Embryology 1&2 (2024) PDF

Lec. 1&2 Dr. Abdulsattar salim Embryology Embryology: is the study of embryo and its development. it also includes the study of the prenatal development of gametes and fertilization Oral embryology: is the study of the develo...

Lec. 1&2 Dr. Abdulsattar salim Embryology Embryology: is the study of embryo and its development. it also includes the study of the prenatal development of gametes and fertilization Oral embryology: is the study of the development of the oral cavity, and the structures within and around it, during the formation and development of the embryo in the first 8 weeks of pregnancy. After this point, the unborn child is referred to as a fetus. The correct biological term for this time period before birth is prenatal. Period of prenatal development: 1: proliferated period: started from the day of fertilization to the end of two week include fertilization, implantation and formation of embryonic disk. 2: embryonic period: during this period the all tissues develop and organize to form organ systems that completed in the end of eighth week. 3: fetal period: this period extends until birth. The tissues that developed during embryonic stage enlarged differentiate and become capable of function. (fig.1) Figure 1 1 Lec. 1&2 Dr. Abdulsattar salim Human life: begins as fertilized ovum. This single cell multiplies into millions of cells to form the human body. Human cell is made up of cytoplasm and a nucleus, which encloses the chromosomes. Chromosome contains genetic information in the form of deoxyribonucleic acid (DNA). Human somatic cells contain 46 chromosomes and are arranged in 23 pairs (diploid). Out of 46 chromosomes, 44 chromosomes (22 pairs) are similar and are called autosomes; the remaining 2 chromosomes may be similar or dissimilar and are called allosomes or sex chromosomes (X and Y chromosome). One chromosome of each pair is derived from father and the other from the mother. , the germ cells (sperm and ova, collectively called gametes): In contrast to somatic cells, It contain half the number of chromosome (haploid). It is always 22 + X in case of females and it may be 22 +Y or +X in case of males. (Fig.2) Figure 2 2 Lec. 1&2 Dr. Abdulsattar salim Cell division: Cell multiplication is important for growth and replacement of dead cells. mitosis When cells divide, the genetic information which they contain is passed on to both the daughter cells, where the number of chromosomes and the genetic information are identical in both the daughter cells as of the mother cell. This process is known as mitosis and this occurs in somatic cell. Meiosis In case of germ cells or gametes, this process is slightly different, called meiosis. Here the cell undergoes two successive divisions called first and second meiotic division, with the resultant daughter cells have half the number of chromosomes, genetic information that differs from one another. FERTILIZATION: Fusion of sperm and ova is called fertilization. This occurs in the ampulla of uterine tube, and results in formation of zygote. fig.3 Fertilization results in the following three events: (A) achievement of chromosomal diploidy, (B) determination of sex of the embryo, (C) initiation of cleavage or cell division 3 Lec. 1&2 Dr. Abdulsattar salim Figure 3 CLEAVAGE: The fertilized ovum is a large cell, which becomes smaller as it divides further. This zygote undergoes series of cell division to become two-cell stage, four-cell stage, eight-cell stage, and sixteen-cell stage. This process of cell division is known as cleavage. (Fig.4) Figure 4 4 Lec. 1&2 Dr. Abdulsattar salim Morula: During 16-32 cell stage, this clump of cells looks like mulberry and is called the morula. Blastocyst: Fluid seeps into the morula and makes it into a fluid-filled sac known as blastocyst or blastocoele. The fluid seepage into the blastocyst leads to realignment of the cells of the morula to form two distinct population of cells. The lining of the blastocyst is formed by trophoblasts and the smaller inner cell mass is called embryoblast. (Fig.5&6) Embryoblasts will form the embryo proper (basic cells for formation of fetus) while trophoblasts form the supporting structures (placenta) and help in the implantation of the embryo. From fertilization till the end of 8th week it is called embryo, further from 8th week till birth, the fetus The correct biological term for this time period before birth is prenatal. 5 Figure 5 Lec. 1&2 Dr. Abdulsattar salim Figure 6 Implantation: the blastocyst attaches to the dorsal surface of uterus that lined by sticky endometrium membrane. The blastocyst digests this membrane and allows deep implantation to form future placenta. ALL THAT OCCURE IN FIRST WEEK OF PRENATAL LIFE 6  Lec. 1&2 Dr. Abdulsattar salim SECOND WEEK OF PRENATAL LIFE: FORMATION OF BILAMINAR GERM DISK: After first week of intrauterine life, the embryoblasts differentiate themselves to form a two layered circular structure known as bilaminar germ disk. The upper layer comprised of columnar cells known as epiblasts (ectoderm) and the lower flattened cells, which become cuboidal later, are called hypoblasts (endoderm). (Fig.7) The bilaminar germ disk divides the blastocyst cavity into two cavities: A: The upper amniotic cavity B: The lower yolk sac