General Embryology 1 Lecture 3 PDF

General Embryology 1 Lecture 3 Prepared by Fatma M. Abdelmaksoud Dept. of Anatomy and Embryology Fac. of Vet. Medicine Assiut University 2024-2025 Developmental anatomy is the science dealing with the study of the grow...

General Embryology 1 Lecture 3 Prepared by Fatma M. Abdelmaksoud Dept. of Anatomy and Embryology Fac. of Vet. Medicine Assiut University 2024-2025 Developmental anatomy is the science dealing with the study of the growth and differentiation of the newly formed organism from the single fertilized egg stage (zygote) till the old age. Embryology is confined to the prenatal period in mammals [before birth] and the pre-hatching period in birds. The prenatal period in mammals includes: 1- Zygote or fertilized ovum period: It begins at the time of fertilization and lasts for 2 weeks in human. At which all cells resulting from cell division of the zygote are similar. 2- Embryo period :It begins from the end of the 2nd week to the end of 8th week. In human or 6th week in horse & cattle. At which organs of the body are differentiated. 3- Fetus period : at which the fetus enlarges both in length and weight, this stage continues till birth.  Stages of Development: 1- Gametogenesis: means the formation of the male and female gametes (sperms & ova), which have haploid number of chromosomes [1 N]. 2- Fertilization: means the fusion of male and female gametes to form the zygote that contains diploid number of chromosomes [2 N]. 3- Cleavage: It means the division of zygote to form a mass of cells called blastula. 4- Gastrulation : It means the arrangement of the cells of the blastula in 3 germinal layers (ectoderm, mesoderm & endoderm) to form a structure called the gastrula. 5- Organogenesis : It means the formation of different organs of the body. This takes place during the embryonic period. 6- Growth and histological Differentiation: It means the increase in length and weight of the developing organs with complete histological differentiation of the different tissues. This takes place during the fetus period  1. Gametogenesis - Gametogenesis is the first stage in the sexual reproduction. - It means the formation of male gametes (sperms) or female gametes (ova), therefore it is named spermatogenesis in males & oogenesis in females. (NB : The male and female gametes are the most highly specialized and species-specific cells in the body). - The mother cells of both male and female gametes are derived from the primordial germ cells (P.G.Cs.). The P.G.C. appear very early during embryonic life in the wall of the yolk sac and migrate to become localized in the growing undifferentiated gonads [testis in males or ovary in females] - Within the gonads, the P.G.C. differentiates into spermatogonia [mother cells of spermatozoa] or oogonia [mother cells of ova].  Spermatogenesis - It means the formation of sperms. - It takes place within the seminiferous tubules of the testis. - It occurs simultaneously from the time of puberty till the old age. - It includes 2 stages : A. Spermatocytogenesis Germ cell Spermatids (immature sperms) B. Spermiogenesis: Maturation & differentiation Spermatids Sperms (immature sperms) (mature sperms) A. Spermatocytogenesis: - It starts by spermatogonia and ends by the formation of spermatids. - It consists of 3 periods : Multiplication, Growth & Maturation. 1- Multiplication : - During which the spermatogonia Spermatogonia divide by mitosis into spermatogonia Mitotic divisions type A & type B. spermatogonia spermatogonia - Spermatogonia type A enters a period type A type B of growth while spermatogonia type B Growth ,after a short period of rest, divides into primary spermatogonia spermatogonia type A & type B. spermatocyte (2N). type A type B 2- Growth: During which the spermatogonia type A increases in size to form the primary spermatocyte (2N). 3-Maturation: The period of maturation is characterized by 2 maturation or reduction [meiotic] divisions, where the diploid number of chromosomes (2N) is reduced to a haploid number (1N). The first maturation division is meiotic, through which the primary spermatocyte (2N) divides into 2 secondary spermatocytes (1N). In the 2nd maturation division each secondary spermatocyte divides mitotically into 2 spermatids (1N). B. Spermiogenesis: - It means transformation of the non-motile spermatids into a motile spermatozoa through the following changes : - The nucleus of the spermatid shrinks and forms the head of the sperm. - The Golgi apparatus forms the acrosomal cap at the anterior end of the head. - The mitochondria form the mitochondrial sheath in the middle piece of the sperm. - The centriole is located just caudal to the nucleus and detaches the axial filament (flagellum or tail). - The cytoplasm forms only a thin layer around the nucleus and the rest is discharged. Abnormal sperms : giants, dwarfs, double heads, double tails, very long, very Short Giant short.  Oogenesis - It means the formation of ova. - It takes place in the cortex of ovary. - It consists of 3 stages: multiplication, growth & maturation. I- Multiplication: - It occurs only in the prenatal life and stops after birth. - The Oogonia cells (which originate from the P.G.C) proliferate by mitotic cell division and form egg nests. II- Growth : This stage starts at or just before birth. Oogonia develop into larger primary oocytes which stay in rest until puberty. III- Maturation: - The stage of maturation is characterized by 2 maturation or reduction [meiotic] divisions, where the diploid number of chromosomes (2N) is reduced to a haploid number (1N). - In the first maturation division (meiotic) the primary oocyte divides into a secondary oocyte (1N) & the first polar body. - In the second division both secondary oocyte and first polar body divide, the secondary oocyte gives ootid (Ovum) and the second polar body while the first polar body gives 2 polar bodies. The end result is an ootid and three polar bodies. (This division is usually completed just after fertilization). NB: Due to the unequal