Mesoderm lateral to the notochord forms blocks called somites • Lateral to the somites, the mesoderm splits to form the coelom ECTODERM MESODERM ENDODERM Epidermis of skin and its derivatives (including sweat glands, hair follicles) Epithelial lining of mouth and anus Cornea and lens of eye Nervous system Sensory receptors in epidermis Adrenal medulla Tooth enamel Epithelium of pineal and pituitary glands otochord Skeletal system Muscular system Muscular layer of stomach and intestine Excretory system Circulatory and lymphatic systems Reproductive system (except germ cells) Dermis of skin Lining of body cavity Adrenal cortex Epithelial lining of digestive tract Epithelial lining of respiratory system Lining of urethra, urinary bladder, and reproductive system Liver Pancreas Thymus Thyroid and parathyroid glands How is the characteristic body plan for any organism developed • Gastrulation: the first step in the process of body formation. It transforms a complex sphere into 3 basic germ layers from which all other tissues develop. • • • Ectoderm is the outer layer = forms epidermis and nervous system. Mesoderm is in the middle and forms a many structures (i.e., heart, muscles). Endoderm is the inner layer and forms the ‘gut’ and related organs. • The first change is to generate the rudiment of the digestive tract, hence the name gastrulation (gastric = stomach). Gastrulation is the first step of morphogenesis • Morphogenesis is the process whereby individual cells undergo complex movements that generate the organ rudiments. Gastrulation generates the three basic germ layers from which organs arise. • • How do sheets of cells (epithelia) move during gastrulation? 5 methods. Invagination: Invagination is the local inward movement of cells from a cavity Involution is similar, but more dramatic. It is an inward expansion of epithelial cells around an edge such as the blastpore. • Convergent extension is elongation of an epithelium in one direction while it shortens in the other direction (stretching taffy). The cells can keep their relative positions and elongate or they can interdigitate. Epiboly is spreading movement of an epithelium to a deeper or thinner layer. • Delamination is the splitting of one layer into two different layers. Different combinations of these basic movements yield a variety of changes that characterize gastrulation • Epithelial cells are well-differentiated. They compose skin and line the body cavities (ie, the digestive tract). They are polarized. Their apical surface faces out and their basal surface rests on the basement membrane (extracellular matrix that supports cells). Epithelial cells are closely connected with adjacent cells by specialized attachments including tight junctions, gap junctions, and desmosomes. Mesenchymal cells are poorly differentiated and have the potential to develop into many different tissues, including epithelial cells. They have a leading edge with lamellipodia, and a trailing edge. They are not connected to adjacent cells but they are in contact with the extracellular matrix. The second phase of gastrulation is caused by convergent extension of cells into the blastocoel • The invaginated cells of the vegetal plate extend to form a long thin archenteron. It is unclear as to how this occurs. • Stretching model: the extension could result from the cells changing shape to become long and thin. • Cell movement model: invaginated cells could move to extend the length of the tube. • Secondary mesenchymal cells: cells at the tip of the archenteron guide the progress. They send filopodia (thin extensions) to find the correct area of the roof. The roof cells send back other filopodia to direct the archenteron where to go. • This allows the mouth to hook up with the gut. Gastrulation(1) • Gastrulation transforms flat two-layered blastula(epiblast,hypoblast) into threelayered gastrulaectoderm, endoderm, mesoderm. Mechanism of gastrulation consists of: 1. Formation of primitive streak(PS) 2. Involution of PS 3. Regression of PS. 4. Formation of primitive streak, marked by: --expansion of epiblast cells and caudal convergence(A). --Primitive streak(PS) forms as longitudinal ridge in midline(B) --PS elongates, cranial tip widened as primitive node(C) Gastrulation(2) 1.Involution of primitive streak(PS) --epiblast cells leave PS --primitive groove in midline --first group of cells form intraembryonic endoderm(A) --second group of epiblast cells intercalate between endoderm and ectodermform mesoderm 3. PS regresses caudally Enlarged tip Of PSHensen’s node/primitive knot, moves to posterior region(C). 4. Function of PS is to form three germ layers Formation of the Notochord • The Notochord --a rod-shaped aggregate of chordamesoderm cells extending along entire length of embryo. --notochord cells formed of migrating cells from Hensen’s node • Functions of notochord. 1. Defines cranial-caudal axis of embryo. 2. Serves as primary inducer at neurulation.Induces formation of the neural tube and somitogenesis. 3. Transient, remnants in intervertebral disc as nucleus pulposus.