Chapter 12 Early Development in Mammals

Chapter 12 Early development in mammals Cleavage The mammalian cleavage has been the most difficult to study because: 1. Mammalian eggs are among the smallest in the animal kingdom. 2. The human zygotes are about 100 µm in diameter. Therefore, they are barely visible to the eye. 3. Mammalian zygotes are produced in a fewer numbers. 4. The development of the embryos occurs inside the mother. Cleavage Most research in mammalian development has focused on mouse because: • Mice are relatively easy to breed. • Have large litters. • Easily housed in laboratories. Cleavage The unique nature of mammalian cleavage • Cleavage in mammalian cells is among the slowest in the animal kingdom. ➢ The time between one cleavage and the other is 12- 14 hrs. Cleavage The unique nature of mammalian cleavage • During cleavage, the mammalian blastomeres have unique orientation. ➢ Rotational cleavage: in mammals, the first cleavage is a normal meridional division, while in the second cleavage, one of the two blastomeres divides meridionally and the other divides equatorially. Cleavage The unique nature of mammalian cleavage • The mammalian cleavage is asynchronous at early cell division. • Unlike the genome of rapidly developing animals, the mammalian genome is activated during early cleavage. Cleavage compaction • One of the most crucial events of mammalian cleavage is compaction. • Cell adhesion proteins such as E-cadherin are expressed on the surface of blastomeres, and the blastomeres are huddle together to form a compact ball of cells. Cleavage compaction • Most of the descendants of the external cells of the morula becomes trophoblast (trophectoderm) cells, whereas the internal cells give rise to the inner cell mass (ICM) • Trophoblast cells form the tissue of the chorion: an extraembryonic membrane that forms embryonic/ fetal portion of the placenta in mammals. Cleavage compaction • Most of the descendants of the external cells of the morula becomes trophoblast (trophectoderm) cells, whereas the internal cells give rise to the inner cell mass (ICM) • Trophoblast cells form the tissue of the chorion: an extraembryonic membrane that forms embryonic/ fetal portion of the placenta in mammals. • Functions of the chorion: 1. It enables the fetus to get oxygen and nourishment from the mother. 2. It secretes hormones that cause the mother’s uterus to retain the fetus. 3. It produces regulators of the immune response so that the mother will not reject the embryo. Cleavage compaction • The ICM gives rise to the embryo and its associated yolk sac, allantois, and amnion. • The earliest blastomeres are totipotent: can form both trophoblast cells and the embryo precursor cells of the ICM. • The cells of ICM are pluripotent: that is each cell of the ICM can generate any cell type in the body but is no longer able to form the trophoblast. These pluripotent cells are the embryonic stem cells. Cleavage Cavitation • By the 64- cell stage, the ICM and the trophoblast cells have become separate cell layers, neither of which contribute cells to the other. • At this stage, the ICM is composed of approximately 13 cells. • During a process called cavitation, the trophoblast cells secrete fluid into the morula to generate a blastocoel. (F) Trophoblast cell Na+/H+ Na+/K+ Cleavage Cavitation • As the blastocoel expands, the inner cell mass becomes positioned on one side of the ring of trophoblast cells, resulting in the formation of mammalian blastocyst that is unique to mammalian cleavage. (F) Cleavage Escape from zona pellucida and implantation • During the passage through the oviduct, the zona pellucida prevents the blastocyst from adhering to the oviduct walls. • When the embryo reaches the uterus, it must “hatch” from the zona pellucida so that it can adhere to the uterine wall. • The mouse blastocyst hatches from the zona pellucida by digesting a small hole in it by the action of trypsin- like protease secreted from the trophoblast cells. Cleavage Escape from zona pellucida and implantation • The endometrium has made an extensive extracellular matrix that is composed of complex sugars, collagen, laminin, fibronectin, cadherins, hyaluronic acid, and heparan sulfate receptors. • The extracellular matrix of the endometrium “catches” the blastocyst and makes the initial binding with it. Cleavage Escape from zona pellucida and implantation • After the initial binding, several adhesion systems act to keep the blastocyst tightly bound to the uterine lining. 1. The trophoblast cells synthesize integrins that bind to the uterine collagen, fibronectin, and laminin. 2. The trophoblast cells synthesize heparan sulfate proteoglycan prior to implantation. 