Fertilization After breeding, the sperm and egg unite to form a zygote. After the nuclei have fused the zygote becomes an embryo. Embryos do not have any anatomical form.

Fertilization Fetus is the potential offspring still within the uterus. Conceptus is the product of conception which includes: Embryo Membranes placenta

Development of zygote After fusion of nuclei, the zygote undergoes a series of mitotic division. The first division creates a two celled embryo. These cells are called blastomeres. Each blastomere undergoes divisions yielding 4, 8, and 16 daughter cells. Twins are derived from blastomeres of a 2-celled embryo that divide independently.

Development When a solid ball of cells is formed and blastomeres can’t be counted, the embryo is called a morula. During this stage, fluid fills the cell and creates a cavity called a blastocoele. When a distinct cavity is recognized, the embryo is called a blastocyst. After growing, the blastocyst starts to crack the membrane and now becomes a free-floating embryo in the uterus.

Development

Development As the hatched blastocyst continues to grow, it forms three different layers known as the endoderm, ectoderm, and mesoderm. Ectoderm CNS Sense organs Mammary glands Sweat glands Skin Hair Mesoderm Circulatory Skeletal Muscle Reproductive tracts Kidneys Urinary ducts Endoderm » Digestive » Liver » Lungs » Pancreas » Thyroid gland » Other glands

Break time

Placenta The placenta is a transient organ of metabolic interchange between mother and offspring. Also produces a variety of hormones. Placentas have 4 different membranes: Allantois Yolk sac Amnion Chorion

Placental Membranes Yolk Sack In birds to nourish embryo In mammal a source of blood cells and primordial germ cells Amnion Non-vascular, fluid filled Fluid produced by fetus Protective cushion Ruptures at birth

Placental Membranes Allantois Blood vessels Fuses with chorion Brings blood vessels to chorion Chorion Outermost membrane Attachment to mother

Chorion Allantochorion (Chorioallantois) Allantois Cavity Cotyledon Amniotic Cavity Amnion Yolk Sack Yolk Sack

Placenta The functional unit of the placenta is the chorionic villus. Placentas are classified according to their chorionic villi. The four classifications are: Diffuse Zonary Discoid Cotyledonary

Placenta Diffuse: Pig & horse Characterized by having uniform distribution of the villi covering the surface. Microcotyledons are the functional interface between mother and offspring

Placenta Zonary: Dogs & cats A prominent region of exchange that forms a zone around the middle of the conceptus.

Placenta Discoid: Rodents & primates Characterized by having one or two distinct adjacent discs. These discs interface with the endometrium and provide nutrient and metabolic waste exchange.

Art

Placenta Cotyledonary Ruminants (cows, sheep, goats, deer, etc.) Characterized by having numerous, button-like structures called cotyledons. Cotyledons are the placental unit for blood vessels and connective tissue. A placentome consists of a fetal cotyledon and a maternal cotyledon.

Re-cap

Break Stand up and shake off your hands

Parturition A complex cascade of events Triggered by the fetus Separated into three stages: 1. Initiation of contractions 2. expulsion of fetus 3. expulsion of membranes

Stage 1 “Labor” Initiated by fetus Fetus becomes stressed due to lack of Nutrients Oxygen Space This stress leads to a series of hormone secretions initiating contractions and dialation.

Stage 1 comparison Cow – 2 to 6 hours Dog 6 to 12 hours Horse – 1 to 4 hours Alpaca – 2 to 6 hours Human – 8+ hours

Stage 2 Expulsion of the fetus As the hormones increase so do the contractions As the contractions increase, the feet and head of the fetus put pressure on the membranes causing them to rupture. As the fetus enters the birth canal, they become hypoxic causing stronger contractions. Positive feedback loop

Stage 2 comparison Cow – 30 to 60 min Dog – 6 hours Horse – 12 to 30 min Alpaca – 5 to 90 min Human – 2 hours

Stage 3 Membrane expulsion In most species, stage three quickly follows stage 2. Easy process Retained placenta Can cause disease, need to be removed manually

Stage 3 comparison Cow – 6 to 12 hours Dog – 15 minutes Horse – 1 hour Alpaca – 45 to 180 min Human – 1 hour or less

Dystocia Difficult birth Difficulties usually occur in the second stage due to the fetus not positioned correctly. Other causes Size of fetus Improper rotation (head comes first) multiple births

Puerperium The period after birth in which the reproductive tract returns to non-pregnant conditions. 4 events of puerperium Myometrial contractions Endometrial repair Resumption of ovarian function Elimination of bacteria