Early Embryogenesis

Gametogenesis

diploid germ cells undergo chromosomal and morphological changes to form mature gametes

Meiosis

the cell division process through which animals produce gametes in the gonads

Spermatogenesis

male gametogenesis; production of sperm in testis

Oogenesis

female gametogenesis; production of ova in the ovaries

Steps of Spermatogenesis

1. spermatogonia divide by mitosis

2. primary spermatocyte divides by meiosis I into 2 diploid secondary spermatocyte

3. secondary spermatocyte divide by meiosis II into 4 haploid spermatids

4. spermatids become mature sperm cells through cellular differentiation

Cellular Differentiation

the final step of gametogenesis of the male

Steps of Oogenesis

1. oogonium divides by mitosis into primary oocyte

2. primary oocyte divides by meiosis I into a diploid secondary oocyte and polar body

3. secondary oocyte divides by meiosis II to form 1 haploid ovum and 1 polar body

4. Ovum undergoes cell growth and becomes mature egg cell

Polar Body

a product of meiosis I and II in oogenesis that has less cytoplasm than the ovum or secondary oocyte so it degenerates

Cell Growth

final step of oogenesis

Products of Spermatogenesis

4 daughter cells

Products of Oogenesis

1 daughter cell and 2 polar bodies

Acrosome

cap that covers the sperm head containing hydrolytic enzymes that break through outer layer (jelly coat) of the ovum

-initiates developmental changes

Head Abnormalities

-crater defect

-tapered head

-ruffled acrosomes

-knobbed acrosomes

Tail Abnormalitites

-coiled tail

-double midpiece

-folded tail

-detached head

Fertilization

fusion of gametes to create a new individual with genetic potentials derived from both parents

Activities During Fertilization

-combining of genes derived from the two parents

-creation of new organisms

Functions of Fertilization

1. transmit genes from parent to offspring

2. initiate reactions in egg cytoplasm that proceed development

-also restoration of diploid number of chromosomes reduced during mitosis

Steps of Fertilization

1. sperm contacts egg

2. sperm on nucleus enters egg/regulation of entry

3. sperm and egg nuclei fuse

4. egg becomes activated

--developmental changes begin

Acrosomal Process

allows nucleus to penetrate cytoplasm

Cortical Granules

released into perivitelline space when plasma membranes of sperm and egg fuse, stop other nuclei from entering egg

Polyspermy

when multiple sperm nuclei enter 1 egg that results in early embryo loss

Zygote

fertilized egg, sperm and egg -> 1 cell

Embryo

early period of development, no distinct anatomical structures formed

-multiple cells

-does not resemble anything

Conceptus

product of conception; consists of embryo, extra-embryonic membranes during implantation

-fetus and placenta later

Fetus

a potential offspring, alive in uterus, recognizable as a member of the species

-more advanced than embryonic

30d

After how many days is an embryo considered an "advanced embryo" in cattle?

Step 1 of Embryonic Attachment

development of embryo while confinement of zona pellucida

Step 2 of Embryonic Attachment

hatching of embryonic cells (blastocyst) from ZP

Step 3 of Embryonic Attachment

formation of extra-embryonic membranes (placenta)

Step 4 of Embryonic Attachment

maternal recognition of pregnancy

Zona Pellucida

called ZP, outer shell restricting embryonic growth

Blastomere

each cell of the embryo

Zygote to 2-cell

mitotic division of zygote into 2 blastomeres

-no cell growth, division of cytoplasm

2 cell to 4 cell to 8 cell embryo

blastomere division yielding 4 and then 8 daughter cells

-no cell growth

-blastomeres are 1/4 and 1/8 of original size

Morula

a solid ball of cells (too many to count) that is usually between 16-32 cells

-blastomeres become progressively smaller but size of embryo remains (no embryonic growth)

-flushing occurs at this time

6 days

When is the morula formed in cattle (days after fertilization)?

Why is flushing not advisable until day 6 after fertilization?

at this point the embryo is a morula size, it is difficult for the initial replication to occur ex vivo

late morula stage

In which stage do the blastomere cells differentiate into two populations?

Populations in the Late Morula Stage

-embryoblast

-trophectoderm

Embryoblast

inner layer of blastocyst, becomes embryo

-gap junctions for communication

Trophectoderm

outer layer of blastocyst (OCM), forms extra-embryonic membranes

-tight junctions for fluid accumulation

-forms chorion and contributes to placenta

False

When the blastomeres differentiate into the embryoblast and the trophectoderm, embryonic growth occurs.

Function of Gap Junctions

allows for cell to cell communication

Function of Tight Junctions

alters permeability allowing for fluid accumalation

Ectoderm Layer Organs

-skin and associated glands

-nervous system

-hair

-mammary glands

Mesoderm Layer Organs

-muscle

-cardiovascular system

-skeleton

-reproductive system

-excretory system

Endoderm Layer Organs

-digestive system

-endocrine system (glands)

-lungs

-liver

-pancreas

Blastocoel

the hollow fluid filled cavity of a blastula

Blastocyst

first stage in which the embryo grows in size

-passage of time depends on species

-hatched out of ZP and is no longer constrained for growth

Vegetal Pole

the portion of the blastocyst where there is a thick layer of ICM

-grows quickly

Animal Pole

the portion of the blastocyst where there are only trophoblastic cells

-grows slowly

Gastrulation

rearrangement of the cells of the blastula to form a a3 layered embryo, the primitive gut and the archeteron

Germ Layers

-ectoderm

-mesoderm

-endoderm

Ectoderm

outermost germ layer, forms exterior tissues

-i.e. nervous system, mammary glands

Mesoderm

middle germ layer, forms structural tissues

-i.e. muscle, circulatory system, and repro system

Endoderm

innermost germ layer, forms internal tissues

-i.e. lining of digestive tract, liver, and endocrine glands

When are organs formed in a pig fetus?