3. P- cadherins on the trophoblast and uterine endometrium also help dock the embryo to the uterus. Cleavage Escape from zona pellucida and implantation • Once the blastocyst is in contact with the endometrium, Wnt proteins instruct the trophoblast to secrete a set of proteases including collagenases, stromelysin, and plasminogen activator. • These protein digesting enzymes digest the extracellular matrix of the uterine tissue, enabling the blastocyst to bury itself within the uterine wall. Mammalian gastrulation Modifications for development inside another organism • The mammalian embryo obtains its food directly from the mother and does not rely on stored yolk. • The trophoblast divides into cytotrophoblast, which forms the villi, and the syncytiotrophoblast, which ingresses into the uterine tissue to form the chorion. Mammalian gastrulation Modifications for development inside another organism • The trophoblast develops through several stages, to eventually become the chorion: the embryonically derived portion of the placenta. • The trophoblast cells also induce the mother’s uterine cells to form the decidua: the maternal portion of the placenta which becomes rich in the blood vessels that provide oxygen and nutrients to the embryo. Mammalian gastrulation The primitive endoderm: the mammalian hypoblast • The first segregation of cells within the inner cell mass forms two layers: the primitive endoderm (hypoblast) and the epiblast. Functions of the primitive endoderm: 1. Positions the site of gastrulation. 2. Regulates cell movement in the epiblast. 3. Promotes the maturation of blood cells. Mammalian gastrulation The primitive endoderm: the mammalian hypoblast • Whether a mouse ICM cell becomes epiblast or primitive endoderm may depend on when the cell became part of the ICM. • Cells that become internalized in the morula in the division from 8 to 16 cells appear to become pluripotent epiblast cells. • While the primitive endoderm cells may be generated by cells entering the ICM during the division from 16 to 32 cells. Mammalian gastrulation The primitive endoderm: the mammalian hypoblast • The epiblast and primitive endoderm form a structure called bilaminar germ disc. • The region of primitive endoderm where the cells contact the epiblast in the mammalian embryo is called visceral endoderm. • The cells of the primitive endoderm that contact the trophoblast of the mammalian embryo are called parietal endoderm. • Amniotic fluid: a secretion that serves as a shock absorber as well as preventing the developing embryo from drying out. Mammalian gastrulation The primitive endoderm: the mammalian hypoblast • The cells that originate from the node give rise to the notochord. • The cells that form the mouse notochord become integrated into the endoderm of primitive gut. • These cells form a band of small ciliated cells extending rostrally from the node. • They form the notochord by converging medially and budding off in a dorsal direction from the roof of the gut. Mammalian gastrulation The primitive endoderm: the mammalian hypoblast • In humans, the migration of cells that forms the mesoderm start on day 16, around the time the mouse embryo is almost ready to be born. • A single ectodermal precursor cell can give rise to descendants in more than one germ layer. • Thus, the lineages that give rise to different germ layers are not fully separate from one another at the epiblast stage. Mammalian axis formation The anterior- posterior axis: two signaling centers • In mouse, the epiblast is cup- shaped, whereas in human it is disc- shaped like in birds. • The human embryo looks very much like the chick embryo, whereas the mouse embryo looks like a droplet enclosed by the primitive endoderm. Mammalian axis formation The anterior- posterior axis: two signaling centers • The mammalian embryo appears to have two signaling centers: one in the node and one in the anterior visceral endoderm (AVE). • The two signaling centers are located on opposite sides of the cup. • The node appears to be responsible for neural induction and for patterning of the anterior- posterior axis, while the AVE is critical for positioning the primitive streak. (epiblast) Mammalian axis formation The anterior- posterior axis: two signaling centers • The signals that initiate the primitive streak appear to come from the interactions between the trophoblast-derived extraembryonic ectoderm and the epiblast. Extraembryonic ectoderm BMP4 (Embryonic ectoderm) (Wnt3a and Nodal) (Lefty-1, Dickkopf, Cerberus) Mammalian axis formation The anterior- posterior axis: two signaling centers • Thus, Wnt3a in cells of the posterior not anterior epiblast activates the Brachyury gene which causes the generation of the mesoderm cells. • Once formed, the node secretes Chordin, and later when the head process and notochord form they add Noggin. • Chordin and Noggin are inhibitors of the BMPs. Extraembryonic