cavity. At this stage, head or the tail end of the embryo is not determined. Soon, in a localized area near the periphery, the cuboidal cells of the endoderm become columnar and this region is known as prochordal plate, which determines the central axis of the embryo. This enables as to distinguish the right and left halves and head and tail end of the future embryo. (area of for brain) (Fig.8&9) Figure 7 7 Lec. 1&2 Dr. Abdulsattar salim Figure 8 Figure 9 8 Lec. 1&2 Dr. Abdulsattar salim THE THIRD WEEK OF PRENATAL LIFE: PROCHORDAL PLATE, PRIMITIVE STREAK AND TRILAMINAR GERM DISK FORMATION: During the third week, the ectodermal cells at the tail end (caudal) proliferate to form a bulge which proceeds as a linear structure toward the cranial end (rostral) along the central axis to form primitive streak. The rostro-caudal and medial-lateral axes of the embryo are defined by the primitive streak. The rounded primitive node, or Hensen's node, is situated at the cranial tip of the primitive streak, and contains a depression called the primitive pit. The primitive pit is continuous with the primitive groove. (Fig.10&11&12&13) GASTRULATION The primitive streak is a transient structure whose formation, on day 15 of human development, marks the start of (gastrulation), (the process in which the inner cell mass is of blastocyst converted into the trilaminar embryonic disc), which is comprised of the three germ layers (ectoderm, mesoderm and endoderm). The cells alongside of primitive streak proliferate and migrate toward the lateral and cephalic direction in between ectoderm and endoderm sparing the ectoderm from endoderm to form mesoderm, the third germ layer. (Fig. 9&11) 9  Lec. 1&2 Dr. Abdulsattar salim Thus, the bilaminar germ disk becomes trilaminar germ disk. All these events concerned with cellular proliferation and migration occur continuously and on an inherent pattern. Figure 10 Figure 11 10  Lec. 1&2 Dr. Abdulsattar salim Figure 12 Figure 13 11 Lec. 1&2 Dr. Abdulsattar salim Embryonic Derivatives of the Primitive Streak: Ingression Cells from the epiblast migrate into the interior of the embryo via the primitive streak, in a process termed ingression, which involves a cellular epithelial-to- mesenchymal transition (EMT). Wave of migrating: 1: The initial wave of migrating cells streams through the primitive streak, displacing the hypoblast cells to become definitive endoderm, which ultimately produces the future gut derivatives and gut linings. 2: The second wave of migrating cells populates a layer between the epiblast and the definitive endoderm, thereby forming the mesoderm layer. The intraembryonic mesoderm cells later give rise to five subpopulations of cells: A: paraxial mesoderm, B: intermediate mesoderm, C: lateral plate mesoderm, D: cardiogenic mesoderm E: population that forms a midline tube called the notochordal process. (Fig.14) 12 Lec. 1&2 Dr. Abdulsattar salim Figure 14 13 Lec. 1&2 Dr. Abdulsattar salim FORMATION OF NOTOCHORD AND NEURAL CREST CELLS: Notochord is the midline structure. It extends from the primitive streak and ends at the end of prochordal plate. Initially cranial end of the primitive streak thickened to form a primitive knot. The cells from the primitive knot proliferate and migrate cranially between the ectoderm and endoderm to form notochordal process. Later, this process gives rise to a solid rod-like structure known notochord. The central portion of this cord denaturized to form canal that bind the amniotic cavity to the yolk sac. Most of its part disappears later and only a small portion persists in the intervertebral disk as nucleus pulposus. Nervous system develops from ectoderm: The ectoderm at the midline above the notochord thickens to form the neural plate. The margins of the neural plate rise to form a neural fold, this fold elongates and leaves a depression in the midline called neural groove. The folds grow further and fuse with each other converting the neural groove into neural tube. Soon this neural tube separates from the ectoderm. The cells which form the neural tube are called neuroectoderm. The cranial part of the neural tube forms brain and the caudal, the spinal cord. A unique group of cells derived from the ectoderm along the lateral margins of the neural tube are called neural crest cells, which exist only in vertebrates. These cells are multipotent stem cells that migrate to various parts of the embryo to generate broad array of cells and influence the other developing tissues. 14 Lec. 1&2 Dr. Abdulsattar salim Because of this unique feature, it is also considered as fourth germ layer. They migrate beneath the surface ectoderm as sheet of cells. (Fig.15) The mesenchyme of the upper facial region is derived from or influenced by the migrated neural crest cells and is known as ectomesenchyme. In the trunk regio the neural crest cells contribute to development of the sensory ganglion, neuron, and Schwann cells are derived from neural crest cells. In the facial region, the epithelial portion of the tooth and the glands are derived from the ectoderm alone and the connective tissue components namely-osteoblasts, chondroblasts, odontoblasts, fibroblasts, adipocytes, smooth muscle cells, Schwann cells, pigment cells, and neurons are from ectomesenchyme. Figure 15 15