division of the cytoplasm between the secondary oocyte and the polar body, the resulting ovum becomes large cell, containing most of the cytoplasm  1 Polar body Secondary Oocyte n 2 Polar body Ootid(Ovum)  Spermatogenesis Oogenesis Definition Formation of sperms Formation of ova Site of occurrence within the seminiferous tubules In the cortex of the ovary of the testis Stages A. Spermatocytogenesis 1. Multiplication 1. Multiplication 2. Growth 2. Growth 3. Maturation 3. Maturation B. Spermiogenesis: Maturation stage Two successive miotic First miotic division completed just before the ovulation divisions Second miotic division completed after fertilization Beginning time from the time of puberty Prenatal period Ending time Old age Menopause period End results 4 spermatids 1 Ootid+3polar bodies  Ovarian Follicles - The developing ovum becomes surrounded by a fibrocellular capsule, formed from the surrounding ovarian tissue. The whole structure is named the ovarian follicle. - The female mammals born with fixed number of follicles. -Primordial follicle: each germ cell becomes surrounded by a single layer of flattened follicular cells. The germ cell is now called primary oocyte. - Primary follicle :the primary oocytes enlarge in size and follicular cellsbecome cuboidal - Secondary follicle.:-The follicular cells divide forming several layers and secrete a homogeneous material on the surface of the oocyte ( the Zona Pellucida). - Further growth of the secondary follicle leads to the appearance of irregular spaces between the follicular cells. These spaces join each other to form a large follicular cavity (Follicular Antrum). - Tertiary, Graffian or mature follicle: The follicular cavity enlarges and assumes a crescent shape and filled with the follicular fluid which is rich in estrogen secreted by the follicular cells.. The oocyte becomes located in an eccentric position and surrounded by mass of cells called Cumulus Oophorus. The follicular cells are granular and arranged in what is called the Stratum Granulosum. The whole follicle is covered by a sheath of connective tissue, the follicular theca. The theca is divided into: 1- Theca interna: is cellular and secretes estrogen secreting cells. 2- Theca externa: which is more fibrous in nature  The single flat follicular cells The follicular cells divide forming Consists of one primary oocyte surrounding the primary oocytes several layers and secrete a surrounded by a single layer of flat enlarge in size and become cuboidal. homogeneous material on the surface of follicular cells the oocyte ( the Zona Pellucida) -The oocyte becomes located in an eccentric position and surrounded by mass of cells called Cumulus Oophorus. -The whole follicle is covered by a sheath of connective tissue, the follicular theca. The theca is divided into: 1- Theca interna:. 2- Theca externa: Graffian or mature follicle  Ovulation It means the release of the ovum from its follicle after being mature. As in the following steps : - When the follicle becomes mature, it approaches the surface of the ovary and appears as a tense, blister-like elevation. At the apex of this elevation, there is a clear avascular spot named the stigma. - The continuous pressure of the increasing follicular fluid on the weak stigma causes its rupture and the ovum with the zona pellucid and corona radiate escapes. - The infundibulum of the uterine tube, approaches the ovary to catch the escaped ovum. Rupture of the stigma is affected by three factors: 1- The increased pressure in the follicular cavity. 2- Hormonal control of LH [Luteinizing Hormone]. 3- The ischemia which occurs at the stigma due to lack of blood. Types of ovulation: - Spontaneous : not linked to the coitus (in most animals). - Induced : occurs after coitus (in camel, cat & rabbit).  Corpus Luteum - After ovulation, the cavity of the ruptured follicle becomes filled with blood clot forming the Corpus haemorrhagicum (CH). - The cells of the granular layer and theca interna become containing yellow particles (lutein cells). They multiply and fill the collapsed follicular cavity. The newly formed structure is named the yellow body (Corpus luteum(CL)). Fate of corpus luteum : - If pregnancy does not occur the corpus luteum degenerates and forms a white body known as the Corpus albicans (CA). - If conception (pregnancy) occurs the Corpus luteum remains to save pregnancy by secreting progesterone. - Shortly after parturition, the corpus luteum degenerates and transformed into corpus albicans.  Estrous cycle (heat) - The estrous cycle in animals (menstruation in human) - Comprises; ovarian cycle & uterine cycle. -The ovarian cycle comprises the growth and maturation of the ovarian follicles, ovulation and the formation of the Corpus luteum. - The uterine cycle includes the morphological changes which occur in the mucous membrane and glands of the uterus to be ready for receiving the fertilized ovum. According to the repetition of estrous cycle, animals are classified into : 1-Polyestrous animals : The estrus cycle is repeated all over the year at regular intervals as in sow & cow. (every 21 days in cow) 2-Seasonal polyestrous animals: The estrous is repeated many times during certain season of the year as in cat, sheep & mare. 3-Seasonal monoestrous animals: One estrous cycle occurs in the season as in bitch. 4-Monoestrous animals: One estrous cycle occurs in the year such as in wild animals as in deer & wild pig. The Phases of the estrous cycle: 1. Proestrus (follicular phase ): during this phase some follicles matures and become ready for rupture. 2. Estrus : the period during which ovulation (rupture of follicles) occurs. In this period signs of heat appear on the female (she becomes receptive for male) 3. Metestrus (Luteal phase): the period in which the corpus luteum is formed. 4. Diestrus (period of rest) : It is a period of rest which precedes the proestrus of the next cycle in poly-estrous animals. (This period is followed by a very long period of quiescence [Anestrus] in mono-estrous animals)