ectoderm BMP4 (Wnt3a and Nodal) (Lefty-1, Dickkopf, Cerberus) Chordin Noggin from the head process and noochord Mammalian axis formation The anterior- posterior axis: two signaling centers • The AVE and thus the anterior posterior axis of the mammals appear to be generated by the mechanical stress exerted by the shape of the uterus. Mammalian axis formation The anterior- posterior patterning by FGF and RA gradients • The head region of the mammalian embryo lacks the Nodal, BMPs, FGFs, and Wnts signaling. • The posterior region is characterized by having high levels of Nodal, BMPs, Wnts, FGFs, and retinoic acid. Mammalian axis formation The anterior- posterior patterning by FGF and RA gradients • Fgf8 is expressed in the posterior tip of the gastrula and continues to be made in the tailbud. Fgf8 mRNA gradient in the tailbud of a 9-day mouse embryo Newly formed tissue Posterior portion of the embryo Mammalian axis formation The anterior- posterior patterning by FGF and RA gradients • The retinoic acid gradient appears to be controlled by the expression of RA-synthesizing enzymes in the embryo’s posterior and RA-degrading enzymes in the anterior part of the embryo. RA Mammalian axis formation The anterior- posterior patterning: The Hox code hypothesis • In all vertebrates, anterior- posterior polarity becomes specified by the expression of Hox genes. • Vertebrate Hox genes are homologous to the homeotic selector genes of the fruit fly. • The Drosophila homeotic gene complex (Hom-C) on chromosome 3 contains the Antennapedia and bithorax clusters (complexes) and can be seen as a single functional unit. lab: labial dfd: deformed scr: sex comb reduced Antp: Antennapedia pb: proboscipedia Ubx: Ultrabithorax abdominal A: abdA Abdominal B: AbdB Mammalian axis formation The anterior- posterior patterning: The Hox code hypothesis • Because the homeotic selector genes are responsible for specification of fly body parts, mutations in them transform one body part to another. Mammalian axis formation The anterior- posterior patterning: The Hox code hypothesis • All mammalian genomes contain four copies of the Hox complex per haploid set. • The four copies are located on four different chromosomes. Human HOXA • The order of the Hox genes on the chromosome is similar to the order of their expression patterns along HOXD the anterior- posterior axis. Mammalian axis formation The anterior- posterior patterning: The Hox code hypothesis • The equivalent genes in each mouse complex (such as Hoxa4, Hoxab4, Hoxc4, and Hoxd4) are called paralogues. • It is thought that the four mammalian Hox complexes Human HOXA were formed by chromosome duplications. • Also, it is likely that independent gene duplications and deletions have occurred since the divergence between Drosophila and mouse. HOXD Mammalian axis formation The anterior- posterior patterning: The Hox code hypothesis • Hox gene expression can be seen along the mammalian body axis from the anterior boundary of the hindbrain through the tail. • The regions of Hox expression are not in register. • The identity of certain region along the anteriorposterior axis is primarily determined by the expression of the most posterior Hox gene in that region. Mammalian axis formation Experimental analysis of Hox genes • Particular sets of paralogous genes provide segmental identity along the anteriorposterior axis of the body. Evidence of such a code comes from two main sources: 1. Comparative anatomy, in which the type of vertebrae in different vertebrate species is correlated with the constellation of Hox gene expression. 2. Gene targeting (knockout) experiments, in which mice are constructed that lack both copies of one or more genes. Mammalian axis formation Comparative anatomy and Hox genes expression • Although the mouse and chick have a similar number of vertebrae, they apportion them differently. • Mice have 7 cervical vertebrae, 13 thoracic vertebrae, 6 lumbar vertebrae, 4 sacral vertebrae, and a variable (20+) number of caudal vertebrae. • In contrast, the chick has 14 cervical vertebrae, 7 thoracic vertebrae, 12 or 13 lumbosacral vertebrae, and 5 coccygeal (fused tail) vertebrae. Mammalian axis formation Comparative anatomy and Hox genes expression • Does the constellation of Hox gene expression correlate with the type of vertebra formed (cervical or thoracic) or with the relative positon of the vertebrae (e.g, number 8 or 9)? 