GASTRULATION BY DR ANYANWU GE REAVIEW OF FIRST

GASTRULATION BY DR ANYANWU GE

REAVIEW OF FIRST 2 WEEKS

Early embryogenesis 3 �Events in first 14 days after fertilization q First week ü Cleavage ü Blastocyst formation ü Implantation q Second week ü Bilaminar germ disc o Embryos of same fertilization age may not develop at same rate 1/26/2022

Fertilization 3 -4 �This is the fusion of two sex cells to form a new cell containing genetic material derived from both parents. �Restores the diploid number of chromosomes. �Determines the sex of the organism. �Initiates cleavage. �Occurs in the widest part of the uterine tube (the ampulla).

Fertilization 3 -5 �Millions of sperm cells are deposited in the female reproductive tract during intercourse. �Only a few hundred have a chance at fertilization. �Only the first sperm to enter the secondary oocyte is able to fertilize it. �The remaining sperm are prevented from penetrating the oocyte.

FERTILIZATION � Fertilization occurs in the ampulla of the uterine tube and includes three phases. � A. Phase 1: Sperm penetration of corona radiata is aided by the action of sperm and uterine tube mucosal enzymes. � B. Phase 2: Sperm binding and penetration of the zona pellucida � Sperm binding occurs through interaction of sperm glycosyltransferases and ZP 3 receptors � � Penetration of the zona pellucida is aided by acrosomal enzymes, specifically acrosin. � � C. Phase 3: Fusion of sperm and oocyte cell membranes � Male and female pronuclei fuse, forming a zygote ; a new cell whose genotype is an intermingling of maternal and paternal chromosomes. �.

Cleavage 3 -7 �Cleavage is a series of mitotic divisions of the zygote. �Shortly after fertilization, the zygote begins to undergo a series of divisions. �Divisions increase the number of cells in the preembryo, but the pre-embryo remains the same size. �During each succeeding division, the cells are smaller and smaller.

Cleavage 3 -8 �Before the 8 -cell stage, cells are not tightly bound together �after the third cleavage division, the cells become tightly compacted into a ball called a morula (16 cells). �Zygote cytoplasm is successively partitioned (cleaved) to form a blastula consisting of �increasingly smaller blastomeres (2 -cell, 4 -cell, 8 cell, and so on).

9 �Cleavage �Two-cell stage zygote undergo mitotic division �Blastomeres seen ü Cells become smaller ü number increase Fig 18. blastomeres Source: Sadler, 2010 1/26/2022

10 �Compaction occurs after 3 rd cleavage ü Cells held together by tight junction ü Inner and outer cells seen �Morula(16 -cell) seen at 3 days ü Inner cell mass forms embryo ü Outer cell mass forms trophoblast Fig 19 Source: Sadler, 2010 1/26/2022

�Blastomeres form a morula by undergoing compaction, that is, tight junctions are �formed between the cells in the outer cell mass, thereby sealing off the inner cell mass.

12 �Blastocyst formation �Morula at uterine cavity �Fluid penetrate intercelluar spaces of inner cell �Spaces coalesce to form blastocoele �Zona pellucida disappears Fig 20. blastocyst Source: en. wikipedia. org 1/26/2022

� Uvomorulin, a glycoprotein found on the surface of blastomeres, is involved in compaction. � fluid secreted within the morula creates a cavity that is called the blastocyst cavity. � The conceptus is now called a blastocyst. � � The blastocyst now has two cell masses an inner and outer � The inner cell mass is now called the embryoblast (becomes the embryo). � The outer cell mass is now called the trophoblast (becomes the fetal portion of the placenta). � � Zona pellucida degeneration occurs by day 4 after conception. The zona pellucida must degenerate for implantation to occur.

� � The blastocyst usually implants within the posterior superior wall of the uterus by day 7 after fertilization. � � Implantation occurs in the functional layer of the endometrium during the � progestational (secretory) phase of the menstrual cycle. � � The trophoblast proliferates and differentiates into the cytotrophoblast and syncytiotrophoblast. �

Second week of development 15 �Day 8 �Blastocyst partially embedded in endometrium �Trophoblast forms ü Cytotrophobast ü syncytitrophoblast Fig 22. day 8 events source, : Sadler, 2010 1/26/2022

16 q Cytotrophoblast o Inner layer o Mononucleated cell o Mitotic figures seen q Syncytiotrophoblast o Outer layer o Multinucleated cells o No mitosis fig 23. trophoblast Source: courses. stu. qmul. ac. uk 1/26/2022

17 �Embryoblast forms �Hypoblast layer o Cuboidal cells o adjacent to blastocyst cavity �Epiblast layer o High columnar cells o Adjacent to amniotic cavity Fig 24. embyoblast Source: Sadler, 2010 1/26/2022

18 �Day 9 �Blastocyst deeper �Fibrin cover defect �Lacuna stage ü Vacoules appear in syncytium and coalesce �Exocoelomic membrane lines inner surface of cytotrophoblast Fig 25. lacuna stage Source: Sadler, 2010 1/26/2022

19 �Day 11 and 12 �Blastocyst completely embedded �Defect almost covered by surface epithelium �Lacunae meet sinusoids �Maternal blood enters lacunae �Uteroplacental circulation established Fig 26. day 11/12 events Source: Sadler, 2010 1/26/2022

20 �Extraembryonic mesoderm and chorionic cavity appears ü Extraembronic somatopleuric mesoderm ü Extraembryonic splanchnopleuric mesoderm Fig 27. extraembyonic mesoderm Source: Sadler, 2010 1/26/2022

GASTRULATION

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 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 2 different layers. Different combinations of these basic movements yield a variety of changes that characterize gastrulation

How do individual cells move during gastrulation? Seven basic types of cell movement lead to the changes in epithelial sheets that characterize gastrulation. 1. Migration is the movement of an individual cell over other cells or a substrate. 2. Intercalation is wedging of cells between their neighbors. Lateral intercalation involves lateral movements of cells in the same layer between one another = convergent extension. Radial intercalation involves wedging of 2 different layers. This process often leads to epiboly, the surface area of the epithelium increases while thickness decreases.

