General Embryology.pdf

GERM CELL FORMATION AND FERTILIZATION The human somatic (body) cell contains chromosomes (diploid). Two of these are ; the remaining are The sex chromosomes, designated X and Y- is the fusion of male and female germ cells (gametes) to form a , wh...

GERM CELL FORMATION AND FERTILIZATION The human somatic (body) cell contains chromosomes (diploid). Two of these are ; the remaining are The sex chromosomes, designated X and Y- is the fusion of male and female germ cells (gametes) to form a , which commences the formation of a new individual. Prenatal Development  The first phase begins at fertilization and spans the first 4 weeks or so of development- mostly , with some differentiation of cell populations. The second phase spans the next 4 weeks of development and is characterized largely by the differentiation of all major external and internal structures From the end of the second phase to term- mostly and the embryo now is called a INDUCTION, COMPETENCE, AND DIFFERENTIATION the process that initiates differentiation. I - an agent that provides cells with the signal to enter the induction process. Each compartment of cells must be to respond to the induction process. and play crucial roles in development- alters gene expressions. play a fundamental role in A is an , and the appropriate expression of cell-surface receptors bestows on a cell.  FORMATION OF THE THREE- LAYERED EMBRYO The fertilized egg initially undergoes a series of rapid divisions that lead to the formation of a ball of cells called the Fluid accumulates in the morula, and its cells realign themselves to form a fluid-filled hollow ball, called the. Two cell populations now can be distinguished within the blastocyst: (1) those lining the cavity (the primary yolk sac), called (2) a small cluster within the cavity, called the  of the into a. At this time cells differentiate into the (involved in development of the embryo) and the (involved in maintenance).  The cells form the embryo proper, whereas the cells are associated with implantation of the embryo and formation of the placenta. At about of gestation, the cells of the embryoblast differentiate into a two-layered disk called the. The cells situated dorsally, or the ectodermal layer, are columnar and reorganize to form the. Those on the ventral aspect, the endodermal layer, are cuboidal and form the roof of a second cavity (the secondary yolk sac).  During that time the axis of the embryo is established. Represented by a slight enlargement of the at the head ( ) end of the embryo in a region known as the , where ectoderm and endoderm are in contact. During the of development, the embryo enters the period of the germ layers forming the are converted to a The floor of the amniotic cavity is formed by , and within it a structure called the develops along the midline by cellular convergence.  a narrow groove with slightly bulging areas on each side. The of the streak finishes in a small depression called the The cells that pass through the streak change shape and migrate away from the streak in. The cells from the form the , which pushes forward in the midline as far as the prochordal plate. Through canalization of this process, the is formed to support the primitive embryo.  The , infiltrate and push away the extraembryonic endodermal cells of the hypoblast, except for the prochordal plate, to form the true embryonic endoderm. pack the space between the newly formed embryonic endoderm and the ectoderm to form a of cells, called the  Cells also spread progressively forward (apart from spreading laterally), passing on each side of the notochord and prochordal plate. The cells that accumulate anterior to the prochordal plate give rise to the  As a result of these cell migrations, the notochord and mesoderm now completely separate the ectoderm from the endoderm, except in the region of the prochordal plate and in a similar area of fusion at the end of the embryo, called the MARKS THE END OF THE 1ST 3 WEEKS OF DEVELOPMENT.  FORMATION OF THE NEURAL TUBE AND FATE OF THE GERM LAYERS The initial events involved cell proliferation and migration. During the next 3 to 4 weeks of development, major tissues and organs differentiate from the. Key events are: 1. the differentiation of the from the ectoderm 2. the differentiation of the from the ectoderm 3. the differentiation of 4. the in two planes along the rostrocaudal (head-tail) and lateral axes.  The nervous system develops as a thickening within the ectodermal layer at the rostral end of the embryo. This thickening constitutes the , which rapidly forms raised margins (the ). These folds in turn encompass and delineate a deepening midline depression, the. The neural folds eventually fuse so that a separates from the ectoderm to form the floor of the amniotic cavity, with mesoderm intervening.  As the neural tube forms, changes occur in the mesoderm adjacent to the tube and the notochord. The mesoderm first thickens on each side of the midline to form Along the trunk of the embryo, this paraxial mesoderm breaks into segmented blocks called Each somite has components: (1) the which eventually contributes to two adjacent vertebrae and their disks (2) the , which gives origin to a segmented mass of muscle (3) the , which gives rise to the connective tissue of the skin overlying the somite.  In the head region, the mesoderm only partially segments to form a series of numbered which contribute in part to the head musculature. At the periphery of the paraxial mesoderm, the mesoderm remains as a thin layer, the , which becomes the. Further laterally the mesoderm thickens again to form the , which gives rise to: 1. the connective tissue associated with muscle and viscera 2. the serous membranes of the pleura, pericardium, and peritoneum 3. the blood and lymphatic cells; 4. the cardiovascular and lymphatic systems  A different series of events takes place in the head region. First, the neural tube undergoes massive expansion to form the. The hindbrain exhibits segmentation by forming a series of eight bulges, known as which play an important role in the development of the head. FOLDING OF THE EMBRYO A crucial developmental event is the folding of the embryo in two planes, along the rostrocaudal axis and along the lateral axis. Embryo at 21 days, before folding. The arrows indicate where folding occurs.  