Embryological Development 4th-8th Week PDF

Lecturer CEREN ERDEM ALTUN [email protected] ISTANBUL OKAN UNIVERSITY 08.02.2024 2ND WEEK: IMPLANTATION + BILAMINAR GERM DISC Bilaminar germ disc 3RD WEEK: GASTRULATION + NOTOCORD FORMATION ❑ Transformation of bilaminar germ disc into trilaminar germ disc. ❑ It is the beginning of morphogenesi...

Lecturer CEREN ERDEM ALTUN [email protected] ISTANBUL OKAN UNIVERSITY 08.02.2024 2ND WEEK: IMPLANTATION + BILAMINAR GERM DISC Bilaminar germ disc 3RD WEEK: GASTRULATION + NOTOCORD FORMATION ❑ Transformation of bilaminar germ disc into trilaminar germ disc. ❑ It is the beginning of morphogenesis (formation of body shape). ❑ It is characterized by the formation of three germ sheets (ectoderm, mesoderm and endoderm) in the embryo. Each of these three germ sheets forms special tissues and organs. ❑ At this stage, the embryo is also called the gastrula. 3RD WEEK: GASTRULATION + NOTOCORD FORMATION The first sign of gastrulation is the appearance of the "primitive streak" in the caudal part of the embryo. The primitive streak, which is not very prominent at first, appears as more raised regions running along either side of a shallow groove in the 15-16 day embryo. While the primitive streak extends in this direction with the addition of cells to its caudal end, the primitive node (nodus primitivus) is formed by cell proliferation at the cranial (cephalic) end. Meanwhile, a narrow groove, called the primitive groove, develops in the primitive streak. The primitive groove continues with a depression formed in the primitive node, and this depression is called the primitive pit (fovea primitiva). Epiblast cells migrate towards the primitive streak. When they reach the primitive streak, the balloon-shaped cells separate from the epiblast and slide under the epiblast along the primitive groove. This inward movement of cells is called invagination. The epiblast is the source of the three germ sheets (ectoderm, mesoderm, endoderm) from which all the tissues and organs of the embryo will develop. When the primitive streak appears, it is possible to identify the cranial and caudal aspects of the embryo, its dorsal and ventral aspect, its right and left sides. Some of the cells invaginated from the primitive streak enter between the hypoblast cells and enter the embryonic (intraembryonic) endoderm at the ceiling of the yolk sac, Some spread between the epiblast and the newly formed endoderm to form the intraembryonic mesoderm, Cells remaining in the epiblast form the embryonic (intraembryonic) ectoderm. Primitive streak These cells have the potential to proliferate and differentiate into various cell types such as fibroblasts, chondroblasts and osteoblasts. ECTODERM Epidermis including hair, nails sebaceous glands mammary glands Central and Peripheral Nervous System Ear, nose, eye sensory epithelium Pituitary gland tooth enamel ENDODERM Epithelial lining of the gastrointestinal and respiratory tracts Epithelial lining of the tympanic antrum pharyngotympanic or auditory tube Epithelial lining of the bladder and most of the urethra Liver, Pancreas parathyroid gland Thymus Thyroid tonsils MESODERM Smooth muscle layers, Skeletal and Skeletal muscles Connective Tissue, Cartilage and Bone Veins associated with tissues and organs Most of the cardiovascular system Blood cells and Bone Marrow Reproductive and Excretory Organs (kidney, gonad and related ducts) NOTOCORD FORMATION Prenotochordal cells invaginating from the primitive pit migrate in the cephalic direction up to the prechordal plate, and by mixing these cells into the hypoblast, they form a cellular cord called the notochordal plate (notochord extension) for a short time in the midline of the embryo. A lumen is formed within the notochordal plate (notochord extension) in a short time and is called the notochord canal. Primitive streak Primitive streak There are occasional lysis in the notochord canal floor and the underlying endoderm. These melting areas coalesce and the base disappears completely for a while A plate that bends inward from the ceiling ends that remain intact forms the NOTOKORD PLATE. The hypoblast leaves its place to endoderm cells moving inward along the primitive streak, and the cells of the notochordal plate proliferate and break off from the endoderm Starting from the cranial, the plaque curves more and forms the permanent (definitive) NOTOCORD, which is a solid cord of cells. END OF THE 3RD WEEK; It determines the primordial axis of the embryo and gives the embryo perpendicularity. It forms the basic signals for the development of the axial skeleton (cranial bones and spine). It determines the place where the vertebral bodies will form in the future. It stimulates the embryonic ectoderm and induces the formation of the neural plate. FATE OF THE PRIMITIVE STREAK; In normal conditions, the primitive streak undergoes a degenerative change and disappears at the end of the 4th week. The persistence of primitive streak remnants can result in a large tumor called a sacrococcygeal teratoma. Sakrokoksigeal Teratom 4TH WEEK= NORULATION= NEURAL PLATE AND NEURAL TUBE FORMATION When the notochord develops, it induces the overlying ectoderm. With the thickening of the ectoderm, a structure called the neural plate occurs. At the beginning of the 3rd week, the neural plate is in the form of a flat disc that is wide in the cephalic region and narrower in the caudally. The cells of the plate form the neuroectoderm, and the induction of these cells represents the beginning of the neurulation process. As the notochord lengthens, the neural plate expands and moves cranially. At approximately 18-19 days, the neural plate invaginates along its central axis, forming the neural groove that runs longitudinally in