3. Ingression is the movement of individual cells from an epithelium into an embryonic cavity. 4. Shape changes are coordinated changes in cell shape that cause an epithelium to invaginate, buckle or undergo convergent extension. 5. Cell division without growth increases the total number of ‘building blocks’ 6. Changes in adhesiveness: cells adhere to one another and to extracellular material via different types of specialized attachments. Loss of these attachments can facilitate cell movement. 7. Programmed cell death (apoptosis): cells undergo death in a programmed manner in order to sculpt organs into their final appearance (fingers).

How does gastrulation occur in humans? Mammalian embryos have not been studied as extensively as others because they are difficult to maintain in culture after the blastula stage (this is because they implant in the uterus). Surprisingly, mammals undergo gastrulation very similar to birds!! After about 5 days the blastula appears as an inner cell mass and a tropoblast

GASTRULATION; major rd 3 week event 28 �The most characteristic event occurring during the third week of gestation �establishes all three germ layers ectoderm, endoderm mesoderm, and �Begins with formation of the primitive streak �Initially, the streak is vaguely defined �But by day 15 - 16 it is clearly visible 1/26/2022

3 -29

3 -30

Gastrulation… 31 �Primitive streak (at the caudal part of the embryo) The cephalic end-primitive node Primitive pit Cells of the epiblast migrate toward the primitive streak At the region of the streak � They become flask-shaped � Detach from the epiblast � Then slip beneath it – invagination Fig 1. gastrulation Human Embryology 1/26/2022 And Develop

Gastrulation… 32 specification �Cell migration and Controlled by fibroblast growth factor 8 (FGF 8) Synthesized by streak cells �This control of cell movement is by Downregulation of E-cadherin(binds epiblast cells together) �FGF 8 then controls cell specification into the mesoderm by Regulating Brachyury (T) expression. 1/26/2022

�Following invagination some 33 displace the hypoblast �embryonic endoderm, �Others lie between the epiblast and new endoderm �mesoderm. �Cells remaining in the epiblast Form ectoderm Fig. 2 formation of the mesoderm www. daltonstate. ed 1/26/2022

�More cells move between the epiblast and hypoblast layers 34 �They to spread laterally and cranially �Beyond the margin of the disc establish contact with the extraembryonic mesoderm �In the cephalic direction They pass on each side of the prechordal plate. Fig 3. formation of the notochord T. W. SADLER. Medical embryolo 1/26/2022

� The prechordal plate lies between 35 the tip of the notochord and oropharyngeal membrane It is derived from some of the first cells that migrate through the node Important for induction of the forebrain � The oropharyngeal membrane Located at the cranial end of the disc Consists of a small region of tightly adherent ectoderm and endoderm cells Represents the future opening of the oral cavity. Fig 4. Notochord formation Bruce M. Carlson, MD, Ph. D. Human Embryology And Developmental Biology 1/26/2022

FORMATION OF THE NOTOCHORD �Prenotochordal cells 36 Invaginate in the primitive node Move forward cranially in the midline Reach the prechordal plate They become intercalated in the hypoblast shortly Form the notochordal plate �With hypoblast replacement by endoderm notochordal plate cells proliferate detach from the endoderm Definative notochord Fig 5 Transverse section through the embr 1/26/2022 T. W. SADLER. Medical embryology,

�Notochord elongation is a dynamic process The cranial end forms first 37 Caudal regions are added the primitive streak assumes a more caudal position �The notochord and prenotochordal cells Extend cranially to the prechordal plate Caudally to the primitive pit �The pit forms an indentation in the epiblast Neurenteric canal 1/26/2022

�Cloacal membrane Formed at the caudal end of the embryonic disc Similar in structure to the 38 oropharyngeal membrane �Day 16 when the cloacal membrane appears The posterior wall of the yolk sac forms a small diverticulum � Allantois This extends into the connecting stalk It remains rudimentary May be involved in abnormalities of bladder development 1/26/2022

ESTABLISHMENT OF THE BODY AXES �This takes place before and during the period 39 of gastrulation. �The anteroposterior axis Signaled by cells at anterior visceral endoderm (AVE) �Expresses genes for head formation �OTX 2, LIM 1, and hesx 1(transcription factors) �Cerberus and lefty(secreted factors ) �Which inhibit nodal activity in the cranial end of the embryo. These genes establish the cranial end of the embryo before gastrulation Fig 8 genes for body axis determination T. W. SADLER. Medical embryology 1/26/2022

Folding of the Embryonic Disc 3 -40 �Begins late third and fouth weeks �Some areas grow faster than others. �Cephalocaudal folding: Helps form head and buttocks �Transverse (or lateral) folding Helps form trunk

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