Embryonic Development Ch 28

Human Development Vocabulary Pregnancy- an event that occurs from fertilization to birth Conceptus- developing offspring Gestation period- time from last menstrual period until birth (~280 days) Embryo- from fertilization through week 8 Fetus- from week 9 through birth

Stages of Development Pre-Embryonic Development Fertilization Cleavage Gastrulation Organogenesis Fetal Development

Fertilization Facts! Oocyte is viable for 12-24 hrs after ovulation Sperm are able to fertilize oocyte for up to 48 hrs In order for fertilization to occur, coitus must take place 48 hrs before-24 hrs after ovulation Fertilization usually occurs when oocyte is 1/3 way down fallopian tube

Fertilization and Preembryonic Development

Fertilization Corona radiata Zona pellucida

Ovum and Sperm

sperm head

Fertilization

Fertilization first polar body metaphase of second meiotic division

Fertilization Mitotic spindle with chromatids 1st polar body Pellucid zone Perivitelline space Cell membrane of the sperm Kinocilium Nucleus (compact) of the sperm Proximal centrosome of the sperm

Fertilization 1st polar body Nucleus of the sperm Proximal centrosome of the sperm 2nd polar body (being formed) Remainder of the mitotic spindle

Fertilization polar bodies Sperm pronucleus Oocyte pronucleus four hours after impregnation. A nucleic membrane forms around both pronuclei. (In the pronuclei themselves a doubling of the DNA occurs at the decondensing chromosomes.) Sperm pronucleus Oocyte pronucleus Centrosome brought in by the spermatozoon

Approach of Pronuclei Sperm pronucleus Oocyte pronucleus The paternal and the maternal pronuclei move towards each other with the help of microtubules, which begin to be formed immediately after impregnation, i.e., by the penetration of the spermatozoon. They grow in a star-like pattern out of the paternal centrosome directly beside the forming paternal pronucleus (= formation of an aster made of dozens of microtubules). The microtubular proteins themselves arise from the cytoplasma of the oocyte. Sperm pronucleus Oocyte pronucleus Paternal centrosome "Inner bodies" Maternal astral microtubule

Approach of Pronuclei While the microtubules of the aster pull the pronuclei together in the center of the oocyte, the synthesis of the DNA is taking place in the pronuclei. This duplication takes roughly 12 hours. The pronuclei grow in size in this time.

Formation of the Zygote After the two pronuclei have come as close together as they can, no merging of them takes place, i.e., a fitting together of the chromosomes of the two pronuclei within a single nucleic membrane does not happen. It is much more accurate to say that the nucleic membranes of both pronuclei dissolve and the chromosomes of both align themselves on the spindle apparatus at the equator. The zygote, the first cell of a new organism with an individual genome (2n4C) is created by the alignment of the maternal chromosomes together with the paternal ones on a common spindle apparatus. Nucleic membranes of the pronuclei dissolve Microtubules of the mitotic spindle

Formation of the Zygote The mitotic spindle divides the chromosomes that have just been brought together into the two first cells of the embryo. This proceeding towards the two-cell stage occurs on average between 22 and 26 hours after fertilization

Cleavage

Cleavage

Implantation

Implantation Blastocyst floats for 2–3 days Implantation begins 6–7 days after ovulation Trophoblast adheres to a site with the proper receptors and chemical signals Inflammatory-like response occurs in the endometrium

Endometrium Uterine endometrial epithelium Inner cell mass Trophoblast Blastocyst cavity Lumen of uterus (a) Figure 28.5a

Implantation Trophoblasts proliferate and form two distinct layers Cytotrophoblast (cellular trophoblast): inner layer of cells Syncytiotrophoblast: cells in the outer layer lose their plasma membranes, invade and digest the endometrium

Endometrial stroma with blood vessels and glands Syncytiotrophoblast Cytotrophoblast Inner cell mass (future embryo) Lumen of uterus (c) Figure 28.5c

Implantation The implanted blastocyst is covered over by endometrial cells Implantation is completed by the twelfth day after ovulation

syncyticotrophoblast cytotrophoblast Amniotic cavity hypoblast Yolk sac epiblast

The Formation of Primary Germ Layers

Gastrulation Primary Germ Layers Ectoderm Endoderm Mesoderm Gastrulation rearranges the embryo into a triploblastic gastrula. Primary Germ Layers Ectoderm Endoderm Mesoderm

Fates of the Primary Germ Layers Ectoderm hair, nails, epidermis, brain, nerves Mesoderm notochord (in chordates), dermis, blood vessels, heart, bones, cartilage, muscle Endoderm internal lining of the gut and respiratory pathways, liver, pancreas

Gastrulation In mammals, embryo develops from inner cell mass A thickening of cells forms the primitive streak In the middle of the primative streak, cells sink inward forming the primative groove Cells entering the sides of the primative groove become mesoderm and endoderm Cells that stay on outside become ectoderm

Gastrulation

Implantation

Extraembryonic Membranes Amnion: epiblast cells form a transparent sac filled with amniotic fluid Provides a buoyant environment that protects the embryo Helps maintain a constant homeostatic temperature Allows freedom of movement and prevents parts from fusing together Amniotic fluid comes from maternal blood, and later, fetal urine amnion

Extraembryonic Membranes Yolk sac: a sac that hangs from the ventral surface of the embryo Forms part of the digestive tube Source of the earliest blood cells and blood vessels Yolk sac

Extraembryonic Membranes 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: helps form the placenta Encloses the embryonic body and all other membranes Allantosis Chorion

Embryo Pronuclei fuse Male & female pronuclei approach 2-cell stage 1st cleavage 30 hrs after fertilization

Embryo 8-cell stage 2 days 16-cell stage 2.5 days Morula 2.5-3 days

Embryo Blastocyst 4-5 days 4 weeks 5-6 mm 3-4 weeks Neurulation 4-5 mm

Embryo 7 weeks 14-17 mm 6 weeks 10-11 mm 7-8 weeks 30 mm

Phylum Chordata Subphylum Vertebrata

Neurulation A: Embryonic disc accomplished gastrulation - ectoderm thickens B: Neural plate forms neural folds and neural groove

Neurulation C: Neural folds close D: Neural tube detached from surface ectoderm

Embryonic Folding The rapid growth of the neural tube is thought to cause the embryo to fold and bring the developing primitive heart caudal to the brain plate. Early in the third week of development, the germ disk has the appearance of a flat oval disk and is composed of two layers: the epiblast and the hypoplast. The first faces the amniotic cavity and the latter faces the yolk sac. A primitive groove, ending caudally with the primitive pit surrounded by a node, first appears at approximately 16 days of development and extends half the length of the embryo. The primitive groove serves as a conduit for epiblast cells that detach from the edge of the groove and migrate inwards toward the hypoblast and replace it to form the endoderm. After the endoderm is formed, cells from the epiblast continue to migrate inwards to infiltrate the space between the epiblast and the endoderm to form the intraembryonic mesoderm. After this process is complete, the epiblast is termed the ectoderm

Differentiation of primary germ layers into tissues and organs. Organogenesis Differentiation of primary germ layers into tissues and organs.

Development of Reproductive Organs