the midline with neural folds on either side. As a result of the contraction of ACTIN and MYOSIN fibers, which work like a muscle in the cells forming the neural plate, the middle of the neural plate becomes dimpled and the neural groove is formed. While the folding continues, the neural groove edges come together, the opposite cells are adhered by cell adhesives called FIBRONECTIN to form the NEURAL TUBE (primordium of the CNS). FORMATION OF SOMITS When the notochord and neural tube are formed, the intraembryonic mesoderm on both sides of them proliferate to form the paraxial mesoderm columns. Each column continues laterally with the intermediate mesoderm, and the intermediate mesoderm continues with the gradually thinning lateral mesoderm layer. Towards the end of the third week, the paraxial mesoderm differentiates and begins to divide into pairs of cubic bodies called somites. These mesoderm blocks are located on either side of the developing neural tube. During the somite development period (days 20-30), approximately 38 pairs of somites are formed. At the end of the 5th week, there are 42-44 pairs of somites. Since they are very prominent at the 4th and 5th weeks, they are among the criteria used to determine the age of the embryo. They form the dermis of the adjacent skin, with most of the axial skeleton (skull bones, spine, ribs, and sternum) and associated muscles. The neural tube develops on both sides of the somites Wide openings cranial and caudal => NEUROPORE EMBRYO FOLDS The amniotic cavity completely surrounds the embryo in all directions and becomes the dominant cavity. It grows gradually. The yolk sac narrows on all sides and becomes a small sac connected to the midgut by a narrow vitelline duct. It gets smaller. The extra-embryonic coeloma is gradually destroyed by the enlarging amnion and eventually disappears completely. On the 24th day, the first 2 pharyngeal arches are seen. 1. Pharyngeal arch (mandibular) is more prominent. 1. Pharyngeal arch; - for the most part → the mandible (lower jaw) is formed, - from the rostral projection → maxillary protrusion (upper jaw) occurs The embryo is slightly curved due to head and tail folds. The heart is selected as a large protuberance ventrally and pumps blood.. On day 26, 3 pairs of pharyngeal arches are observed in the embryo. The anterior neuropore of the neural tube is closed The forebrain protrudes considerably in the head region, and the curling of the embryo gives it the typical C shape. Upper limb buds 26-27. occurs per day. It appears as a small bulge on the ventrolateral wall of the body. Otic pits with outlines of the inner ear are seen. Ectodermal thickenings called the lens plate, which will form the lens of the eye, are seen on both sides of the head. At the end of the 4th week, 4 pairs of pharyngeal arches and lower extremity buds are evident. The long tail-like caudal protrusion is characteristic at the end of the 4th week. Many organ systems, especially the cardiovascular system, have begun to form. By the end of the 4th week, the posterior neuropore of the neural tube is usually closed.. WEEK 5 OF DEVELOPMENT Less changes are observed compared to the changes in the 4th week. Head growth is higher than other regions. The main cause of head growth is the rapid development of the brain and facial outlines. 2. Pharyngeal arch develops more than the others, covers the 3rd and 4th pharyngeal arches and forms lateral pits on both sides → CERVICAL SINUS The mesonephric ridges on either side are the mesonephric kidneys, which will form the temporary excretory organs in humans. WEEK 6 OF DEVELOPMENT The embryo reflexively responds to touch at 6 weeks. With the formation of hand plates on the upper extremities, a regional differentiation begins. FINGER SHARPS appear on the hand plates, which will enable the fingers to be shaped. Spontaneous twitches are seen in the lower and upper extremities. Lower extremity development occurs 4-5 days before upper extremity development. AURICULA CAPES develop around the 1st and 2nd pharyngeal arches, from which the external auditory canal is formed. The eye becomes prominent with the intense pigment formation in the retina. Due to the bending in the neck region, the head is bent in front of the heart protrusion. The intestines enter the embryonic coeloma proximal to the umbilical cord. WEEK 7 OF DEVELOPMENT Significant changes are seen. Clefts form between finger outlines and fingers can be distinguished The connection between the primitive gut and the umbilical sac (the vitellus sac) decreases and turns into the omphaloenteric duct. At the end of the 7th week, ossification begins in the upper extremities. WEEK 7 OF DEVELOPMENT = LAST WEEK OF THE EMBRIONIC PERIOD At the beginning of the this week, the fingers of the hand are separated, but they are webbed. There are prominent slits between the toes. The scalp vascular plexus is observed and forms a characteristic band around the head. At the end of the 8th week, all contours of the extremities are evident, the fingers are extended and completely separated from each other. WEEK 8 OF DEVELOPMENT; Voluntary movement of extremities seen for the first time Ossification begins in the femur At the end of the 8th week, the caudal process (tail) disappears completely. Hands and feet converge ventrally At the end of the 8th week, the embryo has human appearance. The eyes are well developed and the eyelids are closed to protect the eye from muscle movements of the fingers. AT THE END OF WEEK 8 At the end of the 2nd month, that is, at the end of the embryonic period, the embryo is 21-31 mm in length and weighs 9.5 g After the 8th week, the fetal period begins and the person in this developmental period is called a residual fetus. 06.02.2023