Embryology PDF

# Embryology ## Embryology Embryology is the study of the early development of living organisms. All animals and plants have the capacity to reproduce new individuals to insure the survival of their kind. ## Reproduction * **Asexual:** A single organism splits, buds, fragments to give rise to...

# Embryology ## Embryology Embryology is the study of the early development of living organisms. All animals and plants have the capacity to reproduce new individuals to insure the survival of their kind. ## Reproduction * **Asexual:** A single organism splits, buds, fragments to give rise to two or more individuals. * **Sexual:** Reproduction involves the fusion of two gametes: a male gamete (sperm) fuses with a female gamete (egg) to form the zygote or fertilized egg. ### Asexual Reproduction It needs one parent, i.e. a single organism which splits, buds or fragments to give rise to two or more individuals. #### Types * **Splitting:** The organism splits to two individuals (e.g. Paramecium) * **Budding:** The organism produces a small bud which grows on the mother's body then brakes down to a new organism (e.g. Hydra) * **Fragmentation:** The organism fragments to more than two organisms forming a colony (e.g. yeast). ### Sexual Reproduction * It involves the fusion of two gametes, a male gamete (sperm) fuses with a female gamete (ovum) to form a ZYGOTE or fertilized egg. * In this case the chromosomes (n) of sperm join with the chromosomes (n) of ovum forming the chromosomes (2n) of the zygote. * Zygote undergoes embryonic developmental stages which involve the division of zygote cell to a large number of somatic cells containing diploid number of chromosomes (2n) that form the body of an embryo. * Before we study these developmental stages we have to study the formation of gametes containing haploid number of chromosomes (n) by a process called **GAMETOGENESIS.** ## Gametogenesis It is the production of gametes by parents which involves the reduction of number of chromosomes to haploid sets of chromosomes in the resulting gametes. * The body of an organism contains many organs which all are formed of somatic cells (2n) even the gonads (testis) in male and (ovary) in female, the cells lining these organs are diploid in chromosomes. * The origin of the haploid germ cells is those diploid cells lining the testis and the ovary (the primordial germ cells) which undergo a number of changes in two processes called spermatogenesis (production of sperm) and oogenesis (production of ovum) through three phases in each process. ### Spermatogenesis Includes three phases: multiplication, growth, and maturation phases. * **Multiplication Phase:** The primordial germ cells undergo repeated mitotic divisions to produce a large number of spermatogonia, each containing diploid number of chromosomes 2n. * **Growth Phase:** Spermatogonia grow in size to form larger cells known as primary spermatocytes, which contain diploid number of chromosomes 2n. * **Maturation Phase:** Each spermatocyte undergoes meiosis which consists of two successive nuclear division known as the first and second meiotic divisions. * The first division results in the secondary spermatocytes, the nuclei contain the n chromosomes. * The second division each secondary spermatocyte is divided into two spermatids, each having n chromosomes. * Thus four spermatids are produced from each primary spermatocyte. * The spermatids undergo certain changes known collectively as metamorphosis, to form spermatozoa. ### Spermatozoa The diagram shows a spermatozoa and its parts: * **Acrosome** * **Nucleus** * **Basal body** * **Mitochondria** * **Axoneme** * **Head** * **Midpiece (Body)** * **Principal Piece** * **Endpiece** * **Tail** ### Types of Sperms The shape of a sperm can vary depending on the species: * **Amphioxus:** Long, thin, and pointed with a slightly curved head * **Fisher's Rod:** Long, thin, and rod-shaped * **Septilia:** pear-shaped, with a small head and a thick tail * **Ramals & Male Birds:** Small, round heads with a short, thick tail ### The Testis Testis are formed of a large number of seminiferous tubules in which countless number of sperms is formed: * Walls of seminiferous tubules are lined with the germinal epithelium 2n formed of cubical primordial germ cells from which sperms in are produced. ### The Ovary The ovary of female is lined with the germinal epithelium consisting of primordial germ cells 2n which undergo oogenesis to produce mature ovum in finally The diagram shows a transverse section of the ovary of a cat: * **Tunica albuginea ovarii** * **Cortex** * **Surface epithelium** * **Medulla** * **Hilus** * **Primary follicles** * **Secondary follicles** * **Secondary (vesicular) follicle** * **Primordial follicles**