Exercise 44 Embryology Portland Community College BI 233

2 Zygote A zygote is formed when the nucleus of the sperm and ovum combine to create a cell with 23 pairs of chromosomes. (prior to the start of cleavage) Fertilization occurs in the ampulla of the uterine tube within 24 hours after ovulation (oocyte dies after 24 hours)

3 From Fertilization to Implantation

4 Formation of the Morula Cleavage: Rapid mitotic cell division (without cell growth) that subdivides the cytoplasm of the zygote Cleavage produces daughter cells called blastomeres The zona pellucida remains around the entire group of cells.

5 Morula After 3 days of cleavage, a solid ball of cells has formed that resembles a mulberry (similar to a Marion berry) This Morula contains 16 or more cells

6 Blastocyst By the fourth or fifth day the morula develops into a a fluid-filled hollow sphere called a blastocyst Blastocyst : contains 3 parts Trophoblast: The outer layer of cells Take part in placenta formation Inner cell mass: A group of cells clustered at one side of the blastocyst Becomes the embryo Blastocoele : The fluid filled cavity

7 Blastocyst

8 Implantation By day six or seven the trophoblast will adhere to the endometrium with the inner cell mass side facing the uterine wall The trophoblast cells proliferate and form two distinct layers Syncytiotrophoblast: Cells on the perimeter touching the endometrium. Cytotrophoblast: Cells on the interior of the trophoblast that retain their cell boundaries

9 Implantation

10 Implantation

11 Syncytiotrophoblast At the point of contact between the trophoblast and the endometrium, the dividing cells lose their plasma membranes This creates a syncytium in which there is a layer of cytoplasm containing multiple nuclei The syncytiotrophoblast erodes a path through the uterine endometrium by secreting hyaluronidase The implanted blastocyst will eventually be covered over by endometrial cells

12 Implantation Viability of the corpus luteum is maintained by human chorionic gonadotropin (hCG) secreted by the trophoblast hCG prompts the corpus luteum to continue to secrete progesterone and estrogen Between the second and third month, the placenta assumes the role of progesterone and estrogen production

13 Chorionic Villi Chorionic villi are extensions of the cytotrophoblast covered with syncytiotrophoblast that extend into the maternal blood filled spaces (lacunae) Maternal blood will exchange nutrients through these villi that will eventually contain embryonic vessels.

14 Embryonic Disc The inner cell mass organizes into an oval sheet with two layers (the embryonic disc) Ectoderm: The layer closest to the attachment of the trophoblast Endoderm: The layer closest to the blastocoele

15 Amniotic Cavity The inner cell mass will separate from the trophoblast during implantation The ectoderm cells will migrate and form a fluid-fill chamber (the amniotic cavity) Amnion: Mesodermal cells will soon create the outer second layer of the amnion The fluid and membranes will surround and cushion the developing embryo and fetus

16 Yolk Sac Begins as the endodermal cells spread around the outer edges of the blastocoele. The yolk sac is completed as the mesoderm cells migrate around the pouch. Site of embryonic blood cells and gonadal stem cells

17 Gastrulation This is the term for the process by which a third layer of cells called the mesoderm, forms between the ectoderm and endoderm. Primitive streak : raised dorsal groove that establishes the longitudinal axis of the embryo Gastrulation sets the stage for organogenesis; the formation of body organs

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19 Primary Germ Layers Serve as primitive tissues from which all body organs will be derived Endoderm: Epithelial lining of GI & lower respiratory tract All ducts entering the GI tract Urinary bladder Ectoderm: Nervous system Epidermis Lining of mouth, and anus Sense organs such as eyes

20 Primary Germ Layers Mesoderm: Muscle Bone Cartilage Blood Dermis and hypodermis Kidneys, ovaries, testes Lining of body cavities

21 Embryonic Membranes: Chorion and Allantois Allantois: a small outpocketing at the caudal end of the yolk sac Structural base for the umbilical cord Becomes part of the urinary bladder Chorion Becomes the embryonic contribution to the placenta Derived from trophoblast & mesoderm lining it Contains the chorionic villi

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23 Umbilical Cord Connects the embryo/fetus to the placenta Contains 2 arteries and 1 vein

24 2 Arteries 1 Vein

25 Placenta The placenta is fully formed and functional by the 3rd month of pregnancy Chorion of embryo & stratum functionalis layer of uterus Chorionic villi extend into maternal blood filled intervillous spaces: maternal & fetal blood vessels do not join & blood does not mix Diffusion of O2, nutrients, wastes Stores nutrients & produces hormones Barrier to microorganisms, except some viruses AIDS, measles, chickenpox, poliomyelitis, encephalitis

26 Endocrine Placenta Human Chorionic Gonadotropin (hCG): Maintains the integrity of the corpus luteum and promotes the continued secretion of progesterone Human Placental Lactogen (hPL) and Placental Prolactin: Prepare the mammary glands for milk production

27 Endocrine Placenta Relaxin: which relaxes connective tissue of pelvis and cervix Suppresses the release of oxytocin by the hypothalamus Progesterone and Estrogens: Maintain the endometrial lining

28 Placenta This side faces the babyThis side touches the uterus

29 Gestation Fetal period: Begins at week 9 All major organs have been established This is the time of growth Gestation: 40 weeks measured from the first day of the uterine cycle (day one of menstruation) Actually only 38 weeks from fertilization

30 The End