21 22 14 15 20 21 7 8 Mammalian axis formation Gene targeting • There is a specific pattern to the number and type of vertebrae in mice, and the Hox gene expression pattern determines which vertebral type will form. Mammalian axis formation The left- right axis • The internal organs of mammalian body are not symmetric. Mammalian axis formation The left- right axis • The left- right axis appears to be due to the activation of Nodal protein and Pitx2 transcription factor on the left side of the lateral plate mesoderm, while Cerberus an inhibitor of Nodal is expressed on the right. • In mammals, distinction between left and right sides can be seen in the ciliary cells of the node. • The cilia cause fluid in the node to flow from right to left. • Ciliary motor protein dynein is important in generating asymmetric organ. Mammalian axis formation The left- right axis • The mechanism for fluid rotation appears to be due to placement of the basal body on each of the monociliated node cells. • The basal body that gives rise to the cilium is at the posterior side of each cell and extends out the ventral surface. • The placement of the cilia is governed by the planar cell polarity (PCP) pathway possibly directed by a Wnt. Mammalian axis formation The left- right axis But how does rotation of the fluid generate a body axis? • The crown cells that neighbor the node sense the fluid rotation. • The fluid movement activates the Pkd2 protein on the immobile cilia of crown cells. • A cascade that is initiated by Pkd2 appears to suppress the synthesis of Cerberus and thereby activates the expression of Nodal. • Cerberus which is made by the crown cells on the right side, inhibits nodal expression. Twins • The early cells of mammalian embryos can replace each other and compensate for a missing cell. Human twins are classified into two major groups: 1. Monozygotic: “one egg”. Genetically “identical” twins; form when the cells of a single earlycleavage embryo become dissociated from one another, either by the separation of early blastomeres or by the separation of the inner cell mass into two regions within the same blastocyst. 2. Dizygotic: “Two eggs”. Twins that result from two separate but approximately simultaneous fertilization events. Genetically such “fraternal” (Latin frater, brother) twins are full siblings. Twins • Identical twins occur in roughly 1 in 400 human births. • The chorion is formed on embryonic day 5 and the amnion on embryonic day 9. 1/3 2/3 Small percentage Twins • When ICM cells are isolated and grown under certain conditions, they remain undifferentiated and continue to divide in cultures. • Such cells are embryonic stem cells (ES cells). • ES cells: pluripotent stem cells of the mammalian inner cell mass blastomeres that are capable of generating all the cell types of the body. • When ES cells are injected into a mouse blastocyst, they can integrate into the host inner cell mass. • The resulting embryo has cells from both the host and donor tissues. Twins • Whereas fraternal twins are formed during fertilization and identical twins during cleavage, conjoined twins are formed during gastrulation. • Conjoined twins: monozygotic twins that share some parts of their bodies; they may even share a vital organ, such as a heart or a liver. • It appears that if there is a tear in the marginal zone (which forms a new center of Nodal expression), or if a second region of a marginal zone expresses Nodal, then a second body axis can form. Twins • The different types of conjoined twins depend on where a new axis is formed during gastrulation. • If the new axis (primitive streak) is formed in the anterior region, then craniopagus twins will be produced, where the twins are joint at the head. Twins • The different types of conjoined twins depend on where a new axis is formed during gastrulation. • If the new axis is lateral to the original primitive streak, then Parapagus or thoracopagus twins will be produced. • The parapagus twins have two embryonic axis starting from a common point, whereas the thoracopagus twins are joint at the thoracic area. Twins • The different types of conjoined twins depend on where a new axis is formed during gastrulation. • If the new axis is posterior to the original primitive streak, then pygopagus twins will be produced, where the twins are joint at the posterior part of the body. Week Topics Ch. in Text 1+2 Reproductive organs produce and transport gametes Interplay of tropic and sex hormones regulates mammalian reproduction 45 Campbell Biology 3 Fertilization 4 Early development in Sea Urchin 5+6 Early development in amphibians 7 Early development in birds 8 Early development in mammals 46 Campbell Biology + 7 developmental biology 10 Developmental Biology 11 Developmental Biology 12 Developmental Biology 12 Developmental Biology 9+10 The vertebrate nervous system and epidermis: Neural tube formation and patterning 13 Developmental Biology 11 Paraxial mesoderm: The somites and their derivatives 17 Developmental Biology 12+13 Intermediate and lateral plates mesoderm: Heart, blood, and kidneys 14 The endoderm: Tubes and organs for digestion and respiration 18 Developmental Biology 20 Developmental Biology