Z 101 Embryology PDF

Z 101  Embryology Embryology is the branch of biology that is related to the formation, growth, and development of living organisms from single cell to the fully developed fetus. It deals with the prenatal stages of development beginning from formation of gametes (sex cell...

Z 101  Embryology Embryology is the branch of biology that is related to the formation, growth, and development of living organisms from single cell to the fully developed fetus. It deals with the prenatal stages of development beginning from formation of gametes (sex cells), fertilization, development of embryo and fetus to the birth of a new individual.  Embryonic period vs. Fetal period Embryonic period: The development of the three primary germ layers give rise to all major organs and structures and the features of external body is established (after fertilization till 8th week of gestation). Fetal period: Structures and organs continue to grow and develop (The time from the 9th week till birth).  Importance of embryology The study of prenatal stages of development, especially those occurring during the embryonic period, helps us understand the normal development of body structure and the causes of congenital anomalies. Evolutionary biologists often use embryology to compare species because how an organism develops can tell a lot about its evolutionary history. Embryology is used as a tool to better understand the system or organism they are dealing with, be it the conservation of an endangered species or the reproductive disruption of a pest species.  Branches of Embryology 1. Descriptive Embryology: This field of embryology is associated with pure description of morphological changes of early developmental stages. 2. Comparative Embryology: It embraces the comparative study of embryology of different animal groups. 3. Experimental Embryology: It involves the experimental studies that attempt to understand the various fundamental mechanism in the early developmental stages of different animals. 4. Teratology: It is the branch of embryology concerned with the study of malformations or birth defects. The early developmental stages of an individual include: 1. Gametogenesis (formation of gametes). Sperm Ovum 2. Fertilization (union of gametes). Zygote (fertilized egg) 3. Cleavage (segmentation of the fertilized egg). Blastula Gastrula (Ectoderm, Mesoderm, Endoderm) 4. Gastrulation (development of the primary germ layers). 5. Organogenesis (formation of the organ rudiments).  The two major categories of cells in the body of sexually reproducing organisms Germ cells Somatic cells - Any biological cells that give - Any body cells are not rise to the reproductive cells involved in reproduction. called gametes. - Examples: nerve cell, - Examples: Sperm cell and egg muscle cell, red blood cell cell. …etc Gametogenesis is a biological process by which diploid cells undergo cell division and differentiation to form mature haploid gametes. In males, this process is called Spermatogenesis. In females, this process is called Oogenesis. - The sequence of gametogenesis is the same, but the timing of events during meiosis and the number of the gamete produced differ in the two sexes.  Spermatogenesis Spermatogenesis is the process in which spermatozoa are produced from male primordial germ cells. It takes place inside the seminiferous tubules of the testis. It begins at puberty.  Spermatozoa are motile flagellate cells able to swim in fluids. The majority of sperm are thread-like structures. Their size varies greatly; human spermatozoa are 50 to 70 um, while reaches 2000 um (2 mm) in some toads. Spermatozoa are produced in enormous numbers. In Man it has been estimated that about 200 to 600 million in a single ejaculation. Sperm cell structure: 1) Head: It is composed of the nucleus (containing the genetic material) and a cap-like structure called acrosome is present at the anterior end of the nucleus (containing the hydrolytic enzymes that help the sperm penetrate the egg membrane during fertilization). 2) Mid-piece: It contains the mitochondria which supplies the sperm energy needs to move. 3) Tail: It moves with whip-like movements back and forth to propel the sperm towards the egg. The sperm have to reach the fallopian tube in order to fertilize the egg.  Oogenesis Oogenesis is the process of creation of an ovum (egg cell) from the female germ cells. It takes place inside the ovaries. It begins in the fetal period and then arrested till puberty. The process is continuous with another process called folliculogenesis  FERTILIZATION Fertilization is the process whereby two sex cells (gametes) fuse together to create a new individual with genetic potentials derived from both parents. Have two separate activity: 1. Combining of genes derived from the two parents. 2. Creation of new organisms. Thus The first function is: Transmit genes from parents to offspring. The second is : initiate reactions in the egg cytoplasm that proceed development.  CLEAVAGE (SEGMENTATION) Cleavage a) transformation of the zygote into a population of cells known as cleavage cells or blastomeres. The process takes place through successive mitotic divisions, which starts soon after fertilization, and extends until formation of a multicellular structure called the blastula. b. Blastula formation: By the end of the period of cleavage, the embryo becomes organized as a hollow sphere or blastula composed of about 200 cells, and enclosing a fluid-filled cavity, which is the blastocoel. The cells are structured as a single layer of epithelium known as the blastoderm in which the larger cells (macromeres) are located in the vegetal hemisphere and the smaller ones (micromeres) in the animal hemisphere.  GASTRULATION The process of gastrulation begins as soon as cleavage is completed. Defined as the period of development that concerns with the arrangement and movement of the embryonic cells to the various regions of the embryo, where they will be used. Some cells move to the inside of the embryo, while others remain on the outside surface, with the consequence of the change of the external shape of the embryo. Invagination Gastrulation begins with vegetal area flattening to form the vegetal plate. Invagination is a bending inward of the vegetal plate one-third into the blastocoel which decreased in size gradually till it completely disappeared. The archenteron is the new cavity formed by this invagination. The archenteron is the primitive gut; the blastopore is the opening to the outside. Micromeres at animal pole becomes the ectoderm (outer layer). Macromeres at the vegetal pole becomes the endoderm (inner layer). Differentiation: The gastrula rotates in a manner that the animal pole considers the anterior part of the embryo and the vegetative pole is the posterior part. The dorsal part of the ectoderm is flattened to form the neural plate. Below it, the endoderm is differentiated to form the notochordal plate. At the two sides of the notochordal plate the endoderm is differentiated into two mesodermal plates which will form the muscles and skeleton. The rest of the endoderm will form the gut