Exercise 44

Embryology

Portland Community CollegeBI 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

18

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

22

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 baby This 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