The head fold is critical to the formation of a Ectoderm comes through this fold to line the primitive stomatodeum, with the stomatodeum separated from the gut by the DERIVATIVES OF THE 3 GERM LAYERS THE NEURAL CREST CELLS Neural crest cells comprise a migratory stem and progenitor cell population that forms within the first 3 to 4 weeks of human embryonic development. Derived from the ectoderm during the period of Neural crest cells are essential for embryo development and throughout adult life. There is barely a tissue or organ throughout the human body that does not receive a contribution from neural crest cells.  Neural crest cells in the head region have an important role. They generate neurons and glia within the peripheral and enteric nervous system and the meninges surrounding the brain. They also differentiate to form most of the connective tissue of the head. Embryonic connective tissue elsewhere is derived from and is known as , whereas in the head it is known as , reflecting its origin from.  In a dental context the proper migration of neural crest cells is essential for the development of the and the All the tissues of the tooth (except enamel and perhaps some cementum) and its supporting apparatus are derived directly from neural crest cells, and their depletion prevents proper dental development. NEURAL CREST DERIVATIVES (SUMMARY) 1. Embryonic connective tissue ( ectomesenchyme) in the craniofacial region including the oral cavity. 2. Dental pulp. 3. Odontoblasts, the dentin forming cells. 4. Cementoblasts, the cementum forming cells. 5. Alveolar bone forming cells, namely osteoblasts. 6. Periodontal ligament forming cells, namely periodontal fibroblasts. 7. Spinal sensory ganglia. 8. Sympathetic neurons. 9. Schwann cells. 10. Melanocytes. 11. Meninges. FAILURE OF NEURAL CREST CELLS TO MIGRATE PROPERLY TO THE FACIAL REGION- EG: TREACHER COLLINS SYNDROME BRANCHIAL ARCHES, GROOVES AND POUCHES  PHARYNGEAL (BRANCHIAL) ARCHES: These are 4-6 horse-shoe shaped structures that are present during early development and give origin to most of the facial and the oral structures. They are numbered I, II, III, IV, V, and VI starting with arch I which is closest to the developing head. The last two arches V and VI are rudimentary in humans. BRACHIAL ARCHES AND THE STOMATODEUM Branchial arches form in the pharyngeal wall (which has lateral plate mesoderm sandwiched between ectoderm and endoderm) as a result of lateral plate mesoderm proliferation and subsequent migration by neural crest cells. BA separate the stomatodeum from the developing heart. BRACHIAL ARCHES: It is a wonder of nature that NCCs become transformed into faces & jaws that are functional & species unique. But how do the cells know into which arch they are to migrate to? It has been shown that if you replace 1st arch crest cells by 2nd arch crest cells before they migrate, they will move to the 2nd arch but will form 1st arch structures. Each arch has a unique patterning of neural crest migration controlled by a hox cluster of genes. Each arch has its own address & the crest cells know exactly where they belong. Where does this segmentation come from? The segmented identity of the brachial region ( & indeed the entire body) is determined by a set of regulatory genes called HOMEOTIC GENES (William Bateson- geneticist) Homeotic genes provide each segment with a  unique identity  Positional individuality ANATOMY OF THE BA  Each arch is covered on the external surface by ectoderm Inner surface is lined by endoderm 1st BA is completely derived from ectodermal cells. Each arch consists mainly of a mesodermal core( an inner part of mesodermal mesenchyme surrounded by an ectomesenchyme part). This ectodermal layer forms the lining of the future oral cavity. Each arch has an artery and a nerve which consists of a motor and a sensory parts as well as a cartilage bar, for example arch I has Meckel's cartilage. A- POUCH, B- ARCH;C- GROOVE/CLEFT D- MEMBRANE; E- ESOPHAGUS  STRUCTURES DERIVED FROM EACH BRANCHIAL ARCH  Arch Structures Groove Pouch and cartilag e Muscles Skeletal Ligaments structures 1 Muscles of Mandible and Anterior ligament External Tympanic mastication maxilla of malleus auditory membrane Meckel' meatus s Mylohyoid and Malleus sphenomandibular Tympanic cartilag anterior belly of ligament cavity e digastric Incus Mastoid Tensor tympani antrum Tensor veli palatini Eustachian tube 2 Muscles of facial Stapes Stylohyoid ligament Obliterated Largely expression by the obliterated Reicher Styloid process down- t's Stapedius growth of Contributes to cartilag Lesser cornu of the second tonsils Arch Structures Groove Pouch Muscles Skeletal structures Ligaments 3 Stylopharyngeus Greater cornu of hyoid Inferior parathyroid Lower part of body of hyoid bone gland Thymus Cricothyroid Thyroid cartilage Superior 4 parathyroid Levator veli palatini Cricoid cartilage gland Constrictors of Arytenoid cartilage Ultimobranchial pharynx body Corniculate cartilage Intrinsic muscles of larynx Cuneiform cartilage Striated muscles of esophagus 5-6 Transient Transient Transient Transien Transient t The 5th PA regresses. The cartilaginous components of the 4th and 6th arches fuse to form the cartilages of the  INNERVATION AND VASCULARIZATION OF PHARYNGEAL ARCHES (A) TISSUE FROM ARCH II AND V GROWING TOWARDS EACH OTHER (ARROWS) TO MAKE BRANCHIAL ARCHES AND GROOVES DISAPPEAR (B) RESULTING APPEARANCE FOLLOWING OVERGROWTH (C) CONTRIBUTION OF EACH PHARYNGEAL POUCH FACIAL MUSCLES GROW FROM THE 2ND BRANCHIAL ARCH TO COVER THE FACE, SCALP AND POSTERIOR TO THE EAR MASTICATORY MUSCLES OF THE MANDIBULAR ARCH CRANIAL NERVES GROWING INTO BRANCHIAL ARCHES