35 days

When are the organs formed for a cow fetus?

45 days

Steps of Formation of the Reproductive Tract

1. migration of primordial germ cells from the yolk sac

2. sex chords develop in the gonad, paramesonephric ducts develop

3. sex evident from structures

Genital Ridge

location to which the early germ cells migrate from the yolk sac

Between the 1st and 2nd trimester

When do the male ducts and testes or ovaries and femal ducts form?

2nd trimester

When does the broad ligament form?

When does testicular descent occur in bulls and rams?

2nd trimester

3rd Trimester

When does testicular descent occur in boars and humans?

When does testicular descent occur in colts?

Between 3rd trimester and Partuition

From where does the posterior lobe of the pituitary gland develop?

neural tissue from the brain

Infundibulum

a diverticulum that develops from the floor of the brain downwards to the pituitary gland, forming the pituitary stalk

Rathke's Pouch

a diverticulum that develops from the roof of the mouth and grows upwards, forming the anterior pituitary gland

Adenohypophysis

anterior lobe of the pituitary gland; glandular

Neurohypophysis

posterior lobe of the pituitary gland; neural

Why do the anterior and posterior pituitary glands have different functions?

they originate from different kinds of tissues

Sphenoid Bone

a bone that grows around both lobes of the pituitary gland forming a protective cavity called the sella turcica

Sella Turcica

the protective cavity in the sphenoid bone where the pituitary glands resides

Formation of the Placenta

occurs after hatching and involves massive growth of conceptus (fetus and extra-embryonic membranes)

-used by fetus to attach to uterus

-discarded after birth

-consists of 4 membranes

Filamentous or Threadlike

What is the structure of the placenta in pigs, sheep, and cows?

What is the structure of the placenta in horses?

Spherical

4 Membranes of the Placenta

-chorion

-allantois

-yolk sac

-amnion

Aminion

develops from the trophoblast and the endoderm

-filled with amnionic fluid

-serves to protect the embryo from mechanical perturbations

Chorion

develops from trophoblast and endoderm

-gives rise to the fetal portion of the placenta

-becomes outermost layer

-does not contain blood vessels

Allantois

develops from the splanchnic (visceral) mesoderm

-arises from hindgut

-fuses with the chorion to form allantochorion (chorioallantois)

-becomes fetal portion of the placenta

-contains blood vessels

Yolk Sac

develops from the mesoderm

-arises from the midgut

-site of primordial germ cells

-regresses as allantois develops

-contains blood vessels

chorion, amnion, amnionic, yolk sac, allantois, chorion, chorioallantoic

When the leading edges of the _______ fuse, a complete sac, called the ______, surrounds the embryo and forms the ______ cavity. The _____ _____ continues to regress as the _______ expands, making contact with the ______. The allantois and the chorion eventually fuse to form the ________________ membranes

Vascularity of the Placenta

only the allantois has vascularity

Allantochorioplacenta

also called chorioallantoic placenta protects the fetus and the mother

-can protect the mother from necrotic fetus

Placenta Classifications

-diffuse

-cotyledonary

-zonary

-discoid

Diffuse Placenta

entire surface of the allantochorion involved in formation of placenta

Cotyledonary Placenta

multiple attachments formed, patches of allantochorion with endometrium

-fetal= cotyledons; mother= caruncles; complex= placentome

Zonary Placenta

placenta takes form of a complete, incomplete, or band surrounding the fetus

Discoid Placenta

a single placenta is formed and is discoid in shape

Detached Placenta

level of detachment determines severity- may take baby early to ensure it gets proper nutrients

When does the developing embryo reach the uterus?

Between day 2 and 5

Which process must be prevented in order for the embryo to establish itself?

Luteolysis

2 Events that must take place for Maternal Recognition of Pregnancy

1) PGF2A synthesis/secretion inhibited

2) Progesterone maintained

Caveats to Maternal Recognition of Pregnancy

-conceptus provides timely (before luteolysis) biochemical signal

-signals presence to dam and enables pregnancy contribution

What will happen if the embryo does not deliver a biochemical signal to its dam in a timely manner?

Luteolysis will occur, progesterone will decline, and early embryo death will occur

Maternal Recognition of Pregnancy in Ewes and Cows

Blastocyst secretes trophoblastic protein (an interferon)

- prevents PGF2A from reaching corpus luteum

Between what days must the fetus release interferons in sheep and cows to cause maintenance of pregnancy?

day 13 and day 21

Ovine Interferon tau

Interferon in sheep that enters the uterine lining, cause protein release, blocks oxytocin receptors, and blocks PGF2A function

Bovine Interferon tau

Interferon in cows that enters the uterine lining, cause protein release, blocks oxytocin receptors, and blocks PGF2A function

Method of Action- Interferons

1. bind to endometrium

2. inhibit endometrial oxytocin receptor synthesis

3. pulsatility of PGF2A does not changes but is drained by veins (does not reach CL)

4. Interferons promote protein synthesis crucial to embryo survival