Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 15

Cellular mitosis following fertilization results in transformation of the single‐cell zygote into a cluster of 12–16 cells. With continued cellular proliferation, this morula becomes a blastocyst, containing a fluid‐filled cavity. The cells of the blastocyst will form both the embryo proper and the extraembryonic tissues (i.e., amnion and chorion). At this early stage, the embryo is a bilaminar disc, consisting of hypoblast and epiblast. This embryonic tissue divides the blastocyst space into the amniotic cavity (adjacent to epiblast) and the yolk sac (adjacent to hypoblast).

Gastrulation (formation of the mesodermal germ layer) begins during day 10 of gestation in the dog (day 7 in the mouse; days 15–20 in the human). The primitive streak forms as a longitudinal groove within the epiblast (i.e., future ectoderm). Epiblast cells migrate toward the primitive streak, where they invaginate to form the mesoderm. This forms the three classic germ layers: ectoderm, mesoderm, and endoderm. Gastrulation proceeds in a cranial‐to‐caudal progression; simultaneously, the cranial surface ectoderm proliferates, forming bilateral elevations called the neural folds (i.e., future brain). The columnar surface ectoderm in this area now becomes known as the neural ectoderm.

As the neural folds elevate and approach each other, a specialized population of mesenchymal cells, the neural crest, emigrates from the neural ectoderm at its junction with the surface ectoderm. Migration and differentiation of the neural crest cells are influenced by the hyaluronic acid‐rich extracellular matrix. This acellular matrix is secreted by the surface epithelium as well as by the crest cells, and it forms a space through which the crest cells migrate. The neural crest cells migrate peripherally beneath the surface ectoderm to spread throughout the embryo, populating the region around the optic vesicle and ultimately giving rise to nearly all the connective tissue structures of the eye (Table 1.3).

Table 1.1 Sequence of ocular development in human, mouse, and dog.

Human (approximate post‐fertilization age)	Mouse (day post‐fertilization)	Dog (day post‐fertilization or P = postnatal day)	Developmental events
Month	Week	Day
1	3	22	8	13	Optic sulci present in forebrain
	4	24	9	15	Optic sulci convert into optic vesicles
			10	17	Optic vesicle contacts surface epithelium Lens placode begins to thicken
		26			Optic vesicle surrounded by neural crest mesenchyme
2	5	28	10.5		Optic vesicle begins to invaginate, forming optic cup Lens pit forms as lens placode invaginates Retinal primordium thickens, marginal zone present
		32	11	19	Optic vesicle invaginated to form optic cup Optic fissure delineated Retinal primordium consists of external limiting membrane, proliferative zone, primitive zone, marginal zone, and internal limiting membrane Oculomotor nerve present
		33	11.5	25	Pigment in outer layer of optic cup Hyaloid artery enters through the optic cup Lens vesicle separated from surface ectoderm Retina: inner marginal and outer nuclear zones
			11.5	29	Basement membrane of surface ectoderm intact Primary lens fibers form Trochlear and abducens nerves appear Lid fold present
	6	37	12		Edges of optic fissure in contact
			12	30	TVL present Lens vesicle cavity obliterated Ciliary ganglion present
		41	12	32	Posterior retina consists of nerve fiber layer, inner neuroblastic layer, transient fiber layer of Chievitz, proliferative zone, outer neuroblastic layer, and external limiting membrane
			17	32	Eyelids fuse (dog)
	7				Anterior chamber beginning to form
			12.5	40	Secondary lens fibers present
		48	14	32	Corneal endothelium differentiated
	8	51			Optic nerve fibers reach the brain Optic stalk cavity is obliterated Lens sutures appear Acellular corneal stroma present
		54		30–35	Scleral condensation present
	9	57	17	40	First indication of ciliary processes and iris
				—	EOMs visible Eyelids fuse (occurs earlier in the dog)
	10			45	Pigment visible in iris stroma Ciliary processes touch lens equator Rudimentary rods and cones appear
				45–1P	Hyaloid artery begins to atrophy to the disc
3	12			—	Branches of the central retinal artery form
4				51	Pupillary sphincter differentiates Retinal vessels present
—				56	Ciliary muscle appears
				—	Tapetum present (dog)
5				40	Layers of the choroid are complete with pigmentation
6				—	Eyelids begin to open, light perception
				1P	Pupillary dilator muscle present
7				1–14P	Pupillary membrane atrophies
				1–16P	Rod and cone inner and outer segments present in posterior retina
				10–13P	Pars plana distinct
9				16–40P	Retinal layers developed
				14P	Regression of pupillary membrane, TVL, and hyaloid artery nearly complete Lacrimal duct canalized

Table 1.2 Sequence of ocular development in the cow.

Ocular part or event	Gestational size (mm)
Lens
Optic vesicle	6
Lens placode	6
Optic cup and lens placode	10
Separation of lens vesicle from surface ectoderm	10
Primary lens fibers	15
Lens vesicle cavity disappears	24
Completion of lens capsule	50
Secondary lens fibers	58
Perilenticular vascular mesoderm
Extension of primary vitreous (hyaloid artery) to lens	15
TVL	33
Disappearance of posterior lenticular vascular network	410
Disappearance of TVL	410
Iris
Major arterial circle of iris	90
Iris reaches front of lens	200
Pigment in stroma	200
Sphincter muscle	410
Dilator muscle	410
Ciliary body
Ciliary processes	125
Ciliary processes touch lens equator	230
Pars plana (distinct)	200
Pars plana fully developed	410
Choroid
Choroidal net in posterior pole	33
Choroidal net throughout	50
Outermost large choroidal vessels	40
Choriocapillaris	90
Pigmentation of choroid	90
Retina – posterior third
Inner and outer nucleated zones	10
Multilayer outer cup of optic vesicle forms single cells	20
Nerve fiber layer	20
Optic nerve well formed	24
Inner/outer neuroblastic layers	14
Transient layer of Chievitz	14
Inner plexiform layer	180
Retinal vessels	180
Tapetal cells	410

Table 1.3 Embryonic origins of ocular tissues.

Neural ectoderm	Neural crest
Neural retina	Stroma of iris, ciliary body, choroid, and sclera
RPE	Ciliary muscles
Posterior iris epithelium	Corneal stroma and endothelium
Pupillary sphincter and dilator muscle (except in avian species)	Perivascular connective tissue and smooth muscle cells
Striated muscles of iris (avian species only)
Bilayered ciliary epithelium	Meninges of optic nerve
Orbital cartilage and bone
Connective tissue of the extrinsic ocular muscles
Endothelium of trabecular meshwork
Surface ectoderm	Mesoderm
Lens	Extraocular myoblasts
Corneal and conjunctival epithelium	Vascular endothelium
Lacrimal gland	Schlemm's canal (human)
	Posterior sclera (?)

It is important to note that mesenchyme is a general term for any embryonic connective tissue. Mesenchymal cells generally appear stellate and are actively migrating populations with extensive extracellular space. In contrast, the term mesoderm refers specifically to the middle embryonic germ layer. In the eye, mesoderm probably gives rise only to the striated myocytes of the extraocular muscles (EOMs) and vascular endothelium. Most of the craniofacial mesenchymal tissue comes from neural crest cell.